Celluar Cofferdams

8/4/2019 Celluar Cofferdams

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Cellular Cofferdams

Cellular cofferdams are either segmental or circular type and are used either on a temporary or permanentbasis. External forces are resisted by the mass of the cofferdam.


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Segmental

In this type the diaphragm walls resist the tensile forces in the arcs. The width of the segments is dependent

on the tensile capacity in the sheet piling, which is caused by the earth pressure inside the cells. Adjacentcells are filled simultaneously to prevent the failure of the diaphragm wall.


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Circular

This type of cofferdam comprises a linear group of circular cylindrical cells of diameter D joined with

smaller connecting cells of diameter (0.6D). Dmax = 70 75, Davg = 60. Also, 1.0H D < 1.2H where H =

height of cofferdam wall. Both circular and segmental cells are filled with ballast (granular fill).


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The circular cell is designed as a gravity structure. The design is simplified by using a rectangular section ofwidth b (= 0.6D) and length = 2L.


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Design for:

1. Rupture in tension of the interlocks @ base2. Vertical shear3. Soil bearing capacity at base4. Sliding5. Overturning6. Filtration through base and body

Design methods commonly used (consult appropriate texts):

Terzaghi

TVA

NAVFACCummings

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Segment and Circular (temporary and permanent) - forces resisted by mass segment.

Diaphragm walls resist tensile forces in arcs

If

120o

120o 120o

Diaphragm

Arcs

Then tensile forces in these elements are equivalent width L is dependent on tensile force in sheet piling,which is caused by the earth pressure inside the cells.

Filling of cells is done to prevent the failure of diaphragm wall. Fill adjacent cells simultaneously.


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Static Calculations:

Using the above diagram, these are the design forces and design procedures for a cellular cofferdam.

ssubssoil bhhHbG += )(

22/1 HW w=

011 == TANEactive

21/ 2a sub a E d K =

21/ 2 passive sub p

E t= K

E Lateral confined pressure

2

2

2

1/ 2

cosvertical to lateral stress

2 cos

E h K

where K

=

=


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Angle of internal friction =

Stability against sliding

R

D

FnF

=

FR = Resisting forces

FD = Driving forces

/tan R Ballast Rock F G =

FD = W + Ea Epn = Safety Factor

n = 1.25 Temporary structure

n = 1.50 Permanent structure

Stability against overturning

R

D

Mn

M=

MR = Resisting moment at Ballast

MD = Driving moment at Ballast

21/ 2R M b H =

3 3D a

H d M W E E

= +

3p

t

Vertical shear in cell ballast

38.4

2

MV b

b=

Rupture in tension of interlocks

r

t tot P

( )b soil s sub sP H h K h K = +


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w wP hf=

( )(2

t b w

DP P + )

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Celluar Cofferdams