From mid-2016 to early 2018, CDS were appointed by O’Keefe to provide temporary works design services for both the demolition and construction phases of the basement works for the One Crown Place development near Liverpool Street Station in London.
The demolition phase of the project included the demolition of an existing two level reinforced concrete basement structure. The only part of the basement to remain intact was the perimeter wall at the site boundary. The basement was then to be backfilled with crushed concrete to form a working platform for the piling works, which included the construction of a secant piled wall.
In order to maintain the stability of the perimeter wall during the demolition works and to allow piling works to be carried out during the construction phase, CDS designed hit-and-miss blocks of reinforced earth. These blocks were formed against the perimeter wall, acting as a gravity retaining structure and allowing the removal of the existing column foundations between.
During the piling works, UB king posts were plunged into the secant wall piles to provide the means to support the retained perimeter RC wall during the excavation works required for capping beam construction. The maximum retained height was 3.7m. CDS designed the required steelwork and jacking requirements for this means of wall support.
The construction phase required two tower cranes to be constructed on steel grillages at ground level, supported on plunge columns and the secant piled wall. Main contractor Mace provided the concept design for the grillages and CDS carried out the detailed design. One of the tower cranes was located at the end of a cantilever grillage, which allowed the crane to sit over a footpath packed with services and beyond the outside line of the proposed building above. The grillages were complex, and 3D structural analysis was used to ensure that deflections remained within safe limits.