TUNNELLING CONTRACTOR Delta Civil Engineering Company recently completed a major refurbishment of historic waste water storage tanks in Plymouth.
The firm, working for client South West Water Services, started on site in January 2000.
The contract involved renovating the brickarched West Hoe waste water storage tanks and reducing the 100 litres/s of saline water leaking into the structures to only 5 litres/s. This was achieved by a series of grout injections along the length of the structure.
The tanks were built at the beginning of the last century as three horseshoe-shaped tunnels, 3.5m high and 3.5m wide, with interconnecting access points every 50m.The tanks are next to West Hoe Harbour and are subject to a 5m head of water at high tides.
Access to the 247m long structure was through shafts at either end.To speed up cleaning operations (and to make them safer), two new shafts were sunk near the middle of the tunnels; one 4.5m diameter, spanning the central and outer tunnels; and the other 3m diameter, spanning the inner, landside tunnel.
Sewage flows to the tunnels were temporarily redirected to allow work to be carried out in relatively dry conditions. The tunnels act as a sewage storage during storms, so allowances still had to be made for evacuation and lost time during storm events.
The first job was to remove a 300mm thick layer of septic silt sludge from each invert which, when disturbed, produced high levels of hydrogen sulphide gas.This was carried out by subcontractor Drain Brain using its 'sludge gulping' system, which minimised man entry into the potentially hazardous environment.
Tunnels were then inspected and all ingress points on the inner and outer tunnels marked at high tide. Grout was then injected in these areas to seal the annulus, first on the inner tunnel and then the outer, seaward, tunnel.
Grouting of the seaward tunnel was a twotier operation as the condition of the fill around the tunnel was unknown. Grouting was carefully managed to ensure the cement-based material did not pass beyond the annulus and penetrate the sea wall into the harbour. In areas where this was thought to be a risk, grouting was stopped and resumed the following day.
Grouting continued until infiltration was reduced below acceptable levels. Some minor regrouting of the inner tunnel was needed as predicted, but only 80t of grout was used compared with the original 280t estimate.
Finally, the tunnels were cleaned and some repointing of the brickwork carried out. The original plan was to cast an insitu concrete replacement invert, but this proved unnecessary as the condition of the brickwork in the invert was found to be 'first class'with no signs of infiltration.
Work, which finished in June, achieved target infiltration levels.