Consequently, a number of well-established techniques have been developed to lower the ground water table during excavation. The geology, ground water conditions, and type of excavation all influence the selection of dewatering technology. The most common methods for dewatering include sumps, wells and wellpoints. Sumps work best in tight, fine grained soils, or very coarse, bouldery deposits. Wells work best in soils consisting of sand, or sand and gravel mixtures, and can dewater large areas to great depths.
Wellpoints effectively dewater coarse sands and gravels, or silts and clays.
They have a wide range of applications. However, wellpoints use a vacuum system and their depth is limited to about 25 feet. Wellpoint systems generally cost more than either sumps or wells, and require near-continual maintenance. A number of other dewatering techniques are available including ground freezing and electro-osmosis. However, such techniques are very costly and used only for particularly difficult dewatering applications.