The project involved a bulk excavation to construct the lower ground floor of 5 new dwellings and two basements. The bulk excavation had a perimeter of 226m and would extend to 5.5mbgl. The geology as per the geotechnical investigation was Made Ground over SAND over CLAY with sand laminations over SILT over silty CLAY. Groundwater was encountered at 1.4mbgl.
A perimeter wellpoint system was proposed by PDL. The 152 no. wellpoints were placed at 1.5m C-C and up to 6m deep. The excavation was open-cut and made use of battered slopes for stability. The wellpoints had to be placed within 5m from the formation level of the excavation to remain effective. PDL envisioned some overbleed between the SANDS and CLAY layer and opted to include 2 no. engineered sumps per basement area. The sumps included a large diameter well screen backfilled with filter SAND to 2m below the formation level. PDL opted for wellpoints as our dewatering solution due to their effective use of vacuum-induced pumping. This approach allows them to achieve good drawdowns, particularly in highly permeable soil, while still remaining cost-effective and easy to install. The wellpoint system, together with the sump pumps, ensured the bulk excavation was dry and stable throughout the project.