Drilled Shaft Design and Load Testing in Florida Intermediate Geomaterial and Weak Limestone

Abstract

This case study summarizes the design, construction, load testing, and integrity testing of nonredundant drilled shafts for a bridge project in northwest Florida. The subsurface profile consisted mainly of calcareous sandy and silty clay (marl and marlstone) interfingered with limestone and cemented sand layers. The results of unconfined compression tests revealed significant variability in strength, with strata within the depth of interest holding strength properties that fall within the generally accepted range of cohesive intermediate geomaterial. This paper discusses local geology, geotechnical investigation, design, construction, load testing, and site-specific correlations. Important aspects of the project, including the use of the shaft inspection device to examine the cleanliness of the bottom of the shafts, Osterberg cell load testing, and the site-specific correlations developed from the results, as well as integrity testing with cross-hole sonic logging and a thermal integrity profiler, are presented. The inability of the standard penetration test (SPT) to delineate boundaries adequately between strong and (relatively) weaker layers is presented, as well as nonlinear site-specific relationships between anticipated skin friction resistance and SPT blow count. The nonlinear SPT-based relationships attempt to address the strain incompatibility between strong and weak seams that may exist within a given stratum. Proposed correlations for skin friction and end bearing that are based on laboratory strength testing are presented in terms of revised unconfined compressive strength to address variability in strength characteristics across the site.