utilisation of selected geoingeneering methods for ...

Aktuálne geotechnické riešenia a ich verifikácia, Bratislava 05.- 06. júna 2017

UTILISATION OF SELECTED GEOINGENEERING METHODS FOR PREPARATION OF THE INDUSTRIAL FLOOR SUBBASES Jacek Kawalec 1 Jerzy Sękowski 2 ABSTRACT Industrial floors are important part of warehouses and factories. Owners of buildings with industrial floors do expect their long service life without need for maintenance. To construct long-lasting industraial floor good subbase becomes critical. The paper discusses various methods for improvement of the subgrade and preparation of subbase. Some examples of subbases prepared for floors in weak geotechnical conditions are also discussed. 1. Introduction The issues related to industrial floors have been existing in the technical literature on civil engineering for many years. They are also the subject of papers presented during many scientifictechnical conferences. The interest in the industrial floors, shown by designers and also by contractors, users, and scientific staff, results inter alia from the development of materials and technologies used to execute the floorings, requirements that must be met by them, still insufficient knowledge of the issues as well as the occurring flooring failures, resulting in financial losses and litigations. The basis of positive resolution of this problem includes definitely a proper investigation of the subsoil and the assessment of the conditions of the planned flooring foundation and also a proper preparation of the subbase to this role if it does not meets specified requirements. The basic objective of the paper is to present briefly selected geoengineering methods used in the preparation (strengthening) of the subsoil for the planned floorings, including illustrations of such projects by examples from the engineering 2. Industrial floors A classic design of a floor, because rather such terminology should be used, made on soil, comes down to a three-layer structure [8], i.e. the subsoil, the bearing stratum (subbase), and the cement base (ground slab) (usually reinforced), with an improved top layer (Fig. 1). Usually the insulation (foil) is laid between the cement base and the subbase, also a layer of lean concrete can occur. The cement base is approx. 15 to 30 cm thick, the lean concrete - 10-15 cm, and the subbase from 30 to 60 cm. 1

Jacek Kawalec PhD Eng., Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland, E-mail: [email protected] 2 Jerzy Sękowski DSc PhD Eng., Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland, E-mail: [email protected]

Cement base (ground slab) Bearing stratum (subbase)

Subsoil

Fig. 1. Simplified diagram of structural arrangement of a floor made on the soil Industrial floors made on soil can have additional hydro-insulating and vapour-insulating layers, thermal and acoustic insulations, as well as protective layers - selected depending on the loads, the room type and purpose. The word flooring is commonly used parallel, and perhaps even more often, instead of a floor, however, in accordance with the definition of standard [20], the top, functional layer of the floor is named flooring. In this paper the term flooring will be used. The presented paper concentrates on the geotechnical issues related to the subsoil (investigation, assessment, possible strengthening) with respect to industrial floors laid on the soil, basically in closed premises. 3. Investigation, assessment, and subsoil preparation for floorings 3.1 Subsoil investigation Polish regulation [21] specifies detailed rules for the determination of geotechnical conditions of building structures foundation, consisting among other things in classifying the structure into appropriate geotechnical category. The structure classification into appropriate geotechnical category, depending on the degree of the subsoil and structure complexity, results inter alia in the scope of work related to the subsoil investigation and in the type of documentation prepared for the needs of the investment construction. The geotechnical category is determined by the designer. However, the aforementioned regulation does not mention floorings and does not facilitate the designer's work, like in the case of other structures, related to the geotechnical category determination. It does not mean however, that he is relieved from that. The determination of geotechnical category for a facility, taking into account the specific nature of flooring, resulting from its various functions, is very important in the authors’ opinion. The results of investigations (field and laboratory) of the subsoil and their analysis are the starting point for the determination of geotechnical conditions of the building structure foundation and preparation of appropriate documents. Among the modern, especially useful field examinations, it is worth mentioning:

a) b) c) d) e)

static probing, dilatometric examinations, geophysical tests, and still justified, in particular in anthropogenic soils:

