Study test-boring data along with any available information regarding the existing foundation, including but not limited to drilled shaft or pile driving records. Though often collected with historic TCP drilling methods, 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. Foundation design for new widening structures must consider the historic TCP methods for determining capacity in the original structure.

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. Foundation design for new widening structures must consider the historic TCP methods for determining capacity in the original structure.

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.