GROUND WATER CONTROL

This report which summarizes processes for ground water control, gives details of water lowering processes or techniques for the lowering of water pressures in the zone of soil into which an excavation is required. These techniques involve either drainage or some form of pumping and should be designed to control the water pressures such that excavation can be carried out safely. Water lowering may be effected by conventional sump pumping for excavations in clean gravels and course sands. It should be discontinued whenever fines are being removed in noticeable proportions. Details are given of the method of vertical well pointing in which individual small wells secured to the end riser pipes of appropriate length are washed into the soil at close centers using high pressure water and/or air. On completion of installation, including any "sanding'in" there are connected via a common suction header pipe to a well-point pump. Disadvantages inherent on large installations are listed. In the horizontal wellpointing method one long perforated pipe is laid horizontally at the desired depth using a B.600 machine. Details are given of the excavation. A bored filter well is described which consists of an inner tube having some or all of its length perforated to allow the ingress of water. This is referred to as the screen, and in theory the depth may be unlimited. The screen is surrounded by an annulus of filter media. A deep well is pumped by means of a submersible pump. Settlement due to water lowering, freezing and electro-osmosis are also discussed.