The main steps in the progress of modern solid waste containment systems, ..... waste type at least 1 m thick compacted clay liner with hydraulic conductivity k ...
SOLID WASTE CONTAINMENT SYSTEMS Mario Manassero1, Craig H. Benson2, Abdelmalek Bouazza3,
ABSTRACT A correct waste containment philosophy consists of: (1) optimisation of the landfill location, (2) construction of high performance lining and capping systems, (3) optimisation of waste storage, (4) short and long term careful monitoring, and (5) a convenient re-use of the landfill area after closure. The geotechnical engineer has strong skills about all the aforementioned topics but in particular can effectively deal with the design, construction quality control and monitoring of the lining and cover systems, the waste storage and compaction procedure, and the foundation and improving treatments for constructions above waste deposits. Due to the large number of geotechnical aspects involved into the above listed issues, only the following topics will be discussed within the present paper: (1) lining systems for landfill base and sides, (2) covers and (3) vertical cut-off walls. Each of these topics has been developed referring to their background, recent developments and new trends. In particular, the fundamentals of updated design procedures will be illustrated with particular reference to conceptual and practical modelling and assessment of related input parameters. Moreover the main literature references will be provided. At the end of the paper, a section, devoted to containment systems for polluted subsoil and abandoned landfills, has been developed in order to deal with one of the most interesting areas as far as the present progress and future advancements are concerned. 1.0
INTRODUCTION
The main steps in the progress of modern solid waste containment systems, within the last decade, can be resumed in a rough chronological order as follows: • recognition of the importance of construction procedures on the field scale performances of compacted clay liners (CCLs) and the consequent set up of guidelines for a correct and effective installation (Daniel, 1989, 1993; Jessberger, 1994a; and Daniel & Koerner, 1995). • Current and correct employment of composite barriers consisting of mineral liners (CCLs) or geosynthetic clay liners (GCLs), placed in close contact with a geomembrane (GM). The advantages of these systems have been discussed thoroughly by Giroud & Bonaparte (1989a, b); Daniel (1993); Bonaparte (1995); and Giroud (1997). • Introduction, and in some cases standardisation, of laboratory and field tests for the evaluation of barrier components and monitoring systems for assessing full scale liners performances (Daniel, 1989; Daniel & Trautwein, 1994; Rowe et al., 1995a; and Well, 1997). • Recognition of different types of potential waste deposits stability problems, looking in particular at sliding surfaces involving interfaces of composite barrier systems (Mitchell et al., 1990; and Seed et al, 1990, Stark et al., 1998). • Recognition of the importance of compatibility, diffusive transport and sorption phenomena on the overall performance of barrier systems (Shackelford & Daniel, 1991a, b; Mitchell, 1993; and Rowe et al., 1995a). • Recognition of the importance of biogas migration from landfills and the consequent beginning of a series of research works on gas-barrier interaction (Williams & Aitkenhead, 1991; Jarre et al., 1997; Aubertin et al., 2000 and Bouazza & Vangpaisal, 2000). 1
Professor, Politecnico di Torino, Dept. of Georesources and Territory, Corso Duca Degli Abruzzi 24, 10129 Torino, Italy 2 Professor, University of Wisconsin, Dept. of Civil & Environmental Engineering, 2214 Engineering Hall, 1415 Engineering Drive, Madison, Wisconsin, USA 3 Senior Lecturer, Dept. of Civil Engineering, P.O. Box 60 Monash University, Melbourne, Victoria 3800, Australia
• Recognition of the importance of natural and manmade attenuation layers below waste deposits in order to reduce pollutants impact on groundwater (Daniel, 1993; Rowe et al., 1995a; and Rowe & Badv, 1996). • Recognition of the role played by the deformations and settlements of subgrade layers on the performance of mineral barriers (Jessberger, 1994a; August et al., 1997; and Koerner & Daniel, 1997). • Introduction of geosynthetic clay liners (GCLs) as pollutant containment barriers (Daniel & Boardman, 1995; and Rowe 1998). • Use (in some cases after appropriate modifications) of traditional geotechnical construction techniques to install barriers around polluted subsoils (e.g. simple and composite slurry walls, grouted and jet grouted bottom barriers, reactive diaphragm walls, etc. see Rumer & Ryan, 1995; Rumer & Mitchell, 1995; and Manassero et al., 1995). • Introduction of new barriers types for cover systems (e.g. capillary barrier and natural buffer barriers see Khire et al., 1997; Melchior, 1997; Daniel et al., 1998; and Benson, 1999). • Introduction of performance design and related risk analyses overcoming the prescriptive design procedures. The performance design has become feasible and rather reliable thanks to the goals of the aforementioned theoretical and experimental work and practical experiences. These have led to a substantial improvements in terms of both modelling techniques and knowledge about the different input parameters that are necessary in order to simulate the actual behaviour of waste containment systems (USA, 1994; Estrin & Rowe, 1995; and Manassero et al. 1998). This paper will summarize and develop some of the main geotechnical topics related to modern waste containment systems. Most of the available literature will be quoted in order to give the possibility to gain more insight on all the aspects illustrated in the following sections. In order to remain within an acceptable number of pages, and after a short summary of the current updated practice, the paper will focus on the recent advances in the field of theoretical and experimental research. At the same time and when it is possible, attempts will be made to discuss and promote the use of knowledge advances both in the professional practice and in the regulatory field. 2.0
ROLE AND FUNCTION OF LINING SYSTEMS IN LANDFILLS
In order to approach the design of a containment system for a landfill in a proper way it is important to discuss the role and functions of this fundamental component for a safe deposit of polluting materials. There are various philosophies to approach the design and management of a landfill as pointed out by Rowe (1995a). One (passive) is to provide a cover system as impermeable as possible and as soon as possible after the landfill has ceased operating, so as to minimise the generation of leachate. This approach has the benefits of minimising both the amount of leachate that must be collected and treated, and the mounding of leachate within the landfill. It also has the disadvantage of extending the contaminating lifespan. With low infiltration, it may take decades to centuries before the field capacity of the waste is reached and full leachate generation to occur. An alternative philosophy (active) is to allow as much infiltration as would practically occur. This would bring the landfill to field capacity quickly and allow the removal of a large proportion of contaminants (by the leachate collection system) during the period when the leachate collection system is most effective and is being carefully monitored (e.g. during landfill construction and, say, 30 years after closure). The disadvantages of this approach are two-fold: Firstly, larger volumes of leachate must be treated; this has economic consequences for the proponent. Secondly, if the leachate collection system fails, a high infiltration will result in significant leachate mounding. On the basis of the previous considerations, it should be apparent that the performance of a confinement system is influenced not only by the deposit geometry, type of waste, climatic conditions and lining materials, but also, and in a very important way, by landfilling time history and management activity. The introduction (and in some cases the reintroduction) of performance design and its increasing acceptance by the geoenvironmental community has led to a rethink on how landfill containment systems design should be tackled. It is obvious that with this new trend the design engineer must take into account numerous parameters such as: transport parameters and service life of the mineral barriers, drainage layers, geosynthetics, and the main features of the waste in order to be able to estimate the leachate quality and production over the landfill activity and post closure period. A list of the main parameters that should be defined for evaluating, in a reliable way, the pollutant migration scenario from a landfill, is presented in
Table 1. Today, most of these parameters are rather well assessable whereas, some are presently investigated through research programs and therefore their estimation cannot still be considered fully reliable. In this paper particular attention will be paid to the illustration of the research in progress, devoted to the improvement of the knowledge on design parameters that significantly influence the performance of modern landfill containment systems. Table 1 : Input parameters for the evaluation of confinement barriers effectiveness (after Manassero, 1997) SUBJECTS
MINERAL BARRIERS
COLLECTION DRAINAGE LAYERS
GEOMEMBRANES
WASTE LEACHATE CONTAMINANTS
NATURAL SUBSOIL
CLIMATE CONDITIONS MANAGEMENT MAINTENANCE AFTERCARE TARGETS OBJECTIVES
2.1
PARAMETERS hydraulic conductivity, field capacity, dispersion-diffusion, sorption capacity, mechanical behaviour, compatibility service life hydraulic conductivity, service life (clogging) hydraulic conductivity, diffusion, sorption capacity, service life total mass, density, water content, hydraulic conductivity, field capacity, soluble fraction, actual release, decay, dilution, attenuation present hydrogeological conditions and possible future changes, hydraulic conductivity, trasmissivity, storage coefficient, sorption capacity, dispersivity, mechanical behaviour precipitation, evapotranspiration disposal time history, leachate collection rate history, capping time history limit pollutant concentrations, site vulnerabilities
Types of lining systems and definition of basic components The main sections of the paper deal with the bottom and side lining systems and covers. The basic components of these confining systems are illustrated in Figure 1. The main components of the bottom lining systems are the drainage layers or leachate collection and removal system (LCRS), the manmade barrier and the attenuation layer or geological barrier. Apart from the geological barrier (also listed in Figure 1) which of course can only be of the natural type, the other layers can be foreseen using natural or amended soils and/or geosynthetics and related products.
BOTTOM LINER
SIDE LINER
M IN E R A L C O M P O N E N TS
P O L YM E R IC CO M P O N E NTS
Filter-tran sition layers
Filter-tran sition layers
D rainage layers
D rainage layers
P rotection layers
D rainage pipes
B arrier layers
P rotection layers
A ttenuation layer (G eological barrier)
B arrier layers
COVER SYSTEM
M IN E R A L C O M P O N E N TS
P O LYM E R IC C O M P O N E NTS
M IN E R A L C O M P O N E N TS
P O LYM E R IC C O M P O NE N TS
Filter-transition layers
Filter-transition layers
E rosion control layer (Top soil, cobbles, vegetation layer)
E rosion control layer (B io-grid, geo-cell)
D rainage layers
D rainage layers
B iotic (anim als) barrier (C obbles)
R einforcem ent elem ents (G eogrid, geotextiles)
P rotection layers
D rainage pipes
Frost-desiccation con trol layer
B iotic (anim als) barrier
B arrier layers
P rotection layers
D rainage layer
D rainage pipes
A ttenuation layer (G eological b arrier)
B arrier layers
B arrier layer
Drainage layer
G as collection layer
B arrier layer
G as collection layer
Figure 1 : Components of solid waste containment systems The most relevant advancements in the past few years, referring to manmade barriers, are related to the use of geosynthetic clay liners (GCLs), the modelling of lining system performances (performance design) and the estimation of related parameters via laboratory and field tests and data from monitoring systems. Future trends will concern probably innovative liners such as soil-biopolymer/biofilm barriers. Lining systems for steep to very steep slopes are also receiving lot of attention which stem from the fact that landfills are designed more and more to accommodate two factors: Land saving and increase of landfill capacity. There is also the fact that old quarries, especially those formerly used to mine sand and gravel are still favourite spots for landfills, even though these are usually important sources of groundwater recharge. In The use of GCLs, slurry walls (SW) and compacted clay liners reinforced by the use of mixed cement (Manassero & Pasqualini, 1993) and geocomposites (Hohla, 1995), in order to avoid stability problems, seems to become very popular. On the other hand, the drainage and filter layers is not a problem for steep slope, whereas the protection layer can be very important in particular when geomembranes (GM) and GCLs are employed. In some cases it can be a good practice to use some types of wastes as a protection layer such as tires and big sacks containing waste in a powder form. One of the most active and interesting theoretical and practical research fields within solid waste containment systems concerns the covers. Alternative final cover design considering GCLs, monolithic earthen layers and capillary layers as an alternative to the traditional composite barriers are just few examples of the possible developments in this sector. 2.2
Water balance of landfill During the waste storage activities the performance of landfills is governed by the side and bottom liners. Most of the infiltration due to rain water goes through the waste and reaches the pumping cap through the drainage layers. Therefore, the efficiency of the leachate collection and removal system (LCRS) is of fundamental importance for the subsoil safety versus the pollutant migration. The long term landfill performance is governed by the capping system, since the amount of advective flow through the bottom barriers is mainly a function of the water balance of the top layers of the landfill (Figure 2). A popular method used to estimate landfill water balance is the simulation using the HELP computer model (Schroeder et al., 1994). There is also another procedure proposed by Oweis & Khera (1995). Both these evaluation methods are characterized by simplified assumptions related in particular to the unsaturated
hydraulic seepage through the capping systems.
