FIG. 2.36 DETAILS OF WELL POINT.
FIG. 2.37 LOWERING OF WATER TABLE BY WELL POINT SYSTEM.
The well points are placed in a row or ring, and the riser pipes are attached through a common manifold or header pipe to a special well point pump (Fig. 2.37). For inserting yhe well point into ground by jetting, water is pumped down the well point under pressure from where it emerges with a great velocity through the tip of the drive point. The emerging jet-stream dislodges the surrounding soil and the well point can be lowered to the desired depth. A further advantage of jetting is that water under pressure washes away soil fines from around the well point leaving a relatively coarser material lo settle and form a natural filter around the well point. The hole formed around the riser pipe and the welI point by jetting water is filled with coarse sand. The sand also helps in directing drainage to the well point.
The suction pump used in the well point system has a capacity of bringing water to the surface from a maximum depth of about 6 m. The well points are generally spaced between 1 to 2 m. For dewatering excavations which are more than 6 m below the water table, a multi-sage well point system (Fig. 2.38) is used. Excavations exceeding 16 m depth are preferably drained by deep well system. In the multi-stage well point sysem, the ground is first stripped to the natural water level where the first stage of well points is installed. After excavating about 5 m, second stage is installed to further lower the water table for advancing excavations. The other stages are put successively, upto a maximum depth of 16 m is reached. In the well point system, a round the dock pumping schedule is essential, as the interruption in pumping can have catastrophic consequences. Hence one auxiliary pumps for each two pumps is use should always be available.
FIG. 2.38 MULTI-STAGE WELL POINT SYSTEM.