Geotextiles and Geomembranes 10 (1991) 523-529

Effect of Soil Compaction Conditions on Geomembrane-So i l Interface Strength

Robert H. Swan Jr, Rudolph Bonaparte, Robert C. Bachus

GeoSyntec Consultants, Geomechanics and Environmental Laboratory, 1600 Oakbrook Drive, Suite 565, Norcross, Georgia 30093, USA

Charles A. Rivette & Daniel R. Spikula

Browning-Ferris Industries, PO Box 3151, Houston, Texas 77253, USA

A B S T R A C T

A laboratory investigation was recently undertaken to evaluate the shear strength of the interface between a cohesive soil used for liner construction and a high-density polyethylene (HDPE) geomembrane. In the investiga- tion, the interface shear strength was measured in a direct shear apparatus. The compaction water content and dry unit weight of the soil were varied in each test. It was found that the shear strength of the interface between these two materials is strongly affected by the compaction water content and dry unit weight of the soil. It is concluded from the test results that the soil compaction conditions strongly influenced the interface shear strength and this factor, among others, should be carefully considered during design.

INTRODUCTION

The permeability characteristics of cohesive soils used as liners and final covers are controlled in large measure by the compaction water content and dry unit weight of the soil at the time of compaction. 1 For this reason, design specifications for soil liners and final covers usually require cohesive soils to be compacted to a water content above the optimum water content (determined in the standard Proctor (ASTM D698) or modified Proctor (ASTM D1557) compaction test) and to a relatively high dry unit weight. 2 The conditions under which a soil material is compacted affects not only

523 Geotextiles and Geomembranes 0266-1144/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

524 Robert H. Swan Jr et ai.

the soil's permeability, but also its shear strength. 3 Since both the permeability and shear strength of a cohesive soil are affected by the compaction conditions, it should be expected that the shear strength at the interface between a cohesive soil and a geomembrane liner is also affected.

The purpose of this technical note is to report the results of an investigation into the effect of compaction water content and dry unit weight on the interface shear strength between a cohesive soil and a 1.5-mm thick high-density polyethylene (HDPE) smooth geomembrane. For the investigation, interface direct shear tests were conducted at seven different combinations of compaction water content and dry unit weight. Other test variables, such as the method of compaction (i.e. kneading versus tamping), sample soaking after compaction, rate of shear and length of time between compaction and shearing, were controlled but were not specifically addressed by the investigation.

SOIL P R O P E R T I E S

The material used in the investigation is a cohesive soil composed of a processed mine overburden which contains a well-graded mixture of fine gravels through clay-sized particles. The material has a liquid limit of 36, a plastic limit of 22, and a plasticity index of 14 (ASTM D4318). The material has approximately 35% fines (i.e. finer than 0-074 mm) and about 30% fine gravel. The maximum particle size is about 25 mm, which was controlled by screening during processing of the soil. The soil classifies as a clayey gravel with sand (GC) material according to the Unified Soil Classification System (ASTM D2487). Standard and modified Proctor compaction test results for the soil are shown in Fig. 1.

I N T E R F A C E D I R E C T S H E A R TESTS

The compaction water content and dry unit weight for each of the seven interface direct shear tests are shown in Table 1. For each of the tests, the test specimen was formed from bottom to top in the following manner:

• A dense layer of sand was placed as a bedding layer in a large box. • The H D P E geomembrane was placed on top of the sand and

anchored to the back of the box. • A 305 mm × 305 mm × 200 mm steel box attached to a hydraulic

loading system was placed on top of the geomembrane. • A 50-mm thick layer of cohesive soil, which had previously been

moisture conditioned, was compacted on top of the geomembrane

Effect of soil compaction on geomembrane-soil interface 525

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standard Proctor compaction test (ASTM D698). Test data are given on Table 1.

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Fig. 2. Schematic illustration of direct shear box.

inside the steel box. Compaction was achieved by tamping the soil with a 200 × 200 mm square steel tamper. The compaction energy imparted to the soil was not measured and it was not held constant from test to test. The dry unit weight of the soil specimen was calculated from the weight of material used to form the specimen and multiple measurements of the soil thickness.

• A thin geotextile was placed on top of the soil and the remainder of the shear box was filled with sand. Finally, a neoprene bladder was fitted between the sand and the top of the shear box to serve as the pneumatic normal stress loading system.

