Reinforced soil shear key to mitigate extrusion failure in soft soils under working platforms

Geogrids and geotextiles are widely used in working platforms to increase the allowable surface bearing pressures and reduce the platform thickness required. While analytically based approaches exist for the design of these platforms, these vary in their complexity and the range of critical properties they consider, leading to uncertainty in which ones to suggest. They can be over conservative compared to empirical methods based on historical performance. For projects with combinations of high rig loading and very soft underlying ground (cu < 20 kN/m2), there is greater potential for extrusion and edge circular failures. The edge stability of platforms can be improved with a number of measures such as increased stand-off zones, loading mats, balancing mounds and shear trenches. This paper considers the extrusion mechanism and how a wrapped shear trench can be used to counteract edge insta-bility and extend the suitability of platforms over very soft soils. The effectiveness of this method is demonstrated with a UK case study. This paper covers a brief introduction to working platform design, before comparing a range of approaches. The paper discusses the extrusion failure mechanism and a solution using a wrapped shear key trench.