Methods Of Dewatering

Dewatering is a crucial aspect of many construction projects, mining operations, and other industrial activities. It involves removing groundwater or surface water to create a safe and stable work environment. With a range of dewatering methods available, it can be challenging to determine the optimal solution for a specific site.

Dewatering is the method of removal of excess water from saturated soil. Dewatering of trenches in very previous soils poses the main problem of seepage water entering the pit.

In this article, we will provide a method of dewatering, discuss the importance of a well-designed system, and explore ten effective methods in construction and techniques for creating an optimal dewatering system.

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Methods of Dewatering Used In Construction

Excavation work for the foundation in the waterlogged ground or high water table area poses a great problem for site engineers, because of water entering the trench from the sides, bringing with it the soil from the sides. The timbering, if provided, would become loose and collapse. Excavation can be carried out by dewatering the subsoil water.

The following major methods of dewatering used in construction

Sumps and Ditches
Shallow Well System
Deep Well System
Well Point System
Vacuum System
Cement Grouting System
Chemical Process
Freezing Process
Electro-Osmosis Method

1. Sumps and Ditches

This is the simplest type of dewatering technique used in shallow excavations in coarse-grained soils. In this method, shallow pits, called sumps are dug along the periphery of the area. These sumps are connected by ditches or drains of semicircular in shape and 20 cm in diameter, along the periphery of the excavation. The water from the slopes or sides flows under gravity and is collected in sumps from which it is pumped out.

If the seepage of water is significant, it may cause softening and raveling or sloughing of the lower part of the slope. There is also the possibility of piping in the slump bottom, because of upward flow.

In such situations, the sump can be weighted down with an inverted filter consisting of layers of successively coarser material from the bottom of the sump-pit upwards.

2. Shallow Well System

In a shallow well system, a hole of 30cm in diameter, or more is bored into the ground to a depth not exceeding 10 m below the axis of the pump.

A strainer tube of 15cm, the diameter is lowered in the bore-hole having a casing tube. A gravel filter is formed around the strainer tube by gradually removing the casing tube and simultaneously pouring the filter material, such as gravel, etc. in the annular space.

A suction pile is lowered into the filter well so formed. The suction pipes from a number of such wells may be connected to one common header connected to the pumping unit.

3. Deep Well System or Bored Well System

This system is more suitable when the depth of excavation is more than 16m or where artesian water is present. In this, a system of a 15 to 60 cm diameter hole is bored and a casing with a long screen is provided. A submersible pump is installed near the bottom of the well. Each well has its own pump.

Along with the deep wells arranged on the outer side of the area under excavation, a row of good points is frequently installed at the toe of the side slopes of the deep excavation.

4. Well Point System

In Well Point Dewatering System an excavated area dry continuously kept dry by intercepting the flow of groundwater with pipe wells driven into the ground. The main components of the well-point system are:

The Well Points
The Riser Pipe
The Swinger Arm
The Header Pipe
The Pumps

A WellPoint is a perforated pipe 5 to 8 cm in diameter and about 1 m long covered by a cylindrical wire gauge screen known as a strainer. These pipes are jetted into the ground. All ball valve is provided near the lower end of good point which permits the flow of water only in the downward direction during installation.

The WellPoint is connected to the b tom of the Riser Pipe of the same diameter. The riser pipe is connected to a horizontal pipe known as Swinger Arm. Swinger arms of different Well Points are connected to a horizontal pipe of 15 to 30 cm diameter, known as a header.

Well Point Dewatering System can be installed in a drilled hole, but generally, these are installed by jetting. In this dewatering method, water is pumped through the riser pipe in a downward direction.

5. Vacuum System

Well Points cannot be used successfully for draining silty sands and other fine sands with an effective size less than about 0.05 mm. The coefficient of permeability of such soils is generally between 1 x 10-5 to 1 x 10- 7m/sec. These soils can be effectively drained by using Vacuum Well Points.

For the installation of Well Points, a hole of, about 25 cm diameter is formed around the WellPoint and the Riser Pipe by jetting water under pressure. When water is still flowing, medium to coarse sand is filled into the hole up to about 1 m from the top. The top 1 m portion of the hole is then filled by tamping clay into it.

6. Cement Grouting

Grouting solidifies and strengthens the formation to increase its load-bearing capacity. It also reduces or eliminates the flow of water through a formation. Grouting reduces hydraulic uplift pressure.

