Knowledge of the shearing resistance between soil and concrete,steel, geosynthetics, and wood is needed to evaluate the stability ofmany types of geotechnical structures. Numerous studies have pro-duced useful data on the shearing resistance of interfaces involvingsand on steel or concrete, and clay on polymer geosynthetics, butmuch less information is available on the strength of interfaces be-tween clays and rigid foundation materials. Further, most interfaceshear tests have been conducted at stress levels that are convenientfor laboratory testing and appropriate for typical field conditions.In contrast, there is a lack of experimental data on soil interfacestrengths at low stress levels.

At the University of Texas at Austin, scale models of offshorefoundations are being tested in tanks of normally and overconsoli-dated kaolinite (El-Gharbawy and Olson 1999; Olson et al. 2001;Rauch et al. 2001). The behavior of suction caissons is being stud-ied using acrylic and aluminum models measuring up to 100 mm indiameter by 1 m long, in a 1-g environment. Suction caissons arelarge-diameter steel “piles” that are used in the deep marine envi-ronment to anchor floating structures. They are installed using acombination of dead weight and internal suction. Early suctioncaissons had relatively large diameters and shallow depths, but thesoft clays found in the Gulf of Mexico require deeper penetrationto develop the desired capacity. Knowledge of the shearing resis-tance at the caisson/clay interface is particularly important for de-sign, especially for vertical loading.

Laboratory-scale model tests are being used to study the behav-ior of suction caisson foundations, with the findings used to supportthe development of improved analytical and design methods. Tointerpret these laboratory tests, the interface shearing behavior ofclay at low effective normal stresses must be characterized. Thin-specimen direct shear (TSDS) tests were undertaken to character-ize the drained strength of kaolinite interfaces at low effective nor-mal stresses.

Previous Studies of Soil and Interface Shear Strength

Many engineers accept the presence of an effective stress cohe-sion intercept for a “cohesive” soil, but not for a “cohesionless”soil. However, clean sands can exhibit cohesion intercepts ifstraight lines are fitted to strength envelopes that are actuallycurved. Test results dating back to at least Casagrande (1940) indi-cate that effective stress failure envelopes for sands are curved.Curved failure envelopes for sands have been widely reported(Vesic and Clough 1968; Lee and Seed 1967; Ponce and Bell1971). De Beer (1970) demonstrated that large errors can resultfrom using failure envelopes measured in the usual range of effec-tive stress to interpret small-scale laboratory tests in sand.

For clays, early shear tests of normally consolidated Boston Blueclay indicated no cohesion intercept (Jurgenson 1934). Casagrandeand Hirschfeld (1960) assumed that the cohesion intercept for claywas zero, but the friction angle decreased with increasing normalstress. Jacobson (1953) found an “origin cohesion” for normallyconsolidated natural clays. Bjerrum (1954) also found a cohesionintercept, which he attributed to attractive forces between the clayparticles, while Kenney at al. (1967) and Tavenas et al. (1971)demonstrated cohesion due to cementation. Bjerrum (1973) indi-cated that strengths could increase due to the effects of aging. How-ever, the curvature of failure envelopes for normally consolidatedclays is apparent in the usual stress range (Olson 1974) as well asat elevated stresses (Bishop et al. 1965; Brouillette et al. 1993).

Duncan et al. (1980) found that curved failure envelopes formany soils could be conveniently modeled in the typical pressurerange using a friction angle that decreases linearly with the loga-rithm of effective normal stress. Following this suggestion with thesecant friction angle (��s), a non-linear envelope can be represented(Fig. 1) with:

��s � ��s1 � ���s log (�n�) (1)

where ��n is the effective normal stress, and ��s1 and ���s are pa-rameters used to fit the measured strength data. Like the internalstrength of soil, failure envelopes for soil sheared along various in-terfaces are likely curved and can be approximated in a similarmanner:

��s � ��s1 � ���s log (�n�) (2)

Robert C. Pedersen,1 Roy E. Olson,2 Alan F. Rauch3

Shear and Interface Strength of Clay at Very LowEffective Stress

ABSTRACT: Thin-specimen direct shear (TSDS) tests were conducted to measure the shearing strength of kaolinite, and interface strengths be-tween kaolinite and acrylic plastic and anodized aluminum, at normal effective stresses from 1 to 2400 Pa (0.02 to 50 lb/ft2). At the lowest effectivenormal stresses, curved strength envelopes fitted through the data exhibited no cohesion and high secant friction angles. Accurate information onthe behavior of soil in this low-pressure range is needed to properly interpret the behavior of prototype foundations in laboratory-scale model tests.

