shear strength reduction at soil structure interface

Shear Strength Reduction at Soil Structure Interface

B. Tiwari1, Ph.D., M.ASCE, B. Ajmera1, G. Kaya2 1 Civil and Environmental Engineering Department, California State University, Fullerton, 800 N State College Blvd. E-419, Fullerton, CA 92834 2MTK, 16795 Von Karman Avenue Suite 205, Irvine, CA 92606

ABSTRACT Shear strength of soil-structure interface is very important while designing structures pertinent to geotechnical engineering that include but not limited to design of shallow foundation, pile, retaining wall, and sheet pile. Although the interface shear strength is very important, designers use empirical values while designing such structures. There are numerous literatures available regarding the numerically simulated values of interface shear strength. However, very few researches have been done lately on the interface resistance of different types of soil and structure. Different soil specimens were tested in a multiple-reversal direct shear device to measure the shearing resistance of soil-concrete, soil-wood, and soil-steel interface. The types of soil included in the research were SP, SW, SM, SP-SM, MH, ML, and CL, based on the USCS system. The result shows that skin resistance of soil-structure interface depends on the surface material of the structure and the type of soil. The behavior of dry soil differs from that of saturated soil. BACKGROUND Shear strength between soil and structural material is important while designing various geotechnical structures that include but not limited to deep foundations pile and drilled shaft, shallow foundations spread footing and mat, retaining wall, sheet pile etc. However, very few researches are available that establish soil-structure shearing resistance. Majority of the designs are based on empirical values i.e. ratio of skin friction or adhesion to the internal friction or cohesion of foundation soil. Potyondy (1961) conducted a research to measure the ratio of skin friction and adhesion with soil friction and cohesion, respectively. He conducted direct shear test on the interface of concrete, steel, and wood with sand, sandy silt, cohesive soil, rock flour (called it as silt), and clay. He conducted tests for certain pre-set moisture contents as well as for dry specimen. He concluded that frictional resistance of a soil depends on the proportion of sand in it. He also proposed ratio of designed frictional resistance of construction materials with soil that ranged from 0.4 for saturated loose sand to 1.0 for saturated dense sand. Coyle and Sulaiman (1967) investigated the frictional resistance between sand and steel pile, where as Kulhaway and Peterson (1979) measured the frictional resistance of sand and concrete. Several other researchers that include but not limited to Evgin and Fakharian (1996), Hryciw and Irsyam (1993), Uesigi and Kishida (1988) and Hu and Pu (2004) conducted direct shear tests on the interface between steel or concrete and sand to measure the interface frictional resistance. Other than direct shear device,

1747GeoFlorida 2010: Advances in Analysis, Modeling & Design(GSP 199) 2010 ASCE

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Paikowsky et al. (1995) developed a dual interface apparatus whereas Yoshimi and Kishida (1981) developed a ring shear device to measure interface frictional resistance for a larger deformation. Although there are numerous literature that evaluated the interface frictional resistance of soil and construction material, Potyondy (1961) was the only literature that shows a significant amount of experimental study on soil-structure interface. However, Potyondy (1961) was not clear about several issues that mainly control the behavior of shearing between soil-structure interface. Those issues are:

Although majority of the tests were conducted on clay sand interface, activity of the clay was unknown. Activity is the ratio between the plasticity index and clay fraction.

Although tests were done on different types of soil, most of them fall into a limited number of soil type based on the current Unified Soil Classification System (USCS) classification.

Although tests were done on a controlled moisture situation, effect of metric suction for partially saturated situation was unclear.

The tests were done with a very small sized shear box, which may have a high machine effect.

Majority of the tests were done for two normal stresses only. Because there was no drainage path provided at the interface, proper

drainage between the interface was not assured. This research is conducted to have more systematic study on the interface shearing resistance of various construction materials with different types of soils that are classified with USCS system. Moreover, initial void ratio of the specimen was kept constant so that we could compare the results systematically. The area of the shear box was 3 times larger than the one used by Potyondy (1961) and several drainage holes were made across the structural block to check the effect of drainage on interface frictional resistance. RESEARCH METHODOLOGY For the evaluation of interface frictional resistance between soil and structures, three types of building materials were prepared a) plain concrete, b) plywood, and c) steel. Sizes of the all of those blocks were 100 mm in length, 100 mm in width and 6.25 mm in thickness. All of those blocks were made with sufficient number of drainage holes to ensure proper drainage. Shown in 1 through 3 are the pictures of those material blocks.

