UPLIFT RESISTANCE OF PILES.

Piles are also used to resist uplift loads. Piles used for this purpose are called tension piles, uplift piles or anchor piles. Uplift forces are developed due to hydrostatic pressure or overturning moments as shown in Fig. 15.22.

Figure 15.22 shows a straight edged pile subjected to uplift force. The equation for the uplift force Pul  may be written as

Uplift Resistance of Pile in Clay
For piles embedded in clay, Eq. (15.65) may written as

Figure 15.23 gives the relationship between a and cu based on pull out test results as collected by Sowa (1970). As per Sowa, the values of ca agree reasonably well with the values for piles subjected to compression loadings.

Figure 15.22 Single pile subjected to uplift

Figure 15.23 Relationship between adhesion factor α and undrained shear strength cu (Source: Poulos and Davis, 1980)

Uplift Resistance of Pile in Sand
Adequate confirmatory data are not available for evaluating the uplift resistance of piles embedded in cohesionless soils. Ireland (1957) reports that the average skin friction for piles under compression loading and uplift loading are equal, but data collected by Sowa (1970) and Downs and Chieurzzi (1966) indicate lower values for upward loading as compared to downward loading especially for cast-in-situ piles. Poulos and Davis (1980) suggest that the skin friction of upward loading may be taken as two-thirds of the calculated shaft resistance for downward loading.
 

A safety factor of 3 is normally assumed for calculating the safe uplift load for both piles in clay and sand.