Groundwater control and stability in an excavation in Magnesian Limestone near Sunderland, NE England

A case history is presented of a 22-m-deep excavation for a storm sewage pumping station in the very variable and water-bearing Upper Magnesian Limestone adjacent to the English northeast coast. Considerable effort was expended during several phases of ground investigation to try to characterise the rock in engineering terms prior to construction. In addition to conventional double-barrel cored drilling, valuable data on the condition of the weathered dolomites and limestones and on the localised nature of groundwater inflows were obtained by examining coastal exposures and from two trial shafts and a heading constructed as advanced works on the incoming sewer. These helped in predicting many of the difficulties that might need to be overcame during construction. A full-scale pumping test at the site of the excavation also determined the bulk permeability of the strata and led to the conclusion that preconstruction groundwater control measures would be needed. A peripheral grout curtain and basal grout blanket were constructed by low-pressure techniques and their combined efficacy was tested by further pumping tests. In the event, construction of the excavation was relatively trouble-free and was accompanied by maximum groundwater inflows of about 8 l s−1 at full depth. Induced internal erosion of the weathered rock was minimal. Some difficulties were caused, however, by the presence of a zone of “gash breccia” in one corner of the excavation and by persistent minor instability in the overlying glacial till. These were overcome by special support measures. Overall, the rock mass was less weathered and more competent than predicted by the initial ground investigation. The coastal exposures, subsequent triple-tube coring and the two trial shafts helped to give a more realistic indication of the actual ground conditions.