cohesive soil interface shear behavior

Geotextiles and Geomembranes 13 (1994) 571-590 © 1994 Elsevier Science Limited Printed in Ireland. All rights reserved.

0266-1144/94/$7.00 ELSEVIER

Further Study of Geomembrane/Cohesive Soil Interface Shear Behavior

K.L. Fishman & S. Pal Department of Civil Engineering, University of New York at Buffalo, 244 Ketter Hall, Buffalo, New York 14260, USA

ABSTRACT The results presented in this paper contribute to an existing data base on the shear strength o[ geonwmbrane/cohesive soil interfaces. Three different clay materials are studied, and the interlaces include both smooth and textured H D P E geomembranes. Consolidated drained and consolidated undrained direct shear tests were performed on samples compacted wet o[" optimum under both partially saturated and saturated conditions. The effect of rate of shear is studied through a range j?om 12.7 mm/min to 0.005 mm/min (0.5 in/min to 0.0002 in/min). The shear strength o f textured geomembrane interfaces during drained shear loading was equal to or less than that o]" the clay alone, and the consolidated undrained shear strength was higher than for the clay alone. Deformations during shear were studied and used to explain the variation in shear strength due to the rate of shear loading for textured geomembrane/clay interfaces.

1 INTRODUCTION The regulatory requirements for solid and hazardous waste landfills require the installation of a leachate collection system, and liner, to reduce discharges of landfill leachate into the environment. The leachate collection system and liner typically consist of a combination of geosynthetic materials and natural or processed soil. Double 571

572

K.L. Fishman, S. Pal

composite liner systems are being mandated by many states in the USA for waste disposal facilities. In a typical double composite liner system two composite liners, consisting of a geomembrane placed on top of a relatively impervious compacted clay, are designated as the primary and secondary liner. Drainage materials placed before the primary liner, and between the primary and secondary liners, consist of geotextiles and other free draining materials. Many interfaces between different geosynthetics, and between soils and geosynthetics, exist within the liner system. In particular, geotextile/ geomembrane, geotextile/clay, and geomembrane/clay interfaces, known to be weak in shear, are potentially hazardous, especially when considering earthquake loading. A critical element in designing modern landfills is determining the maximum safe slope and height in order to obtain the maximum disposal volume. The evaluation of slope stability requires a knowledge of the strength characteristics between the interfaces of the soil and the geosynthetic materials. Thus, during the last few years various testing methods have been developed to provide the necessary engineering design information and to describe the engineering properties of geosynthetics, and geosynthetic-soil interfaces. Takasumi e t al. (199l) reviewed state-of-the-art soii-geosynthetic interface testing procedures and the effects these procedures have on the test results. The effects of type and size of test apparatus, and sample thickness, density, moisture content and degree of saturation, consolidation, strain rate, and range of applied normal stress were discussed. A review of the current literature included tests performed with a wide variety of soils including both cohesionless and cohesive soil, and geosynthetic materials which were divided into two groups: geotextiles and geomembranes. The following list of references was adopted from the work of Takasumi et al. (1991) with some additions. Most of these investigations related to the shear strength of geotextilecohesionless soil interfaces and the adoption of a suitable test procedure to simulate the actual field conditions as described by Haliburton et al. (1978), Delmas et al. (1979), Collios et al. (1980), Ingold (1982), McGown and Andrawes (1982), Myles (1982), Rowe et al. (1982), Martin et al. (1984), Akber et al. (1985), Richards and Scott (1985), Saxena and Budiman (1985), Degoutte and Mathieu (1986), Miyamori et al. (1986), Eigenbrod and Locker (1987), Lafleur et al. (1987), Williams and Houlihan (1987), Lauwers (1991). Interfaces between cohesionless soil and geomembranes were also studied by Martin et al. (1984), Saxena and Wong (1984), Akber et al. (1985), Degoutte and Mathieu (1986), Eigenbrod and Locker (1987), Geotek (1987), Williams and Houlihan (1987),

