2.2 Soils 3 DIRECT SHEAR TEST

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2 c) GT TS 50: Nonwoven needle-punched, continuous filament, polypropylene geotextile, with mass per unit area of 200 g/m 2 and thickness of 1.9mm. d) Smooth HDPE geomembrane (GM) with average thickness of 1.5mm. e) Geonet (GN) are high profile mesh structures made by three sets of overlaid intersecting strands (triaxial) and manufactured from extrusion of HDPE. f) Geocomposite clay liner (GCL) containing sodium bentonite and reinforced polypropylene fibers. The lower geotextile was a nonwoven, needle-punched polypropylene geotextile with a mass per unit area of 204 g/m 2, and the upper geotextile was a woven, slit-film, polypropylene geotextile with the mass per unit area of 109 g/m Soils The soils involved for the testing include soil for the compacted clay liner and uniform gravel for the protective layer between the lining system and the waste. The soil for the compacted clay liner was tested for interface resistance together with a geomembrane liner, while the protective sand was used to evaluate the interface strength together with geotextile TS 50. Both type of materials were also evaluated for their internal friction angles and the apparent cohesions with the direct shear apparatus. For the compacted clay liner (CCL), the clay was first compacted in the mould and was sheared with small direct shear apparatus (100 x 100 mm). 3 DIRECT SHEAR TEST The inner dimensions of the shear boxes were 300 x 300 x 100 mm. This direct shear apparatus have a movable lower shear box with a horizontally supported upper box and a rigid load plate to apply the normal stress. The tests were carried out on dry and wet specimens, especially for the cases involving GCL and clay to simulate the in situ conditions. All interfaces were tested with the geosynthetics placed in the machine directions and parallel to the shearing plane. Since shear failures between granular soil and geotextile were considered unlikely, tests was only carried out for dry condition. The normal loads applied in the direct shear tests were kpa, kpa and kpa, which represented the normal pressure applied to the base lining system in Sa Kaeo landfills. The test procedures were carried out in compliance with ASTM D About 100 direct shear tests were carried out in dry and wet conditions for all the side slope interfaces. For geosynthetic to geosynthetic testings, the materials were glued to plywood and set on wooden substrate so that the geosynthetics interfaces accommodate the shearing plane. For the wet conditions, the specimens were sheared with submergence in water. The GCL and the clay samples were hydrated prior to shearing to achieve a fully saturated condition. For other geosynthetics such as geotextiles to be tested in the wet condition, the submerged specimens were soaked in water for 24 hours prior to testing. The displacement rate used for all direct shear tests excluding interfaces involving GCL and clay liner was about 8mm/min since these types of interfaces are independent of the displacement shear rate (Stark et al., 1996). The displacement rate used for all the tests involving GCL and clay soil was about 0.07 mm./min. 4 CRITICAL SURFACE The interface and material shear strengths under dry and wet conditions are tabulated in Table 1. In most of the interface tests, the minimum ultimate or residual frictional resistance was fully mobilized at very small sliding displacement, mostly less than 4 mm, that were likely to be less than the deformations occurring during construction and fill placement operations. Therefore, for stability analyses purposes the frictional resistance can be expressed by the residual angle of friction, δ r. The two most critical interfaces with the lowest frictional resistance in the nonhazardous landfill were the geotextile (GT TS006)/geomembrane (GM) and the geomembrane (GM)/compacted clay liner (CCL). The GT TS006/GM liner and the GM liner/ccl combinations had the interface friction angles of about 7.4 o and 8.2 o, respectively. The GT TS60/GM and GM/GCL interfaces were also potential slippage surfaces, as the frictional resistances for both interfaces were 6.5 o. Similar to the results obtained by Mitchell et al. (1990), Fishman and Pal (1994), and Long et al. (1995), the liner-interface combinations of lowest shear strength were found to be those of GM/GT, GM/geonet (GN), and GM/CCL. Therefore, both of these interfaces are the usual locations of the potential slip failure. The interface shear strength characteristics of the critical interfaces including GT/GM, 508

