Quantification of stability improvement of a dump through biological ...

Geotechnical and Geological Engineering 18: 193^207, 2000. # 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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Quanti¢cation of stability improvement of a dump through biological reclamation S. K. CHAULYA*, R. S. SINGH, M. K. CHAKRABORTY and B. K. SRIVASTAVA

Central Mining Research Institute, Dhanbad, 826 001, India (e-mail: [email protected]) (Received 25 August 1999; accepted 3 August 2000) Abstract. An integrated study on biological stabilisation of a dump slope has indicated that biological reclamation with grass and tree species should be considered for long term stability of this coal mine dump in India.The grasses have greater soil binding capacity and help to control soil erosion and improve dump stability. Native grasses such as Bamboo (Dendrocalmus strictus) and Kashi (Saccharum spontaneum) are the important constituents of grass species which can stabilise the dump slopes. Field observation of growth performance of grasses have indicated that mean grass height, root depth and below-ground root biomass are 185 cm (68), 45 cm (5) and 467 g mÿ2 (170), respectively after three years ofgrass growth on Mudidih overburden dump slope in India. The growth performance of tree species, namely Sisum (Dalbergia sisoo) and Subabool (Leucena lecocephala), in terms of height, diameter increment, below-ground biomass and root depth have shown mean values of 219 cm (94), 48 mm (6), 4.0 kg mÿ2 (1.5) and 1 m (0.1), respectively. This acts as biological fertility which helps in root proliferation and enhancement of dump stability. From the numerical modelling it is suggested that roots of these grass and tree species have signi¢cantly enhanced the factor of safety of dump from 1.4 to 1.8 and therefore have a positive role in maintaining long term stability. Key words: biological reclamation, numerical modelling, overburden dump, slope stability.

Introduction Opencast mining operations involve removal of huge quantities of overburden, dumping and back¢lling of the excavated area. Substantial increase in rate of accumulation of waste materials in recent years has resulted in greater height of dumps to minimise ground cover area. Consequently, this has given rise to the danger of dump failures, gully erosion and various associated environmental problems (Campbell, 1992). Most mine dumps made by dragline result in intermediate slope angles equal to the angle of repose. Poor handling practice has resulted in many overburden dumps which are extremely dif¢cult to revegetate. Mine spoil poses adverse conditions for soil microbe and plant growth, due to its low organic matter and unfavourable soil chemistry, poor structure (either coarse or compact) and high isolation from vegetation (Singh et al., 1996). * To whom correspondence should be addressed.

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Some of the disposal methods such as conical dumping by dragline have resulted in poor conditions of the dump and are subjected to gully erosion and dump slope failure in India (Chaulya, 1993). This results in air and water pollution, reduced aesthetic values, blockage of pit access and ¢lling of sumps, and thereby problems of water pumping leading to £ooding of the working area during rainy months. In the rainy season, soil often creeps down due to the sandy loam texture of certain overburden dump materials and prevents the establishment of early and late successful plant species (Chaulya et al., 1999). Therefore, environmental management of waste dumps is a challenging geotechnical and ecological problem to achieve sustainable development of mining areas. Revegetation is one of the widely used techniques for controlling erosion and stabilisation of dump slope (Akers and Muter, 1974; Singh et al., 1996), and thereby maintaining ecological equilibrium in the area (Jorgensen, 1994). The role of vegetation growth on dump slopes can be described as hydrogeological and mechanical actions (Cherubini and Giasi, 1997). With respect to the hydrogeological action, the roots of vegetation play an important role in enhancing dump stability by controlling interception of rain water and evapotranspiration and the resulting pore pressure reduction (Blight, 1987; Hussain, 1995). Mechanical action on the other hand, reinforces the dump material by roots and enhances the shear strength of the dump material. This action is closely related to root density, depth and strength (Greenway, 1987; Jha, 1989; Suyama, 1992; Hall et al., 1994). However, quantitative evaluation of biological stabilisation is still poorly understood and the subject requires further study. Therefore, to understand the stabilisation of dump by plant roots and to quantify the improvement in stability, numerical modelling technique has been applied. It is a more accurate method and has the £exibility to assign various material properties for different layers to simulate ¢eld conditions (Naylor, 1982). The present study deals with slope stabilisation through revegetation of a coal mine overburden dump in India for the long term improvement of the degraded coal mining environment.