Geomembrane/cohesive soil interface shear behavior

573

Negussey et al. (1989), Seed (1989), O'Rourke et al. (1990), Druschel and O'Rourke (1991), Lauwers (1991). Very few investigations were done to study the shear strength of geosynthetic-cohesive soil interfaces. Geotextile-cohesive soil interfaces were studied by Saxena and Budiman (1985), Eigenbrod and Locker (1987), Fourie and Fabian (1987), Williams and Houlihan (1987), and Lauwers (1991). Some work has also been done on geomembrane-cohesive soil interfaces including Koerner et al. (1986), Williams and Houlihan (1987), Seed et al. (1988), Geotek (1987), Seed (1989), Seed and Boulanger (1990), Lauwers (1991). The topic of this paper shall be focused on the behavior of geomembrane-cohesive soil interfaces. Further details of previous work in this area shall be given, and the need for further data justified. Subsequently, the remainder of the paper will describe the results of a laboratory test program intended to broaden the existing data base and understanding of cohesive soil/geomembrane interface shear behavior. Koerner et al. (1986) conducted tests on a variety of low-permeability cohesive soils in contact with various geomembranes (PVC, CPE, EDPM, HDPE and textured HDPE) at a strain rate of about 0.06 m m / m i n (0-0024 in/min). The soils were placed at a dry density of between 84 and 100% of maximum as determined by the standard Proctor test at a water content wet of optimum. Since the soils were not saturated prior to shearing all tests were considered as drained. Further testing under saturated conditions, drained or undrained was recommended. They used a conventional direct shear box of dimensions 101 m m by 101 mm (4 by 4 in) with the geomembrane in the lower position and the soil placed above it. Koerner et al. (1986) noted that the stress versus strain response curves were rarely strain softening, so that peak values and residual values are approximately equal. Friction coefficients were somewhat lower for the harder PVC and H D P E geomembranes. However both high friction and adhesion values were observed for interfaces involving textured HDPE. This is probably due to the shear plane being in the soil above the geomembrane rather than directly along the soil-geomembrane interface as a result of the geomembrane heavy surface texture. Geotek (1987) reported results for geomembrane interface testing with two different cohesive soils in contact with textured HDPE. The first soil was a clay with liquid limit 48, plasticity index 26, friction angle 17.5 ° and cohesion 76 kPa (11 psi). The second soil was clayey, silty, sand, the finer fraction of which had a liquid limit of 37 and a plasticity index of 21. The friction angle and cohesion of soil number two were 25 ° and 34 kPa (5 psi). Tests were conducted in a large shear device having dimensions of 305 m m

574

K.L. Fishman, S. Pal

by 305 m m (12 by 12 in) at a shear rate of 0.076 mm/min (0.003 in/min). Soil samples were compacted wet of optimum but not saturated prior to shear. The results from testing were consistent with the observations of Koerner et al. (1986). Seed and Boulanger (1990) conducted geomembrane interface tests with two types of soil-bentonite mixtures, one was classified as CL by USCS with plasticity index 19, and the other one classified as CH by USCS with plasticity index 48, in contact with a smooth HDPE geomembrane. Seed and Boulanger (1990) performed tests under as-compacted conditions as well as conditions representing unconsolidated, undrained shear after sustained presoaking under light surcharges. Observations indicated that the interface shear strengths were very strongly a function of compaction conditions. Samples compacted wet of optimum had considerably lower shear strengths than samples compacted at optimum water content or dry of optimum. Mitchell et al. (1990, 1992) and Byrne et al. (1992), reported results from the shear testing of interfaces present within the double composite liner system of the failed Kettleman Hills waste landfill. Of particular interest is the interface between the clay liner and the smooth HDPE geomembrane. The clay liner material was an admixture of claystone, siltstone and 2-5% by weight of bentonite. The material is classified as CH by the USCS having a liquid limit of 60-70% and plasticity index 4050%. 'Quick' undrained direct shear tests were conducted on the interfaces. The clay was prepared at a range of densities and moisture contents against the geomembrane surface, soaked for 24 h under light surcharge and sheared rapidly. It was found that the shear strength of the interface was strongly affected by the as-compacted moisture content of the clay. The peak shear strength was not affected by the level of normal stress during shear, although a weak relationship between residual shear strength and normal stress was observed. Lauwers (1991) tested interfaces between a PVC geomembrane and two different fine grained soil types; glacial till and clay. The glacial till was a sandy silt the finer faction of which had a liquid limit of 23 and plasticity index 3. The clay material had a liquid limit of 44 and plasticity index 17. Soils were compacted wet of optimum and saturated prior to shearing at a rate of 1 mm/min (0-04 in/min). Tests were conducted in a large shear box having dimensions of 305 mm by 305 mm (12 by 12 in) and only low levels of normal stress were applied; o-n < 28 kPa (4 psi). Somasundaram and Khilnani (1991) reported results from direct shear testing of clay/smooth geomembrane and clay/textured g e o m e m brane interfaces. The clay lining material was processed to m a x i m u m clod size of 25 m m and was compacted to a density 90% of maximum

