Methods-pumping, well points, bore wells, electro-osmosis, injections with cement, clays and chemical, freezing process, vibro- flotation.

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PREPARED BY : ASST. PROF. VATSAL D. PATEL
MAHATMA GANDHI INSTITUTE OF
TECHNICAL EDUCATION &
RESEARCH CENTRE, NAVSARI.

2. • Dewatering means removal of excess water from saturated
soil.
• it comes to
when the
Dewatering is a necessary process when many construction
projects, particularly construction is for underground
projects.

3. • Factors such as the type of soil and the nature of the construction site will all influence
which dewatering method will be best suited to the project.
6. Cement Grouting
7. Chemical Process
8. Freezing Process
9. Electro-Osmosis Method
1. Sumps And Ditches
2. Shallow Well System
3. Deep Well System
4. Well Point System
5. Vacuum Method

4. • It is the simplest and most commonly used
form of dewatering.
• In this method, shallow pits, called sumps are dug along
the periphery of the area and connected by drains of
semicircular in shape and 20 cm diameter.
• The water from the slopes
gravityand iscollected in
flows under
sumps from
which it is pumped out.

5.  A hole of 30 cm diameter or more is bored into the ground to a depth not more than 10 m
below the pump level. A strainer tube of 15 cm 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 well so formed.

The suction pipe from a number of such wells may be connected to one common header
connected to the pumping unit.

6. [A typical layout of a shallow well system]

7. This system is more suitable when the depth of excavation is more than the 16m or where
artesian water is present.
In this, 15 to 16 cm diameter hole is bored and a casing with a large screen is provided. A row
of well points is frequently installed at the toe of the side slope of the deep excavation.
A submersible pump is installed at the bottom of the well, of which the casing generally has a
minimum diameter of 150 mm. The discharge pipes from the submersible pumps of a number
of adjacent wells are connected to a common delivery main. The water is raised from the well
by a multi-staged pump.

8. [A typical layout of a deep well system]

10. • The main components of a well-point system are:
1. well points
2. Riser pipe
3. Swinger arm
4. Header pipe
5. pumps

11. • The well point is perforated pipe 5 to 8 CM in dia & 1m long
covered by cylindrical wire gauge screen known as strainer.
Pipes are jetted in the ground 1 to 2mt a part.
Well point → riser pipe → swinger arm → header.
It is suitable for lowering water table by 5 to 6 m in soil.

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14. Multi-stage well point system is suitable for excavations up to 15 m.

15. When water table is greater than 6 m this method willuse.
In this method 2 or more rows of well point are installed at different elevation.
In this method wells are installed in 2 stages.
In 1st stage water table lowered by 5 m.
If required, then 3rd stage of well point can also be installed for further lower water table.
This method useful for up to 15 m. For up to 15th m deep well system will use.

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17. is very rapid. The equipment is reasonably Simple
Advantages:
•Installation
and cheap.
•As water is filtered while removing from the ground, soil particles are not washed
away. Hence, there is no danger of subsidence of the surrounding ground.
•As the water is drawn away by well points from the excavation, the sides of
excavation are stabilized and steeper side slopes can be permitted.

18. Disadvantages:
•Single stage well point system is suitable for lowering water table by 5 to 6 m only. For
deeper excavations, where water table is to be lowered for a depth greater than 6 m, multi-
stage well point system is required.
•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.
•In case of the ground consisting of stiff clay, gravel or boulders, well points are
installed in drilled holes, which increases the installation cost.

20. When draining is required for silt or clay which have size less than
0.05mm. That time vacuum pump system will require.
The process is as follows:
The well-points are driven and 25 CM dia is provided around the well point.
Installed in the ground by forcing a jet of water under sufficient pressure.
The sand of medium to coarse size is then forced into
this hole as rapidly as possible. This sand forms the filter medium.

21. In the upper most 600 mm to 900 mm, an impervious material such as clay is tamped to
form the seal the upper portion.
The pumping is then carried out by means of equipment capable of maintaining a vacuum
in the well-points and the surrounding filter.
In this way, the pressure around the well-points is reduced to a small fraction of the
atmospheric pressure. The ground is acted upon by the atmospheric pressure. Thus the soil
becomes consolidated under a pressure which is very nearly equal to the atmospheric.

22. In highly permeable cohesionless soil, the safety of the side slopes may be endangered
through the application of severe pumping. In such cases, especially if control of the
groundwater is required permanently, the methods of grouting can be used.
The main idea is to insert fine materials or chemicals around the excavation in order to
reduce the hydraulic conductivity of the surrounding soil to a minimum.
In other words, the grouting process creates an almost impervious curtain around the
excavation. The grouting is conducted using movable pipes with holes. The grout material
is injected under pressure as it flows outside the pipes through the holes to fill the voids of
the surrounding soil.
The material used for grouting may be clay, cement or special chemical compounds.

