8.2.2 Well points
The well point method is also called the vacuum well point method.The method is to place the collecting points connected to the pumping pipe inside a small⁃diameter well and have them arranged in a line or in a rectangle.The collecting points are usually separated by 0.8⁃2.0 m.The top of the pumping pipe is connected to the common collecting pipe，which is then pumped out under vacuum，drawing out pore water from the soil and lowering the groundwater level accordingly.Figure 8.2 is a schematic diagram illustrating dewatering the well point method.The method is a type of forced draining method.
Figure 8.2 Dewatering method：the deep well point method
Figure 8.3 Configuration of a well point
The collecting pipes are usually arranged around the vicinity of an excavation site.Collecting points are the main structures of the well point method.A collecting point is usually 100 cm long，has an external diameter of 5⁃7 cm，and many small holes are bored along the side to collect groundwater.On the front end is usually installed a spurting device，which is used to help the collecting point penetrate into the ground using high pressure water.As shown in Figure 8.3，the space between a collecting point and the well side are backfilled with filters to protect the collecting point from obstruction.The well can be enveloped by bentonite near the ground surface to increase the degree of vacuum and the efficiency of the well accordingly.Figure 8.4 is the photos of field application.
Figure 8.4 Well points application and pump for an excavation
Theoretically，the dewatering depth of the well point method can reach as deep as 10.33 m（1 atmospheric pressure）.It is，however，impossible to achieve the condition of total vacuum inside the well.Besides，head loss is unavoidable since there exists friction，which is generated by groundwater flowing through soils，filters，collecting points，pumping pipes，and collecting pipes.As a result，the dewatering depth can at best reach the depth of 5⁃6 m.In fine sands or silty sands，it can reach a depth of only 3⁃4 m.Therefore，when the dewatering depth goes beyond 6 m，it is recommended to carry out the well point method in phases.The deep well method can also supplement the well point method under such conditions.
When the dewatering depth goes beyond 6 m，it is recommended to carry out the well point method in phases，as shown in Figure 8.5.The deep well method can also supplement the well point method under such conditions.
Figure 8.5 Two⁃stage well point dewatering