Geomembrane/cohesive soil interface shear behavior

575

as determined by the modified Proctor test, at a water content 2-5 percentage points above optimum. Consolidated, undrained direct shear tests were performed in a large shear device with dimensions of 305 mm by 305 mm (12 by 12 in). The range of normal stress applied was 0.31.15 MPa (6000-24000 psf). All specimens were saturated prior to shear. Significantly higher shear strengths were realized for textured geomembrane interfaces as compared to smooth geomembrane interfaces. High adhesion components of shear strength were apparent for the textured geomembrane interfaces and the friction angles had efficiencies of 70-80% compared to the frictional component of shear strength for clay alone. Post peak behavior was observed for the smooth geomembrane interfaces only. Mitchell et al. (1992) reported results from the direct shear testing of clay/textured geomembrane interfaces, as part of the stability analysis of the Keller Canyon Landfill. The clay liner materials were native alluvium and colluvium having undrained shear strengths between 70 and 100 kPa (1500-2000 psf) when compacted to densities 91% of maximum as determined by the modified Proctor test. Specimens for interface testing were compacted at optimum moisture content, and at a moisture content three percentage points above optimum. Some specimens were soaked prior to shear, others were not. All direct shear tests were consolidated drained tests. Shear strengths were extremely sensitive to 'wetting' at the interface prior to shear. Friction angles of 31.8 ° and 28.4 ° were realized for unsoaked interfaces compacted at optimum moisture content and wet of optimum, respectively. If the interfaces were soaked prior to shear these friction angles were reduced to 9-5 ° and 8.5 °, respectively. The results of tests performed on geomembrane-cohesive soil interfaces presented in this paper will serve to extend and enhance the current data base. Tests were conducted with both smooth and textured HDPE geomembranes in contact with three different cohesive soil types; two glacial tills and a glacial till-bentonite mixture. The soils were all compacted wet of optimum, however, tests were conducted under conditions of both partial and complete saturation during shear. One of the principal features of this test program was to study the effect of rate of shear on the behavior of geomembrane-cohesive soil interfaces when tested in a saturated condition. The present study is intended not only to evaluate the shear strength of geomembrane-cohesive soil interfaces but also to study the deformation behavior and the effects of strain rates on the shear strength values of these interfaces. The test program presented herein represents results from 70 direct shear tests performed on different interfaces and test conditions.

576

K.L. Fishman, S. Pal

2 SCOPE OF TEST P R O G R A M The shear strength parameters utilized in the stability analysis of a waste containment structure should be determined from laboratory tests with drainage conditions which most nearly conform to the field case. For the study described in this paper both consolidated-drained and consolidatedundrained direct shear tests are performed. These conditions are useful for the long-term stability of a containment structure, and the consideration of seismic forces by pseudo-static analysis, respectively. Firstly considering the long-term stability of a containment structure the generated excess pore pressures have had time to dissipate and pore water pressures have returned to a neutral condition. The stability analysis is in terms of effective stresses and the shear strength parameters must be representative of consolidated drained conditions. Secondly, considering the case of earthquake loading, the containment facility may have been capped and in service for some time. Clay materials within the liner system have had time to consolidate. Upon initiation of earthquake loading imparted shear forces are rapid such that shear failure may occur under undrained conditions. Pore water in the clay liner, and at the clay geomembrane interface, will respond to this lack of drainage and influence the shear strength of the clay and interface. Therefore, consolidated, undrained strength parameters should be employed for the analysis of earthquake loading. 2.1 Test apparatus A square shear box 102 by 102 mm (4 by 4 in), split horizontally at midheight was used for. direct shear testing. Although the new ASTM test standard for direct shear interface tests (ASTM D5321) requires a 300 by 300 m m (12 by 12 in) shear box, results from previous studies indicate that for fine grained soil in contact with most geotextiles and geomembranes a 102 by 102 m m (4 by 4 in) shear box is adequate (Takasumi et al., 1991). For testing only clay material, two porous plates, one at the bottom and the other at the top were used. Figure 1 is a schematic cross-section illustrating the typical sample configuration used for the direct shear testing of geomembrane-clay interfaces. The geomembrane specimens were cut to a square of 102 by 102 m m (4 by 4 in) and were glued to the top of a 102 by 102 mm (4 by 4 in) plexiglass block. The lower half of the shear box was filled with this block-geomembrane assembly. The clay material was then compacted within the upper sample holder, on top of the geomembrane. The proper density of each specimen was maintained by tapping the soil with a small rod-like tamper. All specimens were saturated by immersion

