A deep well typically consists of a borehole fitted with a slotted liner and an electric submersible pump. Deep wells can be installed in a ring around an excavation to lower the construction dewatering and groundwater control pdf level and maintain a safe, dry site. Several equations can be used to design deep well dewatering systems, however many of these are based on empirical data and occasionally fail.
Practice and experience, along with a firm understanding of the underlying principles of dewatering, are the best tools for designing a successful system. Some dewatering situations “are so common that they can be designed almost by rule of thumb”. Wellpoints are typically installed at close centers in a line along or around the edge of an excavation. Wellpoints can be installed in stages, with the first reducing the water level by up to five meters, and a second stage, installed at a lower level, lowering it further. The water trickling between the deep wells may be collected by a single row of well point at the toe.
This method ensures a much thicker width free from seepage forces. Wellpoint spears are generally used to draw out groundwater in sandy soil conditions and are not as effective in clay or rock conditions. Open pumps are sometimes employed instead of spears if the ground conditions contain significant clay or rock content. The installation of horizontal dewatering systems is relatively easy. A trencher installs an unperforated pipe followed by a synthetic or organic wrapped perforated pipe.
The drain length is determined by the drain diameter, soilconditions and the water table. In general drain lengths of 50 meters is common. After installation of the drainpipe a pump is connected to the drain. After the water table has been lowered, the intended construction can start. After the construction is finished the pumps are stopped, and the water table will rise again. Installation depths up to 6 meters are common.
High pore pressures occur in soils composed of fine silts or clays. The design of groundwater control systems using the observational method. TOL Roberts and M Preene. On the analysis of dewatering systems. Proceedings of the Xth International Conference of Soil Mechanics and Foundation Engineering, June 1981.