For very deep excavation the multistage well point set up has the disadvantage that the water level is pulled down abruptly at the edge of the excavation. As a consequence, the hydraulic gradient near the excavation bottom becomes quite large and the resulting seepage pressures may lead to the instability of the side slopes. Under such circumstances a deep well pump is suitable as shown in Fig.1.
Deep wells pump are cased holes and generally they have diameters ranging from 30 cm to 60 Cm. The depth of deep well pumps ranges from 15 to 30 m. In the previous zones the casing is perforated. Coarse filter material is placed in the annular space between the casing and walls of the hole. The spacing of the deep well pump depends upon permeability of the soils and the depth of the permeable material. In general the spacing of the deep well pump varies from 6 m to 60 m. A filter of sand & gravel surrounds the screen section and the length of the screen varies from 6 to 7.5 m.
The pumping unit consists of multistage submersible turbine pump and motor mounted on a common vertical shaft and is placed at the bottom of the well. As a result water to any height can be lifted.
Deep wells are located on the outer periphery of an area to be excavated. A row of well points is also installed at the toe of the side slope.
These well points intercept seepage between deep wells and thus maintain the stability of the slope near the toe. Once the pumping is started. It should be continuous till the excavation work is completed. If stopped in between, there would be rapid development of hydrostatic and seepage pressure in the excavation, which could impair the stability of the excavation.