4000 ACTIVE FILLING
3500
COVER INSTALLED
CELL CLOSED
LCRS FLOW RATE ( lphd )
3000 2500 2000 1500 MSW LANDFILL (PENNSYLVANNIA)
1000 500
Jan-94
Jul-93
Jan-93
Jul-92
Jan-92
Jul-91
Jan-91
Jul-90
Jan-90
Jul-89
Jan-89
Jul-88
0
DATE
Figure 2 : Leachate generation rates at a modern domestic landfill in Pennsylvania (USA). Average annual precipitation at the landfill site is 1.0 m/a (from Bonaparte, 1995). Modeling in a reliable manner the unsaturated flow through the capping system is a fundamental but very difficult task (Khire et al., 1995, Shackelford, 1998), some aspect related to these phenomena will be illustrated in the section devoted to cover systems. 2.3
Mass balance of the contaminants The role played by the active and passive management in terms of pollutants mass balance is illustrated in Figure 3 where the relative concentration and the contaminant flux in the aquifer underlying a landfill are shown as a function of time.
Figure 3 : Contaminant concentration and mass balance underneath a landfill
With an active management of the landfill (as described earlier) and in the case of persistent pollutants, the peak concentration in the groundwater can be higher but the total mass of pollutant escaping from the landfill lower than in the case of passive management. On the other hand, it must be also considered that, in the case of active management, a higher amount of pollutant is caught by the LCRS and removed, outside the landfill, toward leachate treatment plants. In any case, these aspects must be considered in the phase of regional planning of landfill systems since the contaminant mass balance should involve the whole regional waste management system rather than only a single landfill. In the case of quickly degrading pollutant both concentration and cumulative contaminant flux can be lower for passive management than for active management even though the degradation of the pollutant can be much faster in wet than in dry conditions corresponding to the active and passive management respectively. On the basis of the previous observation, it comes out the need of a landfill design based on case by case analysis (Rowe, 1998a), which must consider, not only the lining system features, subsoil, climate conditions and waste types, but also the different aspects of landfill management, the waste mass and distribution within the deposit and the time history of the cover construction. 2.4
Functions, performances, modelling and parameters Referring to the three basic components of a containment system for waste deposits i.e. bottom, side and top liners it is possible to briefly summarise their main functions as follows: • the bottom liner must reduce as much as possible the advective and diffusive contaminant migration toward the underlying vadose soil and/or aquifer. The performance of the bottom barriers is fundamentally governed by the following parameters: (1) field hydraulic permittivity and diffusivity, compatibility, sorption capacity and service life. On the other hand the performance of filters and drainage layers are fundamentally governed by the capacity to avoid clogging that in turn is also influenced by the type of waste and landfill management (in terms of temperature and water content of waste). On the basis of direct field observations, it has been realized that the clogging trend in the LCRS is reduced by increasing the seepage velocity of the leachate. Therefore the higher the hydraulic conductivity (HC) and the gradient (i.e. slope of the landfill bottom) the higher the service life and the efficiency of the drains can be. • The drainage component of the side liner is far less demanding than for the bottom liner due to the generally high hydraulic gradients along the side slopes. On the other hand the side lining must be able to control the migration into the vadose zone of the biogas located above the leachate table inside the landfill. This task is particularly delicate since most of the accidents, occurred in the past few years and concerning municipal solid waste landfill, are related to biogas escape (Williams & Aitkenhead, 1991, Kjeldsen & Fisher, 1995). In the case of the capping systems one of the most important challenge of research in the next future is to make available reliable models and related parameters allowing effective design of the liners with respect to gas migration control (Benson, 1999; and Bouazza & Vangpaisal, 2000). • Capping systems are loaded by a lot of tasks, among them it is possible to include: rise of ground elevations in stable conditions, promote good surface drainage, separate waste from animals and insects, separate waste from plant roots, minimize infiltration of water into waste, restrict gas migration or enhance gas recovery (some sites) and serve other specific functions related to post-closure developments on the landfill area. It is obvious to note that the tasks of a cover system are much more numerous than for a base and side liners considering also that in the long term the water and pollutant balance of almost all the landfills are governed by the capping performance. The only advantages of the covers, in comparison with the other two basic components of solid waste containment systems, is that they are reparable rather easily and the monitoring of their performances is much simpler. It is expected to see in the near future that most of the potential advancement will occur in the field of capping systems 2.5
Environmental impact evaluation (risk analysis) Among the other possible types of environmental impact, the minimisation of pollution due to leachate and gas migration in the subsoil represents one of the major goal for the geoenvironmental engineer. The evaluation of the performance of a landfill considering its location, the characteristics of the surrounding
environment, and referring in particular to the existence of a potential pollutant target, is defined as "risk analysis". The potential source of contaminant is the landfill, moreover there should be a path of the pollutant toward the target. Figure 4 shows the conceptual scheme that is considered in a current type of risk analysis for a polluted site or a landfill (Connor et al., 1997; and Di Molfetta & Aglietto, 1999). The barrier and the attenuation layers are generally located between the concentration C0 and Cx reported in the same figure.
Figure 4 : Conceptual scheme for a contaminant impact evaluation and risk analysis (modified from Di Molfetta & Aglietto, 1999) In the absence of a liner (i.e. abandoned landfills or polluted sites) the ratio between C0 and Cx is simply due to a geometrical dilution. The further concentration reduction in the groundwater downstream the landfill is a function of the thickness of the aquifer, its seepage velocity, dispersivity and sorption capacity (Domenico, 1997). Finally the concentration at the point of exposure (the target POE) can be assessed via an advection, dispersion reaction model of pollutant migration in the natural subsoil. Given the concentration at the POE it is possible to see if it complies with regulations or also to evaluate for example the individual excess lifetime cancer risk (ELCR) or other health potential damages and therefore it is possible to evaluate the feasibility of the landfill and the main features of the lining system. The risk analysis carried out for the Barricalla landfill, the most important Italian deposit of industrial waste, can be considered as a good example for showing the importance of a reliable model of the barrier performance (Manassero et al., 1997). The location of Barricalla landfill is very delicate because it is close to a series of wells supplying irrigation water. The cross section of the lining system of this landfill is shown in Figure 5. The scenario of dilution and attenuation factor obtained by a simplified model that considers a constant concentration source of contaminant (i.e. infinite mass) and does not take into account diffusive migration and sorption capacity of the bottom liner but only advective migration is shown in Figure 6. Referring to these results, phenols concentration in groundwater(c=1.65 µg/l) does not comply with the allowable limit given by the regulation (c=0.5 µg/l). The analysis has been repeated improving the model of simulation of the barrier performances and taking into account the finite amount of waste. The results are shown Figure 7 where it is possible to observe that a more accurate model can point out the fundamental role of the landfill liner, the leachate collection system and the finite mass of pollutants. Therefore a correct evaluation of the barrier performance is fundamental to obtain a reliable risk analysis referred to the subsoil safeguard against pollution.
Figure 5 : Vertical section of the final lining system of Barricalla landfill (after Manassero & Pasqualini, 1993)
Figure 6 : Distribution of dilution-attenuation-factor (DAF) after 90 days obtained with the MT3D model (after Manassero et al., 1997)
Figure 7 : (a) Experimental data and simulation model results of phenols concentration in the leachate vs time; (b) Phenols concentration vs time in the aquifer below the landfill bottom barrier; (c) cumulative flux vs time of phenols leaking from the top and the base of the bottom lining system (Manassero, 1997)
Given the importance, for a reliable risk analysis, of a correct modeling of the mineral barrier, reinforces the importance of a reliable assessment of input parameters. Due attention is not always paid to the evaluation of these parameters referring in particular to low permeability materials for which the accuracy of the tests and the appropriate interpretation procedures are fundamental in order to obtain reliable results. 2.6
Rules and regulations As the consequences of improper waste disposal are better understood, there is a trend toward increasing governmental regulation and stronger nation minimum standards. However, regulations for landfill lining systems can vary considerably from country to country and from state to state. The current minimum containment requirements normally imposed by Environmental Protection Authorities in various places in the world require a double liner system with leachate collection and leak detection systems for hazardous waste and a single composite liner with a leachate collection system for non hazardous waste. The more stringent liner, cover, and monitoring requirements, coupled with strong public resistance to new waste facilities and escalating costs, are leading to new emphasis on recycling, waste minimization, incineration, etc.. Because of land availability and costs, government regulations, and public opposition, it has become almost impossible in some geographic areas to obtain permits for new landfills (sometimes termed "green field" landfills because these sites are currently undeveloped). This has led to increased reliance on expansion of existing landfills and/or costly shipping of waste to distant landfills. Landfill expansions may take two forms : vertical expansion (building on top of an old landfill) or lateral expansion (building beside an existing landfill). Another result of the current regulatory and public atmosphere is that many of the companies involved in waste disposal commonly exceed minimum lining requirements in an effort to mollify public opposition and to remain in compliance with possible future regulatory requirements. The design of a landfill barrier system is generally based on either a prescriptive standard or a performance standard. Most of the present regulations around the world belong to the prescriptive design standard type, only recently some countries have introduced performance standards as an alternative to the requirements for a minimum liner system profile (USA and Canada). It is known that the nature of liner design varies, both within and between countries, depending on waste management strategies and practices, public concern and political will. In general, the type of a waste containment facility is dictated by the type of waste to be disposed of. Waste classification can be broadly grouped into three types: • inert • municipal or domestic and industrial non hazardous • industrial hazardous Landfill classification generally divides into inert sites, municipal and industrial non hazardous sites and industrial hazardous sites. Nowadays, most of the industrialised countries have their own classification
system. Typical European waste categorisation is summarised in Table 2. It is obvious to expect that design specifications will vary from country to country and also according to the classification of wastes. Table 2 : Typical waste categorisation in Europe (from Bouazza & Van Impe, 1998) Belgium
Denmark
France
Italy
Holland Germany
United Kingdom
1- Class I industrial waste 2- Class II municipal waste 3- Class III inert waste NB: other wastes, pesticides, toxic and radioactive wastes are not accepted in landfills 1-Hazardous wastes 2-Controlled wastes (sanitary and non hazardous) 3- Inert waste NB: All hazardous wastes are incinerated or treated prior to landfilling 1- Class A (special industrial wastes, SIW), metallurgical wastes, drilling muds wastes, etc. 2- Class B (special industrial wastes, SIW), ashes from MSW incineration, paint industry wastes, etc. 3- Class C (special industrial wastes, SIW) wastes allowed on a case by case basis 2- Class D municipal and commercial putrescible wastes 3- Class E inert and construction industry wastes 1- Municipal waste 2- Toxic harmful and special wastes 3- Hospital wastes 4- Construction and inert wastes 1- Waste material act-non toxic 2- Chemical waste materials act-toxic wastes 1-Class I inert wastes 2-Class II wastes with higher pollutants contents 3- Hazardous solid waste landfills 1- Household, municipal waste 2- Commercial wastes 3- Industrial wastes. 4- Inert wastes
