Cofferdams in History

The first recorded use of a cofferdam goes back to 539 BC. King Cyrus of Persia used cofferdams as a
weapon of war. He diverted the Euphrates River with earthen cofferdams, which allowed his army to
capture Babylon. Cofferdams were used by the Romans in 102AD to build Trajan’s Bridge. A
cofferdam made of wood pilings allowed the Romans to build the bridge across the Danube River.

Cofferdam Uses

There are various construction scenarios where a cofferdam is necessary:

 Building a structure in a riverbed, seashore, or lake.

 Building in an area of course-grained soil, where deep excavations are required.

 Building below the groundwater table.

 Building when trenches would likely collapse – typically during deep excavations.

 To protect adjoining buildings or nearby structures.

 To remove a sunken vessel.

Cofferdam Requirements

There are several basic requirements of any cofferdam. It must be watertight. A cofferdam must
remain standing against the pressure of existing or added water (such as a flood). Existing water
includes water at, above, or below the groundwater table.

Cofferdam Materials

Cofferdams can be made from several different materials, such as earth, rocks, timber, steel, and
concrete. If possible, the selected material should be relatively easy to dismantle and recycle to
reduce construction costs.

Types of Cofferdams

There are various types of cofferdams, including:

Earthen Cofferdams

Earthen cofferdams are the simplest to construct but have somewhat limited use. An embankment
made of earth is used to enclose the work area. Earthen cofferdams are best for areas with a low
depth of water flowing at a low velocity. Typically, the top of the embankment is built approximately
1m above the water. Boulders may be used on the side slopes of the bank on the water side to
prevent erosion.

Rock-Filled Cofferdams

Similar to earthen cofferdams, rock-filled cofferdams have a rubble or stone embankment to

surround the work area. A rock-filled cofferdam is still used in lower-depth water – perhaps 2m to
3m.
Braced Cofferdams

Braced cofferdams consist of a single wall of sheet pile that is supported by struts. This prevents the
wall from collapsing inward. Braced cofferdams are typically used for smaller work areas. You may
see this type of cofferdam used in the repair of bridge piers and abutments.

Timber Crib Cofferdams

Timber crib cofferdams are made of a frame comprised of wooden beams. Horizontal and cross
beams are laid in an alternate course. Timber crib cofferdams are left open at the bottom and filled
in with either earth, gravel, or rock.

Concrete Cofferdams

Concrete cofferdams are used when pile driving is problematic. This includes limited headroom, the
need to reduce or eliminate vibrations, or where boulders embedded in the ground would split steel
sheet piles. Concrete cofferdams are expensive, but the overall cost is reduced by making them part
of the permanent structure. This type of cofferdam is typically used for smaller areas and are often
built of pre-cast, reinforced cement concrete (RCC) piles.

Single-Wall Cofferdams

Single-wall cofferdams are used for small areas with a depth of water typically between 4.5 to 6
meters. Guide piles made of timber are driven deep into the riverbed – below the firm ground
beneath it. The velocity of the flowing water determines the center-to-center spacing of the guide
piles. Wales, also known as longitudinal runners, are bolted to the guide piles. Steel or wooden sheet
piles are driven along the wales and then bolted to them. The sheets on both faces are braced using
struts. Half-filled bags of sand are used to stabilize the single-wall cofferdam.

Double-Wall Cofferdams
A double-wall cofferdam consists of two straight parallel walls made of sheet piling that are tied
together. The space between the two walls is filled with soil. Double-wall cofferdams are used in
water with a depth up to 12 meters.

Cellular Cofferdams

For large excavations, the use of cross-excavation bracing may not be feasible. Cellular cofferdams
consist of interlocking steel plates designed to resist lateral forces without the need for bracing.
There are two types of cellular cofferdams: diaphragm and circular. Diaphragm cellular cofferdams
have circular arcs at the sides that are attached to straight diaphragm walls. Circular cellular
cofferdams have large circular cells that are connected using circular cells that are a bit smaller. The
cells of cellular cofferdams are filled – usually with earth or concrete. A waterproof membrane
covers the cells and the whole cofferdam is placed and secured.

Cofferdam Construction Process

Most cofferdams are constructed using the following 12 general steps:

1. Pre-dredge and level the area for the cofferdam.

2. Drive temporary support piles.

3. Temporarily install a bracing frame on the support piles.

4. Install steel sheet piles.

 5. Drive sheet piles to grade.

6. Block between bracing frame and sheets.

7. Tie sheet piles at the top.

8. Excavate, leaving the water inside the cofferdam.

9. Install internal bracing as the water is removed progressively from the cofferdam.

10. Drive piles as required.

11. Install rock fill.

12. Place tremie concrete seal.

Caisson, in engineering, boxlike structure used in construction work underwater or as a foundation.

