On completion of the raft, sheet piling and contiguous piled wall the excavation of the basement was carried out. With sufficient surrounding area it was possible to batter the sides to form a stable excavation for construction of the basement.
As with all habitable basements it was necessary to provide two systems to prevent ingress of water. These were provided by using and impervious external membrane and waterproof concrete.
On completion the basement was capped with a precast concrete floor to form the new ground floor to the extension.Following completion of the underpinning we installed a row of sheet piles adjacent to the rear wall of the cottages and a contiguous piled wall internally adjacent to the proposed basement in order to prevent loss of soil below the new raft during the basement construction. The tops of the sheet piles were restrained by a steel waling tied into the edge of the raft and the base toed 1.0m into the sand below the underside of the basement excavation minimising to section of sheet pile required.