For several decades, the only practical means of waste containment was the construction of a hydraulic barrier consisting of a layer of compacted clay. Another alternative which emerged in the early 80’s was the use of single layers of geomembranes as liner material for landfill sites. In this case, the geomembrane is laid over the native foundation soil. Over the geomembrane, there is a layer of gravel with perforated pipes for leachate collection and removal. A layer of filter soil is placed between the solid waste material and the gravel. The above concepts emphasised more the attenuation of the pollution migration than its containment. Nowadays, most of the modern landfills are constructed with a composite liner system, in which a geomembrane is placed over a compacted clay layer (CCL). Leakage rates through well constructed composite liners are far lower than through geomembranes or compacted clay liners (CCLs) alone. Indeed, with a clay liner alone seepage takes place over the entire area of the liner. On the other hand, it is difficult to install an individual geomembrane without defects on a given site and over a large area. In this case, the fluid will move freely through the defects. With a composite liner, the fluid will still move freely through the defects but then will encounter an impervious medium. Thus, leakage through a defect in a geomembrane is minimised or reduced if a low permeability soil (e.g., clay or silty clay) is placed beneath. Test and theoretical analyses carried out by Giroud and Bonaparte (1989a, b) have shown that the leakage rate through a composite liner, due to a hole in the geomembrane, is several orders of magnitude less than the leakage rate through the same hole in the geomembrane alone. As stated earlier, design specifications change from country to country, a summary of typical liner requirements is provided in Table 3 (see also Manassero et al., 1998). In Germany, the liner for MSW landfills consists of several lifts of earthen material with a minimum thickness of 0.75 m and a thick geomembrane. The geomembrane is protected by a thick geotextile with high puncture resistance. The
composite base system is also used in Austria (Brandl, 1992) with a drainage layer ≥ 50 cm and in France. However the French regulation still considers the possibility of using a single geomembrane even if there exists a natural underlying formation of unrestrictive permeability value provided that the formation thickness exceeds 5 m. In Japan, a double geomembrane liner with a drain layer in between is usually used as lining system for MSW landfills. This is due to the fact that clay is not available in most part of Japan (Tsuboi et al., 1997). However, other options are also available (GCLs, sand-cement-bentonite mix, or asphaltic layer). In Australia, landfill legislation is mainly the responsibility of the States and therefore liner design tends to be state-specific. As a consequence there have been differences in the rate at which liner design has developed in each state. Also, the variation in geology and hydrogeology in different parts of Australia has influenced the way in which landfill regulation has developed and the way in which liner design is practised. Generally, the requirement is to have a 0.6 m to 0.9 m clay liner for putrescible waste landfills. Inert waste landfills may also require lining depending on the hydrogeology of the site. In the United States, the minimum liner consists of a single composite liner (geomembrane/compacted clay) with a leachate collection system. In this case the drainage layer is typically required to have a hydraulic conductivity ≥ 1 cm/s, the geomembrane must be at least 0.76 mm thick. The European GLR (ETC 8, 1993) recommends also the same design but with a slight difference concerning the minimum thickness of the compacted clay and protection of the geomembrane. The new European recommendation requires for inert waste type at least 1 m thick compacted clay liner with hydraulic conductivity k ≤ 10-7 m/s. As far as municipal waste is concerned, the prescription is for a simple compacted clay liner with a minimum thickness equal to 1 m and hydraulic conductivity k ≤ 10-9 m/s. In developing countries the design of lining systems is still in its infancy. Very few data or information are avalaible in literature. However, Ashford & Husain (1996) were able to collect a comprehensive list of liner designed in Thailand. The liners were part of landfills currently in operation, being contructed, or scheduled for construction. A summary of their findings are presented in Table 4. Only few of the landfills have a lining system in line with internationally accepted standards. It is worth nothing that the lining system at the Pathum Thani site consists of compacted clay placed over a geomembrane. It seems in this case that the excessive stress induced by the compaction of the clay has not been taken into account in the design. However, it must be kept in mind that, until only a few years ago, open dumping was the standard practice. Giroud et al. (1995) queried whether a developing country should adopt or adapt landfill regulations and designs from countries with stringent environmental regulations. Their answer was that waiting to construct a landfill with a state of the art system in a developing country is likely to result, over time, in more pollution of the environment than accepting lower standards and immediately constructing a landfill with a liner built using local materials. Bouazza (1998) reported recently that using local materials can achieve an acceptable performance and present a viable economique alternative. However, one has to be cautious about the long term behaviour of the different liner components. More importantly, Rowe (1997) pointed out that countries without regulations should be cautious when adopting one of the existing system of regulations. He stressed the fact that a careful technical consideration should be made before deciding on the adequacy of a given design. Hazardous wastes can originate from a wide range of industrial, agricultural, commercial, and household activities. They represent a very high risk potential to the environment and population health. Most industrialised nations are finding out that they are confronted with very acute hazardous contamination problems. In Western Europe, it is estimated that 70% of hazardous waste is still deposited in landfills (WHO, 1995). The legacy of the former Soviet Union also leaves us with a myriad of soil and groundwater contamination issues to be dealt with. In the Russian Federation, it is estimated that 75 million tonnes of hazardous waste were produced in 1990. Only 18% of the waste is treated or recycled; most is deposited or stored on site or in sites not designed for storing hazardous waste, including those for domestic waste (WHO, 1995). To deal with this reality, landfilling of hazardous waste is nowadays under strict regulations. Hazardous waste landfills in industrialised countries that follow today’s restrictive regulations is a highly engineered storage/disposal facility that has been carefuly designed to reduce environmental and health risks to
Table 4 : Liner system composition for MSW landfills in Thailand (from Ashford & Husain, 1996). Province Chonburi
Hua Hin
Type of Liner Composite
Composite
Krabi
Single
Pathum Thani
Composite
Nakhon Sawan
Single
Rayong
Composite
Material Coarse sand Geotextile Geomembrane Compacted clay Coarse sand Geotextile Geomembrane Compacted clay Coarse sand Compacted clay Compacted clay Geomembrane Gravel Geotextile Geomembrane Gravel Geotextile Geomembrane Compacted clay
Thickness 0.30 m 1.5 mm 0.6 m 0.30 m
Foundation soil Medium to very dense fine sand
Fine to coarse cemented sand
1.0 mm 0.30 m 0.30 m 1.0m 0.30 m 1.5 mm 0.40 m
Very soft to soft clay and silty clay Dense silty sand
1.5 mm 0.30 m
Loose fine sand
Clayey silt
1.5 mm 0.60 m
reasonable levels. The liner system for this type of landfills is generally different from a liner for a MSW landfill and will also vary from country to country. In USA double liner systems are widely used, they are illustrated in Figure 8. They comprise, generally, a filter zone, which separates the waste from a free draining zone called the primary leachate collection and removal system (PLCRS) which in its turn lies on the primary geomembrane barrier or what is called the primary liner. In a perfect system, the leachate is stopped by the geomembrane barrier and is collected for treatment. However, studies have shown that leakage through the PLCRS can occur (Bonaparte & Gross, 1990). A leak detection system or what is called the secondary leachate collection and removal system (SLCRS) underlies the primary liner and has the same function as the PLCRS. Beneath the SLCS is the secondary liner, which is a composite liner made of a geomembrane with a compacted clay layer below it.
Figure 8 : Cross section of double liner systems
The new European recommendations prescribe that the bottom lining system for hazardous waste must consist of at least 5 m thick compacted clay liner with k ≤ 10-9 m/s. It is immediately apparent that European countries rely on mineral liners much more than on geosynthetics and related products even though allowance is given for equivalent alternative liners but the definition “equivalent” is not clearly specified. One should also point out that the European recommendations are not adopted by all European countries. Given the aforementioned worldwide scenario which considers the very different conditions of industrialized and developing countries a tentative layout of an updated and optimized regulation should include the following main items: 1. prescriptions for minimum landfill liner profiles should be given possibly referring to some typical climate, subsoil and hydrogeological reference conditions and considering different kinds and amounts of waste (see for example the recommendations for municipal and industrial waste of Italy, CTD, 1997; Brazil, CETESB, 1992; and Canada-Ontario, MOEE, 1996); 2. rules for performance design are established in terms of limitation for contaminant concentrations and fluxes in time and space domains referring to different type of location (local environment) and subsoil conditions; 3. final liner design must comply with both the aforementioned points. Only in the case of very special projects and/or environmental conditions a proposal not complying with point one can be accepted but only after a design peer review by an independent committee of experts; 4. case histories emerging with time from the procedure suggested at the third point will enhance updating and modifications of the rules of the first and the second point leading up to a possible clearing of point one once a reliable framework of experiences has been established with particular reference to the assessment of input data of the models and full scale and long-term performances of the different liner components; 5. if the structure of the lining system is different from a well proved structure which was found efficient in the practice, or the structure of the lining system does not correspond with the existing regulations, the equivalence of the new lining system is to be proved. The equivalence tests must prove the hydraulic conductivity equivalence and, more in general, the same contaminant retention capacity. Looking at the previous considerations it is clear why emphasis will be given in the following part of this paper to the parameters assessment and to the modelling of landfill liners and covers. 3.0
BOTTOM AND SIDE LINING SYSTEMS
The two basic components of a landfill bottom liners are: the drainage or leachate collection and removal systems (LDRS) and the pollutant barriers. Both systems are fundamental for a good performance of the waste containment. There are a large number of factors controlling their performance; in the following points some of these factors will be analysed looking in particular at the results of recent research and to their practical applications. 3.1
Leachate collection and removal system In order to optimise the working conditions of a barrier for pollutant containment, the internal piezometric level (i.e.the hydraulic head on the liner system) must be kept as low as possible. In the following section a short review of the basic aspects related to the layout and design of the LCRS is presented, followed by the presentation of the recent findings, from experimental research, field monitoring data and evidences from exhumations of LCRS. In this way, lessons can be learned from weak points and failures discovered in field. 3.1.1 General lay-out and design Leachate collection systems typically take the form of perimeter drains around the edge of the waste and/or underdrain systems below the waste which may include either “french drains” at some spacing (typically perforated pipes embedded in granular material), granular drainage blankets with perforated pipes at some specified spacing, and geosynthetic (geonet) drainage layers. Geotextiles are often used as filters between the waste and the drainage layer, especially when either coarse drainage material (e.g. gravel) or geonets are used to provide a drainage blanket. Key issues in the design of these systems are: (1) the need to provide adequate drainage, (2) prevent structural failure (e.g. crushing, or other pipe failure) and (3) to
