RSA Geotechnics were recently instructed to undertake a ground investigation at a site in north London that had encountered severe surface cracking during an exceptionally dry summer. The investigation area was located behind a wooden retaining wall on a sloped embankment. Cracks up to 100mm wide had opened up along the bank affecting the gardens of six newly constructed residential homes.
Due to the inaccessible garden areas, hand held equipment was used to drive the window sample tubes to depth and samples were taken to create a soil moisture profile. A small inspection pit was made directly behind the retaining wall to establish whether the backfill material had been compacted according to the Manual of Contract Document for Highway Design. Whilst undertaking the inspection pit a whole glass milk bottle and whole bricks were encountered which suggested that the backfill material had not been suitably compacted and was unsuitable as a fill material.
The investigation found that desiccation had been evident on the site from a previous investigation prior to the removal of a large high water demand tree. Desiccation was still evident during the investigation after the tree had been removed. Along with the backfill material not being suitably compacted a drain was found to be running approximately 1.5m beyond the retaining wall and the desiccation cracks that had opened up were all located beneath the line of drainage.
It was recommended that the material behind the retaining wall was removed and replaced with a more suitable material compacted to the correct guidelines as stated in the ‘Specification for Highway Works’.