Limerick Tunnel PPP

Dewatering

AGL were geotechnical designers for the temporary works for the tunnel casting basin which was 550 m long x 33 m wide and 9 m deep in soft fine-grained alluvial soils up to 12 m deep on the River Shannon floodplain.

• We carried out 2D Finite element analyses of the staged construction sequence to design the sheetpile walls and prop forces.
• We carried out a consolidation analysis to assess the anticipated settlements of the cast tunnel sections and the temporary support requirements prior to float out. The results were used to demonstrate that pile-supports were not required.
• We designed the groundwater control system of relief wells to prevent basal heave in the excavation due to hydrostatic uplift from a layer of gravel below the soft fine-grained alluvium.
• We also carried out a groundwater seepage analysis to check the stability of the northern approach ramp against uplift due to hydrostatic pressures in an extreme tidal surge.