minimise clogging. Of these, the greatest challenge is minimising clogging and prolonging the service life of the leachate collection system. A typical leachate collection and removal system (LCRS) recommended by environmental agencies is shown in Figure 9 and consists basically of the following:
Figure 9 : Typical LCRS layout • a drainage layer, usually 30-50 cm thick and made of granular soils resting on a lining system; when geomembranes are used in the lining system, a protective bedding layer, usually consisting of a geotextile, may be placed between the drainage layer and the geomembrane, to minimise the risk of puncture; geocomposites also may be used, particularly in steep slopes, where granular material would not be stable. • Collecting pipes within the drainage layer to direct leachate to a collecting sump; the collecting pipes should be surrounded by gravel to protect the pipe from clogging. • A filter layer above the drainage layer to minimize clogging of the drainage layer and protect it from damage by sharp objects and from equipment loads. • A leachate collection sump where leachate is collected and stored temporarily. Depending on regulations and site conditions, a double LCRS (Rowe et al., 1995a), as shown in Figure 10 is sometimes required. The first part of the LCRS is located directly below the waste and above the primary
Figure 10 : Leachate collection and leak detection systems (after Rowe et al., 1995a)
liner and usually is called the primary leachate collection and removal system (PLCRS). The PLCRS must be designed and constructed on a site specific basis. The second part of the LCRS is located between primary and secondary liners and is usually called the secondary leachate collection and removal system (SLCRS). The main purpose of this system is to determine the degree of leakage through the primary liner. Therefore, the SCLRS is sometimes called leak detection system. Although the SCLRS may collect just negligible amounts of leachate, it must be designed on the basis of the worst possible case scenario. Most of the literature on LCRS is for solid municipal waste and the recommendations noted above refer to these situations which are usually by far the most critical in comparison with almost all the industrial solid waste landfill types. The first criterion required to design a LCRS is to estimate the flow of leachate into this drainage layer. Leachate generation and flow rates may be estimated by water balance methods. However, as flow rates are largely affected by precipitation events, a decision should be made on an average yearly, monthly or daily basis. Alternatively, the flow rate may be based on a 10, 25, or 100 year design storm or a historic precipitation record. The worst possible scenario to be used for design may be that by which the design storm occurs when only one lift of refuse is in position. The final decision will depend on location, specific site considerations and local regulatory requirements. For deposits with low solids contents (e.g., red muds, etc.) the estimate of flow can be made based on analytical models of reservoir filling (variable-load, large strain consolidation models), largely independent of precipitation. Following the estimate of the leachate flow rate, the design should focus on minimising the leachate head above the liner (second criterion). The usual criterion is to allow for a permanent maximum height of leachate not overcoming the thickness of the drainage layer. It is important to avoid the contact between leachate and waste body in order not to induce temperature increase in the waste body and the related problems that will be pointed out in the following sections. Only in particular climatic conditions hydraulic heads in excess of LRCS thickness are allowed for a reduced time in the case of extreme events. The third important design criterion is the long-term functioning of LCRS. Therefore, due account should be given to physical, chemical and biological clogging of the LCRS. The proposed design methods recommended to minimise clogging are discussed below. LCRS components are usually selected to be larger or more permeable due to clogging and maintenance considerations. Recommended HCs of granular drainage layers range from 10-2 to 1 cm/s, pipe diameters are typically 15 to 20 cm and base slopes should be at least 2-3% minimum. 3.1.2 Design procedures The first input data of a LCRS is the amount of leachate per unit time and unit area that reach the landfill base. A water balance must be performed to estimate leachate generation. Although the analytical basis of water balance methods lies in the field of hydrology, the geoenvironmental engineer must have some understanding of water balance models to be able to use the leachate generation input in the design of final covers, LCRSs and base and cover lining systems. Several models are available to analyse the flow through LCRS due to leachate collection. Some of these models (Moore, 1980, 1983; and Giroud & Houlihan, 1995) assume an impermeable liner and can be used easily. Other models (Wong, 1977; Kmet et al., 1981; and Dematracopoulos & Korfiatis, 1984), are less straightforward. These models apportion the leachate flow into leakage and collection components and thus are most applicable to clay liners, as the HC of the liner is an input parameter. Apportionment models also attempt to account for the real-time variation in leachate flow rates. The models assuming an impermeable liner provide conservative flow rates and have the advantage of simplicity over apportionment models and, therefore are recommended for practical applications. The geometric variables in the design of LCRSs shown in Figure 11 are as follows: • L, the horizontal length of the leachate collection layer, at the bottom of which a pipe is located; • Tmax, the maximum leachate thickness inside the drainage layer; • β, the angle of the slope of the LCRS with the horizontal; • qi, the percolation rate, computed with water balance methods; • k, the HC of the drainage layer. Typically, two issues are addressed in the design of leachate collection systems:
Figure 11 : Definition of parameters for leachate collection models • firstly, maximum leachate thickness, Tmax, is to be determined. If the calculated Tmax is excessive, the design engineer should decrease the horizontal length, L, increase the base slope angle, β, and/or increase the HC of the leachate collection layer material; • secondly, average head of leachate on the liner is to be determined. This average head, Tavg, is used to calculate the rate of leakage through the liner underlying the leachate collection layer. Giroud & Houlihan (1995) presented a simple analytical method to calculate the thickness of drainage layers. Using an accurate but impractical numerical solution as a reference, the proposed method appears to be more accurate than other previously published methods. The first equation proposed by Giroud, as an alternative for the numerical solution, is presented below: tan 2 β + 4 qi /k − tan β Tmax 1 + 4 λ − 1 tan β = = L 2 cos β 2 cos β
(1)
where:
λ=
qi /k
(2)
tan 2 β
Values of Tmax calculated with equation (1) were up to 15% greater than the more rigorous numerical solution; thus it provides conservative values. Giroud’s modified equation presented below produces an error less than 1% compared with the rigorous solution: Tmax 1 + 4 λ - 1 tanβ =j L 2 cosβ
(3)
where:
[
]
2 j = 1 - 0.12 exp - log (8 λ/5 )5 / 8
(4)
λ is given by equation (2). The average leachate Tavg thickness on the liner can be determined with λ values using Table 5. Details on the numbers in Table 5 can be found in Giroud & Houlihan (1995). The equations presented above are for steady-state conditions that appear to take an infinite time to develop(Giroud & Houlihan, 1995). Thus, equations to compute Tmax for unsteady-state conditions also have
Table 5 q i /k
qi /k
Tavg
Tmax
tan β
Tmax
0.00 0.002 0.005 0.01
0.500 0.50 0.51 0.52
0.50 0.53 0.57 0.62
0.73 0.74 0.75 0.76
0.02 0.03 0.04 0.05
0.53 0.54 0.55 0.56
0.67 0.73 0.80 0.87
0.77 0.78 0.79 0.80
0.07 0.08 0.09 0.10
0.57 0.58 0.59 0.60
0.95 1.05 1.16 1.32
0.81 0.82 0.83 0.84
0.12 0.14 0.15 0.16
0.61 0.62 0.63 0.64
1.58 2.0 3.2 5.5
0.85 0.86 0.87 0.86
0.17 0.18 0.20 0.23
0.65 0.66 0.67 0.67
8.5 13 19 30
0.85 0.84 0.83 0.82
0.25 0.35 0.40 0.45
0.69 0.70 0.71 0.72
55 135 1000 ∞
0.81 0.80 0.79 0.785
λ=
tan β 2
Tavg
λ=
2
been developed. Giroud & Houlihan (1995) noticed that steady-state conditions provide conservative values. Equations 1 to 4 are also applicable to final cover systems. The design of leachate collection pipes includes analysis of flow in pipes but also the performance of these pipes under loads. LCRSs are gravity flow systems designed for partial flow within a perforated pipe. This situation corresponds to an open channel condition commonly analysed using the Manning equation. If the velocity derived from Manning’s equation (Gupta, 1989) is multiplied by the cross-sectional area of flow, the Manning equation can be defined in terms of flow as follows: Q= where:
1 A Rh2/3 S 1/2 n
= the flow (m3/s) = Manning’s roughness coefficient = slope of the pipe, (m/m) = hydraulic radius (m) defined as: A Rh = P where: A = is the area of the pipe section occupied by the leachate (area in flow; m2) P = the related wetted perimeter (m)
(5)
Q n S Rh
(6)
The leachate inflow (Q) to a LCRS pipe can be estimated using the water balance methods previously discussed. Once the required flow through the pipe is known, equations (5) and (6) can be solved to determine the required pipe diameter. Generally, rigid pipes absorb much of the applied overburden loads and distribute the loads within the pipe section. In contrast, flexible pipes deflect, causing much of the applied load to be transferred to the surrounding soil in an arching mechanism. Therefore, flexible pipes may perform better than rigid pipes. High density polyethylene (HDPE) pipe is currently the most commonly used pipe for LCRS applications due to the strength and chemical resistance of HDPE to leachate constituents. Polyvinyl chloride (PVC) pipe may also be used, but due to the superior chemical resistance of HDPE to leachate, HDPE pipes have become the industry standard for LCRSs. The three factors most critical for the structural performance analysis of buried flexible pipes are deflection, wall crushing and wall buckling. A comprehensive performance analysis of pipes including a discussion of these three factors is presented in Sharma & Lewis (1994). Mechanical damages of the pipes were reported in one third of a series of 29 sites surveyed in Germany by Brune et al. (1991). It appears that the pipes most prone to damage were stoneware pipes. Also leachate collection pipes located within canyon landfills can be prone to structural failures due to deformation induced by the creep of the waste body along the landfill side slopes that can induce localised folding and buckling. These evidences were discovered during a mobile camera inspection of HDPE leachate collection pipes (50 cm in diameter) carried out in one of the main MSW landfill located along the sea cost of northern Italy. Recently Witte & Schulz (1996) carried out a series of short and long term tests under static and cyclic loads on perforated HDPE and polypropylene (PP) drainage pipes with different geometry in terms of diameter and drainage holes. The tests were carried out simulating the landfill environment conditions as far as chemicals and temperatures are concerned. The main conclusion of the research can be summarized as follows: • diametrical load tests, to determine the influence of apertures on pipes deformation, showed that the hoop stiffness reduction was a maximum of 20% up to a 3.8 % of water intake area referred to the pipe’s external shaft surface. • The reduction of stress occurring at 3% hoop strain, in correspondence of 20° and 40° of temperature lay between 0.4 and 0.5 for HDPE pipes and between 0.57 and 0.59 for PP pipes. • Neither crack formations nor crack propagation was found with the various aperture geometries. • The results of 1000hours load tests in sand boxes essentially agree with the results of the diametrical load tests. Moreover, pipe design must expect a 5% hoop strain under current waste loads. • The use of reduction factors for both strength and stiffness in the determination of pipe’s stress and deformation states in the landfill may underestimate the influence of local stress increase (notch effect) around the perforation areas. Full scale field research has been planned in order to further improve the knowledge on actual behaviour of these important landfill components. 3.1.3 Filtration and clogging One of the paramount problems for the bottom drainage system of a landfill is clogging which, generally, can be due to a combination of clogging by particles, clogging due to chemical precipitation, and biofilm growth (Rowe et al., 1995b). In principles, filtration criteria against particles clogging, for the design of granular filter or geotextile, can be the same used in the current geotechnical practice and illustrated in various literature (e.g. Sherard, 1984; and U.S. Army et al., 1971, etc.). Also the experimental procedures, such as the gradient ratio test carried out in order to assess the gap-graded soils behaviour versus physical clogging, could be used. However, these design approaches may not clearly indicate the actual long-term clogging behaviour in case of LCRS, looking in particular to landfills containing organic waste. Chemical clogging is due to insoluble chemical precipitates, such as calcium carbonates, causing blockage or cementation of granular or synthetic drainage materials and filters. Biological clogging is due to biological growth in the LCRS caused by organic and nutrient materials in leachate.
