Bridge Foundations

Bridge foundations are classified into two main categories based on their depth and load distribution mechanism: shallow foundations and deep foundations.

Shallow foundations are placed near the surface and are typically used when strong bearing strata are available at shallow depths.

1. Spread Footings – These are among the most common types of shallow bridge footings and foundations. They consist of a widened base (or footing) that spreads the load over a larger area of soil. Spread footings are economical and suitable for smaller spans or bridges built on firm ground.
2. Raft Foundations – Also known as mat foundations, raft foundations consist of a continuous slab that supports multiple columns and walls. This type of foundation is particularly useful in bridge construction where soil bearing capacity is low, as it distributes the load uniformly and reduces the chance of differential settlement.
3. Grillage Foundations – Use layers of steel or reinforced concrete beams laid in orthogonal directions, topped with a slab or deck. This system allows the distribution of heavy loads over a wide area and is particularly useful in cases where load concentrations need to be minimised.

When surface soils lack the strength to support a structure, engineers turn to deep foundations that transfer loads to stronger strata deep below the surface.

1. Driven Piles – Long, slender columns, called piles, are hammered into the ground using pile drivers. They are typically made of concrete, steel, or timber. They work by friction and/or end bearing and are ideal for soft soils or sites with fluctuating groundwater levels.
2. Drilled Shafts (Caissons) – Also called caissons or bored piles, they are constructed by drilling deep holes and filling them with reinforced concrete. Ideal for large bridges that require significant load-bearing capacity. A key advantage of drilled shafts is their ability to be tailored precisely to subsurface conditions, and they often serve as both the foundation and substructure component.
3. Caisson Foundations – Watertight structures that are sunk into the ground and filled with concrete. Caisson foundations are commonly used for large-span bridges and allow for deeper placement compared to other methods.
4. Well Foundations – deep, large-diameter cylindrical structures traditionally used for bridges over rivers. Constructed by sinking a well-shaped shaft into the ground, this method supports heavy loads and resists scour in river bed conditions.

Constructing bridge foundations is a complex process that typically involves the following steps:

1. Site Assessment – Soil testing and geotechnical analysis determine the appropriate foundation type.
2. Excavation – Depending on the depth and water table, excavation may require dewatering or cofferdams.
3. Foundation Installation – Foundations are installed using heavy machinery, pile drivers, drills, or sinking techniques.
4. Reinforcement and Concreting – Reinforcing steel is placed as per design, and concrete is poured to complete the foundation.
5. Inspection and Testing – Load testing, pile integrity tests, and soil settlement monitoring ensure that the foundation meets engineering standards.

Safety protocols are crucial during construction, especially in waterlogged conditions, to avoid accidents and ensure structural reliability.

Bridge foundation design and construction face numerous challenges:

• Unpredictable Subsurface Conditions – Soil variability can lead to unexpected settlement or load-bearing issues.
• Scour and Erosion – Water flow can remove supporting soil from around the foundation, compromising integrity.
• Seismic Activity – Earthquakes require special consideration in foundation design to prevent liquefaction and structural collapse.
• Environmental Regulations – Construction in water bodies often necessitates the use of cofferdams and must comply with environmental protection laws.
• Logistical Constraints – Remote or urban sites may limit equipment access or working hours, impacting project timelines.