Geomembrane/cohesive soil interface shear behavior

577

APPLIEDVERTICALLOAD / _ ~ ~

I N

POROUSSTONE GEOMEMBRANE

~ 1/1

CLAY e l l

/

I

APPLIED SHEAR

I

I"

WATERHOLDER PLEXIGLASSBLOCK

Fig. 1. Direct shear testing of clay/geomembrane interfaces.

in water for 24 h prior to testing. To avoid swelling of the soil samples a small amount of normal stress was applied during soaking. The specimens were consolidated under normal stress for 24 h before shearing. All tests performed were strained controlled under constant levels of normal stress which ranged from 10 to 345 kPa (5 to 50 psi). A description of the test series is provided in Table 1. The rate of loading for a drained test must be slow enough such that excess pore pressures generated during shear loading have enough time to dissipate prior to failure. The following equation, recommended by the US Army Corps of Engineers (USACE, 1980), was used as a guide in determining the minimum time required from start of test to shear failure: tf = 50t50

(1)

where tc is total elapsed time to failure in minutes, and t50 is time in minutes required for the specimen to achieve 50% consolidation. Consolidation tests were performed on samples of each of the clays tested. A comparison between the time to failure and that required for drained conditions during shear will be presented in the section Discussion of results.

3 D E S C R I P T I O N OF M A T E R I A L S Table 2 summarizes the results of standard geotechnical engineering tests including Atterberg limits, specific gravity, unconfined compression, consolidation and permeability tests, performed on samples of clay No. 1, clay No. 2 and clay No. 3. All testing was performed according to procedures detailed by the US Army Corps of Engineers (USACE, 1980). Samples for unconfined compression, consolidation and permeability tests were compacted to the same dry density and moisture content as used for shear testing. Further details relative to each clay type will be given in the following paragraphs.

35

(c) Clay No. 3 x Clay Smooth Textured

Clay Smooth

(b) Clay No. 2

(a) Clay No. I Clay Smooth X Textured X

IO

x

69

& = 12.7 (mm/min J

X X X

34

x

138

X X

100

G (kPa)

x x x

35

X

145

Partially saturated

x x x

69

x x x

138

/Ii = 1.27 fmmjmin J

X

320

X X

35

x x x

IO

X

345

x x x

34

x x x

35

X X

69

X

x x x

69

x x x

138

145

/j = 0.127 (mmlmin)

Saturated

x x x

100

Saturated Q, = 1.27 mmlmin 6, (kPuJ

Table 1 Test Program

X X

X

35

138

X

320

X X X

X

69

138

X

34

fi, = 0.0127 (mmjmin j

X X

IO

X X

x x x

35

100

x x x

69

/Il, = 0.005 (mmjmin)

X

145

Saturated b, = 0,015 mmlmin u,, (kPa)

x x x

138

X

320

Geomembrane/cohesive soil interface shear behavior

579

Table 2 Physical and Index Properties of Three Different Clays

Permeability, k Coefficient of consolidation, Cv Specific gravity, Gs Unconfined compression strength Liquid limit Plastic limit Plasticity index

