Description and Overview of Project
Following reports of unpleasant odours in the area of the entrance and conservatory seating area at the Hospice, Exjet were contacted by the management to investigate the possible cause. Initial CCTV surveys indicated a defect in the foul water sewer beneath the floor which was causing a partial blockage in the pipe. Further investigation revealed that this was due to a foundation pile, installed during the building of the new conservatory, which had come into contact with the pipe and dislodged it.
The pile was seen to penetrate almost the full diameter of the sewer pipe, and Exjet were asked to propose a repair solution. With only limited visibility with CCTV cameras inside the pipe, the extent of any external collateral damage could not be confirmed. It was agreed that a trial excavation would be needed, but at nearly 3m deep, this could only be done after an engineering review of floor construction and other structural details so that the implication for the building could be properly understood.
The floor slabs were shown as cast in-situ reinforced concrete, and to be supported on a piled foundation with encastré ring beams. This suggested that there could be a void or un-compacted fill beneath the floor slab, making access to the damaged pipe uncertain. The piles were found to be skin-friction bearing and approximately 10m in length, so that the top 3-4m would not necessarily contribute to their structural strength.
An initial 1m x 1m trial hole was opened, but concerns were raised when 400mm depth of concrete was found rather than the 225mm shown on the drawings. Careful excavation showed that the trial hole was clear of the ring beam and that the floor slab had been mass filled. Excavation below this found more mass concrete, which was believed to be from a previous structure. With these obstructions in the trial hole, it wasn’t possible to expose the pile and damaged pipe as originally envisaged, so the decision was taken to remove the bulk of the suspended slab by cutting out to the nearest edge, and to drive a short heading under the adjacent ring beam to expose the pile from the side rather than from above.
This hadn’t been the preferred method due to the impact on the nearby planted bed, which had to be removed to allow access, but given the frustrations encountered in the initial works, all parties agreed it was the prudent way forward.
Utilising our skills at hand excavation in difficult openings, we were able to dig toward the damage and identify the cause. Once revealed, the repair was made quickly using a combination of new uPVC pipe fittings and the installation of a Cured In Place Pipeline between the downstream and upstream manholes.
The void created by the excavation beneath the floor was reinstated with foamed concrete, and the reinforced concrete floor slab reconstructed, dowelled into the surrounding existing slabs. The planted beds were reinstated, and the project handed back to the main contractor for replacement of the final finishing’s.
During the 4 week construction programme, Exjet maintained unimpeded access to the Hospice for patients, staff and visitors at all times, and developed good working relationships with staff and the other stakeholders who have agreed the project has been a success