A schematic illustration of the direct shear apparatus used in the investigations is shown in Fig. 2. For the tests, the upper steel box was lined with a greased smooth plastic sheet to minimize sidewall friction. Upon application of the normal stress, the upper shear box was raised about 3-5 mm above the geomembrane, as shown in Fig. 2. The procedure for conducting each direct shear test was as follows:

• The sample was prepared as described above. • A normal stress of 140 kPa was applied to the soil using the pneumatic

loading system by pressurizing the space between the neoprene bladder and the top of the shear box. The normal stress was maintained for approximately 12 h to allow consolidation.

528 Robert H. Swan Jr et at.

• The sample was sheared at a rate of 0.5 mm/min by applying a horizontal force to the shear box through a hydraulic loading system designed to minimize the overturning moment acting on the box. During the test, the back end of the geomembrane remained anchored to the back of the shear box.

• Shearing was continued until a peak shear force had been achieved and a relatively constant or decreasing value of shearing load was observed. The shear displacement in each test was approximately 30 mm, whereas the peak shear stress was typically achieved at a displacement of 1-2 mm, or less.

Shear tests were also conducted at normal stresses higher than 140 kPa. However, as the purpose of this technical note is to address the effects of compaction conditions, test results at only one normal stress are reported.

INTERFACE DIRECT SHEAR TEST RESULTS

The interface direct shear test results are summarized in Table 1. The compaction conditions of each test specimen are reported, as is the peak shear stress measured during each test. The results shown in Table 1 are superimposed on the standard and modified Proctor test results in Fig. 1. Inspection of Table 1 and Fig. 1 shows that the peak shear stress during the tests ranged from a low value of 17 kPa for a sample compacted to a relative compaction of 93% (based on ASTM D698) and a water content of 3.7 percentage points wet of the optimum water content, to a high value of 49 kPa for a sample compacted to a relative compaction of 102% and a water content of 1-4 percentage points wet of optimum. Thus, for the conditions tested, the peak shear strength of the soil-geomembrane interface had a range of almost 32 kPa. It also appears that for the test conditions, the peak shear stress increased with both increasing water content and increasing dry unit weight. These results also indicate that for the test conditions the peak shear stress increases with increasing compactive effort.

COMMENTS ON TEST RESULTS

This study was of a limited scope and addressed only the compaction conditions of a specific cohesive soil against a specific geomembrane (smooth HDPE). The compaction conditions were controlled to approxi- mate the range of potential conditions that could be anticipated on a project in which the cohesive soil would be used. Under the limited range

Effect of soil compaction on geomembrane-soil interface 529

of test conditions reported herein, the peak interface shear stress varied by a factor of almost three. If only test conditions meeting the requirements of the project specifications are considered (i.e. compaction to a relative compaction of at least 95% based on ASTM D698 and a water content wet of the optimum water content), the potential range in interface shear test results exceeds a factor of two. This range could be even larger if the effects of the other test variables cited previously are considered.

To understand the impact of the test results on design, the results can be interpreted in terms of a total-stress interface friction angle, ~bi, using the equation:

(~i = tan-l (ri/cr,) (1)

where r~ = peak interface shear stress measured in the direct shear test, and O'n = normal stress in the direct shear test. Total stress interface friction angles calculated using eqn (1) are reported in Table 1. From Table 1, it can be observed that for the range of test conditions, the total stress interface friction angle ranged from 7 ° to 19 ° . If these two values of friction angle were used in an assessment of slope stability, they might result in very different conclusions as to the stability of the slope.

From the test results, it is concluded that the conditions under which a cohesive soil specimen is compacted have a significant effect on the results of cohesive soil-geomembrane interface direct shear tests. This effect should be expected from previous knowledge of the behavior of compacted clays. As a result, it is recommended that for design, an interface testing program be selected that includes tests which cover an appropriate range of compaction conditions, as well as the appropriate range of the other important test variables.

REFERENCES

1. Mitchell, J. K., Hooper, D. R. & Campanella, R. G., Permeability of compacted clay. Journal of the Soil Mechanics and Foundation Division, American Society of Civil Engineers, 91 (SM4) (1965) 41-65.

2. Daniel, D. E., Summary review of construction quality control for earthen liners. In Waste Containment Systems: Construction, Regulation and Perform- ance, American Society of Civil Engineers, Geotechnical Special Publication No. 26, Nov. 1990, pp. 175-89.

3. Seed, H. B., Mitchell, J. K. & Chan, C. K., The strength of compacted cohesive soils. Proceedings, Research Conference on Shear Strength of Cohesive Soils, American Society of Civil Engineers, Boulder, June 1960. ASCE, pp. 877-964.