The ASP excavation was to be carried out within tight spatial constraints, with existing structures on two sides and existing roads present on the others. This meant that expensive sheet pile walls around the excavation (in combination with a dewatering system and pressure relief system), was the most obvious method of providing a safe excavation.
From the available Site Investigation information, it was apparent to OGI that a properly designed dewatering system would not only provide a dry excavation but would also strengthen the ground, to such an extent that a steep embankment alternative to sheet piles was achievable.
OGI designed a series of steep 1:1 batters for each side of the excavation, in accordance with Eurocode 7 design criteria. The design was based on the geology encountered, soil strengths, water table levels, site geometry (including location of access roads and ramps into the excavation and pinch points around the excavation), vehicle loads etc.
At certain locations, for example in one corner of the excavation, where there was not sufficient distance between the new structure and the existing road to allow a 1:1 batter, a small number of sheet piles were required. Across the site, the number of piles required was kept to a minimum.
OGI’s design incorporated a structural stone toe, connected to an array of drainage channels beneath the foundation of the new structure which enabled the release of any groundwater pressure beneath the structure. This was necessary to relieve uplift pressures until the ASP was filled with water, to provide drainage for rainwater, and to provide drainage of any groundwater entering the excavation through the embankment.
The project proved very successful and led onto further similar works on the site and elsewhere for our Client.