Load and Resistance Factor Design, Cost, and Risk: Designing a Drilled-Shaft Load Test Program in Florida Limestone

Abstract

Currently there are few if any guidelines on estimating the number of load tests in the design of drilled-shaft foundations in Florida limestone. For instance, for many sites there may be a similar number of field load tests but a significantly different number of design shafts. Moreover, little if any information exists on risk or reliability versus cost of drilled-shaft foundations or on the cost of field load testing. The collection of a large database of drilled-shaft tests (more than 25 with Osterberg and Statnamic devices), in situ laboratory data, drilled-shaft construction costs, and field load testing costs for Florida limestone is reported on. From the field load tests, the average unit skin friction for various sites was found, as well as the predicted values based on the Florida Department of Transportation recommended design approach. Next, using load and resistance factor design (LRFD), the resistance (ϕ) values were found for various reliabilities (risk or probability of failure). Once the factored design loads were known (from plans), drilled-shaft lengths were estimated on the basis of the computed LRFD ϕ-values for different reliabilities (i.e., risk). From the linear length of the designed shaft as well as the expected cost per meter, a plot of total foundation cost versus reliability (risk) was generated for each site. On the basis of the latter plot, acceptable risk, and the cost of field load testing (bid and itemized), the designer can identify the cost savings of load testing and the appropriate number of tests to be performed.