test excavations, trial subsoil loading. In the case of point subsoil investigation by penetration holes (boreholes, probing) their arrangement and depth remain important. For the directly founded structures, so-called active zone decides about the investigation depth, adopted acc. to standard [19]. In the subject literature (e.g. [1], [17], [18], [19]), one can find recommendations for the depth and arrangement of holes for enclosed and linear structures, but there is no detailed information on the rules for the flooring subbase investigation. The arrangement and number of test boreholes and their depths should be selected in such a way as to unequivocally enable separation of geotechnical layers with a precision corresponding to the foundation calculation requirements. Also the experience of people documenting the subsoil and their geotechnical intuition is important here. The authors are convinced that the mentioned recommendations for enclosed structures, in which the existing floorings are exposed to an average load - up to a few kN/m2 (public buildings), are sufficient. In such cases the depth of investigation not smaller than 3 m below the level of flooring (if load bearing soils exist in the subsoil) is sufficient. However, when organic soils or man-made fills exist in the subsoil, this investigation should be extended to the depth of at least 2 m below their floor. In the case of floorings, designed as an independent structure, with an average load, it is recommended to adopt the number and arrangement of penetration holes in a way similar to that above. In practice there are also structures, in which floorings are loaded with a pressure of up to a dozen or so or even up to 30 kN/m2, spread during a longer period on a substantial area and not necessarily evenly (e.g. high-bay warehouses, open storage yards). The depth of subsoil interacting with the load is then definitely higher and a shallow investigation is insufficient. In such situations the depth of subsoil investigation should be absolutely increased, at least to the depth equivalent to the replacement height of an earth embankment, for instance. The high-bay warehouse constructed in Tychy may be an example here, for which the point investigation depth reached approx. 50 m. However, it is important that the subsoil for enclosed structures is investigated taking into consideration floorings and the functions, which they are going to fulfil.  The static probing consists in the measurement of resistance for a probe tip sunk in the ground. The determination of the resistance on the cone base and of the friction on the sleeve qc, fs (CPT) enables to define many geotechnical parameters. The introduction of measurements of the pore water pressure uc (CPTU), the velocity of shearing wave propagation Vs (SCPTU), and also of the conductivity Cs (CCPTU) by appropriate measuring sensors installed on the tip and/or sleeve, significantly expands the scope of determined parameters (e.g. the soil type, ID, IL, w, , c, and OCR) [14], [15]. The dilatometric examinations should be considered among contemporary most interesting research techniques, parallel to the aforementioned static probing. The results obtained during the field tests provide the basis to determine many parameters significant for the assessment of the

investigated soil, like the OCR and K0 coefficients, M, E, G modules, compression strength in undrained conditions, as well as the cohesion and the value of the angle of internal friction [7].  Geophysical tests are used successfully in the subsoil investigations, using inter alia electrical resistance, seismic and electromagnetic methods. The essence of investigation by those methods comes down to generating by a transmitter of a specific field in the subsoil (e.g. electric) and to measuring it by a receiver situated in another point, observing a determined procedure [1]. In the case of geophysical methods the subsoil is subject to continuous investigation. It is justified to combine the results of investigations obtained by means of geophysical methods and by classical point methods. The results of geophysical investigations enable the determination of e.g. various anomalies, including fissures, voids, and caverns. Hence they are especially helpful in the case, where old, not registered foundations and cables exist in the subsoil. These issues become particularly important in the areas of shallow mining operations.  Test excavations and foundation pits are used to determine more precisely the arrangement of soil layers, to carry out trial loading and to survey the existing foundations, and also to determine the inhomogeneity of soils grain size and compaction, in particular of anthropogenic soils (Fig. 2a) [11].

Fig. 2. a) foundation pit, b) surface loading of anthropogenic subsoil with a pile of precast pavement slabs.  The trial loading of the soil with e.g. a stiff plate [16], allows to determine the deformation parameters of the subsoil and to estimate its ultimate bearing capacity. It is performed most often, when non-typical soils exist in the subsoil (e.g. fills of mineral or anthropogenic soils and organic soils). Because for such soils, being increasingly often interesting to investors and designers, it is difficult to obtain appropriate geotechnical parameters in the available subject literature. Within recent years many structures have been constructed on the grounds considered generally as not suitable for development. The authors mean here e.g. landfills and heaps. The soils that contribute to them are frequently the subbase of both structures and floorings. Apart from a stiff plate loading, another solution is to use for this purpose e.g. pavement slabs, laid