3 SLOPE 1.5H:1.0V WASTE GEOSYNTHETIC CLAY LINER 600 mm PROTECTIVE SAND COVER NON WOVEN GEOTEXTILE FILTER (200 g/sq. m) 50 mm SAND mm THICK 3/4" WASHED ROCK NON WOVEN GEOTEXTILE FILTER (500 g/sq. m TS006) 1.5 mm THK (HDPE) GEOMEMBRANE NON WOVEN GEOTEXTILE (500 g/sq. m TS006) 5mm GEONET (GSE HyperNet) NON WOVEN GEOTEXTILE (500 g/sq. m TS006) 1.5 mm THK (HDPE) GEOMEMBRANE 1000 mm COMPACTED CLAY LINER EXISTING SOIL RECOMPACTED TO 90% STANDARD Figure 1. Sections of Hazardous Landfill Project Table 1. Summary of interface shear strength parameters Dry Wet/Submerged No. Material/ δ p δ D peak δ p δ D peak Interfaces ( o ) 50mm ( o ) (mm) ( o ) 50mm ( o ) (mm) 1 Protective layer NT NT - 2 Drainage layer NT NT - 3 Gravel/GT TS (curved) 4 GT TS50/GN GN/GT TS GT TS006/GM GN/GT TS GT TS60/GM GM/GCL GCL/GT TS GT TS006/GN GN/GM GM/CCL [0.53] [0.47] [0.41] [0.41] 14 CCL Internal friction (σ n = kpa), φ p = 33.0 o (curved) Internal friction (σ n = kpa), φ p = 19.2 o Cohesion, c = 46.4 kpa Note : 1) NT = not tested 2) All tests are carried out for normal stress, σ n ranging from kpa. 3) δ p = peak angle of friction 4) δ r = residual angle of friction 5) D peak = Displacement at peak shear stress (mm) 509

4 =155.6 kpa =258.1 kpa =401.5 kpa =155.6 kpa =258.1 kpa =401.5 kpa Shear stress, τ (kpa) Horizontal displacement, D (mm) Figure 2. Shear Stress versus Horizontal Displacement at the Geotextile (GT TS006)/Geomembrane (GM) Interface =155.6 kpa Dry σ n =258.1 kpa =401.5 kpa =155.6 kpa =258.1 kpa Wet σ n =401.5 kpa 50 Shear stress, τ (kpa) Horizontal displacement, D (mm) 6 Figure 3. Shear Stress versus Horizontal Displacement at the Geomembrane (GM)/Geosynthetic Clay Liner (GCL) Interface 510

5 GM/CCL, and GM/GCL, are discussed in the subsequent sections. Since the interface of GN/GM was mentioned as a critical interface in the literature, its interface shear strength characteristics is also discussed. 4.1 Geotextile/Geomembrane Interface The shear stress versus horizontal displacement curves of GT TS 006/GM is illustrated in Fig. 2. The peak shear stresses were mobilized at displacement less than 2 mm. According to the Stark et al. (1996), generally lower unit weight and thinner geotextile gives greater peak shear stresses for geotextile/geomembrane interface compared to the thicker geotextile, but this phenomenon was only observed in tests at wet condition. However, the friction resistances obtained by Stark et al. (1996) using thicker and thinner geotextiles with smooth HDPE geomembrane almost yield similar results with the difference of less than 0.5 o to 1 o, which corresponded to the data as tabulated in Table 1. The interface in the wet condition displayed slightly higher peak stresses than the corresponding dry condition. 4.2 Geomembrane/Geosynthetic Clay Liner Interface The shear stress versus horizontal relationships for the geomembrane (GM)/geosynthetic clay liner (GCL) are illustrated Fig. 3. No distinct peak appeared and the residual peak stresses were mobilized at horizontal displacements of less than 2.5mm. Under dry and hydrated conditions, the shear resistances are 8.9 o and 6.5 o, respectively. Bentonite was extruded through the GCLs woven geotextile and was smeared onto the geomembrane. This interface posed the critical slip surface due to the low residual frictional resistance in hydrated condition, which was also noted by Gilbert et al. (1996). 4.3 Geomembrane/Compacted Clay Interface From the shear stress-displacement curves, no strain softening behavior was observed except for the results at σ n = 401 kpa. The shear displacements to reach peak shear stresses were of the order 1.5 mm to 4 mm. Since the residual shear stresses were mobilized at relatively small displacement, the attainment of fully drained conditions during shearing is difficult to achieve, unless at a shear rate of mm/min (Fishman and Pal, 1994). Therefore, it can be assumed that the shear friction angle obtained in this study represents partial drained condition only. The peak friction angle in dry condition (10.5 o ) was consistent with the values noted by Orman (1994) and Fishman and Pal (1994). Prior to soaking, the peak shear resistance was reduced to 8.2 o. The peak and residual shear resistance at wet condition were 2.3 o and 1.1 o which were lower than the shear resistance at dry condition, respectively. 4.4 Geonet/Geomembrane Interfaces No peak was observed in the shear stress versus displacement curves in both wet and dry condition and the mobilized residual shear stresses were attained at displacement 7 to 9 mm. The frictional resistance obtained in this study was 13.2 o, which closely resembled the average values given by Lydick and Zagorski (1991) and Koutsourais et al. (1991). However, the peak and residual angle of friction were higher than those obtained by Mitchell et al. (1990). In addition, the wet condition shear resistance was 1 o to 1.5 o lower than corresponding dry condition shear resistance. 5 AXIAL TENSILE TEST The stress-strain curves are presented in Fig. 4. To analyze liner stability, the ultimate axial tensile strengths, T ult, were taken at the strain of the first geosynthetics above slip surface to display strain softening or failure. For nonhazardous and hazardous landfills, the geonet was the first geosynthetics above the critical interface to show strain softening at 19% strain. Therefore, the ultimate axial strength, T ult for the other geosynthetic components above the slip surface were taken corresponding to 19% strain. 511

