The building of bridges over large expanses of water, such as lakes and oceans is an engineering marvel which requires innovative building techniques, precise engineering, and extensive coordination.
While every condition is different, depending on the type of bridge, the body of water over which it is being built, and the soil conditions on which the bridge piers rest, here is the general approach to building bridges underwater:
- A foundation is drilled.
- The surrounding water is removed.
- Water is removed with a pre-fabricated frame.
- A battered pile foundation is laid.
- A bridge is constructed offsite and installed onsite.
Let’s take a closer look now at the sequence of bridge construction over bodies of water. You’ll find out how each method or technique works, and how it’s possible to build underwater.
1. A Foundation Is Drilled
Large and long piles are drilled into the floor of a water body. A massive drill makes a hole deep enough to form a sturdy base. A foundation needs good bearing soil or a hard solid surface, like the earth’s rocky crust.
Hence, a drill is used to excavate underwater until it reaches good-bearing soil or a rocky base. The type, design, and expected total load of the bridge decide the depth of the hole inside the sturdy base and the dimensions of the pile.
By load, we mean the following:
The excavation and drilling are followed by pile installation. Modern piles are usually made of reinforced concrete. A particular bridge may warrant other materials depending on the variables at play.
Drilled foundation construction has three broad phases:
The essential equipment and materials are drill rig, crane, reinforcing steel cage, concrete, and tremie. Spoil handling tools such as loaders and skip pan are necessary, too.
Some types of construction require casing and slurry handling, particularly when excavated and dug holes are at risk of sloughing:
The process is repeated for every pile or pier.
Meanwhile, a part of the crew or another team may start working on the rest of the foundation, such as pile caps. Then, the crew will build the substructure atop the already constructed piles and caps, collectively known as bents.
The substructure generally includes abutments, wing walls, piers, and caps.
Finally, the crew constructs the superstructure comprising beams or girders, bearings, and decks. The bridge design dictates other critical elements of the superstructure, such as trusses, arches, suspensions, cables, and barriers.
Many substructures and superstructure components are designed differently to suit predefined objectives. For instance, there are around half a dozen types of bearing:
Types of Drilled Pile Foundation
The diameter, height, composition, and type of pile used in drilled foundations depend on many elements. The immediate environment weighs heavily on the specifications, bridge type, design, and total expected load.
The type of drilled pile foundation depends on the following factors:
Straight Shaft End Bearing Pile
The straight shaft end-bearing pile foundation is suitable for rock, hardpan, and good bearing or firm soil. The end bearing pile is firmly installed into the hard rock or soil. It is the simplest form of drilled pile foundation deemed stable and safe, and the construction cost is relatively less.
Straight Shaft With Sidewall Shear Pile
A drilled pile needs sidewall shear support when a bottom or end support is unavailable or unreliable. The base of the pile still needs a rock or good-bearing soil, but a hard and reliable bottom may not be desirably accessible.
Hence, the sidewall is used to provide shear support to the pile base.
Straight Shaft With Sidewall & End Bearing Pile
This foundation type uses both sidewall shear support and end-bearing. Also known as a socketed pier, the straight shaft with sidewall and the end-bearing pile is costlier to construct. However, there is more support at the pile’s bottom and around the base. This means that it may have a greater load-bearing capacity.
Belled or Under-Reamed Pile
A belled pile is not a typical cylindrical or large cuboidal pillar, but instead, the base is bell-shaped. Hence, the bottom or end-bearing is broader and endures more pressure. This drilled pile foundation type is more commonly chosen for good-bearing soil, especially when a rocky bed is unavailable.
2. The Surrounding Water is Removed
A cofferdam is an enclosure used to dewater an area or zone. The objective is to build a waterproof barrier for the demarcated area and pump out any remaining water to create a dry working zone.
Large barriers require bracing or support, or else the walls may collapse due to the surrounding water pressure. Engineers use different materials and construction techniques to build cofferdams.
A cofferdam is like an embankment. Visualize a river, and imagine pouring enough earth, rock, or both at two places spanning the full width of the river, thus blocking the entire water flow. You will have a patch of dry riverbed between the two embankments.
The downstream river flow should be diverted to avoid flooding. Engineers and planners use dams or reservoirs, tunnels, and other means for this purpose.
However, it is not necessary to build embankments covering the entire span of the river for a bridge foundation. Limited enclosures suffice in most cases.
Types of Cofferdam
Earth or soil is not a perfectly reliable embankment material, as rocks can be unsuitable in many settings. Besides, it is easier to use steel sheets or other materials such as wood and concrete.
Boxed and Braced Cofferdam
Sheets are erected on the water body floor to form an enclosure, such as a box. A crew may use more sheets to make a larger enclosure. Large sheets must have braces at the base to provide additional support, so the walls don’t collapse.
Earth, soil, sand, or rocks can provide a drier base on the water body floor. A pump removes all water from inside the enclosure. If some water seeps through, the pump on standby should swing into action as and when necessary.
