In order to properly assess the soil and heave pressures to be applied to the walls and base slab of the four storey basement and to predict the likely ground movements of the nearby buildings, brick sewers and tube & train tunnels, an extensive series of 2D and 3D finite element ground models were built. These models perform a time history analysis starting with ‘Greenfield’ conditions (stresses) in the ground on which is imposed the various development pressures including the changes of clay stresses resulting from the construction of the underground tunnels. These models continue the time history analysis through the basement formation, the construction of the building and into the future until the dissipation of excess pore water pressures in the clay has been achieved.
The retaining wall type and the numbers, levels and stiffnesses of any temporary propping are incorporated into the analytical models thus the excavation/construction sequence is determined by Tier and must be strictly adhered to by the contractor on site.
A ‘stiff response’ (minimised movement) to the excavation was required to limit the movement of , in particular, the Northern line tunnels and the listed Chapel of St. Barnabus. Three levels of propping were installed to provide this stiff response.
To allow the formation of the ground level walkway, under the main building above and the formation of the main entrance to the retail in the north east corner, the team designed a storey height vierendeel truss, cantilevered at each end, supported on four prefabricated box section transfer structures. This allowed the columns in the north-east and south-east corners of the building to stop at level 2 (the top of the truss) and the intermediate columns between them to continue down to basement level on a new in-set line within the building and behind the new pavement line.
The requirement for column free space necessitates the development of relatively deep long span beams with regular holes in the webs to allow the integration of building services within the depth of the beam.
The requirement also gives rise to the potential for a dynamic response to footfall excitation that is uncomfortable for certain building users. A design based on limiting each floor’s Response Factor ( a measure of floor acceleration) to a specific level has been carried out.
The transfer structures to facilitate the set-backs are created by enhancing the weight of the affected beams and reducing the perforations through the beams between the transfer column and the supporting column below.
Interestingly, whilst the strength requirement of the transfer beams is increased to support the column load above, the mass supported by the column and applied to the beam improves the dynamic performance of the floor plate in this area.