What are the type of well point System?
There are two types of well point system, namely single stage well point system and multi-stage well point system. These systems are briefly described in the followings.
Single Stage Well Point system – A well point consists of a pipe about 1 m long and 50 mm in diameter. It has perforations, which are covered with a screen to prevent clogging in. At the lower end, a jetting nozzle is provided. This nozzle also acts as a drive point. A ball valve provided near the lower end allows the water to flow only in downward direction during installation of the well point. A sketch of the well point is shown in Fig.1 below. A riser pipe having the same diameter as the well point connects the well point at its bottom end. The riser pipes are connected to a horizontal pipe, known as header pipe, at the ground level. The header pipe is connected to a pumping unit. The well points are spaced every 0.6 to 1.5 The well points are installed to the required depth by jetting water through riser pipes in the downward direction. Once installed pumping is started continuously till the excavation work is completed.
Single stage well point system is used when the depth of excavation is less than 4.5 m. They are effective in granular soils. By Single stage well point stage method the water table is lowered by about 4.5m. This method is suitable to soils having coefficient of permeability between 10-2 to 10-4 cm/sec.
Multistage Well Point system – These systems are used if excavation exceeds 4.5 m and there is a chance of slope failure. The set up of the system is shown in Fig.2.In this system the well points are installed in stages. In the first stage well points are installed to a depth of around 4.5 and put into action till the water table is lowered by about 4.5 m. Then the area is excavated up to this 4.5 m depth. After this second stages of well points are installed within the area, which is excavated already. The water table is further lowered by another 4.5 m.
In this way the excavation is carried out till the excavation reaches to about 15 m. By well point system method excavation greater than 15 m cannot be made.