Practical method for load and resistance factor design (LRFD) of deep foundations at the strength and service limit states

Abstract

The strength limit state design of deep foundations is usually performed using static capacity equations to predict the ultimate side and tip resistance. The service limit state requirements are typically verified after the strength limit state design is completed and, if required, the design is revised as necessary. The strength and service limit state design resistances can be verified utilizing field developed load-settlement curves from load tests along with a number of failure prediction methods. However, rational integration of load test data into static capacity design methods to optimize deep foundation design is often difficult. In addition, most static capacity design methods are based on allowable stress design (ASD) principles and therefore incorporate an arbitrary factor of safety to account for design uncertainty, while service limit state design typically neglects design uncertainty. In this paper, a practical Load and Resistance Factor Design (LRFD) method is developed for the simultaneous design of deep foundations at both the strength and service limit states using a "t-z" model design approach. The developed "t-z" design procedure is compared with a static capacity preliminary design as well as at the final design stage based on field load test data.