Monday, November 5, 2012

EXCAVATIONS: Well point system.

A more complicated dewatering system based on gravity flow is the installation of well points. A well point is a perforated pipe, about ½ to 1 m long and 5 to 8 cm in diameter, covered by cylindrical wire gauge screen. In an expensive type well point, the steel tube is covered with two brass screens, the inner of fine mesh and the outer perforated (Fig. 2.36). A conical steel drive point is attached to the lower end of the pipe, with a neoprine ball valve fitted in the point to allow jetting of water to pass through it for driving it. When operating on suction, the ball is in the position shown and the soil water enters the outer screen, through the mesh, and down the flutes of the inner tube. Holes near the bottom of the latter and just above the shoe admit water to the inside where it is drawn up the riser, along the header to the pump for discharge through the pipes to a drain.



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.


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