down directly on the investigated subsoil (Fig. 2b). Another, very widespread form of trial loading, used mainly in the road engineering, at the construction of car parks, storage yards, airfields and floorings, is so-called static plate load tester, and recently also a dynamic plate load tester. Both devices apply a plate D=300 mm in diameter and are used to assess the quality of consecutive layers of the formed embankment and also of the subsoil, in the context of its usability as a flooring subbase. The field tests are supplemented by laboratory tests of soils. The scope of tests depends on so-called geotechnical category, determined by the designer. 3.2 Assessment of subsoil usability for the needs of industrial floors The values of geotechnical parameters of soils building the subsoil decide about the its usability for the planned development. In the quantitative assessment these are the subsoil’s load bearing capacity and deformability. Each of the soil layers situated down to a certain depth, including the structure and considering e.g. its size, stiffness, sensitivity to uneven settlement, decides about the subsoil’s load bearing capacity and deformability When assessing the subsoil usability for development special attention should be paid to the existence of weak soils in the subsoil; generally soft cohesive soils, organic soils and loose sands are considered as such. Moreover, the existence of man-made fills should be analysed in the documented subsoil, including so-called anthropogenic soils, which may comprise mining, power plant, metallurgical, building and municipal waste [12]. The assessment of the subsoil usability for the planned floorings, at least in terms of qualitative assessment, does not differ substantially from that presented above, where its basic element consists of results of subsoil deformability determinations by means of a static or dynamic load plate tester, 300 mm in diameter, and of the deformation index I0. In this case the static load plate testing is performed to the level of unit loads 0.25 MPa, and the measure of the aforementioned assessment is the value of the secondary module E2 within the range of loads from 0.05 to 0.15 MPa (for soils improved with binders the maximum load is increased to 0.35 MPa, and the modulus is calculated from the 0.15÷0.25 MPa range). It is worth drawing attention here to a certain significant issue. The assessment of subsoil usability for a flooring, based on results of trial loading by means of a static load plate tester may fail in the case of stratified subsoils, in which weak soils exist and the planned floorings will be loaded with a significant pressure on large areas (e.g. in high-bay warehouses, on storage yards). If as a result of carried out calculation analysis the designer concludes that in the existing geotechnical situation the flooring execution on the existing subsoil is impossible or too costly, then we have a weak subsoil. In this situation such subsoil may be strengthened or the flooring should have a spread foundation. Usually the first option is selected. 3.3 Strengthening of the subsoil for floorings The modern geoengineering has many methods available, which enable strengthening of a weak subsoil [2]. From among a numerous group of weak soils strengthening methods, the authors recommend the following for the subbases for the planned floorings:  strengthening of the superficial layer, performed via compaction (static, dynamic, vibratory), addition of solidifying agents (lime, cement, ashes, blast-furnace slags, bitumens, resins, water glass, binders), addition of admix (aggregate, gravel, shale, cobblestone, etc.),

 replacement of weak soils (partial or total),  subsoil stabilisation using geosynthetics,  and also:  vibro-flotation and vibro-replacement,  deep soil stabilisation: by heavy compaction, by dynamic replacement,  classical injection, jet grouting, and deep mixing (DMM, DSM). Weak soils may be strengthened by permanent lowering of groundwater table and in the mining areas or karst areas by filling the existing voids e.g. with water and fly ashes with various additions. Both the solutions concentrating on the superficial layers - to 1.5 m (surface compaction, replacement of a weak soil, ground improvement) and solutions strengthening the subsoil to a few or even to a few dozen metres (dynamic consolidation, driven stone columns, injections) are successfully applied now. The hitherto experience shows that none of the methods cannot be referred to as universal, the more so the best. Therefore the final choice is difficult. The application of geoengineering methods in construction has increased in recent years. In the further part of the paper the authors will focus on a few methods for strengthening the subbase for the planned floorings and the specific nature of the strengthened subsoil - related to post-mining areas, characteristic of a large part of the Upper Silesia - will be the premise for their selection. These are the following methods: heavy compaction, filling the voids, and reinforcement by means of geosynthetics. 

