A large 1.8m ID tunnel runs through Bournemouth carrying the combined foul and surface water to the various sewage treatment works across the network. The local geology of the Bournemouth area consists mainly of fine sand.
A defect was spotted in the road, which caused it to be closed and emergency works began to repair the defect from within the tunnel itself using trenchless technology rather than trying to access a defective structure with a potential void above it and loose wet sand surrounding it using traditional methods.
These emergency works were implemented to prevent a likely collapse in the residential road, which would cause major issues with the sewer network in Bournemouth. The solution was developed by MDA, Wessex Water and checked/approved by Atkins.
The first stage of the works was to establish an overpumping route to keep the section of sewer dry which would enable the Miners to carry out repairs. The route included 2km of pipework that had to be installed by hand over parkland, across roads, over a 20m head difference and along main roads and residentials streets.
There were also several alterations to existing manholes to install weir walls, baffle walls and stop logs to ensure that the overpumping system could be monitored and maintained throughout the works.
Once the overpumping was put into place, MDA surveyed the condition of the tunnel internally as well as Wessex providing million-point cloud surveys of location, position and size. The tunnel showed numerous signs of water and sand ingress that needed to be addressed as part of the works.
Through probe drilling and investigation, the worst areas of voids, water and sand ingress were identified and sealed through pressure grouting.
Once the tunnel was sealed internally, works began to clean the tunnel, install the bespoke steel cage with HEB curved steel beams laced with reinforcement bars and circumferential helical bars to create a rigid structure throughout an 80m section of the tunnel. The main area of repair stretched over a 20m length with an additional 30m repair either side of the defective area.
With the rigid steel structure in place, a GRP liner was installed through the middle of the steelwork and fixed into place using hardwood folding wedges. Upon completion the entire annulus of the composite structure was grouted using a polymer modified grout with fibres to give the most structurally sound solution.
The site team worked extremely hard on this scheme to deliver the works to the client’s satisfaction ahead of programme and without a single accident or incident. There has been no further signs of deterioration, proving a success for the scheme.