.

A Seminar presentation

on

Methods of Dewatering

On Construction SiteBy

Gaikwad A. A.

Guide: Ms. Pujari P.D..

CONTENTS

Introduction

Lesson Objective

Purpose of Dewatering

Factors controlling Selection

Methods of Dewatering

Conclusion

.

Introduction

4

GWT

Deep Foundation

Lowering WT

Definition

If Overlooked??

Lesson Objective

5

To know the concept of Dewatering

To know Importance of Dewatering

To know various Methods of Dewatering

6

Purpose of Dewatering

During construction stage:-

dry excavation

proceed efficiently

Reduce lateral loads

Improve characteristics of foundation materials

Increase stability of slopes and side-hill fills

Prevent piping and frost heaving in pavements

77

Purpose of Dewatering

Post construction stage :-

Reduce uplift pressures on bottom slabs

Reduce lateral pressures on retaining structures

Control embankment seepage in all dams

Purpose of Dewatering

Factors Controlling Selection

Nature and Permeability of Ground

Geologic conditions of soil

Extent of area to be Dewatered

Depth of Water table below ground level

Amount by which it has to be lowered

Proposed methods of excavation and ground support

Proximity of existing structure

8

9

Soil Permeability

Coefficient(k)(cm/sec)

Relative Permeability

Coarse gravel Exceeds 10^-1 High

Clean sand 10^-1 to 10^-3 Medium

Dirty sand 10^-3 to 10^-5 Low

Silt 10^-5 to 10^-7 Very low

Clay Less than 10^-7 Impervious

Permeability of Soil

10

Methods of Dewatering

Surface water control

Sump pumping

Wellpoint systems with suction pumps.

Ejector system

Ground freezing

Deep (bored) wells with pumps.

11

1.Sump Pumping

12

13

1.Sump Pumping

What is Sump?

Slope

Pump

Fines

Lined

14

15

Widely used method

Most economical method for installation and maintenance

Can be applied for most soil and rock conditions

Most appropriate where boulders or massive obstructions

j are met with in the ground

Advantages of Open Sump and Ditches

1616

Due to more flow of GW towards excavation risk of

collapse of sides occurs.

In open or timbered excavations there is risk of

instability of the base due to seepage towards pumping

sump.

Note:- Greatest depth to which the water table can be

lowered by this method is about 8 m below the pump.

Disadvantages of Open Sump and Ditches

2.Wellpoint System

17

Fig. Cross section of a typical well-point system

18

2.Wellpoint System

Perforated Well

Material

Sandy

Foot – Orifice

Series of Closely Placed

Dimension

Spacing

Construction steps

1919

2.Wellpoint System

19

Well point arrangement

20

Well point System(Spacing)

soil spacing

Silty sand 1.5-2.0

Sandy gravel 1.0-1.5

Fine to coarse gravel 0.5-1.0

21

Single Stage Well-point system

21

22

Multi - Stage Well Point System

22

2323

Multi - Stage Well Point System

23

24

Installation is very rapid

Requires reasonably simple and less costly equipment

Water is filtered hence not carries soil particles.

There is less danger of subsidence of the surrounding

gj ground than with open-sump pumping

Advantages of Well point System

2525

A lowering of about 6 m is possible, beyond which

excessive air required, resulting in the loss of pumping

efficiency.

If large gravel, stiff clay or soil containing boulders is

available then it is not possible to install well points.

Disadvantages of Well point System

3.Eductor Well

26

Pump

27

3.Eductor Well

Vacuum / Ejector

Control pore pressure

Fine soil

Extract GW , Generate vacuum

Drainage system

Working

28

29

They are flexible in level and layout

Stable in operation

Able to run dry without damage

Not limited by depth. Also effective to greater depths

Best in low-yielding wells

Energy intensive

Venturi in base of well creates vacuum

Advantages of Eductor Well

3030

4.Ground Freezing

Figure – Formation of frozen earth barrier in different soils

31

32

Pipes prior to freezing

Following initiation of freeze

Closure of frozen earth wall

Complete frozen earth wall

33

5.Deep Well

34

5.Deep WellGreater than 8m

Pump

Size

Perforated as well as Imperforated pipe - Material

Suitability

Artesian water

Chance of settlement

Pumping capacity (3000 to 60,000 gallons/min)

35

Summery of dewatering methods

Method Suitability

Sump pumping Gravel or well graded sandy gravel, partially cemented

material , porous rock formation etc

Well point systems Sandy soil

Drawdown limit 15 ft

Ejector system Fine sand condition

Dewater up to depth of 100 ft

Ground freezing groundwater cutoff, stabilization of earth for tunnel

excavation,

arrest landslides and to stabilize mineshafts

Deep (bored) wells Artesian water

Pumping capacity (3000 to 60,000 gallons/min)

Dewater up to depth of 300 ft

36

Books: DEWATERING AND GROUNDWATER

CONTROL(HQ.DEPARTMENTS OF ARMY, AIR

FORCE, NAVY WASHINGTON)

Construction Dewatering and Ground

Freezing (University of Washington)