The most difficult problems related to the efficiency of a drainage system occur in the municipal landfill (MSW) because of the high organic content in the waste and microbial related activity. There is a growing body of evidence indicating that a major component in the clogging process is microbiological and chemical. The studies of Brune et al. (1991); Cullimore (1993); Rowe et al. (1995b and 1997a); and Rittmann et al. (1996) have shown that the clogging of drainage systems is the results of a mobilisation process involving fermentative bacteria together with iron- and manganese-reducing bacteria. This is followed by precipitation processes involving primarily methane-and sulphate-reducing bacteria. The clog typically has a soft (organic) and hard (predominantly CaCO3) component. Figure 12 (Rowe, 1998a) shows the relationship between reduction in porosity (due to clogging) and the reduction in hydraulic conductivity of a granular medium with 6 mm diameter particles. This data were obtained by passing actual and synthetic leachates through a column of glass bed (Rowe et al. 1997a, b). The clog composition was typical of that encountered in the field by Brune et al. (1991); and Fleming et al. (1997), more than 50% of the clog material (by dry weight) was calcium carbonate when the tests were terminated.
Figure 12 : Variation in measured hydraulic conductivity of a granular medium (6 mm diamter particles) with decreasing drainable porosity due to biologically induced clogging resulting from permeation with landfill leachate (after Rowe, 1998a)
The rate of clogging can be related to some main factors i.e. flow rate, void size and available surface area as well as the temperature in the collection system (higher temperature implies faster clogging) and the leachate chemistry (especially BOD5, COD and Ca). Recent research has demonstrated a clear relationship between the grain size distribution and clogging. For example, Brune et al. (1991) obtained samples from clogged leachate collection systems and from laboratory studies. They demonstrated that a graded sandy gravel with grain size in the range of 1-32 mm and fine gravel (2-4 mm size) experience severe clogging in laboratory column tests conducted using high strength leachate under anaerobic conditions. Coarser material (medium gravel 8-16 mm size) experienced clogging of the smaller pores, but the larger pores remained open at the end of the experiment (16 months). Coarse gravel (16-32 mm) in direct contact with waste was covered with a thick film and locally clumped together; however, the large pores remained unclogged throughout the 16-month test and maintained its HC. It is particularly important to observe that the temperature of the waste in close contact with the lining system of the landfill can have a significant effect on the rate of clogging of the leachate collection system and then on its service life. In turn the temperature of the waste body appears to be related to the water content of the waste and the level of leachate mounding (Figure 13). Therefore an inefficient LCRS allows the increase of leachate head on the barrier system that in turn induces an increase of the temperature of
waste body and thereafter an increase of microbiological and chemical activity that finally lead to a further reduction of the LCRS efficiency. This potentially very fast and invasive negative trend highlights, once again, the fundamental importance of the leachate collection system effectiveness.
Figure 13 : Variation in temperature at landfill base with leachate head for a number of landfills (modified by Rowe, 1998a from Barone et al., 1997)
The build-up of a leachate mound within the landfill can result in an impact on surface water by leachate seepage from the side slopes of the landfill as well as an increase in contaminant migration through the barrier system and into the groundwater. In fact, “clogging” of a drainage layer is not synonymous with it becoming impermeable (Rowe et al 1995a). On the contrary, a clogged sand blanket may still be substantially more permeable than, say, an underlying clay liner. “Clogging” of a drainage layer becomes significant when the HC of the blanket drops to, or below, the HC of the overlying waste. Under these circumstances, the drainage blanket does not contribute to the hydraulic performance of the barrier, but it will become part of the “diffusion barrier” and the diffusion profile may begin at the interface between the more permeable waste (where lateral flow dominates) and the less permeable (clogged) granular blanket. An example of this is given, with respect to the observed field diffusion profile, at the Keele Valley Landfill discussed by Reades et al. (1989); and Rowe et al. (1995a). Other examples of clogging of leachate collection systems can be found in a number of existing landfills including Toronto’s large Brock West Landfill where a 20 m high leachate mound built up during the first eleven years of its operation (McBean et al., 1993). They noted extensive clogging which resulted in excessive leachate mounding and leachate seeps. Both these landfills were supplied with localized LCRS consisting of “french drains” with pea gravel (5-10 mm) pipe bedding at wide spacing (50-200 m). Very useful and practical indications about the performance of LCRS, looking in particular at the geotextile filtering performances, can be found in Table 6 (Rowe 1998a) were a summary of observation from exhumation of collection systems in North America are presented. Cases 1 and 2 refer to two different sectors of the well known Keele Valley Landfill which are supplied with a relatively uniform stone blanket LCRS including or not geotextile filter between waste and stone. Cases 3, 4 and 5 described by Koerner & Koerner (1994, 1995) show excessive clogging of the geotextile in two cases were the geotextile was wrapped either around the perforated pipes or around the stone in a drainage trench. The reduction in hydraulic conductivity for these three cases and a fourth case (#6) involving geotextiles around well casing from a gas extraction system are summarized in Table 7. Brune et al. (1991) reported the findings from field investigations involving 29 German landfills. More than a half of the LCRS showed evidence of significant encrustation material. At seven of the sites, exhumation of the collection system was performed and it was found that layers of waste above the drainage systems had become “consolidated” and relatively impermeable. This indicates that clogging is not restricted to the drainage layers but can also occur in the waste near the bottom of the landfill. Among others some observations about the German investigation have been summarised by Rowe (1998b):
Table 6 : Summary of observations from exhumation of collection systems in North America (after Rowe, 1998a) Waste Type 1,2 Age; Leachate 1. MSW & LI ∼ 4 years COD=14.800 mg/L BOD5=10.000 mg/L pH=6.3 Performance: •Adequate at time of exhumation 2. MSW & LI As for #1 above
3. MSW & LI; LR ∼ 10 years COD=31.000 mg/L BOD5=27.000 mg/L pH=6.9 Performance: •No flow in LCRS •High leachate mound
Collection System Design3 •Blanket underdrain: Waste over 50 mm relatively uniform stone; 200mm, SDR 11 HDPE pipe; 8 mm holes •Pipe never cleaned •No geotextile between waste and stone
Key Observations •30-60% loss of void space in upper stone •50-100% loss of void space near pipe •Permeability of stone decreased from ∼10-1 m/s to ∼ 10-4 m/s •All lower holes in pipe blocked •Large clog growth inside pipe
•Blanket underdrain: Waste over geotextile over 50 mm stone (rest as above) (GT: W;MA=180 g/m2, AOS=0.475 mm; tGT=0.6 mm, ψ=0.04 s-1) •Toe drain only •Trench with 600 mm of crushed stone (6 to 30 mm) around geotextile wrapped 100 mm SDR 41 perforated PVC pipe (GT:HBNW; MA=150 g/m2, AOS=0.15 mm; tGT=0.30 mm, ψ=1.1 s-1)
Substantially less clogging than observed in #1 above where there was no GT •0-20% loss of void space in upper stone below geotextile •Flow reduction noted after 1 year •Pipe crushed (likely due to construction equipment) •Substantial reduction in void space and cementing of stone. k reduced from 2.5*10-1 m/s to 1.2*10-4 m/s •Sand (SW; AASHTO#10) layer above GM was clogged and leachate drained on top (not through this layer).k reduced from 4*10-4 to 2*10-7 m/s •Excessive clogging of GT •Only small reduction in k of gravel from 5.3*10-1 m/s to 2.8*10-1 m/s •Marginal clogging of GT
•Perimeter drain to control leachate seeps geotextile wrapped trench with 618 mm gravel and 100 mm SDR 30 HDPE perforated pipe (GT:W; MA=170 g/m2; POA=7%, AOS=0.25 mm; tGT=0.41 mm, ψ=0.9 s-1) 5. ISS (included slurried fines •High leachate mound •Blanket underdrain: Waste over protection sand (0.075- •Upper geotextile functioning 70% finer than 150 µm) 0.5 years 4 mm) over geotextile (AOS=0.19 •Pea gravel relatively clean COD=3.000 mg/L mm) over pea gravel (1-20 mm) •Geotextile wrapping around BOD5=1.000 mg/L drainage layer; 100 mm diameter perforated pipe excessively clogged pH = 9.9 geotextile wrapped HDPE •Once geotextile sock removed, Performance: perforated pipe; 12 mm diameter leachate flowed freely holes (GT: NPNW, MA=330 g/m2, •Heavy geotextile sock clogging at •No flow in LCRS AOS=0.19 mm; tGT=2.7 mm, location of perforations in pipe ψ=1.8 s-1) 1 MSW≡Municipal Solid Waste; LI≡Light Industrial; ISS≡Industrial Solids and Sludge; LR≡Leachate Recirculation 2 References: Cases 1 and 2 - Rowe et al., 1995b; Fleming et al., 1997; Cases 3, 4, 5 - Koerner & Koerner, 1995 3 GT≡geotextile; W≡woven; HBNW≡heat bonded, nonwoven; NPNW≡needle punched, nonwoven; SDR≡Standard Dimension Ratio 4. MSW & LI; LR 6 years COD=10.000 mg/L BOD5=7.500 mg/L pH ∼ 7.5 Performance: •Drain functioning adequately
Table 7 : Summary of hydraulic conductivity (kn) and permittivity (ψ) changes for geotextile exhumed from field application in landfills (after Rowe, 1998) Case
31 41 5a1 5b 6a2 6b 6c
Leachate COD (mg/L) 31,000 10,000 3,000 3,000 24,000 24,000 24,000
TS (mg/L) 28,000 3,000 12,000 12,000 9,000 9,000 9,000
Geotextile
MA (g/m2)
AOS (mm)
tTG (mm)
Initial kn (m/s)
Final kn (m/s)
Initial ψ (s-1)
Final ψ (s-1)
HBNW W NPNW NPNW NPNW NPNW NPNW
150 170 220 220 176 176 176
0.15 0.25 0.21 0.21 0.21 0.21 0.21
0.38 0.41 2.7 2.7 2.2 2.2 2.2
4.2×10-4 3.7×10-4 4.9×10-3 4.9×10-3 2.3×10-3 2.3×10-3 2.3×10-3
3.1×10-8 1.4×10-4 8.5×10-5 4.4×10-8 3.7×10-5 1.6×10-7 7.5×10-7
1.1 0.9 1.8 1.8 1.1 1.1 1.1
8.2×10-5 3.3×10-1 3.1×10-2 1.6×10-5 1.7×10-2 7.3×10-5 3.4×10-4
(1) Refer to Table 6; (2) Geotextile around gas collection wells at depth of 3 m, 7.5 m and 15 m respectively • the main components of encrustation materials were the cations of calcium and iron combined with carbonate and sulphides. • The highest concentration of organic and inorganic substances in the leachate and the greatest annual amount of drain encrustation were associated with the landfill that was most rapidly filled (10-20 m/a). • Once a landfill has reached the stable methane phase with its lightly loaded leachate, there is very little incrustation. • Excavation of a sewage sludge deposit, over large areas, revealed that the drainage system had become more or less impermeable due to massive incrustation. • Limestone gravel is absolutely unsuitable as a drainage material. It decomposes under the milieu conditions prevalent on the bottom of sanitary landfills. In addition to the field examples aforementioned clogging of geotextile in MSW leachate has been demonstrated by numerous laboratory studies (Cazzuffi & Cossu, 1993; Fourie et al., 1994; Koerner & Koerner, 1995). These studies show that the magnitude of the decrease of hydraulic conductivity depends on the geotextile features (e.g. openness of the pore structure), the flow rate and the concentration of the leachate. The main outcomes of these experimental researches can be summarised as follows (Rowe, 1998a): • the clogging potential is directly related to the leachate mass loading that is a function of the cumulative leachate flow per unit area of the geotextile perpendicular to the leachate flow and of the mass of COD (Chemical Oxygen Demand due in particular to the concentration of fatty acids) and metals (in particular calcium) that are present in the leachate (Rowe 1998a). • Laboratory test data (Figure 14 and Table 8) have demonstrated that the best performances against clogging, when permeated by MSW leachate, come from monofilament woven geotextiles (Giroud, 1996). Whereas the lower level is represented by non woven needle punched geotextiles characterised by lower hydraulic conductivity (in particular under high confining pressure) and higher specific surface area allowing an easier biofilm growth. • Tests conducted using a nonwoven geotextile filter above the drainage material indicated that the geotextile played a sacrificial role, substantially reducing clogging of the underlying drainage material with the formation of a cake of clogged waste in the upper portion of the geotextile and above the geotextile. The beneficial role of the geotextile was most evident for the coarse gravel (16-32 mm), which remained uncemented, with only a very light biofilm around the gravel particles. 3.1.4 Input Parameters and Design Considerations The likelihood of clogging occurring can be minimised by an appropriate layout and design of LCRS (Rowe et al., 1995a). In particular, considering the aforementioned aspects of filtration and clogging, it is possible to list the following three main goals for getting an actually well working LCRS in the long term: 1. maximising the flow velocity in the drain; 2. maximising the void size;
3. minimising the surface area available for biofilm growth.
Figure 14 : Compatibility tests of non-woven (a) and woven (b) geotextiles with a landfill leachate (Cazzuffi & Cossu, 1993)
Table 8 : Decrease in hydraulic conductivity of geotextiles permeated with leachate (average COD: 30004000 mg/L; BOD: 2000-2500 mg/L; 300-600 mg/L) at a rate of 2×10-5 m/s (620 m3/a/ m2) (modified by Rowe, 1998 from Koerner et al., 1994) Type of Filter
Uniform Sand Well Graded Sand W: Monofilament W: Multifilament W: Slit Film W: Monofilament Special NW/W NPNW NPNW NPNW HBNW NPNW
Unit Mass MA (g/m2)
200 270 200 250 740 130 270 540 120 220
POA
AOS
(%)
(mm)
32 14 7 10 0.3 0.21 0.18 0.15 0.165 0.12
Thicknes s tGT (mm)
0.7 0.8 0.4 0.6 6.3 1.1 2.4 4.7 0.4 2.0
Initial kn (m/s) 4×10-3 6×10-4 3.4×10-3 1.9×10-3 1.6×10-4 6.4×10-4 1.5×10-2 2.3×10-3 3.6×10-3 2.4×10-3 2.4×10-4 3.2×10-3
Initial Permittivity ψ (s-1)
4.8 2.4 0.4 1.0 2.4 2.1 1.5 0.5 0.6 1.6
EquiliBrium kn (m/s) 2×10-6 4×10-7 2.5×10-6 7×10-6 8×10-8 5×10-8 3.5×10-6 6×10-8 1×10-7 2×10-7 4×10-8 1.5×10-7
Flow to EquiliBrium (m3/m2) 119 170 51 51 43 76 102 76 93 85 76 68
By maximising the flow velocity, one reduces the residence time for leachate in the collection system, thereby reducing the amount of sedimentation of particulates and chemical precipitates that can occur. Clearly, the flow velocity can be increased either by increasing the gradient (e.g. by increasing the slope of the landfill base) and/or by increasing the HC of the granular layer. Ideally, both the slope and HC should be as high as practicable.
Maximising the void size within the drainage blanket tends to increase the initial HC and reduces the likelihood of the voids becoming blocked. Biofilm growth is related to the surface area available on the particles forming the drainage layer. Since the surface area is proportional to the square of a particle’s diameter, and the volume is proportional to the cube of a particle’s diameter, it is evident that the surface area can be minimised by increasing the diameter of particles used to construct the leachate collection system. For a given set of landfill conditions, it is possible to establish some basic recommendations for the design and a first evaluation of the service life of the system. For example, based on studies by Rowe et al. (1994, 1995b), the Ontario Ministry of Environment and Energy in Canada has developed a design standard and related service life for the system defined in the Annex 1 of MOEE, (1996). In particular the considered landfill LCRS have been characterized by 100, 75 and 60 years design service life. The main features of these LCRSs can be resumed as follows: • leachate collecting pipes must be embedded in a continuous layer of stones that extend completely across the base of the waste fill zone with a minimum thickness ranging between 30 and 50 cm depending on the required design service life. • The stones must have a D85 of not less than 37 mm, a D10 not less than 19 mm, a uniformity coefficient D60/D10 ≤ 2. Moreover the fine content (passing #200 sieve) must be less of 1% by weight. • A suitable geotextile or graded granular separator must be installed between the stone layer and the overlying waste and between the stone layer and any underlying soil or liner. • The perforated leachate collection pipes must be made of high density polyethylene (HDPE) with a minimum internal diameter of 150 mm and with perforation not less than 12 mm in diameter located along and around the pipes so that: (1) the hydraulic capacity of the perforations can readily accommodate the expected quantity of leachate, (2) leachate that enters the pipes can readily flow within the pipes, (3) blockage by sedimentation is minimised and, (4) the structural integrity of the pipes is maintained. • The perforated leachate collection pipes must be bedded in the stones so that there is at least 250 mm of stones above the pipes and at least 50 mm of stone below the pipes. Local variation of these requirements can be accepted only in the case the design service life of LCRS is equal or less than 75 years. • The perforated leachate collection pipes must be placed across the base of the landfill and spaced so that the drainage path before leachate can potentially intercept a collection pipe is not more than 25-50 m depending on the design service life. • Leachate collection pipes must be inspected at least annually for the first 5 years after placement of waste over the top of each pipe and then as often as future inspections indicate to be necessary. Moreover leachate collection pipes must be cleaned whenever an inspection indicates that cleaning is necessary. • The base of the waste fill must be contoured to provide minimum surface grades of 0.5% toward the leachate collection pipes. • Leachate must be systematically removed from the LCRS in order to avoid obstruction of some system components. • Sludges must not be deposited in the waste zone in a manner that would allow sludge to move into the leachate collection system and promote biological clogging. It should, however, be emphasized that the aforementioned characteristics and the related service life are based on typical Ontario leachate strength and collection system temperatures. The leachate strength and temperatures in other countries (e.g. Germany, see Brune et al., 1991) may both be significantly greater than that found in Ontario, Canada, and this may reduce the service life of the proposed design. Rowe et al. (1994) proposed a preliminary methodology for calculating service life of collection systems with respect to clogging in order to be able to generalise the indications given for the Canadian conditions. For more details on design and service life predictions of LCRS see Rowe et al. (1994, 1997c, d). Up to now the traditional mechanical cleaning (flushing and milling), that is limited to the drainage pipes, has been occasionally used in some well managed landfills but this maintenance procedure should become a current operation during the landfill activity and the post closure period. An interesting German research project is now in progress with the aim of investigating the possibility of dissolving and removing incrustations and clogging materials across the complete LCRS, not only inside the
drainage pipes, and to develop disinfection methods to prevent or delay the formation of incrustation and clogging (Turk et al., 1995; August, 1997). Relevant results of this research work for practical applications can be resumed as follows: The redissolution of oxidises and reduced incrustations is feasible at least in the laboratory scale. Among those chemicals tested, peracetic acid, HNO3, HCL, and HCLO4 had particularly high redissolving capability. Reduced incrustations allow faster redissolution than do oxidised ones. In addition to being the most effective redissolution agent, peracetic acid behaves in a positively environmentally friendly way, in as much as it does not cause salt pollution. Furthermore, the by-products from its reactions can be broken down completely in the sewage works. Also in terms of incrustation prevention by managed disinfection, the peracetic acid solution at 0.2% concentration coupled with a 4% of hydrogen peroxide is the most suitable agent. One positive side effect of oxidising disinfectants is that a non specific oxidation of leachate components take place which results in an additional cleaning of the leachate. To sum up the results of a numerous series of laboratory tests, it seems possible to say that both intermittent and continuous maintenance methods (permanently installed dosing equipment) using acids and disinfectants can maintain the efficacy of a LCRS. However, it is not yet possible to determine unequivocally how frequently maintenance in landfills should be carried out. Moreover the laboratory test results must be validated via large scale tests. In spite of operational methods to prevent and reduce incrustation, it is a common opinion among worldwide experts that biological waste pre-treatment is indispensable, at least for municipal solid waste landfills. Moreover the landfilling of sewage sludge, should be done, as much as possible, using lime free conditioning agents (e.g. polymers), and the sludge must be extensively biologically stabilized in the course of sewage works sludge treatment, and mechanically dehydrated. In conclusion, the basic observations and suggestions for leachate collection systems according to the present state of knowledge can be summarised as follows (see also Figure 15):
Figure 15 : Details of a basal leachate collection system (modified after ETC 8, 1993) • a drainage layer around 0.3 to 0.5 m thick must cover the entire bottom of the landfill and must be constructed using clean cobbles with a diameter of about 20-50 mm.
• In general it is convenient to use a geotextile layer having a high percentage of open area to cover the drainage layer in order to avoid penetration of the finer portion of the waste. • On the top of the geotextile, a secondary 16 to 32 mm cobble layer could be provided as an alternative leachate collection system in the case of geotextile clogging. • The geotextile main features for an effective filtering action minimising clogging potential can be singled out from Table 9 which is based on the suggestions of Giroud (1996) and Koerner and Koerner (1995). Table 9 : Recommended minimum values for geotextile filters for use with mild leachate (TSS & BOD5 ≤ 2500 mg/L) and select waste over the geotextile (no hard or coarse material; for coarse or hard material over GT, the strength requirements may need to be increased) Woven Monofilament Nonwoven NeedleMonofilament woven punched geotextile Property Koerner & Koerner Koerner & Koerner Giroud (1995) (1995) (1996) 2 MA (g/m ) 200 270 POA (%) 10 15-30 AOS (mm) 0.21 0.5 Grab strength (N) 1400 900 Trapezoidal tear (N) 350 350 Puncture strength (N) 350 350 Burst strength (kPa) 1300 1700 • Geotextile filters surrounding pipe networks must be avoided and, moreover, pipe networks should be inspectable and cleanable. The internal diameter should be at least 150 mm and the pipes should have the maximum possible percentage of open area and the maximum opening dimensions considering the structural resistance of the pipes and the grain size distribution of the surrounding drainage material. • Some authors observed that the presence of a geomembrane underlying the drainage layer improved significantly its efficiency. Certainly this is a soundy intuition but still it is not proved and quantified by any experimental investigation. A large scale field test pad could give interesting and useful results for future design applications. • As far as the LCRS design procedures previously illustrated are concerned, it is important to outline that the input parameters of equations (3) and (5) must be evaluated referring to the worst possible case scenario since the need of a redundant design for this fundamental component of the landfill. Therefore as a first recommendation the leachate production should be assessed on the worst monthly based evaluation and taking into account a reduction of 3-4 orders of magnitude of the initial hydraulic conductivity of the granular drainage blanket. Also the long term hydraulic conductivity of the geotextile filters must be checked (on the basis of the data of Tables 7 and 8) in order not to cause excessive perching leachate mounding at the landfill bottom. Rowe (1994) analyzed a number of hypothetical landfill scenarios in which primary and secondary leachate collection systems are used. The analysis indicated that outward advective movement is controlled for most cases and that diffusion of contaminants is a major transport process. These results and also those of Rowe et al. (1995a) suggest that there can be significant diffusion of contaminant through HDPE membranes even for negligible leakage of leachate through the geomembrane. The major potential barrier for diffusive transport for the system shown in Figure 10 is the secondary leachate collection system. Rowe (1994) also suggested PLCRS and SLCRS design options with overall negligible impact on groundwater quality. These design options could involve the use of a granular layer, which provides a “barrier” by being kept unsaturated or by being maintained at a hydraulic head greater than that on the base of the landfill. This approach is called “hydraulic trap” and will be analyzed in the following part of this report dealing with diaphragm walls (see also Rowe et al., 1995a for more details).
