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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
-------------
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 + )