The foundation is the base of a bridge and a significant structural piece that plays a crucial role in maintaining the structural stability of the entire bridge. This article discusses the major types of bridge foundations used in the construction of bridges.
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Importance of Bridge Foundations
Bridge foundations are the most critical design consideration for all bridge projects as they maintain the entire bridge load and the load of the traffic that will pass over the bridge. A bridge foundation is built under the pier/abutment and over the fundamental soil/rock.
All loads experienced by the bridge, including the loads from the abutments, piers, and wings, are distributed into the supporting strata/transmitted to an extensive bearing region by the bridge foundations.
Additionally, bridge foundations ensure the solidness of the bridge against shifting and settlement. Thus, proper selection of the bridge foundation type is necessary to maintain the structural integrity and stability of the entire bridge structure.
The selection of the proper bridge foundation type depends on the bridge superstructure and site conditions. Site conditions refer to the geologic and soil features at the location of bridge construction.
The bridge foundation must be sufficiently deep to prevent the effect of scour caused by the flow in the river. Additionally, the loads transmitted by the foundation to the subordinate soil must not lead to soil shear failure or harm the settlement of the bridge superstructure.
Different Types of Bridge Foundations
Shallow and deep foundations are the two major types of bridge foundations based on the depth of the foundations extending into the ground. Soft soils require deep foundations, while high bedrock or solid, well-compacted soils need shallow foundations. However, large superstructure loads due to long-span bridges require deep foundations irrespective of site conditions.
Shallow foundations distribute all loads from the bridge across a large area and do not extend much deeper into the ground level. These foundations rely on soils with large bearing capacities to support the loads from the bridge.
Deep foundations extend much deeper into the ground surface compared to shallow foundations. Several types of deep foundations support the bridge loads using friction along their depth, while the rest of the deep foundations extend up to bedrock/hard soil with sufficient bridge load-bearing capacity.
Shallow Bridge Foundations
Spread footings are the most extensively used shallow foundation in bridge construction. This foundation is constructed exclusively using concrete and consists of two components, including the footing and the stem.
The bridge directly rests on the stem, a vertical and solid section built using concrete. Stem transfers load from the bridge to the footings, which are located several feet below the ground surface. Footings are primarily the large base of bridge foundations that distribute the load evenly over a large area.
Open-pit and shored excavations are allowed for spread footing foundation construction. Open-pit excavations are performed when the site is not near/in water, the soil is adequately stable to ensure worker safety, the site can be maintained in dry conditions, and the stability of the nearby built structures can be effectively maintained.
Shored excavations are performed when the excavation is more than four feet deep, the soil is not sufficiently stable to ensure worker safety, and the excavation area is limited due to the presence of adjacent structures.
Spread footing can be constructed easily, quickly, and cost-effectively in different shapes, such as rounds, squares, and rectangles, depending on the application. However, spread footing is only suitable for specific soil structures. Additionally, settlement is a major issue with the spread footing foundation.
Raft foundation is suitable for bridge construction where the watercourse bed consists of silt and soft clay, and the hard soil is unavailable within reasonable depth below the river bed.
A raft foundation is commonly built where the soil has a low allowable bearing capacity or the bridge loads are heavy. The foundation is also suitable when the soil bed has some compressible soft pockets or when there is a risk of unequal settlement.
Driven pile foundations are an extensively used type of deep foundation that uses side friction to support the loads from the bridge. This foundation consists of long shafts of material/piles that are driven deep into the ground.
Piles are composed are steel, concrete, or timber and can extend up to several hundred feet deeper into the ground based on the bridge loads and soil conditions. Large machinery is utilized to drive the piles deep into the ground by hammering them down repeatedly until they reach the targeted depth.
This foundation is cost-effective and versatile as it can be used in different ground conditions. However, noise and ground vibrations are created during the installation of piles. Moreover, short piles can have low lateral resistance/capacity.
Drilled shafts/caissons are another common type of deep foundation that involve drilling a large hole deep into the ground and filling the hole with concrete and reinforcing. Caissons extend up to bedrock/strong soil below the ground that possesses sufficient bearing capacity to support the bridge loads.
This type of foundation can distribute loads across a large area as it is larger in diameter compared to driven piles. The large diameter of drilled shaft foundations compensates the absence of friction resistance that is present in driven pile foundations.
Drilled shaft foundations have very high lateral and axial resistances, low ground vibrations and can be constructed as deep as required for scour resistance/capacities. However, this foundation is expensive, lacks redundancy, and is sensitive to construction procedures.
More from AZoBuild: The Main Bridge Types and Their Structural Design
References and Further Reading
Engineering for Structural Stability in Bridge Construction [Online] Available at https://www.fhwa.dot.gov/bridge/pubs/nhi15044.pdf (Accessed on 02 June 2023)
Bridge Foundations [Online] Available at https://www.oregon.gov/ODOT/Construction/Doc_BridgeInspectorManual/06_bridge.pdf (Accessed on 02 June 2023)
Felty, D., Haight, R. Introduction to Bridge Foundations [Online] https://www.bridgeofflowersmass.org/tufts-bridge-foundations (Accessed on 02 June 2023)
Hall, M., Thompson, P. (2018). Bridge Foundations [Online] Available at https://www.virginiadot.org/business/resources/bridge/Foundation/9-BCIS_Foundations_pg53_add_12318.pdf (Accessed on 02 June 2023)