Using water instead of bentonite as support fluid during piling has reaped benefits on a small, congested site in London's Docklands.
Two more residential towers will be joining the office skyscrapers on the skyline of London's Docklands. Ballymore Properties' No 1 Millharbour development, just north of Millwall Dock on the Isle of Dogs, will consist of 37- and 50storey towers, plus excavation of a single-level basement.
WSP, structural and geotechnical engineer for the project, has designed a combined piled raft foundation to support the towers.
The basement raft sits on the Thames terrace gravels, with typically 1.2m diameter rotary bored piles used to carry a significant proportion of the load.
The basement is being formed by installing an 880mm diameter secant wall around the perimeter.
'Construction of many large diameter piles in London's Docklands involves the use of bentonite slurry to support pile bores during drilling and construction, ' says WSP technical director Matt Sharratt. 'But installing and operating a bentonite slurry system can be expensive, and on a relatively small site, with several activities being undertaken at once, it is not always practical.' This is the problem at No 1 Millharbour, where foundation contractor Expanded Piling began work in November. It is installing both the secant pile wall and the bearing piles for the basement raft at the same time. This means there are three piling rigs and their three attendant cranes on the small and congested site at any one time.
To avoid the problems associated with bentonite slurry, including disposal - it is considered a waste product - Expanded proposed an alternative technique using full length temporary casing and water as a supporting fluid.
Sharratt says several drilling and construction trials showed this method could be used successfully, if various tests and checks were carried out on site to ensure the pile bases were clean.
The ground is typical of the Isle of Dogs, comprising fill over alluvium, river terrace gravels, the Lambeth Group Beds and Thanet Sand.
A Casagrande C800 rotary bored piling rig is installing the casing and removing the spoil from the pile bore. Temporary double walled, 40mm thick, segmental casing in 2m, 4m or 8m lengths is installed as drilling proceeds.
'An important part of the process is to avoid potential ground loss by ensuring drilling does not extend deeper than the temporary casing at any time, ' explains WSP associate Imraan Motara.
'Site trials identifid that piles could be formed to the top of the Thanet Sand without the need for support fluid. Upon reaching the Thanet Sand, water is added to the pile bore to maintain stability before the casing and pile bore are advanced to the design depth. A deflocculent is used to help desand the pile bore water.' Monitoring of the pile base depth and testing the pile bore water support fluid for sand content in suspension is carried out from the time the bore is finished right up until concrete placement.
Motara says a limit of less than 25mm thickness of sand on the pile base was specified, as well as a limit of 1% sand content in suspension - both these criteria have been achieved. Following construction of the piles, base grouting of the piles is undertaken normally.
A preliminary test was done on a sacrificial pile installed using the same method to verify the construction method and pile performance. In fact, Motara says, the test showed the method produced piles with a slightly stiffer shaft and base capacity than predicted.
The secant pile wall, forming the sides of the 8.5m deep basement, was also designed to limit groundwater inflow during construction.
Because of the depth to and limited thickness of the Lambeth Group clay layer, good verticality is essential to ensure the secant is maintained to the bottom of the basement.
To this end, Expanded proposed its cased secant piling system, which uses a high torque Casagrande CFA piling rig with a thick wall casing to form the male piles. The casing cuts into the female piles and keeps the male piles vertical.
Male and female piles are the same diameter, the females unreinforced and formed using the conventional rotary bored method, with segmental casing to improve verticality.
'The overall verticality required is 1:150 and this is being achieved on site, ' says Motara. The piling contract is due to ' nish at the beginning of April.