Nonstandard Bridge Geometry. Size and weight of proposed substructure and superstructure elements can vary greatly between projects and geotechnical engineer must work with planning and bridge engineer to determine the locations of foundations and conditions that would trigger additional design checks.

Design load. The magnitude and type of design loading dictates the required size of the foundations from a structural standpoint. The foundation engineer must work in collaboration with the structural engineer to adjust sizes, quantities, and material types of foundation elements to meet or exceed resistances needed for the loading. Design loads typically will be provided and should be calculated for the strength, service, and extreme event limit states. This is consistent with structural considerations.

Subsurface stratigraphy. The depth and strength of subsurface stratigraphy determine the type of foundation chosen. In general, drilled shafts are well suited to areas with competent soil and rock. While drilled shafts have been successfully installed in soft soil, they may be less efficient than piling. Very hard material at or near the surface makes driven pile installation impractical.

Corrosive conditions. Salts, chlorides, and sulfates are detrimental to foundations. Where these conditions exist, take preventive measures. Use sulfate-resistant concrete as defined in Standard Specification Item 421 for construction in seawater or soils with high sulfate content. Consult the list of recommended corrosion protection areas for specific areas of Texas that may have structures with possible corrosion due to sulfate soil or salt water. Do not use steel piling in corrosive environments without an appropriate protective coating and/or providing additional steel section to ensure proper performance of the foundation elements over the design service life.

Economic considerations. Consider economics in the final selection. Compare the foundation types. The cost of a drilled shaft foundation, for instance, may be less than piling. It may be feasible to use fewer piles at higher design loads, or fewer drilled shafts with larger diameters to maximize economy. If no clear economic difference exists between piling and drilled shafts, consider including both and offer the contractor alternate designs in the contract plans.

Superstructure type. The type of superstructure chosen for the bridges may dictate or eliminate certain foundation types. For instance, short-span structures over streams may work well with trestle piling, but tall, single column flyovers justify footings with multiple shafts or piling.

Special design requirements. Special designs are sometimes necessary to straddle another structure or utilities and may require a different type of foundation than the rest of the structure.