KEYWORDS: shear strength, low stress, interface friction, direct shear, tilt-table

Copyright © 2003 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. 1

Geotechnical Testing Journal, March 2003, Vol. 26, No. 1Paper ID GTJ200310648_261

Available online at: www.astm.org

Received July 17, 2001; accepted for publication April 5, 2002; publishedJan. 17, 2003.

1 Graduate research assistant, Department of Civil Engineering, The Univer-sity of Texas at Austin, Austin, TX 78712.

2 L. P. Gilvin Professor, Department of Civil Engineering, The University ofTexas at Austin, Austin, TX 78712.

3 Assistant professor, Department of Civil Engineering, The University ofTexas at Austin, Austin, Texas, 78712.

where ��s is the secant interface friction angle and ��s1 and ���s arefitting parameters.

Potyondy (1961) measured interface friction angles in thenormal-stress range of 50–150 kPa using direct shear tests. It islikely that Potyondy’s tests with clayey soils involved partialdrainage, making it impossible to interpret the interface shearingresistance in terms of effective stress. Direct shear tests on inter-faces between sand and wood, concrete, or metal have also been re-ported by Acar et al. (1982), Bosscher and Ortiz (1987), Reddy et al. (2000), and others.

Relying mostly on direct simple shear tests of sand on steel orconcrete, Uesugi and Kishida (1986a) investigated the relation-ships between interface shear resistance and particle size, particleshape, surface roughness, and normal stress. They found that theinterface friction varied with surface roughness up to a criticalvalue; on even rougher surfaces, failure occurred in the soil and theinterface strength was determined by ��. The interface friction an-gle was found to decrease slightly between normal stresses of 100to 4000 kPa. They also related interface friction to particle round-ness and a surface roughness index (Uesugi and Kishida 1986b;Kishida and Uesugi 1987). For effective normal stresses in therange of 100 to 500 kPa, Uesugi et al. (1988, 1990) found that lit-tle sliding occurs on the interface at shear stresses below failure. Atthe peak shear stress, sand slipped along a smooth interface, whileshearing was concentrated in the sand on a rough interface.

Fewer interface shear tests have been conducted with cohesivesoils on concrete or steel. In drained direct shear tests, Clark andMeyerhof (1972) and Littleton (1976) found contacts between steeland clays to be entirely frictional (no cohesion). Tsubakihara andKishida (1993) used direct simple shear tests and found that, forclay-steel interfaces, shearing deformations occur in the soil up tothe peak shear stress, followed by sliding along a smooth interface.On a surface rougher than the critical value, deformation and fail-ure occur completely in the soil.

Lehane and Jardine (1996) summarized the results of interfaceshear tests performed with clays, silts, and sands on steel. Theyshowed that the interface friction angle decreased as the soil be-came coarser, consistent with the findings of Uesugi and Kishida(1986b). Tsubakihara et al. (1993) observed three failure modes:shearing within the soil placed against a rough surface, sliding ofthe soil along a smooth interface, and, for clay-sand mixtures, par-tial sliding in localized areas of the interface contact area.

Direct shear tests of clays on geomembranes have been reportedby Koerner et al. (1986), Seed and Boulanger (1991), Fishman andPal (1994), and others. Direct shear tests have also been performedon geosynthetic clay liners, some involving normal pressures lessthan 10 kPa (Gilbert et al. 1996; Merrill and O’Brien 1997; Fox et

al. 1998). Tests of sand on polymer membranes show that the in-terface friction angle increases with higher normal stresses due toparticles becoming embedded in the membrane (Girard et al.1990), an effect that changes with the hardness of the polymer(O’Rourke et al. 1990).

