Desiccation Assessment: Pinner, London

RSA Geotechnics carried out an investigation at a residential property in Pinner, which for some years had shown signs of movement and suffered damage to the superstructure.

The property was situated amongst established shrubs and trees including oak, lime, sycamore and willow. The underlying geology consisted of a thin cover of Head Deposits with London Clay below.

Two previous site investigations had been carried out by others, to determine the ground conditions and reason for the movement exhibited. Desiccated soil was identified to be present to a depth of between approximately 3 and 4m. A piled raft solution was therefore proposed, to counteract the effects of the desiccated soil, however, the previous investigations had only extended to a maximum depth of 5m below ground level.

Further work was therefore required to confirm the ground conditions to a nominal distance below the proposed pile toe depth, anticipated to be at a depth of around 15m.

Access around the property was restricted to the footpath that led to the back garden and did not allow for use of a standard cable percussion drilling rig. RSA Geotechnics therefore mobilised a demountable light cable percussion rig. Three boreholes were drilled at locations around the property and a further borehole was drilled at a remote position within the back garden.  The boreholes extended to a depth of 20m.

Following a review of the two previous ground investigation reports and the findings of the RSA Geotechnics ground investigation, it was considered that use of a piled heave protected raft foundation would be appropriate to counteract the effects of the disclosed desiccated soil and arrest future movement of the property. Restricted access piling techniques would be required for the pile installation and specialised construction methods needed for installation of the reinforced heave protected raft within the existing house.