Control of groundwater

The main purposes of controlling groundwater are to enable the construction of below-ground works in the dry and to improve the stability of the ground below the sides and base of an excavation. In order to plan and design a control scheme, it is necessary to obtain good-quality information on the stratigraphy, the groundwater regime and the mass permeability of the ground. A number of techniques are available, each being applicable to particular circumstances. The principal parameters governing design are permeability and drawdown. Even with good site investigation, the mass permeability of the ground surrounding a proposed excavation is only approximately known. Sensitivity analysis is therefore an important part of the design process, to give the designer an appreciation of the possible range of flows needed to produce the required outcome. Monitoring of groundwater control schemes is essential to ensure that sufficient drawdown or pressure reduction is achieved and maintained. Environmental aspects must also be considered, to check the quality of the water being extracted from the ground and of the water being discharged from the process. Consents are required, both to abstract the water from the ground and to dispose of it.