Interface strength of various geosynthetics and soils from ring shear tests

• Main Author: Effendi, Rustam
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/3535

Interface strength is one of the major parameters controlling geotechnical constructions that incorporate geosynthetics. However there are, at the time of writing, few data on the interface strength of various geosynthetics with soils. On occasion, slope failures have been attributed to a lack of knowledge on this parameter. The main objective of this thesis is to contribute to the available database on the interface strength of geosynthetics, with emphasis on the implications for design and construction of waste containment facilities. A program of laboratory tests was used to observe and interpret the mobilization of shear strength. Limitations of the direct shear apparatus for simulating the large displacements commonly exhibited in the field led to the selection of the UBC ring shear device for testing. Most tests were performed at a strain rate of 0.04 mm/s, for normal stresses in the general range 50 to 200 kPa. Two types of geosynthetic, geomembranes and geotextiles, and two types of soils, a compacted clay and Ottawa sand, were examined in the program of testing. A marked sensitivity of the interface strength of the compacted clay-eosynthetic to variations of moulding water content in the reconstituted clay is apparent from the results. Clay samples compacted to a moisture content of 26% and 27%, which are nominally 1% and 2 % wet of optimum exhibited residual shear strengths of approximately 75 kPa and 50 kP a respectively. Results for the various geosynthetic-compacted clay interfaces gave residual strengths ranging from 13% to 95% of that for the compacted clay alone, which was assumed to have φ = 23.9° and c = 20.9 kPa. The interface strength of the Ottawa sand and various geomembranes is generally governed by the stiffness, hardness, and texture of a geomembrane and by imposed stress level. It was found that the residual interface friction angles, δ[sub residual], for the interfaces varied from 10.5° to 29.1°. The lower value is for a smooth HDPE, while the higher value is mobilized by a soft PVC at higher stresses. The Ottawa sand/nonwoven geotextile interface strength varied from 24° to 27.8° and appeared to be controlled primarily by the type and the arrangement of fibres composing the fabric. Values of δ[sub residual] for the geomembrane-geotextile interfaces were found to be independent of stress level. They vary widely from 6.1° to 33.8°, and are controlled mainly by the texture and stiffness of geomembranes, and the types and arrangement of filaments composing the geotextile. The lower value is for a smooth HDPE with a geotextile comprising glossier filaments, while the higher value is mobilized by the textured HDPE against a geotextile with filaments that are best-interwoven. Of all interface combinations, the ring shear tests with a smooth HDPE geomembrane always resulted in lowest residual interface strengths.