6 Axial tensile strength, T (kn/m) Geonet Geomembrane Geotextile TS 006 Geotextile TS 60 Geotextile TS 50 GCL As-received GCL Hydrated LINER STABILITY ANALYSIS Axial strain (%) Figure 4. Tensile Strength-Axial Strain Behavior of Geosynthetics The minimum residual shear strength either in wet or dry condition was adopted in the analyses. The methods have been employed including the limit equilibrium methods, LEM, (Koerner 1997) the limit method, LM, (Koerner and Hwu, 1991) and simple composite column method, SCC, (Long et al., 1995). 7 THEORETICAL BACKGROUND 7.1 Limit Equilibrium Method The limit equilibrium method, LEM, assumes that the components, including both the soil and geosynthetic materials, above the potential failure surface act as a sliding body along the potential failure surface (Koerner 1997). Tension is induced in geosynthetic materials when the resistance of the slope toe and interfacial shear resistance are fully developed. 7.2 Limit Method The limit method, LM, adopted herein was based on formulations proposed by Koerner and Hwu (1991). Tensile loads were calculated using interface strengths corrresponding to large displacement conditions. For multi-layered liner, the subsequent layer was determined as the difference between the shear resistance mobilized on the upper surface, τ U and that mobilized on the lower surface, τ L. When τ U > τ L, the geosynthetic goes into a state of pure shear equal to t L and the balance of t U - t L must be carried by the geosynthetic in tension. The essential equation for the design is as follows, where T/W is in units of force per unit width. T = [ +γ ω δ δ ] / W (c c ) H cos (tan tan ) L au al U L where c au and c al are adhesion at the upper and lower interfaces, respectively; γ is the unit weight of protective soil; H is the thickness of protective soil; δ U and δ L are friction angle at the upper and lower interfaces, respectively, and L is the length of protective soil. (1) 512

7 7.3 Simple Composite Column The method of simple composite column, SCC, was proposed by Long et al. (1995). The geosynthetic and soil components are assumed to behave as elastic-plastic materials. The soil exhibits an axial stiffness in compression, K c, but no stiffness in tension, K t, while the geosynthetic layer exhibits tensile stiffness but not stiffness in compression. No slippage is assumed at the interface between the two structural layers; therefore, the two columns must strain equally. The SCC is assumed fixed from movement at both ends (at the base and top of the slope). A uniform distributed shear load (f), which represent the driving force, is applied along the total length of the column (L) and is resisted by the interface shear resistance (s LD ) along the interface with the smallest resistance. Axial loads are developed in the columns above the plane of slippage only if the net distributed load (f net = f s LD ) is positive. The maximum compressive load (C soil ) can be determined as equal to f net L - T gs. The maximum tensile load in a composite column can be calculated as: 7.4 Tensile Stress in Geosynthetics ( L) fnet T gs = (2) Kc 1+ K The tensile load predicted in the geotextile TS50 was approximately 25 times greater using the limit method compared to the SCC approach. Moreover, no tensile load was transfer to the geonet underneath the geotextile TS50 using the limit method. Logically, the two layers should strain equally since no slippage was assumed between the geotextile TS50 and the geonet. Furthermore, the geonet and the geomembrane in the lining were predicted to develop no tensile load, regardless of the slope angle, because the shear capacity along the lower interface between the geonet and the geotextile (GN/GT TS006) and between the geomembrane and the compacted clay liner (GM/CCL) exceeded the mobilized shear load at the upper interface between the geotextile and the geonet(gt TS50/GN) and between the geotextile and the geomembrane (GT TS006/GM). Consequently, the limit method predicts behavior that is considered unreasonable and the method is heavily depended on the interface resistance. 7.5 Passive Resistance The passive resistance using the LEM can be estimated by subtracting the total tensile force in the geosynthetics obtained from the LM with the LEM. The LEM methods yielded a higher passive resistance comparing the SCC approach due to the assumption of full mobilization of soil buttress before any tensile load would occur in the geosynthetics. The passive resistance in the protective gravel layer using the LEM and the SCC approach were 20.4 kn/m and kn/m, respectively. The axial loads induced to the lining components and the associated factor of safety (FS) values are summarized in Table 2. The factors of safety for the liner stability were determined to be greater than 1.5 using the SCC approach, which indicated that the downdrag during the placement of protective layer material on the side slope would not rupture the geosynthetic components. Table 2. Comparison of axial loads in the side lining Protective Layer Placement (kn/m) LM LEM SCC Geosynthetic T ult T all T LM FS T LEM FS T SCC FS (kn/m) (kn/m) (kn/m) (kn/m) (kn/m) GT TS Safe GN Safe 0.0 Safe GT TS Safe GM Safe 0.0 Safe 0.0 Safe t 513