It is usually rectangular or circular in plan and may be tens of metres in diameter.

A box caisson, open at the top and closed at the bottom, is usually constructed on land, then
launched, floated to position, and sunk onto a previously prepared foundation, leaving its upper
edge above water level. It serves as a suitable shell for a pier, seawall, breakwater, jetty, or similar
work, remaining permanently in place on the sea bottom

An open caisson, open at both the bottom and the top, is fitted with a cutting bottom edge,
which facilitates sinking through soft material while excavation is carried out inside through a
honeycomb of large pipes, or dredging wells. As excavating proceeds and the caisson sinks,
additional sections are added to the shaft above. This process is continued until the caisson has sunk
to the required depth. A floor, usually of concrete, is laid to provide a bottom seal. The dredging
wells can then be filled with concrete to complete the structure.

Pneumatic caissons are similar to open caissons except that they are provided with airtight
bulkheads above the cutting edge. The space between the bulkhead and cutting edge, called the
working chamber, is pressurized to the extent necessary to control the inflow of soil and water; thus
the excavating can be performed by workmen operating in the working chamber at the bottom of
the caisson.

What is a Caisson Foundation?

A caisson foundation also called as pier foundation is a watertight retaining structure used as a
bridge pier, in the construction of a concrete dam, or for the repair of ships. It is a prefabricated
hollow box or cylinder sunk into the ground to some desired depth and then filled with concrete
thus forming a foundation. Caisson foundation is Most often used in the construction of bridge piers
& other structures that require foundation beneath rivers & other bodies of water. This is because
caissons can be floated to the job site and sunk into place. Caisson foundations are similar in form to
pile foundations, but are installed using a different method. It is used when soil of adequate bearing
strength is found below surface layers of weak materials such as fill or peat. It is a form of deep
foundation which are constructed above ground level, then sunk to the required level by excavating
or dredging material from within the caisson. Caissons (also sometimes called "piers") are created by
auguring a deep hole into the ground, and then filling it with concrete. Steel reinforcement is
sometimes utilized for a portion of the length of the caisson. Caissons are drilled either to bedrock
(called "rock caissons") or deep into the underlying soil strata if a geotechnical engineer finds the soil
suitable to carry the building load. When caissons rest on soil, they are generally "belled" at the
bottom to spread the load over a wider area. Special drilling bits are used to remove the soil for
these "belled caissons". The caisson foundations carry the building loads at their lower ends, which
are often bell-shaped.

Functions of Caisson Foundation

The foundation system of and the soils beneath the building prevent the complex from moving
vertically. When a load is placed on soil, most soils settle. This creates a problem when the building
settles but the utilities do not. Even more critical than settlement is differential settlement. This
occurs when parts of your building settle at different rates, resulting in cracks, some of which may
affect the structural integrity of the building. Conversely, in some rare instances soils may swell,
pushing your building upwards and resulting in similar problems. Therefore, the foundation system
must work in tandem with the soils to support the building.

Types of Caisson Foundations

 Box Caissons

 Excavated Caissons

 Floating Caissons

 Open Caissons

 Pneumatic Caissons

 Sheeted Caissons

Box caissons are watertight boxes that are constructed of heavy timbers and open at the top. They
are generally floated to the appropriate location and then sunk into place with a masonry pier within
it.

Excavated caissons are just as the name suggests, caissons that are placed within an excavated site.
These are usually cylindrical in shape and then back filled with concrete.

Floating caissons are also known as floating docks and are prefabricated boxes that have cylindrical
cavities.

Open caissons are small cofferdams that are placed and then pumped dry and filled with concrete.
These are generally used in the formation of a pier.

Pneumatic caissons are large watertight boxes or cylinders that are mainly used for under water
construction.

