Reliability of Estimated Anchor Pullout Resistance

Abstract

In anchor pullout design, conservative soil and rock shear strength parameters are usually adopted. Presumptive values of soil/grout and rock/grout bond strength are available in different design manuals. In this study, in-situ pullout test data for anchors in soil and rock type worldwide were collected from published sources and information provided by specialty wall contractors. The measured pullout test data were compared to estimated pullout resistance using the Post-Tensioning Institute (PTI) presumptive bond strength values. Statistical analysis was performed to determine the probability of success and the corresponding reliability indices using the minimum, average and maximum PTI ground/anchor bond strength values for cohesive soils, cohesionless soils and different rock types. A minimum safety factor of 2 recommended by the PTI to the ground/grout bond strength was found not conservative where the maximum presumptive bond strength values were used in clays and sands, and the average and the maximum presumptive bond strength values were used in rocks. Based on the results of this study, an average minimum factor of safety was recommended for ground/grout bond strength. Introduction Anchor pullout resistance is determined using soil and rock bond strength parameters based on site specific in-situ and/or laboratory test results. For preliminary design purpose, the designer may adopt presumptive soil and rock bond strength values from available design manuals to determine anchor bond length. However, the designer should account for the uncertainty of soil and rock strength values. Lacasse and Nadim (1996) reported that cohesive soil shear strength had coefficient of variation (COV) ranged between 5% and 30%. Kulhawy and Trautmann (1996) indicated that standard penetration test (SPT) results had COV ranged between 15% and 45%. Note that SPT is a common test to estimate cohesionless soil shear strength. Based on published rock shear strength values (Hoek and Bray 1981), the COV of rock unconfined strength ranged between 30% and 70%. That means, for example, the average rock/grout bond strength adopted in a design could be 70% less than the actual field value. Project Engineer, D’Appolonia Engineering, 275 Center Road, Monroeville, PA 15146; Phone 412-856-9440; Facsimile 412-856-9440; yahegazy@dappolonia.com.