Heavy compaction

This method comes down to the ground improvement via high-energy tamping. This energy originates as a result of mass m impact, freely dropped from height h, on the surface of ground being improved (Fig. 3a). The shock wave generated in the subsoil causes its compaction to the depth of z=α·(m·g·h)½ (where α=0.65÷0.8 acc. to [9]). In practice the improvement is carried out by dropping the mass in one place a few times (5-10 times) , usually in 1÷3 phases, which results in the origination of a clear crater, which is filled with macroclastic material and the whole is carefully surface-compacted [6]. The compaction takes place at points creating a regular grid of approx. 1.215 m dimensions. The method is used in particular in the case of surface-situated anthropogenic soils of substantial thickness, with examples at the construction of Al motorway sections, of residential estates and of development of areas intended for the construction of industrial and shopping facilities. One of the first constructions with the use of heavy compaction in the Upper Silesia was a large shopping facility, situated on a landfill of maximum thickness reaching 17 m. It was formed by filling a post-clay pit with a non-structural fill. The fill featured a diversified physical condition and the occurrence of voids. Albeit the facility itself and the flooring was founded on CFA piles, but the entire area intended for the car park, access roads, parking and service yards of approx. 8 ha area was strengthened with heavy compaction. It was carried out by means of one of the very first pieces of equipment in the country, performing altogether 1702 points in a 5x5 m grid (a 12 t hammer at 15 m of drop height), and the whole was surface compacted by ironing twice. The construction of a car park, roads and yards was executed on the subsoil strengthened in this way (Fig. 3).

Fig. 3. Example of a car park around a shopping centre on a landfill strengthened with heavy compaction [3] 

Filling the voids

The essence of methods utilising the mechanism of changing the nature of bonds in the treated subsoil is to obtain a rock-like material, featuring increased strength parameters. In some of those methods the strengthening consists in the introduction to the treated subsoil of an inject under the pressure of a few or more atmospheres, which penetrates the pores or mixes with the soil. Classical injections and jet grouting are examples of such methods. The strengthening is less often carried out by the introduction of the inject gravitationally or under a small pressure. Although a general effect in the aforementioned treatments is similar, in the last case the point is rather to fill in cases substantial, in terms of volume, voids and caverns, e.g. of mining origin. We face such cases in the Upper Silesia. The reasons should be sought in the shallow (to approx. 80 m bgl) mining operations, carried out already many years ago without filling the roadways and shafts on a current basis. For a number of reasons the recognition of this problem is difficult, frequently it happens already after an occurred failure. The example presented above is related to a hall and office building constructed for the needs of uniformed services, with a footing area of approx. 1200 m2. [13]. Despite a relatively careful subsoil investigation from the geotechnical point of view initially no threat resulting from shallow mining, carried out in this area in the second half of the 19th century, was found. After detailed investigation of the problems from the mining point of view and performance of electrical resistance examinations steps were taken to fill the voids visible in Fig. 4 in the form of anomalies. Finally, after drilling five boreholes to a depth of approx. 70m, a mixture of water, fly ash and cement was injected at the total amount of approx. 7000m3, adapting also the facility design to uneven settlements always possible in such situations. Similar strengthening was performed for a shopping centre recently commission in one of Silesian cities. In this case the subsoil was strengthened to a depth of approx. 80 m in the area of at least 36,000 m2, carrying out at the same time control geophysical tests, In this case the volume of pumped in inject was approx. 66,000 m3.

Fig. 4. Results of electrical resistance examinations at the depth of 30 m under the planned facility Legenda = Key; strefy anomalne = anomalous zones; otwory petryfikacyjne = petrification holes



