WELL POINTS
A
well point system consists of a number of wellpoints spaced along
a trench or around an excavation site, all connected to a common
header which is attached to one or more wellpoint pupms. Wellpoint
assemblies-made up of a wellpoin, screen, riser pipe, and swing
joint with tuning-are genrally installed by jetting. They provide
for entry of water into the system by creation of a partial vacuum.
The water is then pumped off through the header pipe. A wellpoint
pump is a combination of two pumps, one of which pumps water from
the header and the other of which is a vacuum pump to remove air
which enters the system. Control of air is important, as excessive
air causes cavitation which reduces pump efficiency.
GART - Dewatering dewatering pumps are designed specifically for the dewatering function. They are available in capacities from 500 to 2 000 gallons per minute.
Wellpoint systems are frequently the logical and economic choice for dewatering construction sites where the required lowering of ground water level is on the order of 20 feet or less. Greater lifts are possible by lowering the water in two or more stages. The 20-foot lift restriction results from the fact that the water is lifted by difference between ambient air pressure and the lowered pressure created by the pump.
Wellpoint systems are practical and effective under most soil and hydrologic conditions. Among the instances where other dewatering techniques are preferable are where water levels must be lowered greater distance than can be practically handled by the wellpoint systems, where greater quantities of water must be moved than is practical with wellpints, or where the close spacing of wellpoints and the existence of the above-ground header might physically interfere with construction operations.
In a typical system, wellpoints are spaced at intervals of from 3 to 10 feet. Depending on their diameter and other physical characteristics, each wellpoint can draw from 0.1 to 25 gallons and more per minute; total systems can have capacities exceeding 20 000 gallons per minute. The depth to which a wellpoint is sunk into the ground is largely determined by the nature of the subsurface soil.
Systems design - The size and number of pumps, the number and type of wellpoints, the diameter of risers and headers, the number and location of valves and so on-is a more complex problem than it may first appear to be. GART - Dewatering engineers determine such physical parameters only after a thorough study of hydrogeological conditions at a particular jobsite and of conditions imposed by construction plans.
Trench dewatering with wellpoints.
- (a) Uniform soil can be dewatered with wellpoints on one side.
- (b) A clay layer above subgrade may require sand drains on opposite side to handle perched water.
- (c) Clay at and below subgrade may require wellpoints on both sides of trench.