Clay No. 1

Clay No. 2

Clay No. 3

7.1 × 10 11 m/s 1-4 × 10-7 m2/s

2.2 × 10-9 m/s 1-9 × 10 7 m2/s

1-8 × 10-l° m/s 3.8 × 10-8 m2/s

2.78

32 21 11

2.76 217 kPa

2.79 172 kPa

37 25 12

55 24 31

Clay No. 1 was a glacial till material and classified as CL by the Unified Soil Classification System (USCS). Grain size analysis indicated that clay No. 1 consisted of 26% gravel, 23% sand, 35% silt and 16% clay sized factions. The maximum dry density and optimum moisture content of clay No. 1 as determined by the modified Proctor compaction test were 2.07 Mg/m 3 (129 lb/ft 3) and 11%, respectively. Samples for testing in the saturated condition were compacted to a dry density of 1.89 Mg/m 3 (118 lb/ft 3) at a water content of 14%. Clay No. 2 was also a glacial till and classified as CL by the USCS. Grain size analysis indicated that clay No. 2 consisted of 15% sand, 50% silt and 35% clay sized factions. The maximum dry density and moisture content as determined by the Harvard miniature compaction test were 1.76 Mg/m 3 (110 lb/ft 3) and 18%, respectively. Samples for testing were compacted to a dry density of 1.75 Mg/m 3 (109 lb/ft 3) at a water content of 20%. Clay No. 3 was a mixture of clay No. 2 and 5% bentonite by weight and classified as CH by the USCS. The addition of bentonite served to reduce the permeability and shear strength of the natural material. Grain size analysis indicated that clay No. 3 consisted of 12.5% sand, 47.5% silt and 40% clay sized factions. The maximum dry density and optimum moisture content as determined by the Harvard miniature compaction test were 1.75 Mg/m 3 (109 lb/ft 3) and 20%, respectively. Samples for shear testing were compacted to a dry density of 1.68 Mg/m 3 (105 lb/ft 3) at a moisture content of 22%. Two types of geomembrane were used in this study whose specifications are G U N D L E 60-mil/1.5 mm smooth HDPE geomembrane and G U N D L E 60-mil/1.5 mm textured HDPE geomembrane. These two types of geomembrane are very commonly used in the construction of solidhazardous-waste repositories.

K.L. Fishman, S. Pal

580

The textured HDPE geomembrane had a wavy outside layer fully integrated with the inside, barrier layer. The asperities on the outside surface had an average thickness of 0.5 m m (0.02 in) and spacing 2.5 mm (0.1 in).

4 DISCUSSION OF RESULTS Typical plots of shear stress versus shear displacement are presented in Figs 2 and 3. These figures show the results from testing clay No. 3 alone and in contact with each of the smooth and textured HDPE geomembranes. Figure 2 is for a shear displacement rate of 1.27 mm/min (0.05 in/ min) roughly corresponding to undrained shear. Figure 3 refers to a shear displacement rate of 0.005 mm/min (0-0002 in/min). It will be demonstrated subsequently that a shear rate of 0-005 mm/min (0.0002 in/min) may be considered a drained shear test for the clay alone and with textured H D P E interface. The stress-strain behavior during undrained shear may be considered as ductile for the clay and the textured geomembrane interface. For clay in contact with smooth HDPE the behavior is best described as elastic, perfectly plastic, although there is a slight reduction from a peak strength

100.0 I Textured

70.0~

6o.t

'

5

f(-

,,,/

Interface "~

-',-

Clay Alone

30.0t Smooth Interface

0.04

0.0

1:0 2:0 ~ 0

5:0 610-- 7.0 8'.0 9:0 ~ 1 , '

LO

Shear Displacement (mm) Fig. 2. Typical plot of shear stress versus shear displacement; ~,; = 69 kPa; fir = 1-27 ram/ min.

Geomembrane/cohesive soil interface shear behavior

581

100.0

90.080.070.060.050.0

Textured Interface

40.0

/

~

--..

///"

30.0

Clay Alone

~

/ 20.0

;///......

Smooth Interface .............................................................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10.0 O0 I J ~ '0.0

1.0

. . . 210 3.0

.

4.0

5.0

, 6.0

710

, 8.0

, 9.0

10.0

1t.0

1~.0

1 1.0

Shear Displacement (ram) Fig.

3. Typical

plot

of

shear

stress versus shear #r = 0"005 ram/rain.

displacement;

an = 69 kPa;

after a large displacement. The clay/textured HDPE interface exhibits an undrained shear strength which is higher than that of the clay alone while the clay/smooth H D P E interface exhibits low undrained shear strength compared to clay alone. During drained shear the clay behavior may still be considered as ductile, however, the textured geomembrane interface exhibits a more brittle behavior compared to the response during undrained shear. In terms of peak strength the textured geomembrane interface exhibits the highest strength. The residual strength of the textured geomembrane typically is equal to or slightly less than the shear strength of the clay alone. Observations of the textured geomembrane interface after shear indicated that the failure plane passes through the clay material. This observation is consistent with that of Koener et al. (1986) who also tested clay/textured H D P E interfaces under drained, but unsaturated conditions. The behavior of the smooth geomembrane interface during the slowest rate of shear is similar to that observed for higher rates of shear. The shear behavior of the clay/textured H D P E interfaces may be interpreted in terms of the deformation characteristics which were observed during drained and undrained shear behavior. Figures 4 and 5 show the measured vertical displacements which accompany shear displacement for

582

K.L. Fishman, S. Pal

0.50-

0.400.3ONormal Stress= 137.9 kPa r

E,E 0.20i

34.5 kPa

0.10-

.........

