Deep Well System | Civil Engineering Projects

Deep Well System | Building Construction and Design

In our series of articles regarding wells and well Points, we have discussed the following:

• Single Stage Well Points
• Multi-stage Well Points
• Vacuum Well Points
• Shallow Well System

Now we will move on with our further discussion on "Deep Well System".

A deep well is about 30 to 60cm in diameter, bored to a depth of 15 to 30m. It is provided with a casing which is perforated one in the previous zones penetrated. Coarse filter material is placed in the annular space between the casing and the walls of the hole. The spacing of deep wells varies between 10 to 30m, depending upon the area to be dewatered and the location of the water table.

A submersible pump is placed inside the casing near the bottom. The pump is driven by a motor mounted on the top of the casing through a vertical shaft. However, if the motor is submersible, it can be directly attached to the pump.

As the pump is  placed at the bottom, there is no restriction on the height to which the water can be lifted, unlike a shallow well system.

Deep wells are located on the outer periphery of the area to be excavated. A row of well point is also installed at the tow of side slopes is also installed at the toe of side slopes of the deep excavation ti intercept seepage between the deep wells and to prevent sloughing of the slopes near the toe.

As in the case of well point systems, pumping once started must be continued until the entire excavation work is completed. If the pumping is stopped in between, there would be a rapid development of hydrostatic and seepage pressure in the excavation, which may prove to be disastrous.

We will discuss "Horizontal Wells" in our succeeding article.

Vacuum Well points | Building Construction

Vacuum Well points | Civil Projects

In our earlier article, we discussed "Single stage Well points" and "Multi-stage Well Points". Now we will move on with our discussion on Vacuum Well Points.

Well points cannot be used successfully for draining silty sands and other fine sands with an effective size less than about 0.05mm. The coefficient of permeability of such soils is generally between 1x10^-5 to 1x10^-7 m/sec. These soils can be effectively drained by using vacuum well points.

For installation of a vacuum well points, a hole of about 25cm diameter is formed around the well point and the riser pipe by jetting water under pressure. When water is still flowing, medium to coarse sand is filled into the hole upto about 1m from the top.

The top 1m portion of the hole is then filled by tamping clay into it. It forms a sort of seal. Any other impervious material can also be used instead of clay to form a seal. Well point spacing is generally closer than that in a conventional system.

When the header is connected to a vacuum pump, it creates a vacuum in the sand filter around the well point. As the pressure on the water table is equal to the atmospheric pressure, the head causing flow is increased by an amount equal to the vacuum pressure. The hydraulic gradient increases and it overcomes the flow resistance in the soil pores. The ground water flows to the region of vacuum in the well points and drainage occurs.

As the effective pressure on the soil is increased, consolidation takes place. It makes the soil stiff. However, the process is slow and it may take several weeks for the soil to become stiff enough for carrying out the excavation work.

We will also discuss the following in our succeeding articles:

• Shallow Well System
• Deep Well System
• Horizontal Wells