Design load. The magnitude of the design load dictates the required size of the foundation from a structural standpoint.

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. In general, use piling where softer soil is present. Very hard material at or near the surface makes driven pile installation difficult.

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 for specific areas of Texas that may have structures with possible corrosion due to sulfate soil or salt water. The use of steel piling in corrosive environments is not recommended. If steel piling must be used, an appropriate protective coating must be selected, additional steel section provided or a combination of these methods utilized to ensure proper performance of the foundation elements.

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, you may choose to include 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.