Study Site LOCATION AND DESCRIPTION

The site is located in Katras area of Bharat Coking Coal Limited. The dump is situated in Dhanbad district of Bihar state of India with longitude and latitude of 86 180 E and 23 480 N, respectively. The topography of the area is undulating. The dump was formed by back¢lling in 1984 with a shovel-dumper combination. Earlier mining operation was opencasting and now it is worked by underground mining. Maximum dump height and slope angle are 30 m and 35.5  , respectively. Figure 1 shows a 3D view of Mudidih dump on which the study has been conducted.

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Figure 1. 3D view of the Mudidih dump.

CLIMATE

The climate of the area is dry tropical and a year can be divided into the cold winter (December to February), a very hot summer (April to June) and a rainy season (July to September). Mean minimum daily temperature within the annual cycle ranges from 10^28 C and mean daily maximum temperature varies between 26 C and 45 C. The average annual rainfall is 1376 mm of which 1107 mm occurs between late June and September. GEOLOGY AND SOIL

This site is located in the Jharia Coal¢eld, which is a member of the Damodar Valley coal belt, occurring as an `outlier' in the Archaean basement area. The bedrock is formed of medium to coarse grained sandstone and clay stone with ferruginous bands and carbonaceous shales. The soil surface layer is 10^11 cm thick grey brown to very pale brown sandy loam to clay loam with subangular blocky structure. Ferromanganese concretions and clay content are found in the sub soil. The overburden consists of alluviul loose sand, gravel, shale and sandstone.

Methods The methods adopted for the ¢eld and laboratory studies are systematically presented in Figure 2. The dump selected for the study was surveyed by electronic

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Figure 2.

S. K. CHAULYA ET AL.

Flow chart of the methods adopted for the study.

distance meter (EDM) for measuring dump geometry. Field and laboratory studies were conducted to determine the physico-chemical properties of dump material by standard methods (Table 1). In situ shear strength properties of the dump material (before and after revegetation) were carried out by jack shear test as described by Anand and Rao (1967) and Hribar et al. (1986), and subsequently studied by Singh (1992) and Chaulya (1997). These tests have been repeated ¢ve times for both barren dump and reclaimed dump (separately for dump material with grass roots and tree roots). The summary of the laboratory test results of physico-chemical properties of dump materials is given in Table 2. The dump material consists of coarse sandy soil having a bulk density of 18.6 kN mÿ3 and the dump material lacks in nutrient content. Results of in-situ

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QUANTIFICATION OF STABILITY IMPROVEMENT OF A DUMP Table 1.

Standard procedures adopted for dump material testing.

Variables

Procedures

References

Physical Parameters: . Grain size distribution . Moisture content . Bulk density . Dry density . Speci¢c gravity . Void Ratio . Porosity . Liquid limit . Plastic Limit . Permeability

. . . . . . . . . .

Sieve analysis Measurement of weight Measurement of weight and volume Constant weight method Pycnometer Measurement of weight and volume Measurement of weight and volume Liquid limit test apparatus Thread (3 mm) test Falling head test

Jumikis (1995) Desai (1986) Lambe (1977) Afanasyer (1976) Desai (1986) Punmia (1987) Jumikis (1965) Desai (1986) Desai (1986) Desai (1986)

. . . . . . .

Glass electrode Glass electrode Walkley-Black method Devarda's alloy method Olsen method Microkjeldahl method Atomic absorption spectrophotometry

AWWA (1992) AWWA (1992) Jackson (1958) Bremner (1965) Sparling et al. (1985) Jackson (1958) AWWA (1992)

Chemical Parameters: . pH . Electrical Conductivity . Organic Carbon . Available-N . Available-P . Available-K . Heavy metals (Fe, Cu, Mn, Zn)

Table 2.

Physico-chemical properties of dump materials.

Variables Physical Parameters: Grain Size distribution (>4.75 mm Separated) ^ Sand ^ Silt ^ Clay Moisture Content Bulk Density Dry Density Speci¢c Gravity Void Ratio Porosity Permeability Chemical Parameters: pH Electrical Conductivity Organic Carbon Available-N Available-P Available-K Heavy metals ^ Fe ^ Cu ^ Mn ^ Zn

Unit

Value

% % % % kN mÿ3 kN mÿ3 % cm secÿ1

71 22 7 3.95 17.26 16.87 2.44 0.52 34.21 11.810ÿ6

mmhos cmÿ1 % kg haÿ1 kg haÿ1 kg haÿ1

7.52 0.073 1.22 89 8.1 131.2

mg mg mg mg

gÿ1 gÿ1 gÿ1 gÿ1

6.5 1.84 10.05 2.58

198 Table 3.

S. K. CHAULYA ET AL. Results of in situ (jack) shear test. Dump material with

Parameters Cohesion Friction Angle

Unit

Natural dump material

kN mÿ2 degree

64(4) 32(1.5)

