Chapter 5: Foundation Design

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Section 1: Foundation Type Selection

Foundation Selection Factors

The designer is responsible for selecting the appropriate bridge foundation. Consider the following factors in that selection:

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

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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.

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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. 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.

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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.

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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.

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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.

Design foundations of new bridges as either drilled shafts or piling. Study all the available soil data, and choose the type of foundation most suitable to the existing soil conditions and the particular structure.

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Foundation Guidelines for Widening Structures

Study test-boring data along with any available information regarding the existing foundation, including but not limited to drilled shaft or pile driving records. Usually, old test-boring data is adequate for widening the structure. In widening structures, consider special designs to prevent differential movement between the new and the old foundations. This is normally accomplished by founding the new foundations at approximately the same elevation as the existing foundations, if applicable. Do not use piling in widening structures founded on spread footings.

Widening Structures on Piling. Widen structures on piling with piling tipped in the same stratum, when possible. If loads for piling supporting the widened portion of the structure are the same or lower than loads for the original construction, tip the new piling at approximately the same elevation as the existing piling. If new loads are higher, longer or larger piling may be required. Avoid extreme variations between the new and existing tip elevations to minimize differential movement.

Widening Structures on Drilled Shafts. Widen structures on shafts with shafts at approximately the same tip elevations. Often existing structures with belled shafts may be widened with straight shafts tipped at the same elevation due to current higher allowable soil design loads and use of skin friction in drilled shaft design.

Widening Structures on Spread Footings. The most critical situation occurs when widening a structure founded on spread footings. If the existing footings are less than 6 ft. below natural ground and on rock, widen with spread footings at the same elevation. For abutment and interior bents on deep spread footings, widening with drilled shafts is usually more economical with the shafts founded near the existing footing elevation. This is not always practical, as in the case of widening a structure on spread footings with drilled shafts. In a case like this, evaluate the soil for shrink/swell potential.