UPLIFT CAPACITY OF DRILLED PIERS.

Structures subjected to large overturning moments can produce uplift loads on drilled piers if they are used for the foundation. The design equation for uplift is similar to that of compression. Figure 17.22 shows the forces acting on the pier under uplift-load Qul. The equation for Qul may be expressed as

Uplift Capacity of Single Pier (straight edge)
For a drilled pier in cohesive soil, the frictional resistance may expressed as (Chen and Kulhawy, 1994)

Poulos and Davis, (1980) suggest relationships between cu and α as given in Fig. 17.23. The curves in the figure are based on pull out test data collected by Sowa (1970).

Uplift Resistance of Piers in Sand
There are no confirmatory methods available for evaluating uplift capacity of piers embedded in cohesionless soils. Poulos and Davis, (1980) suggest that the skin frictional resistance for pull out may be taken as equal to two-thirds of the shaft resistance for downward loading.

Uplift Resistance of Piers in Rock
According to Carter and Kulhawy (1988), the frictional resistance offered by the surface of the pier under uplift loading is almost equal to that for downward loading if the drilled pier is rigid relative to the rock. The effective rigidity is defined as (Ec/Em)/((d/Ds)^2), in which Ec  and Em  are the Young's modulus of the drilled pier and rock mass respectively, d is the socket diameter and DS is the depth of the socket. A socket is rigid when (Ec/Em)/((d/Ds)^2)  > 4. When the effective rigidity is less than 4, the frictional resistance fr for upward loading may be taken as equal to 0.7 times the value for downward loading.

Figure 17.22 Uplift forces for a straight edged pier