0.0o-

: ........II.T :---LI

I-I ...

I~ -0.10-0.20-0.30-0.40 -0.50 0.0 Fig.

1'.0

2'.0

4. Deformations

310

410 510 6'.0 710 810 9'.0 16.0 1 ~ , ~ Shear Displacement (mm)

during

shear for textured /~r = 0-005 mm/min.

geomembrane/clay

interface

0.500.400.30 0.20-

i ::o:]

..." ...................

,......,

:..'C'~

..................

Normal Stress= 68.95 kPa

....................... ~~:::_Z~L~L/L27__,~,I~ ~34.5

i

. . . . . . . . . . . . . . .

kPa

-0.20 -0.30-0.40 -0.50

0.0

Fig.

1'.0

210

5. Deformations

310 during

410 510 6'.0 710 810 9'.0 16.0 1"~.0 1'~.0 1; I.O Shear Displacement (ram) shear for textured ~r = 1"27 mm/min.

geomembrane/clay

interface;

583

Geomembrane/cohesive soil interface shear behavior

drained and undrained shear, respectively. Vertical displacements are plotted with compression considered as positive values. Shear failure planes must pass through clay material near the interface, and effective stresses in the clay depend on the condition of drainage. During undrained shear a dilative response is observed for the textured geomembrane interface which is not observed for undrained tests with clay alone. D u e to the lack of drainage and the tendency for dilation, a reduction in pore water pressure occurs near the interface. A reduction in pore water pressure tends to increase the effective confining pressure, and thus the shear strength of the clay near the interface compared to drained conditions. F o r drained conditions pore water pressures remain neutral during shear, and deformations are compressive; similar to those for the clay alone. Thus, the shear strength of the clay near the interface of the textured surface during drained shear is similar to that realized when testing the clay alone. 4.1 Time to failure for drained conditions

Table 3 compares the time to failure in shear to the time for 50% consolidation (ts0) of the specimens due to the confining stress. The ts0 is a function of the clay properties and the length of the drainage path, hdr. The time to shear failure, tf, depends on the shear displacement at failure, Af, and the applied rate of shear displacement,/t r. Drained conditions were not achieved for all specimens tested under the applied /~r" This is especially true for the clay/smooth H D P E interfaces. The clay/textured H D P E interfaces tested exhibited a longer tf than for the clay/smooth H D P E interfaces, however tf was still less than 50 ts0. F o r Table 3 Comparison of Time to Failure with 50t50 Material

Clay No. Clay No. Clay No. Clay No. Clay No. Clay No.

1 1/T. HDPEa 1/S. HDPEb 3 3/T. HDPE 3/S. HDPE

aTextured HDPE. bSmooth HDPE. c~ 45t5o. d~ 38t5o.

hat (mm)

tso (rnin)

50t5o (min)

Af (mm)

itr tf (ram~rain) (min)

12.7 17 17 12.25 11 11

3.73 6-70 6.70 12.73 10-64 10.64

186.5 335.0 335-0 636.5 532.0 532-0

2.54 2.29 0.25 5.08 2.00 0-90

0.015 0.015 0.015 0.005 0.005 0.005

169.33 152-66 16.67 1016.0 400 180

Drained

yc N N Y ya N

584

K.L. Fishman, S. Pal

the clay No. 3/textured HDPE interface tr was 38 tso, but the behavior was apparently drained when compared to the behavior of clay No. 3 alone. For the clay No. 1/textured H D P E interface tr was only 23 ts0 and drained conditions may not have prevailed during shear.

