Avoiding settlement and collapse when building a new basement into river gravels

Solving construction problems amongst London’s architecture

The problem was to devise a way to construct foundations and a basement adjacent to a seven-storey listed building without causing potential settlement damage.

The piled capping beam of the new building had to be constructed approximately 1m below the level of the footings of the adjacent block. These concrete footings were founded on non-cohesive terrace river gravels. Excavation into this deposit would have risked collapse of even a short length trench and the likely settlement of the flank wall. STRESS were called in to design a temporary works scheme to strengthen the gravels so that short sections of the capping beam trench could be excavated below the foundation level without risk of collapse and settlement.

The solution

Using our patented ReFORCE permeation engineering system of ground stabilisation our technicians injected the terrace river gravels to a depth of 2.5m below site level for the full length of the flank wall.

Using a hit and miss method the main contractor then excavated into the grout stabilise deposit and constructed the RC capping beam in 2.5m lengths without any signs of the sides of the trench collapsing under the weight of the adjacent seven storey building.

Monitoring showed that not a single millimetre of movement was experienced by the adjacent building during the construction of the 38m long capping beam.