Grasses 108(6) 33.5(2.2)

Trees 134(8) 34.2(2.7)

shear (jack) tests are presented in Table 3. The results indicate a summary of ¢ve tests. It can be seen that grass and tree roots have signi¢cantly enhanced the shear strength properties of the dump material. The major change is in the cohesion intercept from 64 kN mÿ2 to 108 and 134 kN mÿ2 with grass and trees, respectively. Utilising the measured dump geometry and physical properties, stability analysis of different portions of dump slopes was carried out using Sarma's (1979) limit equilibrium method. From the analysis, certain unstable portions of dump slope were identi¢ed for regradation. The regradation was done through manually terracing and benching as per the required design for stable dump analysed by Sarma's (1979) limit equilibrium method and numerical modelling. Then the regarded area was revegetated by grass species on the slope and tree species on the £at portions as per the methodology described below to analyse the enhancement in long-term stability by the proliferation of roots in the dump material. Reconnaissance was carried out in and around the dump site for selection of suitable grass and tree species for biological reclamation of dump slope and £at portions, respectively. Dalbergia sisoo (Sisum) and Leucena leucocephala (Subabool) were selected as the two best native leguminoseae tree species for planting on £at portions of the dump. These species were selected after seeing the best growth performance and root development by pot experiments on the overburden dump material. Five seeds of each tree species were each sown in polythene bags at 1.5 cm depth in the ¢rst week of May 1993 in the nursery to see the germination potential of the seeds. A single plant was subsequently grown in each polythene bag for transplantation on the £at portions of the dump. Pits were dug on the £at portions of dump with a spacing of 22 m and the size of the pits was 303050 cm. Top soil from the surrounding area was brought and mixed with farm-yard-manure with a ratio of 5 : 1 and each pit was ¢lled with this mixture. Tree saplings were collected from the nursery and transplanted to the prepared pits after the onset of rain during July 1993. Height and diameter increment have been measured after 3 years, i.e. July, 1996. The plant biomass (above and below ground) was also estimated after three years by the harvesting technique (Jha, 1989). It was dried in an oven at 80 C till the weight became constant. Growth performance of the tree and grass species was analysed by statistical methods following Scedecor and Cochran (1968).

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Dendrocalmus strictus (Bamboo) and Saccharum spontaneum (Kashi) which are the two dominating native grass species were selected for revegetation of the dump slope after pot experiments. Both the grasses have good soil binding capacity by their roots. Grass tillers were collected from the laboratory plant nursery and transplanted on the dump slopes (0.250.25 spacing) after the onset of rain during July, 1993. Grass root biomass was estimated by digging monoliths (Jha, 1989) of 0.50.50.5 m at the time of peak biomass, i.e. October 1996. Physico-mechanical properties of the dumps was also studied again in the month of October, 1996 in the laboratory for index and shear strength properties. To get the in situ shear strength of the dump materials (with and without grasses and trees) in situ jack shear tests were carried out at Mudidih site by the standard method (Anand and Roa, 1967; Singh, 1992; Chaulya, 1997). Two models were formulated to simulate the ¢eld conditions (with and without grasses on slope and trees on £at portions of the dump) and analysed by the Finite Difference Method (FDM). Numerical modelling was carried out by assigning the dumps geometry, material properties and boundary conditions to the simulated models. It was assumed that the dump formation gravity loaded and no external load applied on the model. Nodal displacements (i.e. movement of an element due to gravitational loading) for each zone was calculated ¢rst. From the nodal displacements, strains were calculated and from the strains maximum shear stresses calculated. By utilizing the Mohr^Coulomb constitutive relation, the factor of safety (FOS) was calculated for each zone. Contours of FOS were drawn by the Kriging method for the whole domain (Davis, 1973). NUMERICAL MODELLING

Numerical modelling for the problem was simulated by the Finite Difference Method (FDM). In this method, the whole domain is discretised into small two-dimensional zones (elements) which are interconnected with their grid points (nodes). Over each zone the differential equation of equilibrium is approximated. This result in a system of simultaneous equations which are generally solved by iteration methods. A two-dimensional FDM package, FLAC version 2.27 (developed by Itasca Consulting Group Inc., USA) was utilised for the analysis. CONSTITUTIVE MODEL

The Mohr^Coulomb Plasticity Constitutive Model was used to represent the behaviour of dump materials. This model assumes an elastic, perfectly plastic solid in-plane strain which conforms to a Mohr^Coulomb yield condition and nonassociated £ow rule. The yield surface is given by: f ˆ s1 ÿ Nf s2 ‡ 2c…Nf †1=2

