Passive Relief Systems

It is sometimes possible to control excess pore water pressures in a confined aquifer below the base of a proposed excavation by using passive relief well systems.

The passive wells are drilled in the base of the excavation before the excavation has reached the piezometric level in the aquifer. As excavation continues below the piezometric surface, the wells will start to overflow, providing pressure relief. Water is then removed by sump pumping The effectiveness of the system is often proven with a pumping test carried out from a couple of passive wells that were lined during construction.

In practice passive relief wells are generally only used for shafts or excavations in stable soils, stiff clay or weak rock, where there is only a marginal risk of base heave caused by sand lenses, fissures or a confined aquifer.