Engineering and Construction

A downdrag load is defined as a force induced on deep foundations resulting from the downward movement of adjacent soil relative to the foundation element. Downdrag is also referred to as negative skin friction.

Conditions that could cause downdrag loads are as follows:

• Above the end-bearing strata, there is a soil layer that is in the process of settling due to consolidation or collapse.
• Above the end-bearing strata, the original groundwater table is permanently lowered causing an increase in effective stress and resulting in settlement of the soil strata.
• A fill layer is placed at ground surface that causes settlement of the soil strata, such as shown in Fig. 4.17.

The following equations can be used to calculate the downdrag load QD for a single isolated pile or pier:

For a pile or pier in cohesionless soil subjected to downdrag:


For a pile or pier in cohesive soil subjected to downdrag:

Example Problem 6.8 Assume a site consists of an end-bearing pier founded on solid rock.

The diameter of the pier is 3 ft (0.9 m) and a 40 ft (12 m) long zone of the pier is embedded in clay that is in the process of consolidating. Assuming the clay has a uniform undrained shear strength su of 500 psf (24 kPa), calculate the downdrag load on the pier.

Solution Using a total stress analysis, therefore, su = cohesion (c). From Fig. 6.13, the value of cA = 480 psf (23 kPa) for a concrete pier embedded in clay having c = 500 psf (24 kPa).

Using Eq. 6.22: