Substruck Ltd. were employed to address localised subsidence on a newly purchased property in West Cork. A pre-purchase survey identified foundation movement to the front corner of the building and the requirement for underpinning. As per BRE Digest 251, this cracking could be described as Category 2 damage – up to 5mm in width.
In order to control risk, the design of any underpinning solution should be based on the underlying subsoils and the required loadings. Therefore, Substruck completed a trial hole and dynamic probe at the area of distress. A control was employed to the rear where no movement was noted.
The trial hole identified an adequate foundation but the soil at formation level was described as very soft to soft and could be easily probed with a crowbar. Dynamic probing showed very soft soil with the probe travelling under self-weight and N10H values not exceeding 1 until refusal was met abruptly at 2.6m. There was no evidence of water in the excavation. The movement was attributed to this soft spot which was not appropriately addressed during construction.
Considering the cracking and the soil conditions, the risk of further movement was considered high. A traditional pile and beam underpinning solution, straddling the corner, was designed and constructed.
BS 5930:2015 outlines the requirement for continuing investigation through controlling and monitoring during construction works. During the works, water was noted to be trickling into the excavation from the rear. This may have been a contributory factor in the foundation movement. It was recommended that land drainage be provided to the rear elevation and diverted to the local stream to mitigate the risk to the remainder of the building over the long term.
The site was left neat and tidy on completion and the client even sent us a new ‘Thank You’ note. Another happy client.