As a result of the phase 1 Desk Study, a first round of site investigation was undertaken including two cable percussive boreholes to confirm the composition of the underlying geology and gain strength profiles of the chalk. Geophysical methods of site investigation were not considered to be suitable at this stage due to the limited area available for investigation prior to demolition works. The results of the cable percussion drilling revealed the Winter Hill Gravel Member to be ~ 3.00 – 3.50m thick overlying the Seaford Chalk Formation and Newhaven Chalk Formation (Undifferentiated) to the base of each borehole at 20.00m below existing ground level (bgl). A soft horizon was recorded within one of the boreholes at the boundary between the overlying gravels and chalk where an ~1.00m thick band of soft clay, with SPT “N” Values of five, were recorded. Due to the classification of the site, further dynamic probing was carried out at six locations across the site, to determine the extents of the soft horizon at the boundary of the superficial and bedrock deposits.
The results of the probing highlighted two areas with soft spots/suspected dissolution features at varying depths between 3.00 – 13.50m bgl. The suspected dissolution features were identified by the extremely low blow counts recorded, between 0 – 3 (see image to the left for example log). Subsequently after discussions with the client and NHBC, it was determined that further dynamic probing would be carried out post demolition across the site in a 5m by 5m grid to a depth of 15.00m bgl to identify the locations of dissolution features and the competent chalk below.