Building bridges over deep water involves complex engineering techniques primarily focused on establishing stable foundations in the waterbed. A key method, as described in construction practices for such environments, is the use of specialized structures to create dry or stable working areas.
Foundation Techniques for Deep Water Bridges
For most bridges constructed over deep water, the initial challenge is reaching the underlying riverbed or lakebed to build robust supports for the bridge deck. This is achieved through proven methods:
- Cofferdams: These are temporary watertight enclosures built within a body of water. By pumping out the water from inside, a dry work area is created on the riverbed or lakebed, allowing construction crews to build the bridge's foundations or piers directly in a dry environment.
- Caissons: Similar to cofferdams, caissons are prefabricated structures, often box-shaped, that are sunk into the water. Different types of caissons exist, some allowing work from the surface while the structure sinks, and others creating a sealed chamber where workers can excavate the seabed below water level under compressed air (open caissons or pneumatic caissons). Like cofferdams, they provide a stable platform or enclosure for constructing the bridge's concrete towers or supports.
As stated in the reference: "For most bridges built over deep water, construction crews must build cofferdams or lower caissons into the water to create a dam and platform for the concrete towers to stand on."
Ensuring Stability
Building foundations in deep water presents challenges beyond just reaching the bottom. The nature of the waterbed itself is crucial.
- Unstable Beds: Lakebeds and riverbeds can often be soft, muddy, or otherwise unstable. A simple foundation placed on top might settle or shift over time.
- Driving Piles: To counteract instability, crews often need to drive piles deep into the earth beneath the waterbed. These are long columns made of materials like steel, concrete, or timber, which are hammered or vibrated down until they reach a stable soil layer or bedrock. The reference highlights this need, noting, "Lakes and riverbeds can be unstable, so crews may need to drive piles deep into the earth to achieve stability." Piles transfer the weight of the bridge and its piers down to stronger sub-surface layers, ensuring the entire structure remains stable.
The Process Overview
Constructing deep-water foundations typically involves these steps:
- Site Preparation: Analyzing the water depth, currents, and seabed composition.
- Installing Cofferdams or Caissons: Placing and securing these structures to create the protected work area.
- Dewatering (for Cofferdams): Pumping water out to create a dry space.
- Piling (if needed): Driving piles deep into the seabed inside the protected area to ensure foundation stability.
- Foundation Construction: Building the concrete base (footing) and the main support piers (towers) within the cofferdam or caisson.
- Removing Temporary Structures: Once the foundation is complete and cured, the cofferdams or caissons are typically removed (though some parts of caissons may become permanent components).
This meticulous process ensures that even over vast depths and potentially soft ground, bridges have the solid, reliable base required to stand for decades, supporting significant loads and resisting environmental forces like water currents and seismic activity.
