| Interface Friction |
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The weakest plane of failure within a geofoam fill under load can be at the interface between geofoam blocks and between geofoam blocks and interfacing materials. The friction factor or interface friction coefficient is the ratio of the resisting shear stress to the applied normal stress. Interface friction strengths for the various construction materials that will be placed in contact with the geofoam will be required for design. There has so far been limited study of interface strength between geofoam to geofoam as well as geofoam to other commonly used construction materials. The results of an investigation aimed at generating additional information related to geofoam interface friction behavior are summarized below (Sheeley, 2000). |
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The magnitude of peak and residual shear strengths, at corresponding stress levels, for wet geofoam interfaces are not much different than for dry conditions. A friction factor of 0.6 is a reasonable lower bound value for design. At lower normal stresses of about 25 kPa or less and typical of stress levels commonly applied on geofoam fills, the interface friction factors are higher and generally in the range of 0.7 and 0.9 for residual and peak conditions, respectively. |
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The friction coefficient for rough HDPE and geofoam surfaces is in the range of 1 at normal stresses of 25 kPa or less and reduces to about 0.8 at about 45 kPa. Smooth HDPE and geofoam interfaces resulted in a low friction factor of about 0.25 regardless of normal stress level. There is not much difference in evidence between residual friction factors of less than 0.5 for rough and 0.4 for smooth PVC. The friction factor for rough PVC decreases only slightly with normal stress. The friction factor for smooth PVC is relatively independent of normal stress. These results indicate that the interface resistance between geofoam and geomembranes, with the exception for rough HDPE, is considerably lower than for geofoam to geofoam interfaces. These results suggest need for careful consideration when using a geomembrane cover in seismic areas and on sloping geofoam surfaces. |
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Fresh geofoam and cast in place concrete interfaces develop a strong adhesion bond. A strong adhesion bond does not develop as readily between cast in place concrete and geofoam degraded by extended UV exposure. Both the peak and residual strengths for these degraded geofoam interfaces are lower than for fresh geofoam and cast in place concrete. Pressure washing in the field re-established the interface strength performance to that of a fresh surface. The adhesion bond and higher residual interface strength for cast in place concrete and geofoam has not been recognized in design practice of geofoam fills. |
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Interface |
Peak Factor |
Residual Factor |
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Foam-Foam, 20 kg/m3 (dry) |
0.85 |
0.70 |
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Foam-Foam, 20 kg/m3 (wet) |
0.80 |
0.65 |
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Foam-Foam, 30 kg/m3 (dry) |
0.85 |
0.65 |
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Foam-Foam, 30 kg/m3 (wet) |
0.75 |
0.65 |
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Foam-Cast in Place Concrete |
2.36 |
1 |
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Foam-Textured HDPE Membrane |
1.0 |
~1.0 |
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Foam-Smooth HDPE Membrane |
0.29 |
0.23 |
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Foam-Smooth PVC Membrane |
0.70 |
0.40 |
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To facilitate free drainage and prevent uplift due to buoyancy as well as to provide level and uniform support, geofoam blocks are normally placed on a base of granular soil. Individual soil particles tend to penetrate and lodge in the surface of the geofoam. For design against sliding, the interface friction can be set equal to the internal friction of the coarse soil in contact with the geofoam. Coarse angular or sub angular soils should be preferred to rounded or sub rounded aggregates. |
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References |
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Sheeley, M., (2000). "Slope Stabilization Utilizing Geofoam", M.S. Thesis, Syracuse University, Syracuse, N.Y. |
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