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.

Downdrag (or negative skin friction) is an additional force acting on an installed driven pile or drilled shaft foundation which tends to drag or pull the foundation downward. Conditions where it should be evaluated are outlined in AASHTO LRFD Bridge Design Specifications Article 3.11.8. In-situ behavior depends on top of pile/shaft loading, soil stiffness and interface friction properties, lateral stress, and time-rate effects such as consolidation and soil set-up. The downdrag force, or dragload, typically develops by consolidation of soft soils underneath embankments. As soil consolidation progresses, shear stresses (“drag” forces) are induced between the relatively fixed pile or shaft and the adjacent, downward moving embankment soil. Site conditions, which might promote a modest to large dragload effect include:

Sufficient penetration into natural soil is required to counteract all the anticipated negative friction plus dead and live load forces. Disregard side resistance in all fill material when designing in strength limit state during conditions of potential downdrag. Also disregard an additional 5-10 feet of the natural soil under fill to compensate for the weight of the fill imposed on the load carrying stratigraphy.

At the geotechnical strength limit state, the entire shaft or pile is moving downward relative to the soil and therefore negative skin friction is not present. Foundation settlement is concern especially for friction piles and drilled shafts. Downdrag or dragload should not influence the geotechnical strength limit state analysis, rather the concern is at the structural strength limit state and geotechnical service limit state. AASHTO LRFD Bridge Design Specifications Articles 10.7.1.6.2 (for driven piles) and 10.8.3.4 (for drilled shafts) provide guidance on downdrag assumptions and the “neutral plane method” to calculate dragload for use in service limit state analysis as described in FHWA-NHI-16-009.

Methods to consider for addressing potential downdrag include: