Client: Oliver O’Connell & Son Limited
Location: Englefield, Surrey
Duration: February – May 2018
Redevelopment work for The Savill Court Hotel incorporated a large two-level basement under the entire footprint of the building. Ground Conditions consisted of River Terrace Deposits overlaying Bagshot Beds and Claygate Beds.
Dewatering assessment was based upon dealing with differing conditions within the upper unconfined and lower confined aquifer. Excavation works was within a sheet pile cofferdam. To enable bulk excavation to B2 basement level (plus additional lift pits) we proposed an internal perimeter vacuum wellpoint system installed from –B1 level.
Our design assessment indicated that groundwater pressure within the lower confined aquifer posed a risk to the stability of the base of excavation when reducing the excavation –B2 level. To mitigate the risk of basal heave generated from the sub-artesian groundwater head within the lower confined aquifer, we propose the installation of 9no internal passive pressure relief wells.Scope of Works
An internal perimeter of vacuum wellpoint together with a temporary central line of wellpoints (pumping for ~2-3 weeks) to aid removal of residual groundwater within the excavation area. The perimeter vacuum wellpoints were installed within every internal sheet pile pan (behind wailing beam) from reduced –B1 ground level by conventional water jetting techniques. 3no Electric (7.5 kW 415V) piston dewatering pumps were used with vacuum hearer main located behind the sheet piles. All discharge water flowed via a v-notch settlement tank placed on an earth bund to enable gravity drainage to the pond.
9no x 20m deep passive pressure relief wells are installed using cable percussive drilling methods in a grid across the site at nominal 20m centres. Within each 200mm Ø bore a 50mm Ø well liner would be installed with graded silica gravel from base to ground level.