Murphy Ground Engineering constructed 1200mm diameter bored piles to form a deep, circular secant pile shaft for Thames Water. The shaft would ultimately become a pumping station, forming part of the Deephams Sewage Upgrade being completed by AMK – the joint venture between AECOM, Murphy and Kier. The aim of the upgrade is to clean up the nearby River Lee and surrounding environment.
Ground conditions consisted of gravels overlying London clay and water-bearing Thanet sand. This meant that the temporary segmental 1200mm diameter casing had to be taken the full 29m depth of the shaft. This would provide full bore support during pile construction. The casing weight for these piles was approximately 45T plus skin friction along the full casing length. This meant a large Bauer BG40 rig with significant torque was required to install and extract the casing whilst also meeting stringent tolerance requirements.
The length of the piles and design requirement for full length reinforcement provided a challenge. A reinforcement cage at a length of 29m could not be delivered in one complete section. The cages were therefore delivered in two sections to site. This meant lifting the first section into the pile before lowering the second and splicing them together using a safe splice system. This intricate operation required an experienced team that had dealt with similar crane movement, lift plan and logistics challenges before.
Each reinforcement cage weighed a total of 4.2 tonnes when fully spliced and constructed. This large weight combined with the weight of the concrete meant there was potential for the reinforcement to push into the base of the pile; comprising cage placement levels. To counteract this, the team welded steel plates to the bottom of each cage to reduce pressure at the pile toe and keep the cage in position once placed.
This was Murphy’s first project for the brand new Bauer BG40 rig. Having a fleet of Bauer rigs already, Murphy were confident in the ability of Bauer plant and equipment. The team worked closely with Bauer to ensure that the rig they chose was correct for this and future project requirements.
Approximately 35m3 of concrete per pile (on average) was required and a tremie placement of concrete methodology had to be used in every pile. This allowed piles to be concreted from the bottom up; ensuring ground water did not compromise concrete quality and strength.
The shafts were designed by Byrne Looby & Partners. The male piles used C32/40 strength concrete and the female piles used C8/10 strength concrete. The concrete strength selected was critical for achieving the 1:200 vertical tolerances required by the client. The concrete in the female piles had to be of low strength in order to be augured without deviation while maintaining sufficient strength to comply with design requirements. Factoring in short and long term strength gain in concrete mixes is an important factor in any successful project, especially when the depths are as deep as they were here. Murphy had previously installed secant piles using similar mix designs and the team were able to reference historic strength gain results; giving the client full confidence in this approach.
The tolerances required for the piles were extremely tight with 25mm in any direction specified in the plans. A concrete guide wall was constructed prior to boring, which along with a 1:200 tolerance meant that there was an accuracy of within 5mm per metre for all piles.
In order to speed up the programme, Murphy brought a second set of casings on to site. This allowed the crane on site to complete the concreting operations on a pile while the Bauer BG40 rig began installing casing and boring the next pile. Using this procedure, the team reduced the programme by five weeks.