Comparison of preloading by fill surcharge and ground water lowering based on a case study

Preloading by fill surcharge is a widespread and rather simple ground improvement method for the anticipation of settlements brought on by additional loads from new constructions. The magnitude as well as the area of surcharge is adapted to the final construction, whereas in general about 120–140 % of the final load is applied as surcharge load. Both factors have an influence on the effective depth for which soil improvement and an anticipation of settlements is possible, respectively. In case of large construction areas with significant loads, for which a greater depth effect needs to be considered, a large earth volume for surcharge is necessary. It is a cost-intensive and environmental-relevant aspect, when earth material is not available in sufficient amount and near distance of the construction site. In order to avoid this, it was investigated whether lowering the in situ groundwater level and thus increasing the effective stresses over depth could be a possible alternative. Therefore, a case study was carried out to examine these two different types of preloading techniques and their efficacy based on known subsoil conditions. Furthermore, the influence of the possible anisotropy of hydraulic conductivity in the weak soil layer on the preloading process was investigated.