Pull Out Resistance Of Geosynthetic Straps From Shale And Silty Sands

This paper presents results from a study of the pull out resistance of geosynthetic (ParawebTM ) straps from two fill materials. The first is sand with variable silt contents and the second is Ashfield Shale. For both materials shear box tests of the fill material, shear box interface (fill material – geosynthetic) tests and pull out tests have been performed for a range of confining stresses.

There has been evidence of the poor performance of reinforced shale fills in Sydney, but the reasons for this are not well understood and many different opinions exist. These include weathering/breakdown of the shale, wetting/pore pressure development, creep and arching. Tests have been performed to evaluate which of the proposed mechanisms is responsible for the poor performance. It is shown that the tendency for even well compacted shale to compress when sheared contributes to arching, which leads to low pull out resistances. It has been found that when the shale is crushed to <75 m the pull-out resistance is greater than when a uniform material with gravel sized lumps is used. This is explained by the arching mechanism and the impact of the grading on the stiffness, density and compressibility of the fill material.

It is widely accepted that reinforced earth walls (REWs) need to contain ‘good quality’ fill i.e. sands and gravels that are well graded. For example, the RTA QA specification R57 (2007) states that soil properties should be granular and contain less than 15% fines <75 m. Tests have been performed to assess the effects of increasing non-plastic fines content by mixing silt with sand. Addition of silt affects maximum and minimum densities, frictional and dilatational characteristics and the pull-out resistance. It is shown that addition of non-plastic fines significantly improves the pull- out resistance for low fines contents and as further fines are added the pull-out resistance reduces. However, even with 26% fines the pull-out resistance is greater than for clean sand.