A cofferdam is a temporary watertight structure that lets crews work in the dry inside a river, along a coastline, or beside an existing bridge pier. Most engineers meet their first one on a bridge project, but the same idea turns up in dam repairs, outfall construction, pipeline crossings and foundation pits that sit close to open water. After a decade of supplying sheet pile walls for water-related works, we have watched how often the choice of cofferdam type decides the schedule, the budget and the safety of everyone on site. This guide walks through the main types of cofferdams, what each one does well, and how to narrow the list down to the right option.

What a Cofferdam Is Really For

A cofferdam is not a dam, and it is rarely a permanent wall. It is a temporary enclosure built to hold back water, and often soil as well, so that the area inside it stays dry enough to work in. Once the permanent structure is finished, whether that is a pier, a culvert, an intake or a foundation slab, the cofferdam is normally removed.

That temporary status shapes every decision. Because the structure comes out again, engineers accept risks they would never tolerate in a permanent seawall. In practice the choice of type usually comes down to four questions: how deep the water is, what the ground is like, how much dry space the work needs, and how long the enclosure has to stay in place.

The Main Types of Cofferdams

Cofferdams generally fall into two families: those that hold water back with mass, such as earth, rock and timber structures, and those that hold it back with a driven wall, such as sheet pile systems. Both families have been in use for more than a century, and both still appear on modern sites.

Earthen Embankment Cofferdams

This is the simplest option, and often the cheapest where there is room to spare. Soil is pushed into a bank around the work area, sometimes with a clay core to cut seepage, and the water is held back by the sheer width and weight of the embankment. Earthen cofferdams suit shallow water, gentle ground and sites where fill material is close at hand. They struggle in fast currents, in deep water, and wherever the footprint has to stay small. If vehicles need to travel along the crest, that crest has to be wide, and the volume of fill grows quickly.

Rock-Fill Cofferdams

Where fill is scarce but rock is available, a rock-fill embankment does the same job with a steeper and more stable profile. A rubble core is usually capped with an impermeable layer to reduce seepage. Rock-filled cofferdams cope better than earth banks with rougher water and higher flow velocities, and they can be built up in stages as the water rises. The trade-offs are haulage cost, weight, and a much harder job when removal time comes.

Timber Crib Cofferdams

Timber cribs are an older solution that still appears in repair and heritage work. A box framework of timbers is floated into position, filled with rock or earth to sink it, then stacked until the required height is reached and sealed. They work well in narrow work areas and shallow water where timber is locally available. Few contractors reach for them today, but understanding them helps when reading older project records or upgrading structures that were built with them.

Single-Wall Sheet Pile Cofferdams

This is the type most people picture: a continuous line of interlocking sheet piles driven into the ground, with the enclosed area then pumped dry. In shallow water or on land the wall can be cantilevered, supported only by the soil below the excavation. In deeper water, or where the excavation is wide, wales and struts are added across the opening, producing the classic braced cofferdam used for bridge piers, pump stations and deep foundation pits. Because the whole system depends on the piles locking together, the profile and the clutch matter as much as the driving method, which is why piles for foundation pit support are usually specified with a continuous interlock rather than a simple lapped joint.

Double-Wall Sheet Pile Cofferdams

Two parallel sheet pile walls, tied together and filled with soil between them, produce a much stiffer structure than a single wall. Double-wall cofferdams handle deeper water and longer spans without internal bracing, which keeps the excavation clear for plant and machinery. The price is material volume, driving time, and tie-rod design that has to be right the first time.

Cellular Cofferdams

Cellular cofferdams are made from interlocking sheet piles arranged into closed cells, which are then filled with sand, gravel or soil. The fill provides the weight and stiffness, so the cells resist water pressure and lateral loads without any internal bracing. Two layouts are common: circular cells, where each ring is complete and tied to the next, and diaphragm cells, where arcs are joined by straight connecting walls. Cellular types are chosen for deep water on large rivers and are used for bridge piers, locks and dam works. They are among the most robust temporary structures available, and also among the slowest and most expensive to build.

Comparing the Main Types at a Glance

The table below is a starting point rather than a design tool. Real selection always depends on site-specific hydraulic and geotechnical data, and the figures move once a project's own loading conditions are applied.

How to Choose the Right Type

Working from the outside in, six points tend to settle the argument faster than any technical debate:

1. Water depth and head. Shallow, still water opens up mass structures; deeper or faster water pushes you towards a driven wall.
2. Ground conditions. Soft silt, dense gravel and rock all change what can be driven and how far a wall must penetrate.
3. Size of the dry area. A wide excavation needs bracing or a stiffer wall system, and bracing eats into working space.
4. Duration. A short enclosure can be built lightly; one that stays through a wet season needs more robustness built in from the start.
5. Removal. Some sites must be returned to their original condition, which makes extraction a design requirement, not an afterthought.
6. Access and logistics. Fill, rock and heavy steel all have to reach the site. On constrained sites, lightweight materials often tip the balance.

It also pays to study the fundamentals before specifying anything, because small detailing mistakes in a temporary wall are expensive to correct once the pumps are running. Our own team keeps a working reference on cofferdam design principles that covers exactly these checks.

Where Modern Sheet Pile Materials Fit

For decades the driven wall meant steel. Steel is strong and well understood, but it is heavy to transport, it needs coating or cathodic protection, and it corrodes in salt water unless it is carefully maintained. Those drawbacks are why polymer alternatives have taken a growing share of shallow and moderate-depth work.

Vinyl sheet piles, also called PVC sheet piles, are extruded profiles with an interlocking edge that forms a continuous wall once driven. Because the polymer does not rust, the wall keeps working in brackish and saline conditions without a coating programme, and there is no repainting, no sacrificial anode and no ongoing maintenance budget to defend. The panels are light, so they can be handled by smaller plant, transported in greater numbers per load and installed with lighter equipment than a comparable steel section. For projects where carbon and site disruption are on the agenda, the lower embodied impact compared with steel is a genuine advantage.

We produce these panels at our own facility in Haiyan, Zhejiang, on more than twenty piling lines, with an in-house testing laboratory and a warehouse on site. That setup lets us supply standard profiles and customised sections for the kind of work our customers bring us most often: small watershed management, anti-seepage and anti-erosion ecological protection, flood control, channel lining and coastal or harbour retaining walls. If you are at the stage of comparing cofferdam types for a specific site, our team is happy to talk through profiles, lengths and driving conditions before you commit.

The best cofferdam is rarely the most impressive one. It is the one that matches the water, the ground, the schedule and the crew you actually have. Work through the types above, check them against your site data, and the shortlist usually writes itself.