Most of the interface shear tests described in the literature wereconducted using either direct shear or simple shear devices. Tor-sional shear devices are sometimes used to achieve larger shearingdeformations (Yoshimi and Kishida 1981; Stark and Poeppel1994). Almost all of the interface tests cited here were conductedat normal stresses exceeding about 15 kPa.

Direct Shear Tests with Tilt Tables and Thin Specimens

A simple means of conducting shear tests at very low stress lev-els involves the use of a tilt-table, where shear stresses are gener-ated by inclining the specimen. Tilt-table devices have been usedto measure low-pressure friction along rock joints (Cawsey andFarrar 1976; Hencher 1976; Bruce et al. 1989) and geosynthetic in-terfaces (Girard et al. 1990; Shan 1993; Lalarakotoson et al. 1999).Tilt-table shear tests have the following disadvantages (Hencher1976; Shan 1993; and Gilbert et al. 1995): displacements are notcontrolled and the post-peak response cannot be measured, testpressures are limited by toppling of the surcharge weights, andnonuniform normal stresses develop on the interface as the deviceis inclined. Advantages include the elimination of internal machinefriction, not forcing failure to occur along the interface, and theability to perform tests under low normal stresses.

Thin soil specimens can be used in tests to measure the strengthof soils. Terzaghi (1925) performed direct shear tests on specimensas thin as 3 mm and found that ��s for his “yellow clay” decreasedfrom 38 to 15° as the normal stress rose from 3 to 90 kPa. respec-tively. Over the same range of pressures, he measured ��s for clayon glass of 33 to 12°. Although Terzaghi’s samples were initiallyconsolidated, full drainage may not have occurred during shearing.Jurgenson (1934) performed direct shear tests on remolded clayspecimens as thin as 2 mm and measured friction angles that wereindependent of specimen thickness. Casagrande and Hirschfeld(1960) reported obtaining the same failure envelope using standardthick specimens and specimens as thin as 3 mm, in the stress rangeof 10 to 30 kPa. Kenney et al. (1967) performed simple shear testson specimens as thin as 1 mm and found similar friction anglesfrom thin or thick specimens. Chandler and Hardie (1989) studiedresidual friction angles using specimens as thin as 2 mm.

Although not suitable for testing undisturbed specimens, usingthin specimens in direct shear tests on remolded soil is an estab-lished technique with many advantages: thinner specimens reduce

2 GEOTECHNICAL TESTING JOURNAL

FIG. 1—Model of curved strength envelope using the secant friction angle.

the time needed to dissipate excess pore pressures and eliminate theneed for lateral confinement.

Thin-Specimen Direct Shear Tests on Kaolinite

In this study, submerged kaolinite specimens, 0.3 to 0.5 mmthick, were sheared to failure in what will be called “thin-specimendirect shear” (TSDS) tests. The tests were conducted using a tiltingtable apparatus in a water bath (Fig. 2). Specimens were preparedby pouring kaolinite slurry on top of a 13-mm-thick by 150-mm-diameter porous stone that had a temporary slurry retention bandaround the sides. A top platen fabricated from acrylic, aluminum,or porous stone was then placed on top. The slurry was allowed toconsolidate, with drainage through the lower stone, while the bandwas allowed to slide down over the lower stone. Consolidationpressures ranged from 1 to 2400 Pa (0.02 to 50 lb/ft2). The testswere set up, then submerged in the water bath, and the retainingband was removed to leave the specimen unconfined.

Shearing stresses were applied by elevating one end of the table(Fig. 2) in increments of one degree, with the apparatus locked inposition for 1 min after each increment to allow for full dissipationof excess pore water pressures (Pedersen 2001). The angle of incli-nation (�) was determined optically using a cathetometer. Shearingdisplacements were not measured.

The average normal (�) and shear () stresses were:

� � (W / A) cos � (3)

� (W / A) sin � (4)

where A is the area of the shear plane, W is the submerged weightof the top platen and surcharge weights, and � is the angle of rota-tion. The estimated accuracies of the test measurements are sum-marized in Table 1. Rotation of the assembly of soil, platens, andweights converts the applied normal force to an inclined and ec-

centric load (Hencher 1976; Shan 1993; Pedersen 2001). Flatweights were used to minimize this problem. The maximum incli-nation of the test was limited by the tendency for the weights to tip.

