With no margin for error, Cementation had to get everything right first time when it installed foundations for the Moorhouse development. Paul Wheeler also discovers why it needed some of London's deepest base-grouted piles.
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 & Partners' Moorhouse development in London's 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 some of 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.
These are designed to isolate Moorhouse's foundations from ground movements of up to 100mm - the amount the construction of CrossRail is predicted to cause. The project will include running tunnels and a station box to be installed below the pavement immediately outside the Moorhouse building.
It is because the top sections of the piles are frictionless - and so carry none 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 costs, it is no doubt considerably cheaper than installing complex compensation grouting measures which would otherwise be necessary when the tunnelling eventually takes place.
Foundation subcontractor Skanska Cementation is rotary boring 54 basegrouted piles through the London Clay and into the Thanet Sand. Piles are up to 56m in length and 1.8m in diameter - and the biggest take four days to construct.
One of the key issues is the length of shift on the last day of pile construction, explains the company's area manager Martin Kenwright.
Established bored piling procedure is to bore to within 2m of the pile base on the penultimate day, leaving an overnight plug of soil 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.
Cementation originally estimated it would take 14 hours of continuous work - way beyond the permitted working hours.
However, the contractor had conducted trials at Stratford and Canary Wharf, in which a series of penetration tests at the base of pile shafts showed overnight degradation at the bottom of the deep pile bores in Thanet Sand was only 500mm. Fortunately, on the strength of these results, Arup reduced the thickness of overnight base plug to 0.5m.
This meant Cementation could start the final day using the basecleaning tool to remove the plug, and then clean the base without having to change tools. Critically, this shaved a couple of hours off the programme.
Furthermore, on the basis that Skanska 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 granted a Section 60, under which piling operations could continue to 10pm for three days a week.
The other big problem was space. The site is barely big enough to hold the three piling rigs and two cranes used for most of the six-month piling operations. As there was not enough room for a standard bentonite farm, Cementation decided to house it in a series of containers supporting a temporary steel deck, approximately at road level. This created a piling platform, working area and delivery platform.
'Because space is so limited our approach has been to prefabricate as many of the components as possible off site, ' Kenwright explains. The company is also using just-in-time delivery, he adds. 'The success of the project is as much about logistics as engineering.'
Piles feature full-depth reinforcement, complete with base grouting/sonic coring tubes.
They were preassembled off-site using couplers, which meant the completed cages could be easily broken down into lengths for transporting to site and then fully reassembled in a 'rat hole' or redundant pile shaft.
Cementation had a site-based management team of eight shared between the site and the reinforcement cage fabrication yard. 'It's no good having justin-time delivery if there is a problem with the cages when they arrive on site.'
Access to the site was another logistical nightmare, complicated by the fact that the City's anti-terrorist 'ring of steel' runs through it. Although there were two potential access positions only one could be used. 'In principle, if we had used both entrances we would have had to physically erect a steel barrier across the site, ' says Kenwright.
While piles were located to avoid known obstructions, Cementation inevitably found itself drilling through heavily reinforced concrete blocks and even piles remaining from the 1960s development previously occupying the site. In fact, together with perimeter sheet piling, Cementation spent the first eight weeks of the contract on what were essentially enabling works in advance of the load-bearing piles.
Cementation cored through the obstructions using a very high torque Bauer BG30.
Progress was slow but effective, reports Kenwright. During the contract Cementation took delivery of an even more powerful BG36 and both rigs have been used to great effect on the deep Moorhouse bores.
Given the complexity and scale of the piling work, the job has gone incredibly smoothly and much to Cementation's credit it recently completed the work on schedule. 'This was a site where a minor complication would lose a day - and there was no chance ever to make that time up again, ' says Kenwright.
'To be successful on this project, we had to get everything right first time - there was absolutely no margin for error.
The consequences of not being perfect were dire.'
Environmental benefits of less recycling Cementation used the Moorhouse project to pioneer a new type of bored pile drilling bucket - one that uses side cutting blades. The company claims the bucket saves time by significantly reducing the need to recycle bentonite and polymer drilling fluid.
The barrels of standard drilling buckets are generally 100mm smaller in diameter than the pile. They rely on the outer pair of teeth being set at the correct radius to produce the right pile diameter combined with reamer blades fitted to the head of the buckets, to ream the bore to the correct diameter.
However, the reamed soil generally falls into the annular gap around the bucket and is not collected. This contaminates the drilling fluid which then requires cleaning before concreting can take place - a process which takes several hours for large piles.
These reamer blades are also vulnerable to damage.
With Cementation's new design (UK Patent in application 2373801) side cutters at the base of the bucket have vertical edges that cut the full pile shaft accurately to the required diameter. Spoil generated by the cutters is drawn into the bucket along with the spoil from the base cutters.
When the bucket is full, it is rotated anti-clockwise against the base of the hole. This causes the swivel base plate to rotate relative to the fixed base plate assembly, covering both side and base openings and trapping the spoil for removal to the surface, while the vertical side cutters act as covers for the side openings.
The new drilling bucket provides major environmental benefits. With contamination rates low, only minimal quantities of fluid need be replaced during the piling phase, says Cementation. This not only reduces material costs, but saves massively on disposal costs - a big concern with new environmental regulations imposing high premiums.
Client: Moorhouse Property Developments in association with the Moorhouse Ltd Partnership, a partnership of Hammerson, Greycoat and AMP Pearl
Quantity surveyor/employers agent: Davis Langdon & Everest
Main contractor: Skanska Construction UK
Foundation subcontractor: Skanska Cementation
Foundations Architect: Foster & Partners
Structural electrical and mechanical engineer: Arup