CrossRail may still be in the planning stages, but it is already having a big impact on building work in the City. The foundations for Foster and Partners’ Moorhouse development in Moorgate are being constructed on the assumption that CrossRail will run almost directly below it. As a result the 20-storey building – which is big but by no means the biggest in the capital – has the deepest base-grouted piles yet constructed in London.
Skanska Construction UK is the scheme’s design and build contractor. Its CrossRail-related work includes a 50m-deep draught relief shaft below the building, and the shell for a station ticket hall within its basement. But the biggest controlling factor on the design and construction of the building’s foundations is the provision of frictionless pile sleeves to a depth of up to 26m.
They are designed to isolate Moorhouse’s foundations from any future ground movements associated with the CrossRail running tunnels and a planned station box that runs below the pavement immediately outside the building.
It is because the top sections of the piles are frictionless – and so don’t carry any of the structure’s load – that they need to be so deep. From a pile capacity point of view, the top 26m of the piles do not exist.
While this has unquestionably added significantly to the project’s costs, it is no doubt considerably cheaper than installing the complex compensation grouting measures that would otherwise be necessary when the tunnelling eventually takes place.