Shortcomings of the Davisson Offset Limit Applied to Axial Compressive Load Tests on Cast-in-Place Piles

Abstract

Since its introduction in 1973, the Davisson Offset Limit has been widely used in the United States for interpretation of axial compressive load tests on pile foundations and is one of three methods explicitly accepted by the 2006 International Building Code. An examination of the fundamental assumptions of the DOL shows that its application to cast-in-place piles lacks scientific basis and leads to over conservatism. Interestingly, these are the same reasons given by Davisson for its development. The results of the examination indicate that there are notable fallacies in the Davisson Offset Limit, including the assumptions that: (1) the cast-in-place pile behaves as a “fixed-base, free-standing column.”, (2) an elastic line is a dependable reference line for interpretation of load tests on cast-in-place piles, and; (3) an offset of 3.8 mm + D (inches)/120 from the elastic line represents the movement necessary to mobilize toe resistance of cast-in-place piles. Considering these results, the authors present suggestions ranging from modifications to the Davison Offset Limit that recognizes the greater movement required to mobilize the toe resistance to codification of a more rational criterion better suited to interpret the axial compressive capacity of cast-inplace piles. INTRODUCTION The 2006 International Building Code explicitly accepts three interpretive methods for axial compressive load tests on piles. These three interpretative methods (by the names they are used in common vernacular): (1) Davisson Offset Limit (Davisson, 1972); (2) Brinch Hansen 90% Criterion (Brinch Hansen, 1963), and; (3) Butler-Hoy Criterion (Butler and Hoy, 1977). Generally speaking, the Davisson Offset Limit (DOL) is frequently used in the United States because it provides the lowest estimation of axial compressive capacity of the aforementioned methods. Also, it is most likely to provide an ultimate axial compressive pile capacity from the actual load-deflection curve. That is, the Brinch Hanson 90% Criterion and the Butler-Hoy Criterion often require extrapolation to extend the load-deflection curve in order to establish an ultimate axial compressive capacity. In the absence of established guidelines for extrapolation, there is a reluctance, and even opposition, to the extrapolation of load test results. 568 DEEP FOUNDATIONS