A fully automated direct shear device was used for the study. Size of both the upper and lower shear boxes were 100 mm x 100 mm x 6.25 mm each. Vertical displacement, horizontal displacement, and shear force are recorded automatically in separate data acquisition channels through vertical Linear Variable Differential Transformer (LVDT), horizontal LVDT and load cells, respectively. The loading arm in the device is set in such a way that a 10:1 mechanical advantage can be achieved in the normal stress. The lower box was totally blocked with the building materials concrete, steel and wood. Different types of soils were prepared to measure the shearing resistance of soil at the interface with building structures. To compare the


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strength, shear strength of soil specimens were also measured by removing the construction material block from the lower box. The soil specimens used for the study were a) a poorly graded sand (SP) (Ottawa sand was used for this purpose), b) a well graded sand (SW) obtained from concrete aggregate, c) a silty sand (SM) made by mixing silt in the construction sand, d) a poorly graded sand with silt (SP-SM) made by mixing silt in the construction materials, e) an elastic silt (MH), f) a silt (ML), and g) a lean clay (CL). MH and CL materials were prepared by mixing appropriate proportion of kaolin and bentonite with sand. Shown in Figure 4 are the grain size distribution curves of SP, SW, and SM materials. Similar diagrams can be made for other specimens too.

Figure 1. Photograph of wooden blocks used for the study.

Figure 2. Photograph of concrete blocks used for the study.


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Figure 3. Photograph of steel blocks used for the study.

Figure 4. Grain size distribution of a poorly graded sand (SP), well graded sand (SW) and silty sand (SM) material used in the study.


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To address a fully saturated situation, direct shear tests were conducted for the fully saturated specimens. In all tests, initial void ratios of the specimens were kept constant to 0.7. All soil specimens were prepared to have the pre-calculated void ratios and mixed thoroughly with sufficient amount of water to ensure a fully saturated situation. SM, SC, SP-SM, and SW soil specimens were mixed with distilled water equal to 120% of the liquid limit. Pre-saturated materials were poured into the direct shear device (with empty lower block). Proper care was taken to minimize particle segregation. The entire box was submerged under water for all tests. Then, the shearing resistances of all specimens were measured at the shearing rfate calculated through vertical settlement (or consolidation) data. The method specified by ASTM for the drained direct shear test (ASTM D-3080-04) was strictly followed. The computer software used for the test can capture the data and plot the real time consolidation curve and stress strain curve. For each specimen, tests were done at least for 4 different normal stresses and some of the tests were repeated for conformity. The same procedure was repeated several times to measure the friction resistance of soil at the interface of concrete, steel, and wood by blocking the lower shear box with the respective materials. In most of the cases, shearing speed was reduced due to the mechanism of nearly one side drainage. However, shearing rate was still calculated based on the consolidation data. TEST RESULTS Shown in Figure 5 are typical stress strain curves obtained for different normal stresses for a SP material interfaced with a wooden block. Tests were terminated after constant shearing resistance was received. Similar diagram can also be drawn for other materials. Applied effective normal stress range was from 50 to 200 kPa, although higher stresses could also be applied. A minimum of 4 tests were done for each specimen and its corresponding interfaces with different materials. Shown in Figure 6 through 9 are the shear envelops for soil, soil-concrete, soil-steel and soil-wood interface for SP material, SW material, SM, and SP-SM materials, respectively. Similar envelops can also be plotted for other materials. Shown in Table 1 are the friction angles and the skin friction angles of all types of materials used in the study with concrete, wood, and steel blocks. The ratio between the skin frictional resistance and the frictional resistance is also calculated in the corresponding column. Likewise, % reduction in shearing resistance from the original soil-soil shearing resistance is also calculated and presented in the corresponding columns. The results obtained from the tests and plotted in Table 1 are based on the average secant friction angle. Best fit line exhibited very small or no cohesion. That is because the samples were reconstituted. Although similar tests were conducted and a database was prepared for the interface between all types of materials covered in this study and concrete, wood and steel blocks for dry condition as well as for different void ratio for both dry and submerged conditions, the results could not be presented here because several confirmative tests are ongoing. Tests are also going on to measure the similar properties for soil with different activities ranging from 0.2 through 4.5. Shown in Table 2 are the ranges of the ratio of skin frictional resistance with frictional resistance of soil and concrete, wood, and steel. Likewise, range of the


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reduction in shear strength from the soil-soil friction angle (in %) is also presented in Table 2.

Figure 5. Typical stress-strain diagram obtained for the interface between SP material and a wooden block.

Figure 6. Shear envelope obtained for the interface between SP material and different blocks of construction materials.


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Figure 7. Shear envelope obtained for the interface between SW material and different blocks of construction materials.

Figure 8. Shear envelope obtained for the interface between SM material and different blocks of construction materials.


