Substructure configuration typically results in a compressive load being applied to all of the shafts or piles for the service and strength limit states. However, some load combinations may require some of the members within a foundation system to resist an uplift (tension) force. Although this can occur when checking strength or service (due to wind or centrifugal force, etc.), this most commonly occurs in the extreme event case designing for impact loading.

The second source of uplift in deep foundations is often referred to as uplift pressure from swelling clays within the subgrade. This uplift force on the shafts and piles due to swelling of any active clays can be approximated by assuming a uniform swell pressure (from swell testing) acting over the perimeter of the shaft or pile to an ‘Active Zone’ depth of 10 to 20 feet, depending on professional judgement of engineer. Provide reinforcing in shafts to resist the uplift forces.