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

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. Adjacent cells are filled simultaneously to prevent the failure of the diaphragm wall.

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).

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

Design for:

1. Rupture in tension of the interlocks @ base 2. Vertical shear 3. Soil bearing capacity at base 4. Sliding 5. Overturning 6. Filtration through base and body

Design methods commonly used (consult appropriate texts): Terzaghi TVA NAVFAC Cummings ------------- 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.

Static Calculations:

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

ssubssoil bhhHbG γγ +−= )(

22/1 HW wγ=

011 == µφTANEactive

21/ 2a sub aE d Kγ=

21/ 2passive sub pE tγ= K E – Lateral confined pressure

2

2

2

1/ 2cos vertical to lateral stress

2 cos

E h K

where K

γ

φφ

=

= −−

Angle of internal frictionφ =

Stability against sliding

R

D

FnF

=

FR = Resisting forces FD = Driving forces

/tanR Ballast RockF G φ= FD = W + Ea – Epn = Safety Factor n = 1.25 – Temporary structure n = 1.50 – Permanent structure Stability against overturning

R

D

MnM

=

MR = Resisting moment at Ballast MD = Driving moment at Ballast

21/ 2RM b Hγ=

3 3D aH dM W E E⎛ ⎞ ⎛ ⎞ ⎛= + −⎜ ⎟ ⎜ ⎟ ⎜

⎝ ⎠ ⎝ ⎠ ⎝ 3pt ⎞⎟⎠

Vertical shear in cell ballast

3 8.42

MV bb

= −

Rupture in tension of interlocks

rt totPσ ≥

( )b soil s sub sP H h K h Kγ γ= − +

w wP hfγ=

( )(2t b wDP Pσ ≥ + )