…1†

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and the plastic potential function is given by: g ˆ s1 ÿ Nc s2 ‡ 2c…Nc †1=2

…2†

Where, NˆRankine co-ef¢cient where Nx ˆ …1 ‡ sin x†=…1 ÿ sin x† cˆcohesion fˆfriction angle cˆdilation angle s1 ˆmajor principal stress s2 ˆminor principal stress

‰x ˆ f or cŠ

The strain increments are assumed to be composed of elastic and plastic parts: De1 ˆ Dee1 ‡ Dep1

…3†

De2 ˆ Dee2 ‡ Dep2

…4†

The plastic strain rates are given by the non associated £ow rule: e
p 1 ˆ l @g=@s1 ˆ l

…5†

e
p 2 ˆ l @g=@s2 ˆ ÿlNc

…6†

where l is the multiplier which is determined from the stress state.

SHEAR STRENGTH PROPERTIES

Shear strength properties of dump material play a vital role in dump stability. Determination of reliable shear strength values is a critical part of any dump slope design and small variations in strength can result in a signi¢cant change in the dump slope stability. For most of the evaluation regarding the stability on dump slope, the Mohr^Coulomb failure law was used. This is expressed as: t ˆ c ‡ s tan f Where, t ˆ Shear strength, kN mÿ2 ; c ˆ Cohesive strength, kN mÿ2 ; s ˆ Normal stress, kN mÿ2 and f ˆ Angle of internal friction, degree.

…7†

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201

FACTOR OF SAFETY

The critical approach for evaluating the stability of slopes is to evaluate the factor of safety (FOS). The factor of safety is generally de¢ned as the ratio of available shear strength of the dump material to the shear resistance required to maintain equilibrium. The factor of safety is then expressed as: FOS ˆ

Shear strength available to resist sliding Shear stress mobilized along failure surface

…8†

FOS may also be de¢ned as that factor by which the shear strength parameters must be reduced in order to bring the potential failure mass into a state of limiting equilibrium. When the material has both cohesion (c) and friction (f), it is usual to apply the same factor to c and tan f. Denoting the reduced parameters by an asterisk (*) and the factor by l: c ˆ c=l tan f ˆ tan f=l lˆFOS, when c* and f* are associated with incipient failure.

FORMULATION OF MODELS

Stability analysis by the limit equilibrium method (Sarma, 1979) had revealed that the dump was unstable in its original geometry at the place demarcated in Figure 1. Total height of this portion of this dump was measured to be 26 m with an average overall slope angle of 35 . The factor of safety (FOS) for the portion of the dump had calculated to be 1.15. Therefore, the unstable portion of the dump was regraded by terracing and benching as shown in Figure 3. Overall dump slope angle of the benched portion was 31 while the individual slope angle and the height of the two benches were 33 and 13 m, respectively with a berm width of 3 m. Observation in the ¢eld had indicated that the average maximum depths of grass roots and tree roots were 0.5 m and 1 m, respectively after 3 years of revegetation. To study the effect of grass on slopes and tree plantations on £at portions of the regraded dump area the following two models were formulated and run separately: (i)

The whole domain was assigned with same properties as measured in the ¢eld to simulate the natural dump material, i.e. without grasses and trees (Figure 4); and (ii) A modi¢ed layer with c and f values as measured in the ¢eld of 0.5 m thick along the slope and 1 m along the £at potions of the dump have been assigned to represent dump with grasses and trees (Figure 5). Geometry of the regraded portions of the dump has been considered as input model for numerical modelling (Figure 4). Base length of 90 m has selected considering

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Figure 3.

Geometry of the regarded portion of the dump.

Figure 4.

Geometry of the dump used for numerical modelling with boundary conditions.

QUANTIFICATION OF STABILITY IMPROVEMENT OF A DUMP

Figure 5.

203

Area of interest with di¡erent dump material properties.

the geometry of the dump and in£uence of stress. Whole domain has been discretised into two different size of two-dimensional elements. Near the slope (area of interest) small elements of 0.50.5 m size and for the rest area 0.52 m size elements have been selected. The boundary conditions applied include roller boundary (i.e. displacement in vertical direction is allowed and horizontal direction is ¢xed) along the rear side of the dump and ¢xed boundary (i.e. no displacement by horizontal and vertical directions) along the base which are shown in Figure 4.