The materials commonly used for grout include:

Cement and water
Cement rock flour water
Cement, clay and water
Cement, clay, sand and water
Asphalt
Clay and water
Chemicals, etc.

Generally, the most satisfactory rout is the stiffest mix that can be injected effectively when a grout of cement and water only is to be injected into fine seams, it may be necessary to use_ as much as 2 parts of_ water to 1 part of cement, by volume in order to obtain penetration.

When the seams are large, the grout may be as dry as 3/4 part of water, or less to 1 part of cement, For most grouting operations, the ratio will vary from 1 to 2 parts of water to part of cement.

Rock flour and clay may be added to cement grout in the interest of economy, if the seams are small, while sand may be added if the seams are large enough to permit the sand to penetrate. Grout made of neat cement will give a higher strength than grout containing clay or sand.

O – Primary injection holes

X – Secondary injection holes

7. Chemical Process

In this dewatering method chemicals in the liquid form can be pumped into and through very small openings, which other grout cannot penetrate. The chemicals form a barrier to the flow of water through a formation by changing from a liquid to a gel at a predetermined time after it enters the formation.

The gel time may be varied from 3 sec to several hours. Fewer grout holes are required. The time required to inject the chemical is usually less than compared of other grouting materials.

Generally, two chemical solutions of sodium silicate and calcium chloride are used for this purpose. Recently an aqueous solution of 2 acrylic monomers and catalyst dimethyl-amino-propio-nitrate mixed with an aqueous solution of catalyst ammonium persulphate has been used just before it is pumped into the ground.

This process is useful and successful in clean and relatively homogeneous sand having an effective size greater than 0.1 mm. This process is very much under the most favorable conditions expensive and not guaranteed even.

8. Freezing Process

This process is suitable for excavations in water-logged soils like sand, gravel, and silts. It is advantageously used for deep excavations such as for bridges, etc. Especially, when the excavation is to be made adjacent to an existing structure or near some waterways.

This types of dewatering method consist of forming a wall of ice by freezing the soil around the area to be excavated. Freezing pipes encasing smaller diameter inner pipes are Sunk about the one-meter center to center along the periphery of the area to be excavated. The layout of the pipes should preferably be such that the area enclosed is circular in plan.

The freezing liquid is then supplied to the freezing Pipes by the refrigeration plant. This makes the ground around the pipes to freeze and form a thick wall of frozen earth around the area to be excavated.

9. Electro-Osmosis Method

Electros Osmosis Method of drainage is used for line-grained cohesive soils (such as clays) which can be drained or stabilized using electric current. This method was developed by L. Casagrande (1952). If a direct current (D.C.) is passed between two electrodes drib en into natural soil mass, the soil water will travel from the positive electrode (anode) to the negative electrode (cathode).

The cathod is made in the form of Well Point or metal tube for pumping out the seeping water. A steel rod, pipe, or steel piling of excavation can serve as the cathode. !the arrangement of electrodes is done in such a way that the natural direction of the flow of water is reversed away from the excavation, thereby increasing the strength of the soil and stability of the slope.

This dewatering method demands a system that requires about 20 to 30 amperes of electricity per well at a voltage of 40 to 180 volts. The spacing of electrodes is generally 4 to 5 m. This method helps in increasing the are the strength of cohesive soils.

10. Vibro Floatation

Vibro flotation is a technique in which granular soil is vibrated, extra granular soil is added and the density of soil is increased. This method was invented in Germany in 1930 and was first used in the USA in 1940.

The compactor produces a centrifugal force of 10 s to vibrate granular soils like sand and gravels
10% of extra soil is added.
A cylinder of compacted soil of about 7 to 9 feet diameter is formed in which a central cylinder of about 3 feet diameter is formed by the soil added from outside.
The average depth of com action is about 15 feet.
The weight of the vibrator is about 2 tons, 17-inch diameter, and 6 feet long.
One vibrator can compact about 300 to 600 m3 of soil in a shift of 8 hours. Vibroflot penetrates into the ground by 15 to 25 feet in -just 2 minutes.
The total expenditure is about 1 to 3 °A of the total project cost. Various steps in the Vibro flotation -the process is Vibroflot is lowered and placed on the ground where the soil is to compact. The lower water jet is opened.

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Methods Of Dewatering