26. • The material commonly used for grout include:
1) Cement And Water
2) Cement, Rock Flour and Water
3) Cement, Clay and Water
4) Cement, Clay, Sand and Water
5) Asphalt
6) Clay And Water
7) Chemicals

27. • The desirable properties of chemical grouts:
1)It must be able to modify the properties of soil as desired
2)It may either increase the strength or decrease the permeability of soil
3)It must be cheap, non-toxic, non-explosive
4)It must be in the form of a liquid with low viscosity so that it can be readily placed
in the soil
5)It must be non-corrosive, so that it can be handled with common pumps and
piping
6)It must be possible to control the gel time by suitable means.

29. • Inorganic chemicals
Sodium silicate
Calcium chloride
Ligno-chrome
Ligno sulphate
They are called silicate grouts
They are cheaper
• Organic chemicals
Epoxy resins
Polyester resins
They are also called resin grouts
They possess advantage of low viscosity, precise control of gel time and high
strength

30. Soils that will not drain using conventional methods Typically a ground freezing system
consists of an array of freeze pipes.
That are installed into the ground around the perimeter of the excavation, usually in a
circular pattern.
A supermodel brine solution is pump through to freeze the pipes, which freezes the water
bearing soils around the pipes to create a frozen wall. Extreme care must be taken to make
sure that the freeze is complete because any groundwater seepage though the wall or from
below the excavation depth will have a sliding effect.

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33.  The procedure:
A refrigeration plant of required installed near the site of work.
The large pipes of 100 mm to 150 mm diameter.
The distance between the pipes is about 1 m to 1.50 m.
The pipes are closed at the bottom.
The small pipes of 25mm to 50mm diametre are inserted into the large pipes open at the
bottom.
The cold liquid at a temperature of about -23°C to -30°C is then circulated through the
circuit.
The liquid comes through the small pipe and goes up through the large pipe.
This causes the ground to freeze around the pipes.

35. • This method is used for fine grained cohesive soils (such as clay), which can be drained or
stabilized using electric current. The method was developed by
L. Casagrande (1952).
• If direct current is passed between two electrodes driven into natural soil mass, the soil water
will travel` from the positive electrode (anode) to the negative electrode (cathode). The
cathode is made in the form of well point or metal tube for pumping out the seeping form of
well point or a metal tube for pumping out the seeping water.
• A steel rod, a 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 flow of water is reversed away
from the excavation, thereby increasing the strength of the soil and stability of the slope. The
potentials generally used in the process are from 40 to 180 volts, with electrode spacing of 4
to 5 meters.

36. The vibroflot is inserted into the ground and typically can be used to improve soil up to depths
of 150 feet. Vibroflotation utilizes water and the mechanical vibrations of the vibroflot to move
the particles into a denser state. Typical radial distances affected range from 5 to 15 feet (Bauer
Maschinen GmbH, 2012).
The vibroflot is suspended from a crane and seats on the surface of the ground that is to be
improved. To penetrate the material, the bottom jet is activated and the vibration begins. The
water saturates the material to create a “quick sand” condition (i.e. temporarily liquefying the
material), which allows the vibroflot to sink to the desired depth of improvement.
At that point, the bottom jet is stopped and the water is transferred to the upper jet. This is
done to create a saturated environment surrounding the vibroflot, thereby enhancing the
compaction of the material. The vibroflot remains at the desired depth of improvement until
the material reaches adequate density.

37. The density of the soil is measured by using the power input (via the electric current or
hydraulic pressure) as an index. As the material densifies, the vibroflot requires more
power to continue vibrating at which point pressure gauge displays a peak in required
power.
Once this point is reached, the vibroflot is raised one lift (generally ranging from 1 to 3
feet) and compaction ensues until the peak amperage or hydraulic pressure is reached once
again.

39. The vibroflotation process can offer the following benefits:
•When the process is done properly, it will reduce the possibility of differential settlements
that will improve the foundation condition of the proposed structure.
•It is the fastest and easiest way to improve soil when bottom layers of soil will not provide
good load bearing capacity.
•It is a great technology to improve harbor bottoms.
•On a cost-related standpoint, it helps improve thousands of cubic meters per day. It is
faster than piling.
•It can be done around existing structures without damaging them .
•It does not harm the environment. It improves the soil strata using its own characteristic
•No excavations are needed, reducing the hazards, contamination of soils and hauling
material out from the site.

40. METHOD CONDITIONS FOR SUITABILITY
1. Sumps and Ditches For shallow excavations in coarse
grained soils.
2. Well point system Suitable for lowering water table by 5-6 m
in soils
3. Bored well system For coarse grained soils and depth of
excavation more than 16 m
4. Vacuum method Draining silty sands and fine sands
5. Cement grouting For coarse materials or rocks with
cracks
6. Freezing process Suitable for excavations in water logged
soils
7. Electro-osmosis process Suitable for fine grained cohesive soils
such as clays

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