It was clear that an intrusive investigation was required, comprising Window Sampler boreholes, Heavy Dynamic Probing and Hand Shear Vane tests. These were undertaken to assess the surrounding ground conditions where excavations had begun to create the hit and miss underpins, required to construct the basement.
Deep Made Ground to 3.00m bgl was found in one of the boreholes. This contradicted the findings of the previous flight augured (CFA) site investigation. This deep Made Ground pocket suggested the ground in this area and potentially other areas on site, was less stable and much more variable in nature compared with the natural soils. Although no groundwater was found during the original intrusive investigation, perched water was spotted in the base of one of the underpins, seeping out from the base of the deep Made Ground. This water would have softened the soils, and given the inherent variability of Made Ground, excessive settlement had occurred leading to subsidence and ultimately the movement of the building.