(Advanced Construction and Equipment)

Control of Ground Water in Excavation

L.D. College of Engineering, Ahmedabad-15

Semester- 6 Division-C

Guided by..Prof. J.S. Kamble

Prepared by,

Sr. No. Name of Student Roll No. Enrollment No.

1. Patel Keyur 6062013 130280106081

2. Patel Vedant 6062014 130280106089

3. Patel Vishesh 6062015 130280106090

4. Patni Mo.Aamir 6062016 130280106091

Content…

• Introduction

• Dewatering methods

• Suitability of different dewatering methods

Introduction

• Dewatering means removal of excess water from saturated soil.

• Dewatering is a necessary process when it comes to many

construction projects, particularly when the construction is for

underground projects

Dewatering Methods• 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.

1. Sumps And Ditches

2. Shallow Well System

3. Deep Well System

4. Well Point System

5. Vacuum Method

6. Cement Grouting

7. Chemical Process

8. Freezing Process

9. Electro-Osmosis Method

Sumps and Ditches

• 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.

• The water from the slopes flows under

gravity and is collected in sumps from

which it is pumped out.

Shallow Well Dewatering

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.

Shallow Well Dewatering

[A typical layout of a shallow well system]

Applications for Shallow Well Dewatering

• Shallow wells can be installed with effective filters which can allow

them to pump ‘clean’ water with much lower levels of suspended

solids compared to sump pumping. This makes it easier to meet

water quality limits at the discharge point.

• Shallow wells, if installed to sufficient depth, can act as a form of

pre-drainage dewatering, where groundwater levels are lowered in

advance of excavation and can be held significantly below current

dig levels.

Deep Well Dewatering

This system is more suitable when the depth of excavation is more

than the 16m or where artesian water is present.

15 to 16 cm diameter hole is bored and a casing with a large screen

is provided. A submersible pump is installed near the bottom of the

well.

Deep Well Dewatering

[A typical layout of a deep well system]

Applications for Deep Well Dewatering Deep well dewatering is normally suited to relatively deep

excavations where permeability is between moderate (e.g. sands) to

high (e.g. gravels).

Deep wells can be less effective is in low permeability soils. In these

cases the well yields can be very low. Such low well yields can

make it difficult to select suitable submersible pumps that can

operate reliably and efficiently at low flow rates.

Well Point System

• The main components of a well-point system are:

1. well points

2. Riser pipe

3. Swinger arm

4. Header pipe

5. pumps

Well Point System

• Well points are for lowering water table by 5-6 m in soils with a

coefficient of permeability between 10-4 to 10-6 m/sec.

• The riser is surrounded by a coarse sand filter in order to facilitate

the flow of water towards the well points.

Multi-Stage Well Point System

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

Well Point System

Advantages:

• Installation is very rapid. The equipment is reasonably simple 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.

Well Point System

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.

Vacuum System

Grouting of soft soil

Cement Grouting

Cement Grouting

• 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

Grout curtain around excavation

Chemical Grouting• 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

Chemical Grouting

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

Advantage

1) It can be used for sealing narrow cracks of about 0.5 mm width

2) It become hard in cold as well as warm temperature

3) It can bond the moist as well as dry surface

4) It has practically the same viscosity as water and hence it can be

injected into any soil and fine cracks into which the water can be

injected

GROUND FREEZING

Advantages and Limitations

Advantages:

• The freezing process is readily accomplished where other methods

may be difficult or impossible.

• The freezing does not affect the current water table.

Limitations:

• The area to be covered by this process should be as small as

possible.

• The freezing process is suited for work of comparatively short

duration.

Electro-Osmosis Process

Suitability of Different Methods of Ground Water Conditions

METHOD CONDITIONS FOR SUITABILITY1. 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