Dewatering Systems Design | Civil Engineering

Dewatering Systems Design

In the design of dewatering systems, well points or wells play a significant role. While designing a dewatering system, the number, size, spacing and penetration of the well points or wells have to be determined.

The above mentioned parameters depend on the following:

• Expected rate of discharge
• Soil type
• Drawdown in the wells

There should be a proper arrangement made. Collectors and pumps having sufficient capacity are to be installed on the site for the intended purpose.

Darcy’s law is used to compute the rate of discharge. It is of utmost importance to establish a fundamental relationship between the discharge and the corresponding drawdown.

Assumptions:

The stratum is homogenous and isotropic in nature.

The flow of water is continuous and steady.

Equations for the rate of discharge and corresponding drawdown for different types of wells are developed in the following sections.

The well may be either gravity well or artesian well. A gravity well penetrates a homogenous, pervious stratum aquifer in which the water table is located. An artesian well penetrates a homogenous, pervious stratum which is bounded by impervious strata above and below and in which the piezometric surface is an artesian well falls below the top of the pervious stratum.

The equations developed may also be used for the determination of discharge from wells for irrigation and other purposes. However, such wells are constructed for supplying water and not drainage. In this text, the use of wells for drainage is of main concern.

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Drainage and Dewatering 

Electro-Osmosis | Building Construction

What is Electro-Osmosis? | Civil Engineering

Electro Osmosis is a method of drainage of cohesive soils in which a direct current is used. When a direct current Is passed through a saturated soil between a positive electrode (anode) and a negative electrode (cathode), pore water migrates to the cathode.

The cathode is a well point which collects the water drained from the soil. The water collected is discharged, as in a conventional well point system.

Before you proceed with the post on "Electro-osmosis", check out the following articles on Well Points and Well Systems.

• Single Stage Well Points

• Multi-stage Well Points

• Vacuum Well Points

• Shallow Well System

• Deep Well System

• Horizontal Wells

The phenomenon of electro-osmosis can be explained with the help of the electrical double layer. Cations are formed in pore water when the dissolved minerals go into solution. These cations move towards the negatively charged surface of clay minerals to satisfy the electrical charge. As the water molecules acts as dipoles, the cations also attract the negative end of dipoles. When the cations move to the cathode, they take with them the attracted water molecules.

In fact, the entire outer part of the diffuse double layer which is loosely adsorbed to the soil particles gets sheared along a plane.

Anodes are in the form of steel rods located near the toe of the slope of the excavation. Cathodes are in the form of perforated pipes, resembling well points, installed in the soil mass about 4 to 5m away from the slope of the cut.

The electrodes are so arranged that the natural direction of flow of water is reversed and is directed away from excavation. This arrangement is required to prevent sloughing of the slopes. In many cases, mere reversing of the direction of flow helps in increasing the stability of the slope even if there is no significant decrease in the water content of the soil.

The system requires about 20 to 30 amperes of electricity per well at a voltage of 40 to 180. The consumption of energy is between 0.5 to 10 kWh/m³ of soil drained. Because of specialised equipment and high electricity consumption, drainage by electro-osmosis is expensive compared with other methods. The drain water in a cohesive soil of low permeability (k=1x10^-5 to 1x10^-8m/sec)

Electro-osmosis also helps in increasing the shear strength of the cohesive soil.

Horizontal Wells | Civil Engineering Projects

Horizontal Wells | Building Construction and Design

In our previous series of articles regarding Well Systems and Well Points, we have covered the following:

• Single stage Well Points
• Multi-stage Well Points
• VacuumWell Points
• Shallow Well System
• Deep Well System

Now we will move on with our discussion on "Horizontal Wells".

Horizontal wells of about 5cm to 8cm diameter have been used for drainage of hill sides. These wells are drilled into the hill at a slightly upward slope. A perforated casing is installed in the well to collect and discharge water.

Horizontal wells have been successfully installed for horizontal length of 60m.

Large horizontal tunnels have also been successfully used to tap deep aquifers beneath hill sides.

A combined system of vertical wells and horizontal wells can be used to drain stratified soil deposits. In this system, the vertical wells intercept the aquifer and discharge the water into the horizontal well. The water collected by the horizontal well is discharged at a suitable point.

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.

