The project required the replacement of the existing sewer main from Rhyl to Kinmel Bay in Denbighshire, North Wales. Replacement of the sewer main required a 60m bored tunnel under the main railway line between Rhyl and Llandudno, from a Drive Pit to a Reception Pit. The railway line remained live throughout.
To prevent basal heave and soil liquefaction of the ground below the Reception and Drive Pits, a dewatering and artesian pressure relief system was installed within the Pit Cofferdams to lower the piezometric head and artesian pressure head below the Pit formation level within each excavation.
The groundwater control design for both the Drive Pit and the Reception consisted of 8 No. wells, installed to a level of -10m OD. (circa 16m bgl on the Drive Pit side of the railway and 13.7mbgl on the Reception Shaft side).
As a consequence of the dewatering operation, there was also a lowering of the piezometric head in the ground beneath the railway line that had the potential of producing ground settlement due to the increase in effective stress.
OGI was engaged to assess by calculation, the magnitude of the potential ground settlement resulting from dewatering. Based on technical assumptions and computer modelling, OGI established that the settlement was unlikely to exceed 6.2mm as a consequence of dewatering over a period of 4 weeks. However, as conservative assumptions were made, it is most likely that the actual settlement would be less than 6mm.
A considered approach of staged dewatering together with monitoring of drawdown and settlement was set out in the ITM Plan, and helped to ensure the a successful outcome to the project.