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GEOTECHNICAL ENGINEERING
ECG 503
LECTURE NOTE 10
3.0 ANALYSIS AND DESIGN OF
RETAINING STRUCTURES
LEARNING OUTCOMES
Learning outcomes:
At the end of this lecture/week the students would
be able to:
Able to design cofferdams
TOPIC TO BE COVERED
Cofferdam
CELLULAR COFFERDAMS
“A cofferdam is a temporary structure designed to keep water and/or soil out of the excavation in which a bridge pier or other structure is built.”
- Standard Handbook of Heavy Construction
• Used to enable construction works in water bound areas eg. rivers, lake and sea
• Stability depend mainly on interaction of the soil to fill the cell and the steel sheetpiling.
CELLULAR COFFERDAMS
• Used to enable construction works in
water bound areas eg. rivers, lake
and sea
• Stability depend mainly on
interaction of the soil to fill the cell
and the steel sheetpiling.
• Cofferdams are temporary enclosures to keep out water and
soil so as to permit dewatering and construction of the
permanent facility (structure) in the dry.
• A cofferdam involves the interaction of the structure, soil,
and water. The loads imposed include the hydrostatic forces
of the water, as well as the dynamic forces due to currents
and waves.
• Because cofferdams are typically constructed under adverse
conditions in a marine environment, and because significant
deformations of elements may occur at various stages of
construction, it is difficult to maintain close tolerances.
Ample provisions must be made for deviations in
dimensions so that the finished structure may be constructed
according to plan.
• The loads imposed on the cofferdam structure by
construction equipment and operations must be
considered, both during installation of the cofferdam
and during construction of the structure itself.
• Removal of the cofferdam must be planned and
executed with the same degree of care as its
installation, on a stage-by-stage basis. The effect of the
removal on the permanent structure must also be
considered. For this reason, sheet piles extending
below the permanent structure are often cut off and left
in place, since their removal may damage the
foundation soils adjacent to the structure.
Cofferdams
• Types of cofferdam:
– Braced
– Earth-Type
– Timber Crib
– Double-Walled Sheet Pile
– Cellular
• Cofferdam Design Considerations
– Scouring or undermining by rapidly flowing
water
– Stability against overturning or tilting
– Upward forces on outside edge due to tilting
– Stability against vertical shear
– Effects of forces resulting from:
• Ice, Wave, Water, Active Earth and Passive
• Earth Pressures
• Advantages of Cofferdam– Allow excavation and construction of
structures in otherwise poor environment
– Provides safe environment to work
– Contractors typically have design
responsibility
– Steel sheet piles are easily installed and
removed
– Materials can typically be reused on other
projects
• Traditional Sheet Pile Shapes
Z-Type (Z)
Used for intermediate to deep
wall construction
Larson / “U” Type (U)
Used for applications similar
to Z - Type
Flat / Straight Type (SA), (S)
Used for filled cell constructionArch shaped & lightweight
Used for shallower wall
construction
• Typical types of interlocks
Ball & Socket (BS) Single Jaw (SJ) Double Jaw (DJ)
Hook & Grip (HG) Thumb & Finger
one point contact (TFX)
Double Hook (DH) Thumb & Finger
three point contact (TF)
• Braced Cofferdam Construction
– Install Wale and Strut System for Framework
/Template
• Braced Cofferdam Construction– Install Wale and Strut System for Framework /Template (Cont’d)
• Contains three basic types which is :
a. Circular Cofferdam
b. Diaphragm Cofferdam
c. Cloverleaf Cofferdam
• Design consideration:
a. Cell geometry
b. Cell fill materials
c. Sheet piles
Cofferdam is a type of watertight construction designed to facilitate construction projects in areas which are normally submerged, such as bridges and piers. A cofferdam is installed in the work area and water is pumped out to expose the bed of the body of water so that workers can construct structural supports, enact repairs, or perform other types of work in a dry environment. In some regions of the world, a cofferdam is better known as a caisson. Working inside a cofferdam can be hazardous if it is installed improperly or not safely pressurized, but advances in have led to increased safety for workers using this unique work environment.
• A variety of materials can be used to construct a cofferdam, which is truly a feat of engineering. Although a cofferdam is a temporary structure, it must reliably hold water back from the work area and also withstand very high pressures in order to be safe, and the construction of cofferdams is often used as a project for engineers learning their craft. The most basic type of cofferdam uses sheet metal, which is pounded into the bed of the body of water to create a watertight wall. Next, pumps are used to pull water out of the enclosure so that it will be dry. Some cofferdams are built from wood or concrete, while others use a double walled mechanism, with filler made from aggregate materials in between the two walls.
• The walls of a cofferdam can extend all the way to the surface of the water, leaving it open at the top, or it can be built as an enclosed structure. In very deep water, enclosed and pressurized cofferdams are used for worker safety, while in shallower bodies of water, an open cofferdam can be used. Workers access a closed cofferdam through hatches and tubes, and care is taken to make sure that the air supply is consistent and the pressure is kept at a normal level.
