Interface Friction for Geosynthetics
by George Yazdani, P.E.

Interface friction data is used for stability calculations. The industry standard test method is ASTM D 5321, "Determining the Coefficient of Soil and Geosynthetic or Geosynthetic and Geosynthetic Friction by the Direct Shear Method". In summary, the test device consists of two shear boxes capable of testing a 12" x 12" area. The geosynthetic is clamped to the lower stationary box. The soil or the interfacing geosynthetic is placed and conditioned in the upper box so that the two material face each other. A normal load (vertical load) is uniformly applied to the plate overlying the soil or the upper geosynthetic. The upper box is then sheared at a constant rate over the lower box and the shear force is incrementally measured. The test is repeated for three different normal loads. The shear force/displacement data is plotted for each of the three normal loads to produce three curves (Fig. 1). The maximum shear forces are obtained from these curves and plotted against the normal stresses to represent a line (Fig. 2). The angle of this line with the horizontal axis is the interface friction angle and the intersection of this line with the vertical axis is the interface adhesion. Several tests may be required to obtain the friction angles for all interfaces in a liner system. Each test costs approximately $500. The cost is usually higher for testing soils next to synthetic material, if the soils are to be conditioned. The interface friction behavior is influenced by a number of variables such as follows:

Test Variables

• Test Method - There are several test methods such as direct shear (ASTM D 5321), torsional shear, and inclined tables, which often produce different results.

• Soil Type, Gradation, Particle Size and Shape - Well graded sands produce higher interface friction values next to liners than poorly graded sands. Rounded particles, larger than coarse sand, such as pea gravel, exhibit lower interface friction values next to textured sheet than angular particles. This is because the rounded particles cannot cling onto the surface roughness.

• Soil Moisture, Density, and Drainage Conditions - Moisture has little effect on the interface friction between sands and liners, but, it has a significant effect on the interface friction between clay soils and liners. The shear resistance of clay soils decrease with increasing moisture. If the water in clay soils is not allowed to drain during the test, then pore water pressure builds up which reduces the interface shear resistance. Soil density is also an important parameter. Denser soils exhibit higher interface friction than loose soils. The soils should be compacted in the shear box to the same degree as they would be compacted in the field.

• Geosynthetic Type and Surface Condition - The type, shape, size, and strength of a geosynthetic surface affect the interface friction test results. Textured liners produce higher interface friction by digging into the opposing material and mobilizing the internal strength characteristics of the opposing material rather than merely sliding over its surface. The bonding strength between the textured surface and the core material has to be greater than the maximum shear force that the textured sheet will ever experience under field conditions. Otherwise, the textured surface will rub off the core. This should be a consideration for textured sheets manufactured by spray or lamination processes whereby, the textured surface is added to a smooth sheet in a secondary process. Unlike the spray or lamination processes, the blown film coextrusion manufacturing process produces a monolithic textured sheet with no layers between the core and the textured surfaces. The shear resistance between a textured liner and a non-woven, needle punched, geotextile is sharply reduced from its peak because the geotextile fibers either tear or align themselves in the direction of shearing. As a result, the residual interface friction angle can be noticeably lower than the peak.

• Machine or Cross Machine Direction - Some textured or structured geosynthetics do not have the same surface texture, structure, or layout in their machine and cross machine directions. Therefore, they can produce different interface friction test results when tested in different directions. It is important to test materials in the same direction as they would be installed on slopes.

• Normal Stress and Shear Displacement Rate - The shear to normal stress curve, although shown as a straight line in Fig. 2, is not a linear curve. Generally, the interface friction values are higher under low normal stresses. It is important to simulate the site specific loading conditions when running the test. The shear displacement rate is an important parameter and should be such that no pore water pressure builds up at the materials interface during the test.

Interface Friction Data

The following data represent interface friction angles which may be expected for the specified materials. Site specific tests should be performed, under the direction of the Project Engineer by a reputable laboratory, to evaluate all interfaces of a proposed liner system. The following interface friction data should be used for general information purposes only and should NOT be used for design or certification purposes. The residual interface friction values are generally lower than the peak values stated below.

Bidding Strategy

Most project specifications require that the contractor certify that his proposed materials meet the specified friction angle(s) at the time he bids the job. This would require the contractor to do one or more tests with geosynthetics and/or site specific soils which are often not identified or available prior to bid opening. Even if the materials were available, most contractors and manufacturers would not spend hundreds or thousands of dollars to run these tests before they get the job. It is important to clarify this matter by adding a statement to your proposal similar to the following.

"The interface friction tests, if required, should be performed by the Owner (at his expense). Friction test results are highly dependent on the test material, procedure, and conditions. [Liner Installation Contractor] cannot certify to any friction values prior to obtaining satisfactory test results for the site specific materials and test conditions from a reputable laboratory. Test materials, method and conditions should be specified by the Project Engineer."