A 150-mm-diameter porous stone was used as the top platen inobtaining measurements of the kaolinite’s internal shear strength(Jurgenson 1934; Chandler and Hardie 1989). Interface tests wereconducted with a top platen of smooth, unaltered acrylic or an-odized aluminum, the two materials used to make the model suc-tion caissons. The interface platens were larger in diameter (160mm) than the test soil (150 mm) to provide for a constant contactarea as shearing displacements developed.

Failure involved the top platen and weights sliding down the in-clined table. Whether failure occurred within the kaolinite or bysliding along the interface was judged by observing the exposedsurface of the lower porous stone. For sliding along the interface,the top platen should slide off cleanly and the exposed part of thelower stone should remain covered with kaolinite. When failure oc-curs in the soil, some of the soil should adhere to the top platen andsome to the bottom platen, leaving some areas on each that have noadhered soil.

All test specimens were overconsolidated to some degree at fail-ure, because both apparatus submergence and rotation during test-ing reduced the normal stress on the sample (Pedersen 2001). Thenormal stress decreases with increasing specimen inclination (�)and the overconsolidation ratio (OCR) becomes 1/cos�. In thesetests, the maximum inclination achieved was almost 60°, which re-sulted in an OCR of 2 from this effect. In addition, because the testswere conducted by consolidating the soil under the applied weightsbefore submerging the apparatus, buoyancy reduced the effectiveweight of the surcharge and the applied effective stress. For mostof the tests, the combined effects of submergence and rotation leadto an OCR at failure between about 1.2 and 10.

Results from TSDS Tests on Kaolinite

The kaolinite clay tested was “Hydrite R,” purchased as a drypowder from the Dry Branch Kaolin Co. It has a liquid limit inthe range of 54–58 %, a plastic limit in the range of 31–34 %, anda specific gravity of the solids of 2.59–2.63 (El-Gharbawy 1998).El-Gharbawy (1998) used conventional direct shear tests on 100-mm-diameter by 13-mm-thick specimens of this soil and foundthat �� � 28° and c� � 140 Pa in the pressure range of 200–1600Pa (Fig. 3).

The first series of TSDS tests with Hydrite R kaolinite used aporous stone on top of the kaolinite (Table 2), except for two tests

PEDERSEN ET AL. ON SHEAR AND INTERFACE STRENGTH OF CLAY 3

FIG. 2—Tilt-table apparatus used in thin-specimen direct shear (TSDS)tests.

TABLE 1—Estimated accuracy of TSDS test measurements.

Measurement Accuracy Contributing Factors

Inclination (�) 0.1° Accuracy/precision ofcathetometer

Submerged surcharge 2 % 0.03 % in dry weight,weight 2 % in specific gravity

Normal effective stress 4 % 2 % in submerged weight of(��n) surcharge,

2 % in specimen areaSecant friction angle 0.6° at 30° Average over tilt increment

(��s or ��s) of 0.8–1.0° FIG. 3—Failure envelope from drained direct shear tests on kaolinite(El-Gharbawy 1998).

that involved a layer of kaolinite with no stone on top. It was diffi-cult to determine by observation whether failure occurred at thestone/soil interface or in the soil in these tests, but it appears thatthe difference was insignificant. The failure envelope (Figs. 4a and4b) was curved and passed through the origin. The previous directshear tests (El Gharbawy 1998) gave a slightly steeper failure en-velope than the TSDS tests, possibly because of friction in the di-rect shear apparatus.

The measured secant friction angles decreased from about 60° atlow normal stresses to about 23° at the highest normal stresses (Fig.4c). The semi-logarithmic envelope (Eq 1) was fit to the data andis shown as a solid line in all three plots of Fig. 4. A linear strengthenvelope passing through the TSDS data would have given a cohe-sion intercept of about 50 Pa.

