Ten pads required treatment: eight measuring 2 m × 1.6 m × 1 m deep, with their base at 1.9 m below ground level, and two measuring 2 m × 1.8 m × 0.6 m deep, with their base at 1.5 m below ground level. All ten sit beneath the floor slab and a layer of fill, so the ground below them could only be reached through holes cored through the slab and the pads themselves.
A 2006 ground investigation for the depot, comprising four boreholes, recorded Boulder Clay over Upper Chalk. Near-surface soils were typically stiff clay, but one borehole found loose sand and clayey sand to 2.5 m, with SPT N values of 5 and 11, and perched water at 2.0 m. The investigation recommended limiting bearing pressure to about 110 kN/m² to control differential settlement, because of the variable near-surface soils.
Geobear's design assessment estimated an existing bearing capacity of about 140 kPa beneath the pads: adequate for the original 110 kPa loading, but short of the new 200 kPa requirement. Achieving the new loading required the undrained shear strength of the clay below the pads to be raised to about 100 kPa. The works also had to fit around an operational depot, with crane and plant movements nearby and one pad partly obstructed by an adjoining building.