8 7.6 Evaluation of the Analysis Methods As tabulated in Table 2, LM predicted large tensile loads due to the consequence of assuming no contribution to stability from the buttress (i.e. passive resistance), while LEM assumed fully mobilization of the buttress resistance before the tensile load develop in the geosynthetic components. Ignoring the passive resistance result in an overestimation of the tensile forces in the geosynthetics. Assuming full passive resistance would lead to underestimation of the tensile forces needed for maintaining the stability of side liner. Furthermore, in most conditions the passive resistance is not fully mobilized, as being treated in LEM. Therefore, the results obtained using LM and LEM analysis can be treated as the upper bound and lower bound values, respectively. The SCC approach yields satisfactory tensile loads in the geosynthetics especially since the structural components (i.e. compressive column and tensile column) were assumed to strain equally and the distribution of tensile loads is proportion to the axial stiffness of the geosynthetic components above the slip surface. 8 CONCLUSION The critical interfaces were located between the geotextiles (GT) and the smooth geomembrane (GM), the smooth geomembrane (GM) and the geosynthetic clay liner (GCL), and the smooth geomembrane liner (GM) and the compacted clay liner (CCL) with the interface friction angles ranging from 6.5 o to 10.5 o and 6.5 o to 9.5 o in both dry and wet conditions, respectively. In general, the residual frictional resistance was fully mobilized at small shear displacement less than 4 mm, which is probably exceeded by deformations occurring during construction and fill placement operations. In the liner stability analysis, the limit method (LM) and limit equilibrium method (LEM) overestimated and underestimated the tensile stress in the geosynthetics, respectively. The method of simple composite column (SCC) approached reasonable tensile stress. REFFERENCES ASTM D , Standard Test Method for Determining the Coefficient of Soil and Geosynthetics or Geosynthetic and Geosynthetic Friction by the Direct Shear Method, ASTM Standards and Other Specifications and Test Methods on the Quality Assurance of Landfill Liner System, ASTM, Philadelphia, Pa., pp (1994). Daniel, D. E., Geotechnical Practice for Waste Disposal, First Edition, Chapman & Hall, London, UK.(1993). Fishman, K.L. and Pal, S., Further Study of Geomembrane/Cohesive Soil Interface Shear Behavior, Geotextiles and Geomembranes, Elsevier Science Ltd., Vol.13, No.9, pp (1994). Gilbert, R. B., Fernandez, F. and Horsfield, D. W., Shear Strength of Reinforced Geosynthetic Clay Liner, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol.122, No.4, pp (1996). Koerner, R. M., Designing with Geosynthetics, Fouth Edition, Prentice Hall, Englewood Cliffs, New Jersey, (1997). Koerner, R. M. and Hwu, B. L., Stability and Tension Considerations Regarding Cover Soils on Geomembrane Lined Slopes, Geotextiles and Geomembranes, Elsevier Science Ltd., England, Vol.10, No.4, pp (1991). Koutsourais, M. M., Sprague, C. J. and Pucetas, R. C., Interfacial Friction Study of Cap and Liner Components for Landfill Design, Geotextiles and Geomembranes, Elsevier Science Ltd., England, Vol.10, No.5-6, pp (1991). Long, J. H., Gilbert, R. B. and Daly, J. J., Geosynthetics Loads in Landfill Slopes: Displacement Compatibility, Journal of Geotechnical Engineering, Vol.120, No.11, pp (1995). Lydick, L. D. and Zagorski, G. A., Interface Friction of Geonets: A Literature Survey, Geotextiles and Geomembranes, Elsevier Science Publishers Ltd, England, Vol.10, pp (1991). Mitchell, J. K., Seed, R. B. and Seed, H. B., Kettleman Hills Waste Landfill Slope Failure I: Liner-System Properties, Journal of Geotechnical Engineering, ASCE, Vol.116, No.4, pp (1990). Orman, M. E., Interface Shear-Strength Properties of Roughened HDPE, Journal of Geotechnical Engineering, ASCE, Vol.120, No.4, pp (1994). Stark, T. D., Williamson, T. A. and Eid, H. T., HDPE Geomembrane/Geotextile Interface Shear Strength, Journal of Geotechnical Engineering, ASCE, Vol.122, No.3, pp (1996). 514