A serious limiting of well point system is the suction lift. A lowering of about 6m below pump level is generally possible beyond which excessive air shall be drawn into the system through joint in the pipes, valves etc., resulting in loss of efficiency. If the ground is consistently mainly of large gravel, stiff clay or soil containing cobbles or boulders it is not possible to install all points. For dewatering deeper excavations, the well points must be installed in two or more stages as shown in figure 6.8.1c. There is no limit to the depth of draw down in this way, but the overall width of excavation at ground level becomes very large. On the other hand, it is possible to avoid multi-well point stages by excavating don to water level before installing the pump and header. When well points are used in braced excavations, figure 6.8.d, they are placed closed to the toes of the sheet piles. This is done in order to ensure lowering the water level between the sheet pile rows. Well points are provided in conjunction with the sheet piles under the following conditions.
To prevent quick condition of the bottom when the sheet piles are of limited penetration.
To eliminate hydrostatic pressure on the back of a sheet pile coffer dam, thus allowing higher bracing to be used.