4.2 Failure envelopes Figures 6 to 8 depict failure envelopes for clay No. 3 alone and for each of the interfaces with textured and smooth HDPE. Failure envelopes are presented with respect to three different rates of shear displacement corresponding to 1.27 mm/min, 0.127 mm/min, 0-005 m m / m i n (0.05 in/ min, 0.0005in/min and 0-0002 in/min). For clay No. 3 alone and in contact with textured HDPE a trend of increasing strength with rate of shear is evident. The slopes of the failure envelopes do not vary significantly with rate of shear displacement, but higher cohesion or adhesion values are evident for higher rates of shear displacement. As the shear displacement rate approaches that for a drained condition the cohesion or adhesion values approach zero. Figure 8 shows the failure envelopes for the clay No. 3-smooth HDPE. The rate of shear displacement does not seem to have a significant impact on the shear strength. After the mobilization of peak shear strength a reduction in shear resistance was observed for all interfaces tested under both fast and slow rates of shear. For tests involving textured geomembrane interfaces sheared rapidly (1-27 mm/min (0.05 in/min)) the residual shear strengths were not fully 140.0/ 120'01

[]

1.27 mm/min 1.27 mm/min

......... 0.127 mm/min ..... 0.0051 mm/min • 0.127 mm/min x 0.0051 mm/min

~" 10004 ,.-'"m ..., ....,.--'"'""

80.0

....-

60.0 40.0 20.0 0.0 0.0

26.0

46.0

66.0

86.0 1 0 0 . 0 120.0 Normal Stress (kPa)

Fig. 6. Failure envelopes for clay No. 3.

140.0

16b.o

180.0

585

Geomembrane/cohesive soil interface shear behavior 140.0, 120.0t

D

1.27 rnm/min 1.27 mm/min

......... 0.127 mm/min •

1

0.127 rnm/min

[]

80.0-

. . . . . 0.0051 mm/min :~ 0.0051 mm/min

...............

•

60.040.020.00.0 0.0

26.0

46.0

66.0

86.0

100.0

12'0.0

14b.0

16b.0

180.0

Normal Stress (kPa)

Fig. 7. Failure envelopes for textured geomembrane/clay No. 3 interface. 140.0120.0-

3

1.27 ram/rain 1.27 mrnhnin

......... 0.127 rnm/min . . . . . 0.0051 mm/min • 0.127 mm/min ;~ 0.0051 mm/min

100.0-

80.0-

q~

60.0-

0"004.0

20.0

46.0

66.0

86.0

100.0

12'0.0

14b.o

16b.o

180.0

Normal Stress (kPa)

Fi~. 8. Failure envelopes for smooth geomembrane/clay No. 3 interface.

reached given the amount of shear displacement (6-13 mm) (0.25-0.5 in), at the termination of direct shear testing. Development of residual shear strength did occur for smooth geomembrane interfaces sheared rapidly, for all interfaces sheared slowly, 0.005 mm/min (0.0002 in/rain), and for partially saturated clay/geomembrane interfaces. When tested alone the only clay that exhibited significant post peak behavior was clay No. 2, although

to reach residual strength.

rain

. --

ainsufficient displacement

b F o r c l a y n o . 1 /i r = 0 . 0 1 5 m m

24 °

13.5 °

35 °

ff~r

Residual

--

--

--

--

--

.

28 °

20.5 °

35 °

~)p

Clay No. 3/Smooth

--

Clay No. 3

Peak

Clay No. 3/Textured

.

40

0-0

41

Clay No. 2/Smooth

Clay No. 2

Clay No. 1/Textured

1/Smooth

Clay No. 1

Clay No.

(kPa)

C

Part saturated

.

Summary

20

8

5

7

3

7

(kPa)

C

Peak

22 °

11 °

27 °

24 °

14 °

21 °

Cp

',mini

20 °

11 °

27 °

--

22.5 °

13 °

21 °

Cr

Residual

~r =O'O05(mm]b

Saturated

of Test Results

Table 4

48

9

35

7

28

34

0.0

21

C (kPa)

Peak

24 °

14"

16 °

16 °

31 °

19 °

11 °

22 °

4~p

Undrained

ID

14 °

16 °

16 °

ID

ID a

9°

22 °

Cr

Residual

.v,

O~

Geomembrane/cohesive soil interface shear behavior

587

residual strengths were not fully mobilized at test termination. Table 4 summarizes the shear strength parameters from all the tests conducted for this investigation.

