An Approach to Assess LRFD-Φ from Load Test and Borehole Data In and Outside the Footprint of a Drilled Shaft

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Currently (2007––2010), AASHTO’’s LRFD design of deep foundations recommends resistance factors for a variety of scenarios (e.g., method of analysis, number of load tests, etc.). Generally, values account for a desired level of reliability and are based on comprehensive databases (not site specific) of predicted versus measured resistances. Unfortunately, no consideration is given to locations of boreholes with respect to foundation elements, to spatial variability of design properties over the site or the use of load testing on LRFD assessment. The present work considers the case of side friction on axially loaded drilled shafts as assessed from laboratory analysis of rock core samples. Geo-statistical principles are used to rationally account for (1) the presence and amount of site specific load testing with borehole data in the footprint, (2) the availability of off-site load test data, (3) the amount of site specific borehole sampling, and (4) the presence or not of borehole data at production shaft locations. In agreement with previous studies, spatial variability within a site is identified as a dominating contributor to overall prediction uncertainty. Additionally, it is shown that spatial variability may only be eliminated by using borings inside shaft footprints and not through additional load testing or collection of borehole data at other locations on a site. A case study with field data is used to illustrate the approach for practice in Florida limestone.