Bridges come in different shapes, sizes, and locations and so do their foundation needs change. Nevertheless, some basic principles remain the same, while others will apply if a bridge pier is within a water body or not.
The bridge pier superstructure transfers axial, lateral, overturning, and possibly torsional loads on the bridge foundations. Loads can be static (i.e., the dead load of the structure), or dynamic wind, earthquake, or impact. In most cases the pile caps are constructed from reinforced concrete, but some mostly temporary bridges might employ metal. When a shallow foundation system is not possible, a deep foundation supported pile cap is typically provided.
Typical Loading Sources & Conditions
- Selfweight of bridge, weight of wearing surface (pavement).
- Vehicle loads, impact loads, ship impact loads.
- Wind on structure, wind on vehicles.
- Water loading, water flow ice loads, debris flow
- Earthquake loads, lateral spreading, liquefaction.
- Scour (soil erosion within a river or water body due to restricted flow).
Geotechnical Pile Design Basics
First, deep foundations will need to be designed for both axial and lateral loading. The geotechnical axial capacity is usually estimated from empirical or semiempirical approaches for a single pile. SPT or CPT methods can also be used but at the end of the day a load test will likely always be required.
Depending on the pile spacing, interaction factors have to be considered for both the axial and lateral loading, that is if one uses the traditional spring analysis methods for pile groups. Depending on the lateral loading direction, different interaction factors might have to be assumed. Performing a 3D finite analysis is also possible but there is still some question about how to properly capture pile installation effects which play a significant role. Lateral pile load tests are also advised on sacrificial piles.
Downdrag
When the soil around the pile settles more than the pile it can exert significant side shear loads. As a result, the maximum axial load on the pile will be below the pile top.
Volume Changing Soils (Expansive, Ice)
Similarly, to downdrag, expansive soils can induce axial uplift loads when the soils gain moisture. When the soils shrink, they might detach from the pile and increase the unbraced lateral loading length.
Structural Pile Design & Combined Loading
The effects of combined axial and lateral loading will need to be investigated. In reinforced concrete piles, tension conditions will produce smaller bending capacities whereas up to some point increasing axial load can be beneficial. In steel piles, buckling will need to be considered especially if piles have exposed lengths. Local building concrete, steel, and foundation codes usually dictate the applicable method for determining both the axial and bending capacities.
Structural Pile Cap Design
Under all loading conditions bridge pile caps typically experience bending and shear loads. As such, the bending and shear capacity of the pile cap will need to be determined and be greater than the factored loading demands. One would also need to check for one and two-way shear, punching shear checks. Minimum reinforcement will need to be provided both at the top and bottom sides. The reinforcement will also need to be arranged so that conflicts with the deep foundation piles are avoided.
Lateral Pile & Lateral Cyclic Loading
Bridge foundations will undergo cyclic loading and as a result the lateral resistance typically decreases with time. For this reason, when one-directional lateral load tests are performed they need to be compared against the predicted single pile lateral response.
Very often, engineers tend to believe that reducing the lateral soil strength or stiffness in their models is more conservative. While this is potentially true when we have only one soil layer, such an assumption might be significantly wrong when there are significant stiffness contrasts between layers. Unfortunately, some of these recommendations have seeped into the most recent Geotechnical Circular for drilled foundations and can result in potentially unsafe designs.
Other considerations
These are not the only considerations for a bridge deep foundation and pile cap design. Corrosion and deterioration might have to be considered.
Bringing it all together
If anything, deep foundation design for bridges requires extra attention and can be very time consuming and warrants attention to detail. Design errors beyond being costly to resolve can also lead to significant losses in life. For this reason, we created DeepFND to provide as much of a complete design package for deep foundations as possible.
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