Pumping from Well Points:
This system comprises the installation of a number of filter wells, generally 1m long, around the excavation.
These filter wells are connected by vertical riser pipers to a large dia header main at ground level which is under vacuum from a pumping limit.
The water flows to the filter well by gravity &then drawn by the vacuum up to the header main &discharged through the pump.
This system has no advantages that the water is filtered as it is removed from the ground and carries almost no soil with it once steady discharge conditions are attained.
This has a limitation of limited suction lift generally up to about 5m below pump level.
Therefore for deeper excavation, the well points are installed in two or more stages.
The filter well or well points are usually 1m long &60-75mm dia gauze screen surrounding a central river pipe.
The capacity of a single well point with 50mm riser is about 10 lit/min.
The spacing between the two well-points depends on the permeability of the soils and on the time available to affect the drawdown.
A normal set of well points system comprises 50-60 points to single 150 or 200mm pump with a separate 100mm jetting pump.
Type of WellPoint System:
This system is employed when excavation is long.
The header is laid out along the side of the excavation, and the pumping is continously in progress in one length as further point are jetted ahead of the pump down section & pulled up from the completed & back filled lengths &repeated till entire length is completed.
For narrow excavation, like trenches, headers is laid on one side, while for wide excavation, the headers are required to be placed on both sides of the area.
When excavation is done in area of appreciable width, line system is inadequate.
The ring system is used in such conditions, and the header main surrounds the excavation completely.
This system is used for rectangular excavations such as for piers or basements.
A well point system consists of a number of well points spaced along a trench or around an excavation site, all connected to a common header which is attached to one or more well point pumps.
Well point assemblies-made up of a well point, screen, riser pipe, and swing joint with tuning-are generally 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 well point 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. The dewatering pumps used are normally designed specifically for the dewatering function.
They are available in sizes from 4” to 10” with handling capacity up to 500m³/hour.