5.2.3 Movements due to water
Figure 5.7 shows some potential water flow situations that can result in ground movements，water movements in and around an excavation can occur：
•Through flaws in an impervious wall；
•By flow through a wall（e.g.contiguous bored pile wall，leakage through sheet pile interlock and king post wall lagging）；
•By flow under the wall；
•By flow along boundaries between soils of different permeability；
•By flow along the wall if the wall penetrates an underlying aquifer；
•By dewatering.
Figure 5.7 Movements potentially associated with water flow
If piezometric pressures in an aquifer underlying an excavation are not properly reduced，heaving of the base can occur，leading to loss of passive restraint to the wall.Piping may also occur in coarse grained soils.A case study illustrating base instability in a cofferdam excavation is discussed by Preene et al.（2016）Rowe（1986）noted a case study of piping failure at the base of an excavation in interbedded sand and clay horizons due to water leakage through sheet pile interlocks along confined horizons.This illustrates that dangerous situations can arise when excavating below the water table in fine⁃grained soils（sands and silts）.If leaks develop at joints in the wall，then loss of fines can very quickly lead to catastrophic ground movements in this high⁃risk situation.
Consolidation settlements may also occur in fine⁃grained soils due to a combination of these effects.