C Stoewahse, N Dixon, DRV Jones, W Blumel and P Kamugisha. This paper was first published in GE’s February 2002 edition.
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Geosynthetics are increasingly being used for engineering applications. They are typically used in conjunction with soil and other types of geosynthetics and have a number of roles such as reinforcement, separation, drainage and as barriers. They are employed in a range of diverse applications such as landfill barriers and construction of steep soil slopes.
The use of geosynthetics in a structure introduces potential planes of weakness, resulting in a requirement to assess the stability along interfaces between soil and geosynthetic and between geosynthetic and geosynthetic. Figure 1 shows a schematic of a landfill containment system. It demonstrates the range of materials that can be used and emphasises the importance of the soil/geosynthetic systems in controlling slope stability of both the capping system and the side slopes during construction (ie prior to waste placement).
Shear behaviour of side slope interfaces also influences post-waste placement barrier performance. Settlement of the waste can result in over stressing of the geomembrane via transfer of load through shear at the interface and this can lead to loss of barrier integrity.
Figure 2 shows a relatively steep landfill side slope being lined with a geomembrane and illustrates the importance of interface shear strength in ensuring the constructability of such structures.
Assessment of stability requires detailed knowledge of the stress/strain behaviour of interfaces, as post peak shear strengths are often mobilised resulting from the strain incompatibility of soils, geosynthetics and waste materials. It is common practice to measure interface shear strength in a direct shear apparatus (DSA) as used in soil mechanics but with a much larger shear plane. Design parameters are obtained by carrying out performance tests (ie using site specific materials and relevant boundary conditions).
There are three standards in common use that provide guidance on testing procedures; BS6906:1991,ASTM D5321-92 and a German recommendation for landfill design GDA E 3-8 of 1998 (Gartung and Neff, 1998).The final version of a preliminary European standard (prEN WI 00189015) is imminent. In addition, a significant number of research papers have been published on this topic in the past 15 years.
It would appear therefore that there is adequate information and guidance to ensure high quality testing is carried out. However this is not the case. There is growing evidence that tests specified to obtain parameters for design, and those reported in the literature, often lack sufficient control on the key factors affecting the measured values. This results in uncertainty regarding the likely variability of measured shear strengths, and in some instances is leading to the use of over estimated interface strengths in design.
This paper provides a summary of the key factors influencing measured shear strength behaviour. It gives guidance, references key publications on the main issues controlling the measurements and includes test results that illustrate typical observed behaviour.
A companion paper (Dixon et al, 2002, to be published in Ground Engineering March) discusses the selection of characteristic strength parameters from laboratory results for use in design.
While the issues included here are important for the assessment of all geosynthetics, there are specific additional considerations for the testing of geogrids, geonets and geosynthetic clay liners (GCL) that are not covered. Some of the recommendations given are relevant for direct shear tests entirely on soil, although care should be exercised in applying the findings of this study as important issues specific to soil tests have not been included.