STAINLESS STEEL DUTCH ‘WEAVE SCREEN
Use of wellpoints where submergence is small
supplementary method of dewatering if needed. Well – points are more suitable than deep wells where the submergence available for the well screens is small (fig. 2-3) and close spacing is required to intercept seepage.
b. Vacuum wellpoint systems. Silts and sandy silts (Dio = O. Ofmilimetre) with a low coefficient of permeability (к = 0.1 x 10“4 to 10 x 10~4 centimetres per second) cannot be drained successfully by gravity methods, but such soils can often be stabilized by a vacuum wellpoint system. A vacuum wellpoint system is essentially a conventional well system in which a partial vacuum is maintained in the sand filter around the wellpoint and riser pipe (fig 2-5). This vacuum will increase the hydraulic gradient producing flow to the wellpoints and will improve drainage and stabilization of the surrounding soil. For a wellpoint system, the net vacuum at the wellpoint and in the filter is the vacuum in the header pipe minus the lift or length of the riser pipe. Therefore, relatively little vacuum effect can be obtained with a wellpoint system if the lift is more than about 15 feet. If there is much air loss, it may be necessary to provide additional vacuum pumps to ensure maintaining the maximum vacuum in the filter column. The required capacity of the water pump is, of course, small,
c. Jet-eductor wellpoint systems. Another type of dewatering system is the jet-eductor wellpoint system (fig. 2-6), which consists of an eductor installed in a small diameter well or a wellpoint screen attached to a jet-eductor installed at the end of double riser pipes, a pressure pipe to supply the jet-eductor and another pipe for the discharge from the eductor pump. Eductor wellpoints may also be pumped with a pressure pipe within a larger return pipe. This type of system has the advantage over a conventional wellpoint system of being able to lower the water table as much as 100 feet from the top of the excavation. Jet-eductor wellpoints are installed in the same manner as conventional wellpoints, generally with a filter as required by the foundation soils. The two riser pipes are connected to separate headers, one to supply water under pressure to the eductors and the other for return of flow from the wellpoints and eductors (fig. 2-6). Jet-eductor wellpoint systems are most advantageously used to dewater deep excavations where the volume of water to be pumped is relatively small because of the low permeability of the aquifer.