What Is Dewatering?
Dewatering is the method of removal of excess water from saturated soil. Dewatering of trenches in very pervious soils poses the main problem of seepage water entering the pit.
In the case of impervious soils, the piping problem may cause subsidence of the soil surrounding the pit and thereby any structure resting on it. In the case of highly impervious soil ditches, the main factor to be accounted for is its effect of seepage forces.
Therefore, to carry out excavation safely dewatering of sub-soil water, for example, control of groundwater in excavations is necessary.
Read More: Cofferdam – Types of Cofferdam
Methods of Dewatering Used In Construction
Excavation work for the foundation in the waterlogged ground or high water table area possess a great problem for site engineers, because of water entering the trench from sides, bringing with it the soil from sides. The timbering, if provided, would become loose and collapse. Excavation can be carried out by dewatering the sub-soil water.
Main Dewatering systems 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 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.
In Well Point Dewatering System an excavated area dry continously 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 the downward direction.
As it discharges through the nozzles, it displaces the soil below the tip. Jetting is continued until the required penetration of the tip is achieved. The advantage of installation by jetting is the water under pressure. Washes away soil fines near the tip and leaves a relatively coarse. It forms a natural filter around the tip. The hole formed near is filled with coarse sand.
After all the Well Points are installed and connected, the suction pump is t into operation. Due to suction, the ball valve in the WellPoint gets closed and the groundwater is drawn through the WellPoint screen.
The water from the WellPoint is sucked up through the riser pipes flows through the header pipe and is finally discharged away from the site of the work. Each good point lowers the water table around it and forms a cone of depression. Various cones of depression join, and a common drawdown curve is obtained. The water table is thus lowered.
Well Points dewatering method is suitable for lowering the water table by 5 to 6 m in soils. 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.
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 installation Well Points, a hole of, about 25 cm 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 up to about 1 m from the top. The top 1 m 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 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 groundwater 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.
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
- 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.
After a foundation has been explored and tested to determine the extent of grouting required, the size, depth, and spacing of injection holes should be planned so as to get the best results at the minimum cost.
Cement grouting is largely ineffective in sands. In coarse materials or rocks, the excavation is surrounded by ‘grout curtain’ consisting of two rows of primary injection holes at 2.5 m to 5.0 m centers in both directions with secondary holes between them as shown in Fig.
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 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.
This process can be used up to 30 m depth of excavation. The freezing process is suited for work of comparatively short duration. This is because of the fact that the working expenses are heavy and also the stability of frozen earth is not reliable for a long time.
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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