Delta Civil Engineering has just completed rehabilitation of waste water storage tanks under busy Plymouth Hoe on the UK south coast. Client South West Water found the three tanks, built around 1900, were leaking spectacularly, allowing as much as 100litre/s of salt water ingress.
The glazed-brick lined arch tunnels are 3.5m high and 3.5m wide and run for 247m with interconnecting accesses every 50m. But since they are immediately adjacent to Plymouth’s West Hoe harbour, running under the busy Grand Parade, they are subjected to a 5m head of water at high tide. Access was confined to shafts at either end of the tunnels.
Before Delta could begin renovation to reduce leakage to an acceptable 5 lt/sec, a 500mm deposit of septic silt had to be removed from the tunnels. It produced high levels of hydrogen sulphide gas when disturbed and meant operatives had to be trained in the use of breathing apparatus.
Two additional shafts sunk as mid-point caissons for this work were staggered either side of the road to avoid traffic disruption. Specialist subcontractor Drain Brain then cleared the silt using its "sludge gulping" system.
Salt water ingress points were marked, then holes drilled for grout injection to seal the tunnel annulus along their lengths, progressing seaward.
Grouting on the last tunnel was done in two stages to prevent risk of grout penetrating to the harbour through surrounding imported fill. The fill condtion was unknown after been subjected to tides for years.
If there was a risk of grout penetrating the harbour, grouting was stopped early and resumed the next day.
Leakage was eventually reduced only 80t of grout, compared with the 280t allowed for.
Brickwork has been cleaned and re-pointed, and the two new shafts converted into access manholes. The £715, 000 contract was completed for early July.
