Horizontal Passive Pore Pressure Relief System to Reduce Lateral Load on a Combi-Pile Cofferdam

During the widening of the Seaforth Passage, Liverpool, a siphon chamber was constructed within a steel combi-pile cofferdam. To reduce the magnitude of lateral differential stress on the outside of the combi-pile wall during the temporary works period, groundwater pressures were lowered outside the combi-wall, until the siphon structure was completed and cofferdam flooded.

External dewatering wells using borehole pumps were installed initially to lower the groundwater pressures in the surrounding ground. However, due to the variable nature of the ground, the external dewatering system only partially reduced groundwater pressures to the target levels. To enable the groundwater pressure to reach the target level required by the structural engineers, a system of additional internal horizontal passive pressure relief wells was constructed during excavation of the cofferdam.

The construction sequence enabled the internal groundwater passive pressure reduction system to be constructed in stages and enabled work to continue without delays so maintaining programme.

The predicted performance of this passive pressure relief system was first numerically modelled using the SEEP/W Finite Element Model. The performance of the system was monitored using external nested piezometers which verified in practice the required lowering of the groundwater pressures to the required target levels.