When an acrylic top platen was used, failure typically involveda gradual displacement of the acrylic platen with shearing dis-placements up to about 6 mm. The leading portion of the acrylicplaten, which moved beyond the edge of the lower stone, usuallyappeared to be clean and transparent, whereas the clay exposed by

the sliding top plate remained along the upper edge of the lowerporous stone, suggesting that failure occurred at the interface be-tween the kaolinite and the acrylic plastic. After failure, rapid dis-placement occurred, and parts of the acrylic plate that had shearedbeyond the underlying kaolinite had small amounts of adheredkaolinite, and patches of bare porous stone could be seen along thetrailing edge where the clay had apparently adhered to the acrylic.

The measured shearing resistances from the acrylic interfacetests (Fig. 5) are lower than the internal drained strength of the soil.Large friction angles and no intercept were observed at low normalstresses, and the failure envelope could be represented using asemi-logarithmic relationship (Eq 2).

For the interface tests with kaolinite and anodized aluminum, theself-weight of the aluminum platen limited the lowest effectivenormal stress to about 40 Pa. The measured interface strength foraluminum (Fig. 6) was lower than that of kaolinite. Hence, eventhough about half of the tests visually seemed to involve failurethrough the soil, the measured strengths suggest that failure oc-curred by sliding at the interface.

4 GEOTECHNICAL TESTING JOURNAL

TABLE 2—Summary of TSDS tests conducted on kaolinite.

Normal Stress, Pa OCR at Failure

Top Platen Number of Tests Minimum Maximum Minimum Maximum

Stone 15 1 2320 1.4 115.1Acrylic 15 10 1980 1.2 17Aluminum 11 40 1840 1.2 2.1

FIG. 4—Drained strength of kaolinite measured in TSDS and direct shear tests.

PEDERSEN ET AL. ON SHEAR AND INTERFACE STRENGTH OF CLAY 5

FIG. 5—Drained strength of kaolinite-acrylic interface measured in TSDS tests.

FIG. 6—Drained strength of kaolinite-anodized aluminum interface measured in TSDS tests.

Data from all of the TSDS tests are plotted together in Fig. 7, alongwith the fitted curved strength envelopes. Even with the scatter in thisvery low pressure test data, it is clear that the interface shear strengthsfor both acrylic and anodized aluminum are lower than the drainedshear strength of kaolinite. Ratios of interface strength to kaolinitestrength ranged from 0.7 to 0.9 for acrylic and 0.6 to 0.8 for alu-minum. The TSDS tests confirm observations previously made dur-ing pullout tests of suction caisson models in the laboratory (El-Gharbawy 1998; El-Gharbawy and Olson 1999) that failure occurredat the caisson interface under drained loading conditions.

Summary and Conclusions

Low-pressure direct shear tests were performed using thin spec-imens of saturated kaolinite on a tilt-table apparatus. These “thin-specimen direct shear” (TSDS) tests were conducted to character-ize the drained internal strength of kaolinite, as well as the shearingresistance of kaolinite on interfaces of smooth acrylic and anodizedaluminum, at normal effective stresses less than 2.4 kPa (50 lb/ft2).Observations from these test results include:

• The TSDS apparatus functioned well, although it was difficultto ascertain from visual observations whether failure occurredat the interface or in the clay.

• The effective-stress failure envelope of kaolinite is curved forpressures below 2.4 kPa. At the lowest test pressures, the mea-sured secant friction angles for the soil and the interfaces, withvalues approaching 60°, were well in excess of the values thatwould be expected based on tests at higher pressures.

• The interface shearing resistance was generally less than thedrained shear strength of the kaolinite.

• Whereas best-fit linear strength envelopes through the TSDSdata give a cohesion or adhesion intercept, the curved en-velopes pass through the origin. There was little opportunityfor aging effects in these tests.

Analyses of laboratory-scale model tests should use soil proper-ties determined in the appropriate stress range rather than proper-ties extrapolated from tests at higher pressures. The difference be-tween observed and extrapolated properties would be considerablefor the kaolinite tested. To allow constitutive modeling of the soil

response in model tests, research is needed to develop a test devicethat would allow measurement of stresses and strains in very softsoil specimens confined at low effective stresses.

Acknowledgments

This work was funded by the Offshore Technology ResearchCenter, which is supported by the U.S. Minerals Management Ser-vice and the OTRC Industry Consortium.

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