5 CONCLUSIONS The conclusions of the above work are as follows: 1. The shear strength of clay-smooth H D P E interfaces is less than that of the clay alone for all cases studied. 2. The shear displacement required to reach peak shear stress is extremely small for clay/smooth HDPE interfaces, and is of the order of 0-25 mm (0-01 in). This makes the attainment of drained conditions during shear difficult for interfaces with clay having low coefficients of consolidation. Either extremely low rates of shear displacement in the order of 10-4 mm/ min (10 -5 in/min), or thin clay samples in contact with the smooth HDPE interface must be employed to achieve drained conditions. 3. The shear strength of clay/smooth H D P E interfaces did not appear sensitive to the rate of shear displacement for the range of shear displacement considered in this study. 4. The shear behavior of clay/textured H D P E interfaces is influenced by the rate of shear displacement. For low rates of shear displacement, whereby drained conditions exist during shear, the shear strength of the interface is equal to or slightly less than that of the clay. For higher rates of shear displacement the shear strength of the interface increases, and exceeds the shear strength of the clay at the same rate of shear. The increase in strength of the interface during undrained shear may be correlated with the deformation characteristics at the interface. Locally, a tendency for dilation during undrained shear at the interface may induce negative interfacial pore water pressure, increasing the effective confining pressure and resulting in an increased shear strength of the clay near the interface. 5. Increased shear strength from testing textured H D P E interfaces under undrained conditions compared to drained conditions was largely manifested through an increased adhesion at higher rates of shear displacement. 6. The behavior of textured geomembrane interfaces during drained shear testing exhibit both peak and residual strength. 7. Tests performed with partially saturated clay indicated that the geomembrane-clay interface shear strengths could far exceed those determined for saturated conditions.

588

K.L. Fis'hman, S. Pal

ACKNOWLEDGMENT Partial funding for this study was provided by Hyland Facilities Associates of Belmont, New York and Buffalo Environmental Consultants Inc. REFERENCES Akber, S.Z., Hammanji, Y. & Lafleur, J. (1985). Frictional characteristics of geomembranes, geotextiles and geomembrane-geotextiles composites. Proceedings, 2nd Canadian Symposium on Geotextiles and Geomembranes. Canadian Geotechnical Society, Edmonton, Alberta, pp. 209-17. Byrne, R.J., Kendall, J. & Brown, S. (1992). Cause and mechanism of failure, Kettleman Hills Landfill B- 19, Unit 1A. Stability and Performance of Slopes and Embankments H. ASCE Geotechnical Special Publication No. 31, pp. 1188-1215. Collios, A., Delmas, P., Goure, J.P. & Giroud, J.P. (1980). Experiments on soil reinforcement with geotextiles. The Use of Geotextilesfor Soil Improvement. ASCE National Convention, Portland, Oregon, April, pp. 53 73. Degoutte, G. & Mathieu, G. (1986). Experimental research on friction between soil and geomembranes or geotextiles using a thirty by thirty square centimeter shearbox. Proceedings of the Third International Conference on Geotextiles, Vienna, Austria, pp. 1251-6. Delmas, P., Gourc, J.P. & Giroud, J.P. (1979). Experimental analysis of soil geotextile interaction. Proceedings of the International Conference on Soil Reinforcement." Reinforced Earth and Other Techniques, Association Amicale des Ingenieurs Anciens Eleves de l'Ecole Nationale des Ponts et Chaussees, Paris, pp. 29 34. Druschel, S.J. & O'Rourke, T.D. (1991). Shear strength of sand geomembrane interfaces for cover system and lining design. Geosynthetics '91, IFAI, Atlanta, GA, pp. 159 73. Eigenbrod, K.K. & Locker, J.G. (1987). Determination of friction values for the design of side slopes lined or protected with geosynthetics. Canadian Geotechnical Journal, 24(4), 509-19. Fourie, A.B. & Fabian, K.J. (1987). Laboratory determination of clay geotextile interaction. Geotextiles and Geomembranes, 6(4), 275 94. Geotek Inc (1987). Direct Shear Testing of Friction Liner Material. Prepared for Gundle Lining Systems, Inc. Haliburton, T.A., Anglin, C.C. & Lawmaster, J.D. (1978). Testing of geotechnical fabric for use as reinforcement. Geotechnical Testing Journal, 1(4), 203 12. Ingold, T.S. (1982). Some observations on the laboratory measurement of soil geotextile bond. Geotechnical Testing Journal, 5(3/4), 57-67. Koerner, R.M., Martin, J.P. & Koerner, G.R. (1986). Shear strength parameters between geomembranes and cohesive soils. Geotextiles and Geornembranes, 44(1), 21 30. Lafleur, J., Sail, M.S. & Ducharme, A. (1987). Frictional characteristics of geotextiles with compacted lateritic gravels and clays. Geosynthetics '87, New Orleans, pp. 205-15.

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