West Green Pumping Station
DAWSON-WAM was contracted by Barhale to design and construct a circular secant shaft for a new pumping station and attenuation tank to facilitate the construction of over 200 new homes. The requirement was for a 9.2m deep, 10.5m diameter shaft to be installed.
The design team was concerned with the ground conditions, which showed sands and gravels over the top 9m overlying mudstone with high water inflows into the boreholes at high levels. This precluded the use of conventional segmental shaft sinking techniques due to the high water inflows and the potential for instability within the excavation.
DAWSON-WAM proposed a circular secant shaft which could be installed down to and into the underlying mudstone to provide excavation support and anti-floatation measures. The Cased CFA technique was proposed as the method to install the secant piles due to its capacity to core into the underlying mudstone whilst preventing flighting and potential for platform instability.
Further analysis by DAWSON-WAM showed that the secant wall could be design using hoop stress to allow the secant piles to self prop allowing a 9.2m deep unpropped excavation. Installation tolerances of greater than 1:150 were assumed in the design as well as positional tolerances of +25/-0mm. This allowed the design to work with hoop stress to 8m below ground level and thereafter piles worked in bending.
To ensure positional tolerance DAWSON-WAM installed a secant guide wall which positioned 630mm diameter piles at 490mm centres to form a secant circle of 11.4m internal diameter. This was cast a week in advance of secant piling commencement.
Secant piles were installed with an ABI TM22 utilising a Eurodrill twin rotary unit (VDW) configured to install Cased CFA piles. The system allows the casing and auger to be advanced together, continually supporting the sides of the bore from over flighting and allowing accurate and timely penetration into the gravels and bedrock. Once the system reaches the pile toe concrete is placed through the centre stem of the auger under consistent pressure to fill the void as the casing and auger is extracted.
To assist with load transfer around the shaft every pile (both primary and secondary) were constructed with structural grade concrete of 28 day compressive strength of 28N/mm2 or better. DAWSON-WAM specialise in this hard/hard wall system which also provide better water retention capacity than hard/firm or hard/soft walls.
After completion of the secant piles the excavation proceeded once piles had achieved a determined level of structural strength. Thereafter the base slab was completed and piles were lined with precast concrete units.