cofferdams & caissons

CONSTRUCTION TECHNOLOGY V QSM602

DR.LILAWATI AB WAHAB 1

TEMPORARY WORKS: COFFERDAMS & CAISSONS

Cofferdams usually refers to structures in water that are constructed on site, often from

standard parts. Identical structures on land are not usually called cofferdams and the name

seems to be falling out of use.

Caissons usually refers to structures which are constructed offsite and then brought to site

in one piece or in a series of independent modules.

What is cofferdam?

i. Temporary structure designed to keep water and/or soil out of the excavation in which a

bridge pier or other structure is built.

ii. The word "cofferdam" comes from "coffer" meaning box, in other words a dam in the

shape of a box.

iii. The word "cofferdam" comes from "coffer" meaning box, in other words a dam in the

shape of a box.

Function:

When construction must take place below the water level, a cofferdam is built to give

workers a dry work environment.

A cofferdam usually consists of sheets of steel driven into the ground to create a walled

chamber.

The cofferdam is then pumped dry to expose the riverbed.

Soil can be excavated to bedrock, or piles can be driven to create the pier foundation.

The cofferdam is removed after the foundation and pier are constructed.

Figure 1: Cofferdam



Factors to be considered in selecting the type of cofferdam

i. Location on shore/off shore/on land/in water

ii. Depth and size of working area required inside the cofferdam.

iii. Accessibility of site.

iv. Nature of the permanent works to be built.

v. Amount of water and earth to be excluded.

vi. Soil conditions.

vii. Water conditions.

viii. Availability of materials and plants.

ix. Possible effect of cofferdams on adjacent structures.

x. Cost.

Cofferdams Construction Sequence

For a typical cofferdam, such as for a bridge pier, the construction procedure follow the listed

pattern :

1. Pre-dredge to remove soil or soft sediments and level the area of the cofferdam (Figure

a).

2. Drive temporary support piles (Figure b).

3. Temporarily erect bracing frame on the support piles (Figure b).



4. Set steel sheet piles, starting at all four corners and meeting at the center of each side

(Figure c).

5. Drive sheet piles to grade (Figure c).

6. Block between bracing frame and sheets, and provide ties for sheet piles at the top as

necessary (Figure c).

7. Excavate inside the grade or slightly below grade, while leaving the cofferdam full of

water (Figure a).



8. Check blocking between bracing and sheets (Figure a).

9. Dewater (Figure a).

10. Construct new structure (Figure a and b).



11. Flood cofferdam; pull sheets; remove bracing; backfill (Figure c)

Types of cofferdam

Cofferdam can be categorised as Gravity Cofferdam, Sheeted Cofferdam and Soil

Stabilisation.

However, the basic types of cofferdam includes:

i. Earth-Type Cofferdam

ii. Crib Cofferdam

iii. Braced Cofferdam

iv. Cellular Cofferdam

v. Double-Walled Cofferdam

i. Earth-Type Cofferdam

Simplest type of Cofferdam.

Formed by banks of soil or rock.

Side slope depends on size and shape of stone.

Consists of an earth bank with a clay core or vertical sheet piling enclosing the

excavation.

Suitable for large area.

Used for low-level waters with low velocity.

Easily scoured by water rising over the top, unless using the rock fill type.

The top of the dam should be at least 2 feet above the water surface and the top width

not less than 3 feet.

Problems occur in removing the earth & rock fill.



Figure 2: Earth-Type Cofferdam

ii. Crib Cofferdam

Also known as Gravity Dam.

Formed by framework of heavy timber or precast concrete units, which are laced

together in a criss-cross pattern to form pockets.

The crib is constructed on land and floated into place.

Units are secured by bolts and pockets are filled with rock or rubble for stability.

The crib side that faces the water can be made water tight by driving steel sheet piling

along it.

The crib can be removed and use repeatedly.

Used in rapid currents or on rocky river beds.

Figure 3: Crib Cofferdam



iii. Braced Cofferdam

Also known as Single-Skin Cofferdam.

Formed from a single wall of sheet piling.

The piles are driven into the ground to form a box around the excavation site.

The "box" is then braced on the inside.

Interior is dewatered.

Primarily used for bridge piers in shallow water (30 - 35 ft depth).

Figure 4: Braced Cofferdam





iv. Cellular Cofferdam

Two main types are circular and segmental.

The cofferdams are made off contiguous bored piles or diaphragm walling.

Can be used on a temporary or permanent basis, but should be designed to integrate

into permanent structures.

Forces are resisted by the mass of the cofferdam.

Are entirely self-supporting and do not require any other form of support (struts, braces

and tie-rods).

Figure 7: Cellular Cofferdam



v. Double-Walled Cofferdam

Consists of two-parallel rows of steel sheet piles driven into the ground.

Tied together with anchors, then filled with soil to give stability.

