Residential Property in Cheshire

The Project

The site for the luxury development, Thornfield Heights, Alderley Edge, was bounded by masonry walls along two elevations that had to remain during the work. Our project remit specified designing and installing a temporary contiguous bored pile retaining wall. Plans estimated retention heights of up to 4.5 m over a 25-m distance. Maximising the excavation footprint meant working up to 750 mm from the wall’s centre line to the perimeter wall’s vertical faces. The limited space on-site and shallow rockhead covered by loose sand required additional planning. Moreover, we had to account for storing materials, concrete deliveries and spoil management.

The Solutions

TGS’s retention solution consisted of installing 450-mm-diameter piles at 600-mm centres using a high-power minipiling rig. This ensured piles reached depths of up to 8 m in the relatively weak sandstone bedrock. Because of the weak ground conditions, we adapted the rig’s safety cages to enable our drillers to install contiguous piles at as little as 700 mm from the existing walls (centre of the pile to the vertical face). We reinforced the piles using full-length H16 cages before the high slump C28/35 concrete was placed directly into the pile bore. Cube test results for the concrete confirmed the design strength was attained before 28 days. After excavating the basement, we surveyed the piles, which achieved the required plan and verticality tolerances.