Case Study of CPT-based Design Methods for Axial Capacity of Driven Piles in Sand

Today the design of onshore axially loaded driven piles in cohesionless soil is commonly made on basis of CPT-based methods because field investigations have shown strong correlation between the local shaft friction and the CPT cone resistance. However, the recommended design method for axially loaded offshore driven piles in cohesionless soil has until now been the β-method given in API. The API-method is based on the effective overburden pressure at the depth in question. Previous studies show deviations between full-scale load test measurements of the axial pile capacity and the predictions found by means of the API-method. Compared to the test measurements, the API-method under-estimates the capacity of short piles (<20m) in dense sand, over-estimates the capacity of long piles in loose sand, and gives a shaft capacity less conservative for piles in tension than for piles in compression. Thus, several CPT-based methods have been proposed for the design of offshore driven piles in cohesionless soil such as the UWA-05, ICP-05, and NGI-99 methods. This article treats a case study where the API-method as well as the UWA-05 and NGI-99 methods are compared using CPT-data from an offshore location with dense to very dense sand. The design of the piles in the jacket foundation shows that API-00 for both the tension and the compression loads predicted much longer piles than the CPT-based methods. Variation of the pile length and pile diameter shows that NGI-99 and UWA-05 predicts almost similar bearing capacities. API-00 predicts much lower capacities than the two CPT-based methods for short piles and much higher capacities for long piles regardless of the pile diameter.