Three principle types:

i. Box: Consists of straight flush walls

ii. Semicircular cells connected by diaphragms

iii. Circular cells connected with tie-rods or diaphragms

Figure 8: Double-Walled Cofferdam

Figure 9: Double-Walled Cofferdam (box type)



Figure 10: Double-Walled Cofferdam (Semicircular cell)

Figure 11: Detail Double-Walled Cofferdam (Semicircular cell)

Design Consideration:

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 and passive earth pressure.



Safety Requirement:

In cofferdam construction, safety is a paramount concern, since workers will be exposed

to the hazard of flooding and collapse.

Safety requires: good design; proper construction; verification that the structure is being

constructed as planned; monitoring the behavior of the cofferdam and surrounding area;

provision of adequate access; light and ventilation; and attention to safe practices on the

part of all workers and supervisors.

Function:

In engineering, a caisson is a retaining, watertight structure used, for example, to work

on the foundations of a bridge pier, for the construction of a concrete dam, or for the

repair of ships.

A watertight, dry chamber in which people can work underwater.

A chamber, usually of steel but sometimes of wood or reinforced concrete, used in the

construction of foundations or piers in or near a body of water.

A caisson is a large cylinder or box chamber that is sunk into the riverbed.

The caisson will be brought down through soft mud until a suitable foundation material is

encountered.

While bedrock is preferred, a stable, hard mud is sometimes used when bedrock is too

deep.

The excavation and foundation work takes place within the submerged caisson.

Some caissons are removed after construction, while others are left in place, filled with

concrete, and used as part of a permanent foundation.

Type of caissons

The basic types of caisson includes:

i. Box Caisson

ii. Open Caisson

iii. Compressed-air Caisson (Pneumatic Caissons)

iv. Monolith Caisson

i. Box Caisson

Box caissons are pre-fab concrete boxes with sides and bottom that are set down on a

pre-prepared base.

Once in place they will be filled with concrete to become part of the permanent works, for

example the foundation for a bridge pier.



The box caisson is fabricated at a plant and then towed a float and submerged on a

preliminarily prepared seabed owing to being flooded with water.

After the box caisson having been installed in place, its double sides are to be filled with

concrete to provide adequate strength, and the intermediate space is to be filled with

sand to provide adequate stability.

Figure 12: Box caisson

a) Box caisson floated into place with ballast as required.

(b) Caisson filled with appropriate material water may be pumped out first.

Figure 13 : Typical Box caisson



Figure 14: Box caisson

ii. Open Caisson

Similar to box caissons except that they do not have a bottom face.

They are suitable for use in soft clays (e.g. in some river-beds) but not for where there

may be large obstructions in the ground.

The open caisson is a cylinder or box, open at the top and bottom, of size and shape to

suit the projected foundation and with a cutting edge around the bottom.

It is sunk by its own weight and by excavation, then filled with concrete.

Figure 15: Open caisson



Open caissons permit excavation or other work to be carried out inside the caisson.

The caisson will sink down into the soil as excavation proceeds.

Sections can be added on top to increase height.

Water can be pumped out to permit dry work.


Suitable for installation in soft subsoil where excavation can be carried out by

conventional grabs, enabling the caisson to sink under its own weight as the

excavation proceeds.

This type of caissons can be completely or partially pre formed; section can be added

or cast on as the structure sinks to the required depth.

When the desired depth has been reached a concrete plug in the form of slab is

placed in the bottom by tremie pipe to prevent further ingress of water.




Could also be installed in land if the subsoil conditions are suitable.

The shoe or cutting edge is formed so that it is wider than the wall above to create an

annular space some 75 to 100mm wide into which a bentonite slurry can be pumped to

act as lubricant and thus reduce the skin friction to a minimum.

The excavation operation is usually carried out simultaneously with the construction of

the caisson walls above ground level.




iii. Compressed-air Caisson (Pneumatic Caissons)

Similar to open caissons except that there is an airtight working chamber some 3000m

high at the cutting edge.

They are used where difficult subsoils exist and where hand excavation in dry working

conditions is necessary.

The working chamber must be pressurised sufficiently to control the inflow of water

and/or soil and the same time provide safe working conditions for the operatives

When the required depth has been reached the floor of the working chamber can be

sealed over with a 600mm thick layers of concrete until only a small space

remains,which is pressure-grouted to finally seal the working chamber.

The access shafts are finally sealed with concrete some three to four days after sealing

off the working chamber.

Figure 19: Compressed-air Caisson (Pneumatic Caissons)



Figure 20: Compressed-air Caisson

Figure 21 :Compress air cassion

iv. Monolith Caisson

Monoliths are as their name suggests larger than the other types but are similar to open

caissons.

They are often found in quay walls where resistance to impact from ships is required.

cofferdams & caissons

Documents

cofferdam figure

word cofferdam

gravity cofferdam

sheeted cofferdam

cellular cofferdam

typical cofferdam

flood cofferdam

crib cofferdam