The foundations of bridges work in a unique way from many other structures. Bridges typically require stable, yet flexible foundations that allow the structure to handle a wide variety of loads.
To ensure that the bridge is useful for its intended purpose, deep foundations are typically used. These deep foundations can come in the form of piles or strip footing, depending on the location of construction and the type of bridge.
Deep vs shallow foundations
Both shallow and deep foundations work in much the same way, with the difference being how deep the foundation itself will be placed into the ground. With a shallow foundation, the weight of the load is generally spread out to soils that are near the surface. This model works well for lighter structures (such as homes) that exert smaller loads on the foundation supporting beams.
Deep foundations extend further into the ground and spread the load capacity to deeper soil layers. This enables the foundation to handle larger loads above the ground. Most bridges use deep foundations due to their additional load-bearing capacity.
Piling as a method of bridge construction
In line with the need for constructing deep foundations for bridges, the piling method is the most frequently used. Piles of concrete, timber or steel are typically driven deep into the ground to provide support for overlying loads. The piles are precast according to required specifications, after which cranes are used to put the piles in place.
A large powered hammer is then used to drive the piles deep into the ground and to keep them secured in position. The further the piles are underpinned, the higher the pressure and friction that builds up around them. These forces play an important role in ensuring the integrity of the foundation.
Selecting the final design for the structure
Constructing a bridge typically starts with deciding upon a design outline. The process begins with a site investigation, where the feasibility of the bridge will be determined. The size, load, and location considerations will be made and put into the site investigation report for future review.
The type of foundation will then be decided (mostly piling is the popular option). With piling (an any other type of foundation), it is important to consider the efficiency of load transfer to the underlying layers. While most of the load should be upheld by the piles, it should not exceed the limit where the surrounding soil can shear. There should be a balance between individual piles and the ability of the surrounding ground to absorb the pressure that is distributed.
For more information, contact underpinning services.
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