3.2
Bottom Barriers At the present state of the art the overall layout and the general design and construction procedures of barriers systems can be considered framed and addressed (Rowe et al., 1995a; Rowe, 1998a; ETC8, 1993; and TC5, 1998). Today the main research streams pertaining to barrier systems are concentrated on the setup of some specific “subroutines” of the “general code” for the assessment of potential contaminant impact on the subsoil environment. Among these “subroutines” or critical issues, that need further research and studies, it can be mentioned the followings: • Evaluation and quantification of some key factors which govern the field scale performances of mineral (CCL and GCL) and polymeric (GM) barrier components; • evaluation of the service life of mineral and polymeric barrier components; • Stability of slopes involving composite liners; • Set up and calibration of some specific models and related parameters for a reliable risk assessment of pollution potential; • Adequacy of the present regulations to address the use of new products and alternative design options. Some of the aforementioned aspects will be discussed in the following, referring to bottom and side barrier components. As reported by Manassero et al. (1996), the design of bottom and barriers of modern landfills should be based on the fundamental principles listed below: • the mineral barrier is, in general, the basic component of traditional sealing systems referring in particular to the long-term performance (t > 200 years). • The requirements and characteristics of the mineral sealing layer in order of importance are: (1) low hydraulic conductivity (HC) at field scale, (2) long-term compatibility with the chemicals to be contained, (3) high sorption capacity, and (4) low diffusion coefficient. • Composite lining systems using geomembranes can give important advantages both in the short and longterm due to: (1) reduction of HC as a result of the attenuation of local defects of both geomembrane and compacted clay as shown by Giroud & Bonaparte (1989a, b); Giroud et al. (1992); and Daniel (1993); (2) better biogas control; (3) minimization of desiccation problems; (4) enhancement of waste degradation; (5) enhancement of flow within the drainage layers toward the collection pipes (i.e. minimization of ponding leachate on the liner) and (6) the geomembrane on the top of the clayey barrier can delay direct contact between clay and leachate long enough for consolidation of the clay portion of the composite system due to the establishment of high effective stresses when the waste is landfilled. In this way it is possible to reduce or avoid compatibility problems (Rowe et al., 1995a). • Construction details play a fundamental role in the final efficiency of the lining system in terms of fullscale HC. 3.2.1 Hydraulic Conductivity at the Field Scale One of the key aspects related to pollutant containment systems is the HC of the mineral barriers at the field scale. This topic has been discussed and developed in the late 80's and 90’s by Daniel and his research group with reference to CCLs. Their main results and recommendations are related to: (1) the effects of compaction procedure, (2) clay water content and (3) pretreatments (Daniel, 1993; Daniel & Koerner, 1995). Uncertainties in construction, flow through macropores and spatial variability of the hydraulic properties of compacted clay liners have been treated statistically and validated via field test results by Jessberger et al. (1993) and Benson & Daniel (1994a, b) among others (Figure 16). The main conclusions of these authors can be summarised as follows: • a mineral sealing layer, consisting of four or more lifts, compensates for the effect of spatial variability of the HC. • There is little benefit if the number of lifts is increased above four to six. • The recommended minimum thickness for compacted soil liners is four to six lifts or 0.6 to 0.9 m. • High quality of compacted soil liners can be considered achieved if the randomly measured HCs using standard tests during construction is 3 to 5 times smaller than the value expected for the full-scale liner. The various kinds of field and laboratory tests for the assessment of compacted soil liners HC have been analysed and described by Daniel & Trautwein (1994). The representative dimensions of samples for reliable tests are given by Benson et al. (1994); and Trautwein & Boutwell (1994). They found that the size
of representative specimen for measurement of HC of compacted soil liners depends on the method and quality of construction. If the soil is compacted poorly, then the representative specimen size should be very large. However, when the soil is well compacted the representative size is small and close or equal to the dimension of standard laboratory test specimens.
Figure 16 : Hydraulic conductivity of compacted soil liners vs thickness: (a) theoretical assessment, (b) experimental trend from field data (after Jessberger et al., 1993; Benson & Daniel, 1994a, b) The above results and related observations have been confirmed recently (Figure 17) by field data presented by Daniel (1997, 1998). The considered database was assembled including more than 120 sites. In order to be included the CCL had to be: (1) constructed in general accord with industries practice for full size liner; (2) subjected to a quality assurance program that was in general accord with industrial practice; (3) constructed with the objective to demonstrate that large scale HC (Kfield) did not exceed 10-9 m/s; (4) reasonably well documented test results; and (5) availability of results from large scale field HC tests such as the sealed double ring infiltrometer (Daniel,1989; Trautwein and Boutwell, 1994). The index properties of the considered CCL together with the limit values, suggested by Jones et al. (1993) and Daniel (1989) in order to succeed in obtaining low HC at the field scale, are reported in Figure 18 together with some data from Italian and Australian sites for comparison. The database of Daniel (1997) shows that there is a little influence of the index properties on kfield. Other factors appear to be far more important for complying with kfield 5 cP -5 -7 10 to 10 Groutable with low viscosity grouts, but difficult with grouts with a viscosity greater than 10 cP 10-3 to 10-5 Groutable with all commonly used chemical grouts 10-3 Requires suspensions grouts or chemical grouts containing a filler material
6.2
Jet grouting Jet grouting is often described as a high velocity erosion process that uses high pressures to impart energy to a fluid, which is used as the soil cutting medium. This energy causes the erosion of the ground and the simultaneous placement and mixing of grout in the soil, which lead to a homogeneous columnar mass. The
technique requires a pumpable grout that can be injected at very high pressures (> 300 bars) through a small orifice (1 to 2 mm diameter). Jet grouting is feasible in virtually all soil conditions ranging from clays to gravels (Kauschinger et al., 1992). There are three general forms of jet grouting that involve injection of a single fluid (grout), two fluids (grout/air), or three fluids (grout/ air/ water), Figure 99, these are discussed in details by Tausch (1992) and Kauschinger et al. (1992).
Air Grout
Air water
Grout Air
Air
Grout
Single Rod
Double Rod
Triple Rod
Figure 99 : Different jet grouting systems The jet grouting technique can be used to form a horizontal barrier below the waste material by drilling on a regular grid pattern to form a system of interpenetrating grout discs, Figure 100. In this application, a borehole is driven to the required depth and a jet grout monitor inserted. The monitor is fitted with a high pressure jet cutting nozzle and can be rotated to cut a disc shaped hole at the required depth. During cutting the monitor is raised to form a disk of the required thickness. Obviously, this technique is not applicable to municipal solid waste type of landfills since drilling through domestic waste with unknown properties can cause all sort of problems. Jet grouting can also be used to form inclined barriers as described by Dwyer et al. (1997). The barrier in this case consists of two rows (honeycombed) of interconnected vertical and inclined portland based grout columns forming a v-shaped trough with the waste pit contents undisturbed on the inside (Figure 101). The inside of the cement v-trough can be lined with a low viscosity chemically resistant polymer to form a secondary barrier to contaminant movement. This approach offers the advantage of not having to drill through the waste material. A further possible approach is to use a horizontal directional drilling technique as described by Sass et al. (1997) to form a barrier under the landfill. However, this technique is still in its infancy and will take probably a long time before the practical side of it is mastered. Nevertheless it offers the potential of overcoming the shortcomings of the vertical or inclined drilling. There is certainly a large scope for development considering that the use of this type of technique has been successful in contamination detection (Katzman, 1996, & Anon, 2000).
Surface grade W aste material
Clay Cap (1.5m) Inclinerator Ash and Landfill Material ( )
Interconnected Columns Min.1.5 Ground water table
Clay Stratum
Figure 100 : Jet grouting construction profile (from Furth et al., 1997).
Figure 101 : V-trough containment (from Dwyer et al., 1997)
6.3
Performance and Monitoring The primary performance criterion for containment is the maintenance of low hydraulic conductivity. Hydraulic conductivity is a function of the barrier material, in the case of a layer or a barrier using grouting, the grout itself must have an initial hydraulic conductivity low enough to satisfy the regulatory agency requirements (usually ≤ 10-9 m/s). In addition possible chemical compatibility issues need to be addressed. These issues are site specific and compatibility tests must be conducted to determine if a barrier option will provide the required performance and longevity needed to meet the design function of the barrier. Table 24 gives a partial indication on the performance of some of the grouts when in contact with a range of chemicals. Consideration has also to be given to the ability of the barrier to control releases such that applicable regulations are met. Typically, barriers are designed to reduce contaminant exposure to acceptable risk levels. Furthermore, the barrier must be durable with regard to natural and environmental degradation. In this respect, integrity of the barrier can be achieved through a QA/QC procedure which can include rigid specifications for grout mixtures, injection pressures, and drilling geometries to ensure barrier continuity by emplacement of multiple or redundant barrier walls. From a regulatory perspective, once the barrier is installed the question remains on how to ensure performance. It is obvious that the large size and deep placement of containment barriers make the detection of leaks a challenging task, this becomes amplified if the allowable leakage from the site is low. To make things more complicated detection of discontinuities (small cracks or gaps) on the order of centimetres at relatively shallow depths is not possible using existing geophysical or geotechnical techniques. Geophysical techniques can certainly identify anomalies in the subsurface but cannot distinguish small variations, such as cracks or gaps because their resolution is insufficient. On the other hand most of the geotechnical techniques are of a destructive type and cannot be used to validate the integrity of the barrier. In this respect a suite and/or a combination of technologies needs certainly to be used to characterise and to assess reliably the performance of the containment barrier. Combination of non-destructive geophysical testing to monitor the installation of the barrier and gaseous tracers to verify the integrity of the barrier is one possible approach. Dwyer et al. (1997) reported on the use of gas tracers utilising perfluorocarbon tracers (PFTs) to locate breaches in a containment barrier. Breaches can be located by injecting a series of PFTs on one side of a barrier wall and monitoring for those tracers on the other side. The injection and monitoring of the PFTs can be accomplished through geoprobe wells placed inside and around the barrier. Other approaches are possible, their utility relative to monitoring objectives is given in Table 25. Table 24 : Key properties of chemical grouts (from Mitchell & Rumer, 1997). Grout Hydraulic Resistance to Resistance to Resistance to conductivity acids bases organics (m/s) sodium silicate 10-7 fair poor fair -9 -11 acrylate gels 10 to 10 poor good fair colloidal silica 10-10 good poor good iron hydroxyde 10-9 poor good good -6 montan wax 10 to 10-9 fair fair fair -12 Sulfur polymer 10 good poor fair cement epoxy 10-12 good good good polysiloxane 10-10 good good good -8 -10 furan 10 to 10 good good good polyester 10-12 good good fair styrene vinylester 10-12 good good good styrene acrylics 10-11 to 10-13 good good good
Expected lifetime (years) 10-20 10-20 >25 >25 25 >25 >25 >25 >25 >25 >25 >25
Table 25 : General application of monitoring approaches (from Mitchell & Rumer, 1997). Monitoring method Barrier monitoring Well network Geophysical ElectroMechanical and Electrical approach methods chemical electro-chemical methods methods methods Barrier integrity U G R C G monitoring Barrier U R G R C permeation monitoring External C G G R G monitoring C= conventional; G= growing application; R= rare; U= unfeasible
6.4
Future Possibilities With respect to future possibilities, Ali et al. (1997) developed a technique for use in weathered clayey soils based on the utilisation of a vibro-probe to form vertical or conical containment barriers. It involves the insertion of a concrete vibrator into the ground assisted with high pressure water if required. The vibrations induced by the probe results in compression and remoulding of the clayey soil. Once the probe is withdrawn, the cavity left by the probe is pressure grouted, thus creating a vibro-seal. However, this technique is still at an early development stage and needs to be assessed on a larger scale to ascertain its feasibility. Another option to install a secure bottom liner beneath an abandoned landfill is to use long wall mining technique to install a liner beneath the waste (Bowders & Gabr, 1995 & Koerner, 2000). The technique would place a geomembrane or a geomembrane/GCL liner and either allow the overlying mass to collapse behind it, or to support the roof with geotextile tubes (Figure 102). Of course, the feasibility of such approach needs to be carefully considered from the cost view point.
