Cofferdams vs. Caissons: Key Differences in Construction Use

In construction, selecting the appropriate method for creating a dry work environment near water is essential for project efficiency and cost-effectiveness. Cofferdams and caissons are two prevalent solutions, each with distinct characteristics suited for different applications and environments. This section highlights the key differences between cofferdams and caissons to guide builders in making informed decisions.

Types of Cofferdams

Cofferdams are temporary structures used to enclose an area, allowing water to be pumped out and creating a dry work environment. They come in various types, each designed for specific site conditions and project needs.

Earthen

Earthen cofferdams are made from locally available soil and clay, making them cost-effective due to reduced transportation expenses. Constructed by compacting layers of earth into a mound with a clay core for impermeability, they require attention to slope stability to prevent collapse. Best suited for shallow water, they need regular maintenance to address erosion or seepage. Their environmental impact is low, as materials can be repurposed or left to integrate back into the landscape.

Rock-fill

Rock-fill cofferdams use large stones or boulders to create a robust barrier against water, ideal for areas with strong currents. Constructed by layering rocks and filling voids with smaller aggregates, they offer durability for long-term projects. The feasibility and cost depend on the availability of suitable rock material. Precision and equipment are needed for placement, making them more expensive than earthen types, but their resilience often justifies the investment.

Single-walled

Single-walled cofferdams consist of a single row of sheet piles driven into the ground, forming a barrier against water. Suitable for projects needing quick installation, they involve driving steel or wooden sheets into the riverbed or seabed, interlocking them to form a continuous wall. Used in shallow waters or low water pressure, they are adaptable to varying site conditions. In deeper waters, additional bracing or anchoring may be required for stability, making them a cost-effective choice for temporary projects.

Double-walled

Double-walled cofferdams feature two parallel walls of sheet piles connected by tie rods, with the space between filled with granular material for support. Suitable for high water pressure scenarios, they offer enhanced strength and resistance. Construction involves aligning and installing sheet piles, followed by backfill placement. More labor-intensive and costly than single-walled types, they provide improved durability for challenging projects, often used in deeper waters.

Cellular

Cellular cofferdams involve interlocking steel sheet piles to form circular cells filled with earth or gravel for stability. Suitable for large-scale projects with significant water pressure, the design distributes forces evenly, reducing structural failure risk. Useful in deep water or wide areas, installation requires precise engineering and heavy machinery. More expensive due to complexity and material needs, they are preferred for critical infrastructure projects.

Types of Caissons

Caissons are permanent structures used to create a dry working environment below water level, often serving as foundations for bridges, piers, and marine structures. Different types are employed based on project requirements, each offering unique advantages and construction techniques.

Box

Box caissons are prefabricated, watertight structures open at the top and closed at the bottom. Constructed on land and floated to the location, they are sunk by adding ballast or pumping out water, then filled with concrete. Suitable for shallow to moderate depths, they are used in bridge piers and dock foundations. Prefabrication allows for quality control, but transportation and placement require careful planning.

Open

Open caissons are hollow structures with an open bottom, sunk by excavating material from within. Useful for deep foundations, they can be extended by adding sections. Construction involves lowering the caisson and removing soil, causing it to sink. Used in bridge piers and marine structures, they adapt to varying soil conditions. However, construction is labor-intensive and requires specialized equipment.

Pneumatic

Pneumatic caissons are similar to open caissons but have an airtight working chamber at the bottom, allowing work in a pressurized environment. Beneficial for deep foundations in waterlogged or unstable soil, the pressurized chamber prevents water and soil entry. Construction involves sinking the caisson and using compressed air to maintain a dry workspace. Used in large bridges and infrastructure projects, pneumatic caissons require careful management for safety and structural integrity.

Cofferdam Construction Techniques

Constructing a cofferdam involves managing water and supporting structural integrity. The process starts with a site assessment, evaluating water depth, soil conditions, and environmental factors. Engineers conduct surveys to understand topography and hydrology, informing design and material selection. This phase identifies challenges and tailors the construction plan.

Mobilizing equipment and materials follows, including machinery for driving piles or placing materials. The construction team coordinates logistics for timely delivery and assembly, minimizing disruptions. Environmental precautions protect local ecosystems, such as silt curtains and noise mitigation.

Installation is precise, requiring skilled labor and supervision. The team assembles the structure, ensuring components form a watertight barrier. Monitoring systems track structural stability and water levels, allowing real-time adjustments to prevent failures and ensure worker safety.

Caisson Construction Techniques

Caisson construction demands meticulous planning and execution. It begins with analyzing geological conditions to choose the caisson type and materials. Engineers assess soil composition and bearing capacity, dictating deployment and anchoring. This stage prevents complications during installation.

Construction starts with fabricating caisson segments, often from reinforced concrete or steel. Precision ensures sections fit seamlessly, maintaining structural integrity. As the caisson descends, ballast and water management systems counteract buoyancy and stabilize the structure, facilitating controlled sinking.

Applications in Construction Projects

Cofferdams and caissons are used in various construction projects, determined by site demands and infrastructure nature. They are employed in bridge construction, marine and port development, and large-scale building foundations. Their adaptability allows customization for complex environmental and structural needs.

In bridge construction, cofferdams create dry work environments for pier and abutment installation. Their temporary nature makes them efficient for rapid deployment and removal. For example, in river bridge construction, cofferdams isolate riverbed sections for secure concrete placement, minimizing ecosystem impact. Controlled environments reduce weather or water condition delays, enhancing project efficiency.

Caissons are chosen for projects needing permanent, deep foundations. In offshore oil platforms or large piers, caissons provide support against structural and water forces. Their robust design and seabed anchoring suit high-pressure environments. In urban settings, caissons support high-rise buildings against wind and seismic activity. Customizable in shape and size, they adapt to site-specific requirements, ensuring safety and longevity.