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Tunnelling techniques help with sewer repair
Photos courtesy of the Ivybridge, South Brent & South Hams Gazette
May Gurney has completed an unusual sewer repair for South West Water using tunnelling techniques learned from local Cornish tin miners.
The emergency repair involved a 45 meter long timber framed "heading" or tunnel dug 11 meters deep beneath the Aveton Gifford bypass section of the A379 Plymouth to Kingsbridge road.
The £490,000 repair of a 225mm diameter combined sewer was needed because the pipe was partially collapsed and heavily silted. This was causing localised pollution to a nearby village stream which feeds directly into a tidal estuary of the River Avon.
A pollution incident was reported to May Gurneys' call centre at Exeter and a sewer maintenance team dispatched to investigate.
After CCTV surveying and attempting to cleanse and line the failing asset, a temporary repair was carried out. However a more permanent solution would be required, due to the potential for major environmental damage, should the line completely fail.
So May Gurney's Environmental Management Team assembled its rapid response unit to deal with the emergency. Over-pumping was immediately installed and left in situ to safeguard against further pollution incidents and a site compound was erected and secured.
A thorough site investigation followed, involving May Gurney site engineers and in-house temporary work designers. These revealed the works would need to be carried out on the boundary of a flood plain. Due to the depth and location of the repair (11 meters directly beneath the A379), it was decided to auger bore by laser guided drilling to facilitate the installation of a new line.
Excavated
Once the access pit was excavated it became apparent that a directional drill would not achieve the desired result due to an unforeseen bedding plane of shellet and the near flat gradient between existing manholes. Also ruled out were pipe bursting or slip lining, due to the presence of a 450mm reinforced concrete slab installed directly along the length of the sewer. After further consultation with South West Water and supporting consultants Hyders and Faber Maunsell, it was decided to construct a timber framed heading to gain access to the collapsed sewer.
Once the location of the collapse was reached further CCTV surveying discovered additional collapses and that the physical construction of the existing system was below standard.
After consultation with South West Water it was decided to replace the existing line in its entirety. The heading was extended to the upstream manhole, a total distance 45 meters, to allow the laying of a new 225mm line, a 300mm overflow line and two service conduits.
The management team worked closely with South West Water, the Environment Agency and South Hams district council to minimise the environmental impact of the works and disruption to the local community.
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