Abandoned Waste
Geotextile tubes
Geomembrane GCL
Figure 102 : Long wall mining to provide bottom liner (from Koerner, 2000).
7.0
CONCLUSIONS
The main aspects of waste containment systems have been developed. In particular this paper focuses on bottom and sidewall liners of landfills, capping systems, vertical cutoff walls and bottom liners for polluted subsoils.
The leachate collection and removal system (LCRS) is a key component of the landfill bottom liner. It must avoid significant leachate mound assuring: (1) low waste temperature to minimize desiccation problems of compacted clay liners and a fast degradation of the geomembrane, (2) a low hydraulic head on the confinement barrier i.e. reducing advective flow. Chemical and biological clogging is the main cause of LCRS failure looking in particular at the municipal waste landfills and any other kind of landfill containing a large percentage of organic waste (e.g. sewage sludge). In order to avoid or minimize this kind of problems the following guidelines must be adopted: • a drainage blanket covering the whole landfill bottom made by stone without fine materials; • a suitable geotextile as a separator between waste and drainage layer; • a large diameter with large drainage openings pipe network that must be inspectable and maintainable; • no geotextiles wrapping pipes or “French drains” must be used; • a pretreatment of organic waste before landfilling to reduce organic contents; • periodical disinfecting and cleaning treatments of the LCRS during and after landfill activity. Basic design procedures have been presented for design of drainage blanket and pipes, with recommendation to reduce by 3 to 4 orders of magnitude the long term permeability of the material used for the LCRS. Moreover, LCRS service life estimation, based on the indications of the Ontario (Canada) landfill regulation, are reported for practical applications. Nevertheless care must be paid to the difference that can occur between the different geographical locations and practice referring to the main factors that can influence the LCRS clogging such as, among many others, leachate composition and temperature. The main factors influencing the landfill bottom barrier performances have been discussed and the fundamental principles, based on the present state of knowledge, for their design have been introduced and analysed. The main conclusions are as follows: • hydraulic conductivity and chemical compatibility of simple compacted clay liners are the most important characteristics for the medium-term efficiency of pollutant containment systems. • The contribution of the geomembrane is significant for decreasing the overall hydraulic conductivity of composite barriers, for the limitation of diffusive transport of some types of pollutants, for improving efficiency of the drainage bottom system and for delaying direct contact between the mineral liner and the leachate, reducing thereby the potential compatibility problems in some cases. Some of these aspects are only qualitatively estimated; further research is necessary for a quantitative assessment. • Geosynthetic clay liners, integrated with an attenuation layer can be considered as a possible alternative to compacted clay liners in composite liners. However, a careful comparison must be carried out between the two alternatives on a case by case basis. The actual boundary conditions into the time and space domains, the different pollutant transport phenomena, the contaminant lifespan and the active service life of the composite barrier materials and other landfill components must be taken into account. • In order to comply with the performance design approach, landfill proposals must provide an evaluation of environmental impact and related risk analysis including the theoretical models of the barrier behaviour. A consistent use of theoretical models must be based on reliable input parameters such as the field scale hydraulic conductivity, compatibility with the pollutants to be contained, desiccation potential, sorption capacity, diffusion coefficients and service life. All these features, of different types of barriers, have been discussed and, in particular, for some of them, indications for correct laboratory test procedures and appropriate interpretation algorithms have been illustrated. The new experimental activities related to soil-biopolymer and soil biofilm barriers have been illustrated pointing out the great potential of this new research field not only related with the new landfills but also with the control and reclamation of polluted subsoils. Cover systems have been described emphasizing the main functions of this type of structure that controls the long term landfill hydrological balance. The performance of traditional and emerging types of capping systems have been illustrated on the basis of recent data reported in the literature. Experience showed that traditional covers, consisting of a simple compacted clay layer overlain by a thin veneer of soil suitable for vegetative growth, are prone to failure as a result of desiccation cracking, frost action, differential settlements or a combination of these mechanism. Field studies have shown that composite covers consisting of a layer of compacted clay overlain by a geomembrane and a vegetated surface layer are far more effective at limiting percolation than compacted clay barriers. The geomembrane provides a barrier to flow and also protects the clay layer from desiccation cracking. Although the geomembrane prevent desiccation cracking of the fine-grained layer, it will not
prevent frost intrusion. Thus, composite cover should include a frost protection layer thicker than the maximum frost penetration depth. The potential problems associated with cover instability along interfaces above the geomembrane have also been discussed, pointing out the importance of the drainage layers and related outlet details in particular in the regions where subfreezing conditions persist and/or snow occur leading to ice plug of the outlet during critical snowmelt events. Geosysthetic clay liners (GCL) have gained widespread popularity as a substitute for compacted clay liners in final covers. GCLs are easier to install, can be less costly but should not be seen as a panacea. Case histories have shown that GCLs that are not protected by an overlying geomembrane are susceptible to exchange of divalent cations typically in natural pore water for the sodium initially associated with the bentonite. Exchange results in less swelling of the bentonite during rehydration, desiccation cracks that do not heal, and large increase in hydraulic conductivity. It is advised to take precautionary measures, incorporating a geomembrane can alleviate this problem. Field studies do indicate that final covers employing GCLs do not need to be protected from frost. With respect to gas migration, on-going studies suggest that it is dependent on moisture content and types of GCLs. Another potential problem of GCLs in final cover is instability due to low shear strength of bentonite. Reinforced GCLs are almost always needed for final covers. Drained shear strength parameters corresponding to peak and “large displacements” (e.g. 50 mm) conditions should be used in the stability analysis. A conservative design should have a factor of safety of 1.3 based on large displacement strength. Monolithic and capillary barrier covers are the common configurations used for alternative earthen final cover (AEFC). Their design is based on providing sufficient capacity to store infiltration during cooler and wetter months without excessive percolation until the water can be returned to the atmosphere via evaporation and transpiration. Selecting the thickness necessary to provide adequate storage capacity is straightforward, but requires the unsaturated hydraulic properties of the cover soils, which are in general silty sands and similar soils. These properties are not always easy to be estimated in a reliable way referring to the field scale. Currently the greatest need is to collect long-term field data regarding physical and hydrological performance of final cover. Only a limited number of field scale studies have been conducted, and only one study (Melchior, 1997) collected data for at least five years. Therefore other studies are needed in Europe, Africa, Asia, Australia and USA. An “Alternative Cover Assessment Program”, which includes long-term field tests of alternative and prescriptive covers, is in progress in the USA and should add other important information about these key landfill components. As far as diaphragm walls are concerned, traditional techniques and new technological proposals have been briefly illustrated. Thereafter, the design of composite slurry cut-off walls has been addressed. A closed-form solution for evaluating the contribution of jointed geomembranes to the barrier efficiency has been proposed. Calculations clearly point out that the geomembrane contribution could even have negative effects with certain boundary conditions (inward hydraulic flow) due to both the pure diffusion through the geomembrane and particular combinations of seepage and diffusion through the joint system. As far as the bottom liners for polluted subsoils and abandoned landfills are concerned, there are at the moment, only few examples of practical applications worldwide. Scarce details are available about their efficiency at the field scale and in the long term. Nevertheless experimental research programs are in progress in order to adapt the traditional geotechnical technologies to this specific field. Grouting, jetgrouting and tunnelling techniques seem to be the more promising tools for an effective use in the construction of this kind of barrier in the near future.
8.0 AEFC CCL GCL GM AL LCRS HC ADRE SCOW
SWCC
RECURRING NOTATIONS alternative earthen final cover compacted clay liner geosynthetic clay liner geomembrane attenuation layer leachate collection and removal system hydraulic conductivity advection-dispersion-reaction equation slurry cutoff wall soil-water characteristic curve
9.0 AKNOWLEDGEMENTS Support for Prof. Manassero's research on waste containment system has been provided by a number of sponsors including the Italian Ministry of the University and Scientific Research, Acna S.p.A., Cengio, Italy, Barricalla S.p.A. Torino, Italy, Studio Geotecnico Italiano, Milano, Italy. This support is gratefully acknowledged. Prof. Manassero wishes to thank the geotechnical research group of the University of Ancona and in particular Prof. E. Pasqualini and Dr. D. Sani for supplying many of the experimental data published in this paper and for the stimulating discussions and important suggestions. Special thanks to Mrs. C. Spanna for the technical review and the careful editing of text and figures. Mr. A. Cofano, E. Olinic and F. Ulini prepared some of the illustration of this paper. Some parts of this paper are based on previous papers of Prof. Manassero written with the contribution of coAuthors, among them it must be quoted in particular Prof. C.D. Shackelford, referring to the General Report on "Classification of Industrial Waste for Re-Use and Landfilling" at the 1st Int. Conf. on Environmental Geotechnics (Edmonton, 1994), Prof. W. Van Impe referring to the General Report on "Waste Disposal and Containment" at the 2nd Int. Conf. on Environmental Geotechnics (Osaka, 1996), Prof. M.S.S. Almeida and Prof. R.K. Rowe referring to the ISSMGE-TC5 Report on "Controlled Landfill Design" at the 3rd Int. Conf. on Environmental Geotechnics,(Lisbon, 1998). Support for Dr. Benson’s research on waste containment systems has been provided by a number of sponsors including the US National Science Foundation, the US Environmental Protection Agency, the City of Glendale, Arizona, the States of Colorado and Wisconsin, Waste Management, Inc., Browning-Ferris Industries, Inc., EQ, Inc., and the National Council of the Pulp and Paper Industry for Air and Stream Improvement. This support is gratefully acknowledged. Thought provoking discussions with graduate students and professional colleagues on waste containment systems have been essential to this research. While the are too many persons to name individually, their input has been of great value. Mr. Xiaodong Wang, Mr. Brian Albrecht, and Dr. Tarek Abichou prepared some of the illustrations in this paper. Support for Dr. Bouazza’s research on waste containment systems has been provided by a number of sponsors including the Australian Research Council, Australian Academy of Science, Monash University Faculty of Engineering, and various Australian and overseas companies. This support is gratefully acknowledged. Special thanks to Professor John Bowders, University of Missouri-Columbia, Dr. Gerard Didier and Dr. David Cazaux, URGC Geotechnique, INSA-Lyon for their invaluable comments,,
stimulating and lively discussions on the topic of waste containment systems and some other topics. Input from post-graduate students and professional colleagues have contributed greatly to various aspects of some of the research reported herein. Part of this paper has been prepared while on sabbatical leave at the University of Missouri-Columbia, USA. The support provided by the Department of Civil & Environmental Engineering, College of Engineering and University Office of Research, University of Missouri-Columbia is gratefully acknowledged. Mr Rob Alexander and Mr. Pok Vangpaisal prepared some of the
illustrations of this paper. This paper is solely a work product of the authors. Endorsement by any of the research sponsors is not implied and should not be construed.
10.0
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