Gwithian Outfall Shaft, Penzance – Cornwall, England

The new Penzance/St. Ives Sewerage Scheme in Cornwall required the sinking of a 6.1-meter diameter shaft to a depth of 60 meters on the shore at Gwithian, to provide access to a 2.1 km long outfall tunnel to be driven out beneath St. Ives Bay. Initial ground support through the upper water-bearing sands, gravels and fractured killas (metamorphosed sedimentary rock marginal to igneous intrusions) was by precast concrete sections.

High water inflows were encountered while sinking through the water-bearing strata. Despite attempts to control water through backwall grouting with cement, the water make increased to over 1560 liters per minute at the base of the water-bearing interval at 35 meters depth. Due to the high inflow rate, cement grouting, even with use of bridging materials, was unsuccessful.

SCEMm66 was modified to yield the desired coagulation characteristics in the fast-flowing, saline water. After injection through the segmental lining over a 9 shift period, the water inflow was reduced from 1560 liters per minute to 114 liters per minute.