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Figure 9. Shear envelope obtained for the interface between SP- SM material and different blocks of construction materials. ANALYSIS OF TEST DATA The test results presented in the figures and tables explained above clearly show a consistent trend that skin frictional resistance between sand and concrete is always higher than that in wood and steel. Steel surface exhibited lowest skin resistance. That can be attributed to a very smooth surface of steel compared to wood and concrete. The trend of reduction in skin friction was consistent in all soil specimens. . The data presented here shows a significantly high skin frictional resistance than that shown by Potyondy (1961). The major difference is due to the possibility of having unequal degree of saturation at the soil side and the soil-structure interaction side in the tests that Potyondy (1961) conducted as there was no proper drainage and the tests were conducted in a partially saturated condition. Because of the limitation of drainage and affinity of concrete, steel, and wood blocks to water, water might have been migrated towards the interface, especially in the specimens which are partially saturated. Without fully saturating the specimen, this effect cannot be eliminated. Therefore, maintaining same initial void ratio in all tests is more appropriate than starting all tests with the same moisture content. Although the data presented by Potyondy (1961) showed a significant variation in the ratio of skin friction with soil friction for the same material tested with different soil (Table 1), the result obtained in this study does not show that high variation. Moreover, the recommendations presented by Potyondy (1961) show overly conservative values. In several cases, he presented that the skin friction is higher than the soil-soil frictional resistance, which is not possible in the type of shearing performed by Potyondy (1961) or this research. However, this research clearly showed that skin frictional resistance is always smaller than the soil-soil frictional resistance.


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Table 1. Secant frictional (or skin frictional) resistance of different types of soil at the interface of concrete, wood and steel blocks

Soil Type Material

friction angle ( or ) degree

/ % strength drop This

Study Potyondy

(1961) SP Soil 31

Concrete 29.1 0.94 0.89 6.13 Wood 27.1 0.87 0.85 12.58 Steel 24.4 0.79 0.65 21.29

SW Soil 33.3 Concrete 32.6 0.98 - 2.10 Wood 32.3 0.97 - 3.00 Steel 28.5 0.86 - 14.41

SM Soil 33.1 Concrete 30.1 0.91 1.00 9.06 Wood 28.6 0.86 1.06 13.60 Steel 27.6 0.83 0.58 16.62

SP-SM Soil 29.3 Concrete 27.2 0.93 - 7.17 Wood 26.9 0.92 - 8.19 Steel 25.4 0.87 - 13.31

ML Soil 27 Concrete 25.2 0.93 1.00 6.67 Wood 23.7 0.88 0.87 12.22 Steel 22.4 0.83 0.68 17.04

MH Soil 30 Concrete 27.7 0.92 - 7.67 Wood 26.2 0.87 - 12.67 Steel 24.3 0.81 - 19.00

CL Soil 8.2 Concrete 7.7 0.94 0.82 6.10 Wood 7.3 0.89 0.61 10.98 Steel 6.6 0.80 0.56 19.51

CONCLUSION AND RECOMMENDATION More than a hundred direct shear tests were conducted in different types of materials to measure the skin friction between those materials and construction materials such as concrete blocks, wooden blocks and steel blocks. Tests were


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conducted at a void ratio of 0.7 and at saturated conditions. The research results show that there is up to 14% reduction in friction angle between the surface of steel and different types of soils. We observed up to 17% reduction in friction angle when a wooden block was sheared against different types of soil specimens. Likewise, we noted up to 9% reduction in frictional resistance at the interface between a concrete surface and various types of soils. In average, concrete block showed an average reduction of 7% frictional resistance from soil-soil friction, where as steel and wooden blocks exhibited the average reduction of 16% and 11%, respectively. Table 2. Ranges of / and % reduction in from for different types of soils

Material range of / range of % strength reduction Concrete 0.91-0.98 2.1-9.1

Wood 0.83-0.92 7.6-17.1 Steel 0.86-0.97 3-13.6

REFERENCES Coyle, H.M., and Sulaiman, I. (1967). "Skin friction for steel piles in sand." J. Soil Mech. Found. Div., 97(12): 1657-1673. Evgin, E., and Fakharian, K. (1996). "Effect of stress paths on the behavior of sand-steel interfaces." Can. Geotech. J., 33(6): 485-493. Hryciw, R.D., and Irsyam, M. (1993). "Behavior of sand particles around rigid inclusion during shear." Soils and Foundations, 33(3): 1-13. Hu, L., and Pu, J. (2004) . "Testing and modeling of soil-structure interface." J. Geotechnical and Geoenvironmental Engrg, 130 (8): 851-860. Kulhaway, F.H., and Peterson, M.S. (1979). "Behavior of sand and concrete interfaces." Proc. 6th Pan American Conference on Soil mechanics and Foundation Engineering, Brazil, No. 2: 225-230. Paikowsky, S.G., Player, C.M., and Connor, P.J. (1995). "A dual interface apparatus for testing unrestricted friction of soil along solid surfaces." Geotech. Testing J., 18(2): 168-193. Potyondy, J.G. (1961). "Skin friction between various soils and construction materials." Geotechnique, 11(4): 339-353. Uesigi, M., Kishida, H, and Tsubakihara, Y. (1988). "Behavior of sand particles in sand-steel friction." Soils and Foundation, 28(1): 107-118. Yoshimi, Y., and Kishida, T. (1981). "A ring torsional apparatus for evaluation of friction between soil and metal surface." Geotechnical Testing J., 4(4): 145-152.


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shear strength reduction at soil structure interface

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