Foster and Partners' 20-storey Moorhouse development is founded on London's deepest ever base-grouted piles.
Paul Wheeler reports.
Crossrail may still be in the planning stages, but it is already having a big impact on building work in the City of London. Foundations for the Moorhouse development in Moorgate are being built assuming that Crossrail will run almost directly below it. As a result the 20-storey building has the deepest base-grouted piles ever built in London.
Architect Foster and Partners has designed the speculative development for Moorhouse Property Developments in association with the Moorhouse Limited Partnership (Hammerson, Greycoat and AMP Pearl).
Skanska Construction UK was appointed as design and build contractor for the £85M, two-year project in March this year and started on site at the beginning of April. Crossrail-related work includes a 50m deep draught relief shaft below the building, and the shell for a station ticket hall in the basement. But the biggest controlling factor on the design and construction of the foundations is the provision of frictionless pile sleeves to up to 26m depth.
They are designed to isolate Moorhouse's foundations from any future ground movements associated with the Crossrail running tunnels and a station box that is planned to run below the pavement immediately outside the building.
Because the tops of the piles are frictionless and so do not carry any of the structure's load, they need to be deep - up to 56m long. In terms of capacity, the top 26m of the piles do not exist.
While this has added significantly to the project costs, it is undoubtedly much cheaper than the complex compensation grouting measures that would have otherwise been necessary when Crossrail tunnelling eventually takes place.
The complexity of the foundations means that piling is scheduled to take six months. Threequarters of the basement will be built bottom-up, but to meet the tight two-year programme, a quarter will be formed bottomdown. This allows work to start on the building's concrete core before the basement is finished.
Cementation Skanska Foundations is rotary boring 54 basegrouted piles through the London Clay and into the Thanet Sand deep below the compact site. The piles are 1.8m diameter: the biggest take four days to construct.
Cementation starts off each pile by driving slightly oversized temporary steel casing, up to 9.5m long, into the top of the London Clay to seal off the made ground.
Soil is augered out of the temporary casing and the bitumen-coated permanent casing is driven through its base, deep into the London Clay, to just below the predicted zone of Crossrail-associated ground movement.
Mirroring the potential ground movements, the depth of the frictionless sleeves is reduced the further they lie from the anticipated tunnelling route.
Next, soil is augered from the permanent steel casing, and Cementation continues the pile shaft to full depth by rotary boring under bentonite.
At full depth the base is cleaned out, a full length reinforcing cage placed in the shaft and the concrete poured. Pile installation is completed by base grouting through tubes fitted to the cages to decrease pile settlement at working loads. The tubes are flushed out so they can be used later for sonic integrity testing.
Although the work is not unusual, the scale of the piles complicates their installation.
Timing is critical, particularly the sequence of work on the day that concreting takes place.
Cementation originally estimated it would take 14 hours of continuous work on the final day to complete each pile. This was taking into account the novel use of 'rat holes', or redundant pile shafts. Within these, it assembles reinforcing cages before lifting them into position.
The situation is made more acute because bentonite-supported bores cannot be left open over night before concreting because the base can soften, reducing pile capacity. And once concreting has started, it is essential to complete the pour - all 24 concrete wagons - in a continuous shift.
Established procedure is to bore to within 2m of the pile base on the penultimate day, leaving a plug thick enough to prevent softening at the pile base. But when a pile is 50m down, it takes a long time to remove the bottom 2m and then change tools to clean the base.
Fortunately consultant Arup had been involved in trials at Stratford on the Channel Tunnel Rail Link, which proved that overnight degradation at the bottom of comparable piles in similar soils is restricted to 500mm.
Arup was prepared to reduce the overnight base plug to this thickness. This meant Cementation could start the final day using the base-cleaning tool to remove the plug, and then clean the base without having to change tools.
This shaved an hour off the programme.
Because Cementation could demonstrate the technical need to complete the piles, but the physical impossibility of doing so within normal working hours, the Corporation of London's Environmental Health Department allowed work to continue to 10pm, three days a week.
The other big problem is space.
The site is barely big enough to hold the three piling rigs and two cranes used for most of the sixmonth piling operations. It is so small that Cementation housed the site offices and bentonite recycling plant underneath a working platform designed to carry loads of 90t.
Cementation area manager Martin Kenwright says: 'Because space is so limited our approach has been to prefabricate as many of the components as possible off site.' The company is also using justin-time delivery. The success of the project, he adds, 'is as much about logistics as engineering'.
Piles were located to avoid known obstructions. But, inevitably, Cementation has found itself drilling through heavily reinforced concrete blocks and even piles left from the 1960s building previously occupying the site.
For this it used a Bauer BG30, which has very high torque.
During the contract Cementation took delivery of an even more powerful BG36, which it says can install piles up to 2.4m in diameter and 60m long.
Given the complexity and size of the piling work, it has gone incredibly smoothly and is winding up on schedule. Once it is complete, work can begin on the building itself and the programme will soar like 'a cliff face' says Skanska Construction project director Matt Cova.
Construction of the Crossrail draught relief shaft from within the building's basement falls comparatively late in the programme.
Cova says: 'We'll still be shifting muck 11 weeks before we hand the building over, which must be some kind of first for a 20-storey structure.'