The piles were to support the headhouse positioned at the top of the shaft. Safety concerns of spoil falling into the shaft prevented the use of long auger strings and masts, while proximity of the shaft also prevented the use of our larger drilling rigs.
Ground Conditions
Made Ground, Alluvium and River Terrace Deposits overlaid the London Clay which was present circa 6m below ground level. Local groundwater seepages were found to occur on isolated pile positions particularly on days of heavy downpour.
Solution
Due to the ground conditions 660mm temporary casing was installed to a minimum of 6m depth in 1m sections toeing into the London Clay. This Temporary casing locked out the groundwater and stabilised the bore in the poor material overlying the London clay.
600mm diameter bearing piles to 16m and 17m depths were then to be placed in a rotary methodology with our 15T SM12 rig. The augers were added in 2m sections until the design depths was achieved. Full length reinforcement of the piles would be spliced together in 3 sections as they were lowered into an open bore.
Finally a tremmie pipe would be inserted through the centre of the cage to prevent segregation of concrete during casting.
Subsequent to installation a static load test to a maximum load of 780kN on single test pile was undertaken. The results showing a deflection of less than 1mm.
CROSSRAIL C360; STEPNEY GREEN PROJECT SHEET
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