Results and Discussion The main input parameters for the study were dump material properties (e.g. c and f), dump geometry and boundary conditions. All these parameters have been generated by ¢eld study as described earlier. Among all the parameters, determination of actual ¢eld data for critical dump material properties (c and f) are very dif¢cult and results of the stability analysis is very sensitive to the accuracy of the input

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parameters. The overall factor of safety calculated by the ¢nite different method of modelling was 1.4. This compares with a value of 1.5 obtained using Sarma's (1979) limit equilibrium method. Field study of growth performance of grasses has indicated that mean grass height, root depth and below-ground root biomass are 185 cm (68), 45 cm (5) and 467 g mÿ2 (170), respectively after three years of grass growth on the dump slope. The root biomass was calculated for total depth of roots. As mentioned earlier, the development of roots helps to stabilise dump material by hydrogeological and mechanical actions, which are closely related to root density, depth and biomass (Cherubini and Giasi, 1997). The value of below-ground biomass is within the range of 455 g mÿ2 (43) which is reported from the natural succession of plant species on a 12 year old dump slope of a dry tropical coal mine spoil in India (Jha, 1989). Root length and below-ground biomass of tree species varied from 0.75 to 1.1 m and 2.5 to 5.5 kg mÿ2 , respectively. Growth performance of trees has indicated that Sisum and Subabool are the fastest grown tree species in Mudidih site after three years of growth. Sisum has contributed 5.4 kg (0.6)/tree as a dry weight plant biomass, while the Subabool has contributed only 2.3 kg/tree of plant biomass. This plant biomass will also contribute to the fertility of the soil and ultimately productivity of the revegetated overburden dump. Consequently the proliferation of roots takes place on the dump which ultimately enhances the stability of the dump. Results of the present numerical modelling study have indicated that maximum displacement of elements (up to 9 mm) occurred near the crest of the dump i.e. the top portion of slope. Therefore, any dump deformation monitoring programme should be planned near the crest of the dump slopes as dump failure generally occurs after signi¢cant movement over a long time (British Columbia Mine Waste Rock Pile Research Committee, 1991). Thus for large dump with large slope angle and height, continuous monitoring of dump deformation is essential and wireline extensometer with continuous recording facility may be used for this. Wireline extensometer is the simplest type of equipment having easily readable and adjustable features (Chaulya, 1997). From the stress analysis of the dump slope it was obsevered that grass and tree roots have reduced the stress concentrations near the surface of dump slope in comparison to barren slope. Contours of Mohr-Coulomb FOS is illustrated in Figure 6 for the dump without and with vegetation. Figure 6 shows that the FOS was enhanced from 1.4 to 1.8 due to plantation of grasses on the dump slope and trees on the £at portions, and thereby enhancement of shear strength of dump material by the root matrix. The path of critical failure surface has also been changed. This is because of the mechanical action of the tree and grass roots, which reinforces the dump material by roots and enhances the shear strength of the dump material (Cherubini and Giasi, 1997; Hall et al., 1994). Therefore, binding of the dump surface together with vegetation and increases in shear strength have lead to an enhanced factor of safety at depth. The depth of critical failure surface (i.e. the sur-

QUANTIFICATION OF STABILITY IMPROVEMENT OF A DUMP

Figure 6.

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Factor of safety for the barren and reclaimed dump.

face along which dump failure occurs) increases for the case of the slope with grasses and trees compared to the barren slope. This is also an important factor for maintaining long term stability of a coal mine overburden dump.

Conclusions It may be concluded from the analysis that grass and tree roots play a very crucial role in the stabilisation of coal mine overburden dump slopes. It creates mechanical reinforcement of dump material by the proliferation of roots. Shear strength of the dump material is also enhanced by the root matrix which in turn increases the long term stability of dump slopes. The factor of safety was enhanced from 1.4 for the barren slope to 1.8 for the case with grass and trees planted on the slope. On most occasions mine dump failure occurs after signi¢cant deformation with prior warning signals. Results of numerical modelling analysis of slope stability have indicated that the maximum deformation occurs near the crest region. Therefore, any deformation monitoring programme should be conducted near the crest of dumps.

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Acknowledgements Authors are thankful to Director, Central Mining Research Institute (CMRI), Dhanbad, for giving permission to publish this paper. Thanks are also due to Dr. B. K. Tewary, Dr. M. Prasad and Dr. V. K. Singh, Scientists, CMRI, Dhanbad, for their immense support during the ¢eld study and laboratory analysis. Grateful acknowledgment is also due to the Ministry of Coal, Government of India, New Delhi, for sponsoring this research work under the project entitled `Environmental Management of Overburden Dumps'. Finally, the help and cooperation extended by the management of Mudidih mine during the ¢eld study is sincerely acknowledged.

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