LEARNING OUTCOMES:LEARNING OUTCOMES:At the end of this topic, the students should be able to:At the end of this topic, the students should be able to:1.1. Describe different types of cofferdamDescribe different types of cofferdam

2.2. Explain design principles of cofferdamExplain design principles of cofferdam

COFFERDAM

COFFERDAM is a series of connecting soil-filled cells around the perimeter of a work area.

Water side

Basin sideLand, inside or dry side

Not intended to be completely impervious but rather to provide sufficient resistance to water flow that the quantity of water that seeps through can be readily pumped.

Smaller connecting

partial cells

3 TYPES OF CELLULAR COFFERDAM

Circular cells Diaphragm cells

Cloverleaf cells

DETERMINATION OF EFFECTIVE WIDTH, B

Cell intersection angle, , is usually 30 or 45.

90 degrees Tee

30 and 120 degrees wyes

STABILITY AND DESIGN OF CELLULAR COFFERDAMS

Terzaghi’s (1945) method or Tennessee Valley Authority (TVA method)

Design principles of cofferdamDesign principles of cofferdam

Stability factor

Reinforced earth retaining wall

Cofferdam

 Forces 

Internal stability  External stability

 Lateral soil pressure

Safe against ;Tensile failure, bond failure Safe against:Sliding failureOverturning failureBearing failure

 Lateral water pressure

 Safe against:Interlocking stressBursting force Safe against:Sliding failureOverturning failureVertical shearBearing failure

SLIDING STABILITY

25.1'

d

wfp

PPPP

FOS

Note : Pa on the water

side is neglected

OVERTURNING STABILITY

• Consider soil cannot take tension force.

• Resultant weight W must lie within the middle one-third of the base.

6B

WyP

HByP

e dd

Then, determine the average cell width, B.Note : Larger cell heights, H require wider B.

• Consider the cell tends to tip over and the soil will pour out at the heel.

• The tipping movement must overcome the friction resistance between the cell fill and the water-side sheet-pilling. The friction force is considered to developed from water force, Pw.

• Summing moment about toe, A, gives;

1

2

yPBP ww tan

tanyB Required average width, B is

25.1

3

3

d

d

pp

HP

hPeW

FOS

25.1tan

yPBPFOS

w

w

SHEAR ALONG PLANE THROUGH CENTRELINE

Shearing resistance along the centre line plane is contributed by:

1. Soil shear resistance, Vs and

2. Friction, Ril in the interlock joints.

The sum, Vr must equal or greater than the shear, V due to overturning effects, Mo.

BVM o 32

BMV o

23

'tanss PV

Ps = area of abcd

VRVV ilsr

Calculated using K’

itil fPR

Pt = area of abcd

Calculated using Ka.

Interlock friction coefficient usually taken as 0.3.

25.1VVFOS r