Sheet Piles Piling

One of the popular use of sheet piles is for the construction of cofferdams. Before that,

What are cofferdams?

From the definition below, you can also note the usefulness of cofferdams on many types of situation such as construction, ship building and even archaeology works.

However regardless of the use, the main function of a cofferdam is to provide a dry area to work and to prevent the entry of water into the working area when work is in progress. Cofferdams can be constructed in many ways but in modern civil engineering works, they are now mostly constructed using sheet piles.

The installation of sheet piles is highly configurable that it can be install on and into water. Off-course there will be a slight differences between land installation and on water.

According to Wikipedia Cofferdams are :-

A cofferdam is an enclosure within a water environment constructed to allow water to be displaced by air for the purpose of creating a dry work environment. Commonly used for oil rig construction and repair, bridge and dam work, the cofferdam is usually a welded steel structure that is temporary, typically dismantled after work is completed. Its components consist of sheet piles, wales, and cross braces.

The cofferdam is also used on occasion in the shipbuilding and ship repair industry, when it is not practical to put a ship in drydock for repair or alteration. An example of such an application is certain ship lengthening operations. In some cases a ship is actually cut in two while still in the water, and a new section of ship is floated in to lengthen the ship. Torch cutting of the hull is done inside a cofferdam attached directly to the hull of the ship, and is then detached before the hull sections are floated apart. The cofferdam is later replaced while the hull sections are welded together again. As expensive as this may be to accomplish, use of a drydock may be even more expensive. See also caisson.

Cofferdams have also been used to recover ships sunk in shallow waters, such as the USS Maine, or Robert de LaSalle’s La Belle.

Why is steel sheet piles widely used for cofferdams?

This is because of sheet piles structural strength, the water tightness due to its ability to be interlocked and also its ability to be driven to deep penetration in most types of ground. The ability to be driven deep into the ground is important as this gives resistance to inward movement “caving in” at the bottom of the excavation and sufficient cut-off to prevent infusions of water into the excavated/working area.

Common sheet piles used construction of cofferdams are Larssen and Frodingham. The Larssen has a U type section are very popular sheet piles used in cofferdams. This is because of their good design which includes interlocking capability and that they can be driven deep with minimal problem.

When used as in a cofferdams, the sheet piles have to resist both the soil and water pressure which acts on it from outside. This pressure increases with excavation depths. Thus to resist these pressures, the internal of the cofferdams are braced. Proper bracing is important to maintain the integrity and safety of the internal working area.

The construction of cofferdams begin by marking out the location and the shape of the cofferdam. This job is done by the surveyor. Thereby the work of driving sheet piles can start. Methods of driving sheet piles are mentioned in this post “the-installation-of-sheet-piles” During driving and pitching of the sheet piles, ensuring verticallity is very important. Guides are used to ensure proper driving. On land guides can be 2 pairs of guide wailing. One pair at ground level while the other at a higher level. This guides can made into a trestle so that it can be easily shifted as sheet piling work progresses.

However for driving over water, the wailing guides are supported by timber or steel piled stagings constructed into the water. Thus comparing to working in ground, more preparation and setting up is involved when working over water. Sheet piles are driven from a floating barge which has to be anchored to prevent it from drifting due to water current.

Single Wall Cofferdam

Sheet piles are driven either one by one successively which is also known as single driving or can be in echelon or in panel methods. Cofferdams can be either single-wall or double-wall design. A single wall cofferdam consist of one layer of sheet piles only as the picture above shows.

Double Wall Sheet Pile Cofferdam

In double-wall, there are 2 layers of sheet piles. The space between the sheet piles are filled with soil or granular materials. A double wall cofferdam can resist greater pressure than a single wall cofferdams.

Some times a very large cofferdam can be constructed from smaller circular cofferdams known as cells. These cells are constructed very close to one another.  One can find such huge cofferdams in used in large project like dams construction where rivers need to be diverted. Attached picture show cellular cofferdams under construction.

Large Circular/Cellular Cofferdams

Below more pictures of circular sheet piling cells formed from interlocking sheet piles and Larssen sheet piles are the popular ones used.

Cellular Cofferdams