Advantages and Disadvantages of Caissons:

Advantages of Caissons:

 Economics

 Minimizes pile cap needs

 Slightly less noise and reduced vibrations

 Easily adaptable to varying site conditions

 High axial and lateral loading capacity

Disadvantages of Caissons:

 Extremely sensitive to construction procedures

 Not good for contaminated sites

 Lack of construction expertise

 Lack of Qualified Inspectors

Drilled Pier Foundations

A drilled pier is a deep foundation system that is constructed by placing fresh concrete and
reinforcing steel into a drilled shaft. The shaft is constructed by rotary methods using either a self-
contained drill unit or a crane mounted drill unit. The hole is advanced through soil or rock to the
desired bearing stratum. Temporary or permanent steel casings may be used to maintain the sides
of the drilled excavation if caving soils or water infiltration becomes a problem. Drilled shafts can be
used to sustain high axial and lateral loads. Typical shaft diameters range from 18 to 144
inches. Drilled shafts (also called caissons, drilled piers or bored piles) have proven to be a cost
effective, excellent performing, deep foundation system, that is utilized world-wide. Typically they
are used for bridges and large structures, where large loads and lateral resistance are major factors.

Concrete Caissons

A 10" or 12" diameter holes are drilled into the earth and embedded into bedrock 3 to 4 feet.
Usually used for the structural support for a type of foundation wall, porch, patio, monopost, or
other structure. Two or more "sticks" of reinforcing bars (rebar) are inserted into and run the full
length of the hole and then concrete is poured into the caisson hole. A caisson is designed to rest on
an underlying stratum of rock or satisfactory soil and is used when unsatisfactory soil exists

Caisson Construction Process

 After some initial form work and concrete pours, the cutting edge is floated to the
breakwater by towboat and fastened to the caisson guide. Concrete is placed (poured) into
steel forms built up along the perimeter of the box. With every concrete placement, the box
becomes heavier and sinks into the water along the caisson guide.

 Forms are also built inside the box around the air domes and concrete is placed in between.
The resulting open tubes above the air domes are called dredge wells.
  When the caisson finally touches the river bottom, the air domes are removed and earth is
excavated through the long dredge well tubes, as shown in the animation below. The caisson
sinks into the river bottom. Excavation continues until the caisson sinks to its predetermined
depth.

 As a final step, concrete is placed (poured) into the bottom 30 feet of the hollow dredge
wells and the tops are sealed.

Straight Shaft Drilled Piers (Caissons)

  Used in moderate to high swelling soils. (This is one of the most effective foundation
designs for use in sites that contain expansive soils.)

 Purpose is to attain required penetration into zone where there is little or no seasonal
moisture variation. Current standard of care in the area is a minimum penetration of 6 feet
into bedrock and minimum length of 16 feet. Dead loads should be as high as practical. This
design requires relatively long spans between piers and more reinforcing in grade beam.

 Caissons into bedrock

 Friction Piers into stiff clays

 End Bearing Belled Piers

 Appropriate Voiding - Should be constructed with void material of appropriate strength and
thickness

Fig: A series of 1.2-metre

thick diaphragm wall panels were joined to form a 24-metre diameter caisson shaft. Four of these
caissons were built to provide a sound base for the foundation of the main structure of the building
tower. The photo shows the excavation work using typical excavating machines inside one of the
caisson shafts.

PNEUMATIC CAISSON

The pneumatic caisson method comprises the on-ground construction of a reinforced concrete
caisson having a working chamber inside at the lower part, pressurized air supply to the working
chamber to prevent underground water from coming in there, excavation work of soils in the
working chamber and finally sinking the whole caisson structure. Pneumatic caissons are utilized for
a variety of structures: foundations of bridges and buildings and main structures of sewage
treatment facilities, underground water regulating reservoirs, shafts for insertion of shield tunneling
machines, underground railways, and tunnels

Pneumatic Caisson Method has the following advantages.

 The compressed air fed in the working chamber is controlled to be so equal to the pressure
of groundwater that the surrounding groundwater or soils may not be affected.

 Since the working chamber inside is kept dry, an elaborate excavation work is possible by
verifying directly the conditions of soil without disturbing the constitution of soil. This also
allows the pneumatic caisson to sink in all soil conditions such as viscous soil, sabulosity soil,
gems and stones mingling grit and bedrocks.

 Since the sinking pneumatic caisson structures themselves become the final underground
structures, no temporary earth retaining works are required. This also allows a good use of
underground space as much as possible.

 Excavation work in the working chamber and construction work on the ground can proceed
at once. This saves construction period.

 Our developed unmanned excavation system and helium mixed gases breathing system
make possible to construct safe and efficient underground structures and spaces in the
great-depth underground.
  Since the complete reinforced concrete caisson structures have a high rigidity, their side
structures and bottom structures jointly bear and resist against heavy loads thereon, the
caisson structures are highly earthquake-resistant.

Pneumatic caisson foundations as compared with steel pipe pile foundations

Pneumatic Caisson foundations as compared with Spread foundations