Improvement with geosynthetics

The subsoil may be improved also using the combination of crushed-stone aggregate and geogrids stabilising this aggregate [4]. The flooring of a shopping facility constructed in Silesia a few years ago may be an example. The flooring is typically founded directly on the subbase, in the discussed case the requirements for the floorings subbase were determined as E2 ≥ 80 MPa. During the earthen works execution a strong inflow of groundwater was observed on the excavation bottom and also local cases of subsoil yielding. The performed tests of subsoil bearing capacity on the excavation bottom have shown that the obtained values of the deformation modulus measured by means of a dynamic load plate tester range from 9 to 12 MPa. This enforces designing a strengthening layer, allowing to obtain the required values of deformation moduli necessary for proper execution of the flooring. The analysis of geotechnical documentation has shown the existence in the subsoil of varying soil conditions, featuring alternate occurrence of cohesive and non-cohesive soils, additionally of differentiated conditions. The compaction index ID of non-cohesive soils ranged from 0.33 to 0.46 and the liquidity index IL for cohesive soils ranged from 0.22 to 0.59. The arrangement of strata featured high variability. An irregular nature of groundwater occurrence found during the investigation carried out in summer time was an additional adverse phenomenon. Strong local yielding of slightly cohesive soils related to the groundwater inflow was found on the excavation bottom. During the site inspection it was also found that the excavation bottom was fed locally with waters from the dismantled heat distribution line and from the old agricultural drainage. It is necessary to emphasise here that the soils situated in the subsoil were very sensitive to the contact with water and in the case of such contact their geotechnical parameters deteriorated. For the analyses of the subsoil strengthening the value of modulus E2 of around 10 MPa was taken as representative.

Having carried out the analysis of archive materials and as a result of calculations a possibility of flooring direct foundation was adopted, after previously executed mattress stabilised with triaxial geosynthetics. The adopted solution assumed the necessity of the flooring subbase excavation to formation to the ordinate by 50 cm lower than the flooring foundation layer, levelling the subsoil in the excavation without compaction, and then to incorporate the subsequent layers, i.e.:     

Laying the bottom layer of stabilising geogrid with large mesh Laying 30 cm of crushed-stone aggregate of 0-80 mm fraction and its compaction Laying the top layer of stabilising geogrid with medium mesh Laying 18 cm of crushed-stone aggregate of 0-63 mm fraction and its compaction. The construction of the flooring slab acc. to the design. The presented improvement of the flooring subbase with geosynthetics is aimed at unification of subbase parameters under the flooring and at the same time at a uniform distribution of stresses transferred by the flooring onto the subbase, which results in uniform settlement. The solution of thickness was based on the assumption to use specific stabilising properties of geogrids, allowing to reduce the deformation of aggregate obtained as a result of grains locking in the apertures of permanent geometry (Fig. 5).

Fig. 5. Interlocking mechanism shows grains being locked in aperture of monolithic geogrid [10] Apart geosynthetics only, also a combination of deep soil improvement and geogrids is used. Geogrids with aggregate fulfil the role of transmission layers transferring the load from the flooring or from the emabnkment onto columns [5].

Fig. 6. Example of strengthening the subsoil with columns and with a geosynthetic mattress. 4. Conclusion Floorings play a significant role in the constructed industrial and shopping facilities. The requirements, to which they are subject because of that, can be met observing the design procedures and discipline as well as principles of careful execution in relation to each of the layers. In the case of subsoil this applies equally to its investigation and assessment of usability for the considered development and to its strengthening, if it does not meet specific requirements. Most often the subsoil strengthening results from the fact of a facility location in areas of weak soils existence, including anthropogenic soils, and also in areas of voids and discontinuities occurrence, related among other things to the mining operations. The presented examples provide a practical illustration of such situations. The non-observance of the aforementioned recommendations can result in floorings deformations and in their damage. The recognition of causes, and especially the repair of the occurred damage, includes costly and burdensome actions. 5. References [1] Bzówka J., Juzwa A., Knapik K., Stelmach K.: Geotechnika komunikacyjna. Wydawnictwo Politechniki Śląskiej, Gliwice 2012. [2] Gryczmański M.: Współczesne kierunki rozwoju geotechniki w Polsce. Inżynieria i Budownictwo. Nr 8/1994, str.339÷347. [3] Gryczmański M., Kawalec B., Kawalec J., Jastrzębska M.: Przykład przywrócenia budownictwu terenu geotechnicznie zdegradowanego. Problemy geotechniczne obszarów przymorskich. 12 Krajowa Konferencja Mechaniki Gruntów i Fundamentowania, Szczecin Międzyzdroje, 18-20 maja 2000. Cz. 1b: Referaty. Politechnika Szczecińska. Wydział Budownictwa i Architektury. Katedra Geotechniki. , s. 7-16 [4] Kawalec J.: „Wpływ doboru parametrów georusztu na skuteczność stabilizacji warstwy kruszywa”, Konferencja Naukowa z okazji Jubileuszu 70-lecia urodzin Prof. Macieja Gryczmańskiego, Gliwice 2007, opublikowano w Zeszytach Naukowych Politechniki

