Abbey Pynford, the restricted access piling and geotechnical specialist, has provided foundation abutments for a new underground service bridge as part of a high profile upgrading project at Tottenham Court Road Station.
The Station upgrade is being performed primarily to provide sufficient capacity for the new Crossrail network, running from Paddington to Liverpool Street station. Existing services will be diverted onto the bridge to allow the construction of the new ticket hall below to be constructed later this year.
Foundation abutments were required for the new underground service bridge, which had to be constructed within a very tight space and with little or no room for error. In addition, a piled wall was required that would later form the first part of the walls for the new ticket hall. Due to granular soil and groundwater, which was well above the proposed station excavation level, secant mini piles were required to form the walls.
Despite the obvious site restrictions, Abbey Pynford installed the secant wall to the very high degree of precision required, ensuring that the 16m deep 350mm diameter piles would remain interlocked and exclude water. The piles were constructed using a 1.4m wide 5 tonne tracked Hutte 203 rig (this kind of piling is more commonly constructed with 50 –70 tonne equipment).
In addition, the individual bearing piles supporting the rear of each abutment foundation had to be considered. The existing passenger crossover ventilation tunnels of the Northern Line presented a risk as the piles are extremely close to and, in some cases, directly above existing tunnels. In one case a pile passed within 250mm of the side of a tunnel at some 15m below ground level. With conventional piling installation tolerances there would have been a high risk of the piles actually hitting the tunnel. This piling work was the closest ever undertaken to a live LU tunnel.
As the client’s paramount concern throughout the project was the safe use of the existing tunnels, Abbey Pynford developed and implemented a new method to conduct the required stringent surveying of the existing tunnel locations and strict checking procedures for the verticality of each pile as it was installed. This new method involved both guiding and checking verticality of the bores and involved the use of high precision boreholes inclinometers. In addition Abbey Pynford worked closely with the inspector who was appointed to be permanently located in the tunnel adjacent to the works to listen for any sign of damage to the tunnel.
The completion was a huge success as Abbey Pynford achieved almost perfect pile tolerance in plan and verticality was achieved for the critical piles. Furthermore the piling was completed a week ahead of an already tight original programme, despite a short lead in period.
Paul Cresswell, Business Development Manager for Abbey Pynford commented, “This project was challenging in every way but proved to be one of the most fulfilling I have worked on. It proved a great success due to our careful assessment of the site restrictions, close proximity of existing services, shops, high volumes of traffic and the general public. Our in-house design department played a key role in this project, as not only were innovative design concepts required, but also detailed scheme design and detailing were necessary for the actual construction.”
“This project demonstrates Abbey Pynford’s field of expertise and its versatility to provide innovative solutions for highly restrictive access requirements”, he adds.