Such cofferdams are temporary structures, not a part of the bridge foundation.
Single and Double-Walled Cofferdam
A walled cofferdam is larger than a box or braced version.
Engineers can work on any shape and size of a single or double-walled cofferdam. A large cuboidal structure can be erected in a lake, river, sea, or ocean to provide the necessary work area per the project’s demands.
Double-walled cofferdams have two walls with some filling inside, which contains soil or sand, but not concrete. Double walling is preferred in large-scale projects to make the cofferdam sturdier. The two walls with filling inside make the cofferdam more impervious.
Multiple sheets are erected alongside one another to form a series of cells, which are interlocked to form a watertight enclosure. The sheets may form a massive circular cofferdam, usually required for enormously large projects.
Cellular cofferdams may have single or double walls with fillers and braces for greater stability. Some cofferdams are integrated into the overall design to be left behind as a part of the finished project.
Gigantic cofferdams provide access to large machines like cranes and drill rigs.
A modular cofferdam can have versatile features. All cofferdams don’t have a watertight base.
Hence, instead of setting up the sheets or piles directly on the ground, a frame is used to host the entire cofferdam, and it can have an impervious base, such as vinyl or other fabric, preventing water seepage.
3. Water Is Removed With a Pre-Fabricated Frame
The caisson technique is similar to the cofferdam method with a few differences. A caisson is not built on-site, but rather, it is prefabricated and then installed at a chosen place. Also, caissons are permanent structures that are designed to be an integral part of the bridge foundation.
Manufactured hollow cylindrical or cuboidal frames are lowered into the water body, down to the bottom. The water is pumped out, then a crew starts working inside the dry environment of the caisson. The bed of the water body is excavated, prepared, and cleaned per the design blueprint.
Once the crew reaches good bearing soil or the bedrock, the caisson is ready for further foundation construction. The rest of the process is similar to pile foundation. A design may use steel reinforced bars, rods, or meshes to form the cage or casing.
Reinforced concrete fills the caisson to form the pile and, in many cases, the pier. The entire caisson becomes the pile foundation for the bridge. The process is repeated for every pile, pier, or pylon.
Like a drilled pile foundation, a caisson may require drilling into the good-bearing soil or rock. Caissons may eventually have similar forms or shapes like drilled piles.
Types of Caisson
Like cofferdams, caissons may be boxed and braced. Similar to drilled pile foundation, caissons may have:
Almost identical to drilled pile foundation, the only exception is the use of a prefabricated caisson that eventually becomes a part of the bridge. Drilled caissons require excavation, digging a hole into the bedrock or incompressible soil, and all other steps or methods involved in drilling for a pile foundation.
A pneumatic caisson uses air pressure inside the hollow inserted structure to protect its integrity and maintain the dry working area’s condition. Also, a modern pneumatic caisson uses the air pressure to pump water and all excavated spoil materials up through a tube and out of the structure.
4. A Battered Pile Foundation Is Laid
A battered pile foundation involves hammering down piers or pylons into the bedrock or firm soil. A pile driver uses brute force or vibration to ground the piles, forming an angle as a result.
The rest of the foundation, substructure, and superstructure may emulate the other methods. The inward or outward angle of the piles and their limited grafting into the earth below render this foundation type a bit unsafe. Most battered piles are micropiles, not ideal for deep foundations.
Also, there is some concern about the response of battered micropiles to seismic events.
5. A Bridge Is Constructed Offsite and Installed Onsite
Like the caisson technique, this method involves bringing constructed and assembled parts of a bridge foundation to the site, followed by onsite installation. The process includes floating and lowering an assembled pile or pier into the water, then installing it using a pre-existing foundation.
This approach requires a base, which could be a drilled pile foundation.
The offsite construction, onsite installation method does not necessarily require a caisson, cofferdam, or other dry working environments. The crew may work from a floating barge near or at the site.
General Challenges of Underwater Construction
There is no standard or universal formula to design and build an underwater foundation of a bridge. Here is what dictates the rules:
Foundations in the sea or ocean must account for salinity. Besides, seas and oceans demand foundations at much greater depths than riverbeds or lake beds.
Shallower bridge construction is easier as dewatering is simpler.
All major underwater construction requires dewatering irrespective of the bridge type, its foundation, and other pressing factors in a location. The scope of underwater construction without dewatering is stringently limited to cutting, welding, and building relatively small structures.
Scuba gears, remote submersible vehicles, and other special equipment are mostly used for repairs and maintenance. Large scale construction such as the foundation and substructure of a bridge spanning the entire width of a river or covering several miles over water requires a dry working environment.
All methods of constructing a bridge underwater are principally based on dewatering or circumventing the water to create a dry working environment. Thus, the different techniques are about how the dewatering is done, materials and equipment used, and safety, viability, efficiency, and cost.
In theory, the standard methods of building bridges underwater are wondrous engineering feats. In practice, the challenges are exponentially complex, at times insurmountable, and there is no room for error. There are many reasons why one cannot build bridges anywhere and everywhere.