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Case study: performance of a geogrid stabilised working platform constructed over extremely soft dredged silt

This case study describes the construction of a working platform forming part of a fabrica-tion yard for offshore equipment in Laem Chabang, Bangkok. Subsoil was extremely soft dredged silt, and loadings were high, coming from heavy cranes and the equipment being fabricated. The working plat-form was required to provide adequate performance for many years. Various solutions were investigated, including the use of geosynthetic materials, and the final design consisted of a granular layer stabilised with two layers of geogrid. To verify the design, a load test was carried out using concrete slabs, which were arranged to apply 115t over a 2m x 2m area, thereby applying almost 300 kPa contact pressure. The test confirmed that this load could be supported by the working platform with minimal settlement, and the observed behaviour also demonstrated how the geogrid stabilised granular layer was acting as a strength-ened mattress, thereby sustaining greater load spread over a large area at the surface of the underlying silt. This type of behaviour from geogrid stabilised layers is generally difficult to quantify, going far beyond simple load spread or membrane techniques. The complex interactions were simulated by finite element analysis to derive a new “surcharge transfer” design method for working platforms. This case study was back-analysed using the new design method for which a good match was obtained considering the site conditions, thereby providing valuable validation of the new design method. Importantly, this working platform has been providing the required performance for more than 10 years since construction.