C502 Liverpool Street Crossrail Station

Expanded Geotechnical delivered the very complex and challenging foundation package for the new Crossrail station at Liverpool Street. The scope of works included secant piled walls, contiguous piled walls drilled under support fluid, bearing piles and plunge column piles across diameters ranging 300mm to 1500mm and depths in excess of 50m, using a blend of standard, heavy-duty and restricted-access rig types. The plunge columns were up to 38m long, which were the longest installed in the UK at the time.

The project required many months of preconstruction planning to successfully overcome the challenges inherent to the small footprint of the Blomfield Box (one of the worksites), where over 300 rotary bored piles needed to be constructed within a 1 in 200 verticality tolerance. Due to the ground conditions, many of the large diameter piles were constructed under bentonite.

To achieve the stringent verticality tolerance of 1 in 200, Expanded designed and fabricated bespoke extended augers and drilling buckets. Mounted on these tools is a verticality sensor management system supplied by Jean Lutz. This system displays in real-time the actual constructed verticality of the pile bore in front of the rig operator. In the event of pile bores moving out of alignment, corrections can made promptly. All piles were constructed within the required tolerance.

Surrounded by buildings and with piles constructed within 2m of the live London Underground line (during normal working hours), Expanded used a Bauer BG30 rig and Liebherr LTR1100 telescopic crawler crane supplied by Select Plant to install the works. This specialist plant was the perfect match for the tight ‘squeeze’ into the 900m2 site footprint. The Bauer machine had the added distinction of being the first piling rig on the Crossrail project to be Euro IIIB emissions compliant.

Detailed logistics and careful planning of site construction were fundamental to constructing the large diameter piles under bentonite to 50m depth, with their full-length reinforcement cages weighing up to 22t, all delivered through a constricted site access that services all the site requirements. Complex reinforcement cages, prefabricated off site and delivered in four elements, consist of up to 20 B50 main reinforcement bars with inclinometers and full-length sonic logging tubes. The cages were installed to extremely tight tolerances as they contained over 12,000 starter bars and 7,000 couplers. Geothermal loops were included within the cage detailing for a number of the structural piles.