Śląskiej, Budownictwo, Zeszyt nr 111: Teoretyczne i praktyczne aspekty geotechniki, PL ISSN 0434-0779, str. 211-218, [5] Kawalec J., Warchał T. "Dynamic Replacement Columns with Aggregate Transition Zone Stabilized by Geosynthetics for Embankment Foundation Over Weak Deposits". XVI European Conference on Soil Mechanics and Geotechnical Engineering, Geotechnical Engineering for Infrastructure and Development, Edinburgh, 2015, ISBN 978-0-7277-6067-8 str. 1511-1516, [6] Kłosiński B., Gawor B.: Wzmacnianie podłoża udarami o dużej energii. Inżynieria i Budownictwo, Nr 4/1983, str.148-152. [7] Lechowicz Z., Szamański A.: Odkształcenia i stateczność nasypów na gruntach organicznych. Wydawnictwo SGGW. Warszawa 2002 [8] Mierzwa J., Biliński W.: Kształtowanie i obliczanie posadzek przemysłowych. XV Ogólnopolska Konferencja „Warsztat Pracy Projektanta Konstrukcji”. Ustroń, 2000, t. III, str.197÷230. [9] Pisarczyk S.: Szybka kontrola zagęszczenia nasypów metodą ugięciomierza dynamicznego. Geotechnika w hydrotechnice i budownictwie lądowym. Monografia wydana z okazji 50 lecia pracy naukowej Profesora Wojciecha Wolskiego. SGGW, Katedra Geoinżynierii. Warszawa, 2006, str.196÷203. [10] Rakowski Z., Kawalec J.: “Mechanically stabilized layers in road construction” Proc. Of XXVII International Baltic Road Conference Via Baltica 2009, Riga, section A2 Road Construction, [11] Sękowski J.: Geotechnika. Przewodnik do badań polowych. Skrypt Politechniki Śląskiej nr 1668. Gliwice 1992. [12] Sękowski J.: Grunty antropogeniczna jako podłoże budowli. Prace Naukowe Politechniki Warszawskiej. Inżynieria Środowiska, Nr 54/ 2007, str. 119÷128. [13] Sękowski J., Jóźwiak.: Niespodziewana awaria podłoża gruntowego podczas wznoszenia fundamentów obiektu, wywołana płytką eksploatacja górniczą. XX Konferencja Naukowo-Techniczna. Awarie Budowlane. Szczecin - Międzyzdroje, 2001, t. 2, str. 531÷538. [14] Sikora Z.: Sondowanie statyczne. Metody i zastosowanie w geoinżynierii. Wydanictwo Naukowo-Techniczne, Warszawa 2006. [15] Tschuschke W.: Sondowanie statyczne w odpadach poflotacyjnych. Zeszyty Naukowy Politechniki Śląskiej, seria Budownictwo, Nr 110/2006. [16] Wiłun Z.: Zarys geotechniki. WKiŁ. Warszawa 1987. [17] Wysokiński L., Kotlicki W., Godlewski T.: Projektowanie geotechniczne według Eurokodu 7. Poradnik. Instytutu Techniki Budowlanej, Warszawa 2011. Standards, instructions, and guidelines [18] PN-81/B-03020. Grunty budowlane. Posadowienie bezpośrednie budowli. [19] PN-EN 1997-2:2009 (Eurokod 7): Projektowanie geotechniczne - cz. 2: Rozpoznanie i badanie podłoża gruntowego. [20] Norma PN-EN 13318:2002. Podkłady podłogowe oraz materiały do ich wykonania Terminologia. [21] Rozporządzenie Ministra Transportu, Budownictwa i Gospodarki Morskiej z dnia 25.04.2012 r. Dziennik Ustaw z dnia 27 kwietnia 2012 r. poz.463.