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. Through Wessex Water’s proactive approach to asset maintenance, a defect was found in the tunnel’s concrete lining. Having recently successfully completed the Percy Road scheme, MDA were called to carried out trenchless technology rather than trying to access a defective structure.
The actual repair to the tunnel was far simpler than the Percy Road works, but the logistics and other associated works to be carried out in close proximity in a confined residential area made this scheme particularly challenging.
Firstly, MDA had to set up over 400m of over and underground 450mm diameter bolted steel pipework overpumping from the main access chamber to a discharge shaft that was 15m deep. In order to do this this, the existing manhole has to be enlarged to accommodate two 8″ hi-line pumps.
While this was being carried out, a 1.5m diameter caisson shaft was being excavated by hand down on top of the CIS 30m downstream of the repair location, this shaft would then allow the overpumped flows to get back into the system without affecting the works.
Several sewer lines were coming into the main shaft at Seafield Road and this meant that they all had to be isolated separately for periods of time to ensure the network was not adversely affected.
A new 200m long drainage network with four new manholes was constructed, along with the re-construction of the main 10m deep, 5.5m diameter shaft. This shaft had been so badly corroded by hydrogen sulphide (H2S), generated by the tumbling and turbulence of foul liquid, that it needed to be demolished back to a solid part of the structure and rebuilt by spraying high strength shotcrete, giving cover to the reinforcement bars, but also by the spraying of Sewpercoat. This is an aluminum cement product that is resistant to H2S and prevents the shaft from further potential decay.
Upon completing all the associated works and diverting the flows, MDA ensured that the tunnel was sealed and began to install 1600mm diameter GRP liners to the affected 200m length of tunnel.
Once the liners were installed, the annulus was grouted in stages to ensure a completely solid structure that would provide the strategic sewer network in Bournemouth with a significantly enhanced life span.