Subglacial water escape structures and shearing; a case study of hidden problems for construction below rockhead in glaciated regions

摘要

Two shafts constructed on the floodplain of the River Ribble in Preston as part of an improvement of urban drainage were excavated through the sands and gravels of the Ribble alluvium overlying sands and gravels of glaciofluvial origin, which covered sandstones of the Sherwood Sandstone Group. Water level was close to ground level. Pumping tests had shown that the hydraulic connection between the drift and the sandstone was poor, thus the design concept for both shafts was to cut off groundwater in the permeable drift with diaphragm walls socketed into the sandstones where groundwater flow was considered controllable by pumping from within the shafts. Despite this, the excavation of both shafts was interrupted by inflows of groundwater carrying sand, so they had to be flooded to protect the stability of the excavation. The problems occurred when the base of each shaft was above the base of their respective diaphragm walls. At the time the inflows appeared to be the result of piping from groundwater that had penetrated through the superficial deposits and the sandstone, on the outside of the diaphragm wall. It was concluded that hydraulic connections of high conductivity existed that permitted groundwater to move from the drift to the bedrock. To recover the shafts jet grouting was used around the outside of the shafts to intercept any such connections. This was backed up with dewatering wells and the shafts were completed successfully. Detailed investigations to establish the cause for the groundwater flows were undertaken in parallel with shaft recovery. These forensic investigations confirmed the integrity of the diaphragm walls, and revealed evidence of glacial shearing and water escape structures within the sandstone. Such features could explain the groundwater inflows encountered if they provided high-conductivity connections to groundwater in the overlying superficial deposits. This case study describes the difficulty in finding these geotechnical hazards during a ground investigation. It is likely that they are more commonly associated with glacial processes than is currently recognized. The study illustrates how easy it is for these features to be overlooked by conventional ground investigation.