Shallow Well System | Civil Engineering

Shallow Well System | Building Construction

In our earlier articles, we have discussed the following well Points:

1. Single Stage Well Points
2. Multi-stage Well Points
3. Vacuum Well Points

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

In a shallow well system, a hole of about 30cm diameter is first bored into the ground, using a casing. A filter tube of about 15cm diameter, covered with a special wire mesh, is then lowered into the casing.

The casing is gradually withdrawn and suitable filter material is added in to the annular space between the casing and the filter tube. This forms a filter well.

A suction pipe is lowered into the filter well. A number of such wells may be installed. The suction pipes of all these wells are connected to a common header. A pumping unit is attached to the header. As the pumping is started, the drainage occurs. The suction lift of the well should not be more than 10m for its proper working.

Shallow well system is rarely used in practice. Well point systems, as discussed earlier, are more economical upto a depth of 10m than a shallow well system.

We will discussion the following topics in our succeeding articles:

• Deep Well System
• Horizontal Wells

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

Multistage Well points | Building Construction

Multistage Well points | Civil Engineering Projects


In our previous article, we discussed "Single stage Well Points". Now we will move on with our discussion on "Multistage Well points".

When the water table is to be lowered for a depth greater than 6m, multistage well points are required. In this method, two or more rows of well points are installed at different elevations.

How is the installation of Well points done?
The installation of well points is done in stages. The first stage well points are located near the perimeter of the area, as in a single stage well point system. These are put into operation and the water table is lowered by about 5m depth is done.

Thus the total depth of excavation becomes about 10. If required, the third stage of well points can also be installed to further lower the water table.

The method is useful for excavations upto 15m depth. Excavations exceeding 15m depth are generally dewatered by a deep well system.


We will also discuss the following in our succeeding articles:

• Vacuum Well Points
• Shallow Well System
• Deep Well System
• Horizontal Wells

Single Stage Well Points | Building Construction

Single Stage Well Points | Civil Engineering

A well point is a perforated pipe about 1m long and 5cm in diameter. The perforations are covered with a screen to prevent clogging. A jetting nozzle is provided at its lower end. A conical steel drive point is fixed to the lower end of the well point to facilitate installation.

A ball valve is also provided near the lower end which permits flow of water only in the downward direction during installation.

The well point is connected to the bottom of the riser pipe of the same diameter. Risers of different well points are connected to a horizontal pipe of 15 to 30cm diameter, known as header.

The header is connected to a specially designed pumping unit. The spacing of the well points depends upon the type of soil and the depth of water. Generally, it varies between 1 to 3m.

Well points can be installed in a drilled hole, but generally these are installed by jetting. Water is pumped through the riser pipe in the downward direction. As it discharges through the nozzle, it displaces the soil below the tip.

Jetting is continued till the required penetration of the tip is achieved. The advantage of installation by jetting is that the water under pressure washes away soil fines near the tip and leaves a relatively coarse material. It forms a natural filter around the tip. The hole formed near the tip is filled with coarse sand.

After the well points have been installed around the area to be dewatered, pumping is started. Each well point lowers the water table around it and forms a small cone of depression and a common drawdown curve is obtained. The water table is thus lowered.

Well points are suitable for lowering the water table by 5 to 6m in soils with a coefficient of permeability between 1 x 10^-4 to 1 x 10^-6m/sec. The screen normally provided with the well points can prevent medium sand and coarse sand from entering the well point. If the stratum to be dewatered consists of finer soils, a sand filter has to be provided around the well point.

It is essential to continue pumping once it has been started until the excavation is complete. If it is stopped in between, it may prove to be disastrous.

Inceptor Ditches | Civil Engineering

Inceptor Ditches

Inceptor ditches are used for excavation of limited depth made in a coarse soil. These ditches are constructed around the area to be dewatered. The ditches must penetrate deeper than the level of the work area.

At suitable locations, sump pits are constructed along the ditch for installation of the pump to remove the water collected.

If the soil is fine sand of low permeability, boiling may occur in sumps and ditches. This may be prevented by placing filter layers on the sides and at the bottom of the ditches and sumps.

Interceptor ditches are most economical for carrying away the water which emerges on the slopes and near the bottom of the foundation pit.

The method can be effectively used for formation, gravel and coarse sand. In fine sands and silts. There may be sloughing, erosion or quick conditions. For such soils, the method is confined to a depth of 1m to 2m.

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Drainage, Dewatering | Building Construction
Conduits
Types of Underground Conduits
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