Development of a new CPT averaging technique and review of existing CPT based methods for the calculation of total pile capacity

Abstract

Over the last couple years, a number investigations into the α factors used for the cone penetration test (CPT) based calculation methods for the base and shaft capacity of driven piles have been carried out. These investigation, express different concerns and limitation of the currently used CPT based calculation methods. Prompting the need for further investigation into the consistency of these methods.For the base capacity calculation methods, the αp factor used is accompanied by a cone resistance (qc ) averaging technique. However, some limitation have been found for the currently used qc averaging technique, which affects the consistency and accuracy of the base capacity calculations when a constant αp factor is used, namely, a dependency with penetration depth. Presentiments by Randolph (2003) & White and Bolton (2005), suggest that a single, constant αp factor can be used when an appropriate averaging technique is applied in combination with the inclusion of the residual loads present in a pile. Hence, a new CPT averaging technique was developed during this study. The goal of this new averaging technique is to find an accurate and consistent averaging technique, which can be used in combination of a single, constant αp factor. The new CPT averaging technique was calibrated using a series of high quality CPT laboratory tests with varyingsoil deposits. Comparisons were then made between the new and existing averaging techniques, as well as, base capacity calculation methods by applying the different averaging techniques to the Deltares pile load test database. Lastly, the effect of residual loads were investigated by applying the averaging techniques to 4 well documented pile load tests where distinctions between residual loads were made. The investigation carried out in this report concluded that the new CPT averaging technique, developed in this study, was more accurate than the other investigated CPT averaging techniques, when comparisons were made with the CPT laboratory tests. This was also the case when comparing the CPT averaging techniques applied to the pile load database. Although the pile load database demonstrated that the new averaging technique had the least spread in results, a dependency with embedment length in the sand bearing layer was still present. This effect was not removed when pile load tests including residual loads were considered, as predicted by Randolph (2003) & White and Bolton (2005). However, the limited number of these tests calls for further research in order to confirm any conclusions.For the shaft capacity calculation methods, the αs factor used by the Dutch norm in particular, NEN-9997- 1, was investigated. This is because the formulation of the NEN-9997-1 calculation method is believed to be too simplistic (van Tol, Stoevelaar, Bezuijen, Jansen, & Hannink, 2013). Comparisons between a variety of shaft capacity calculation methods were made by applying the calculation methods to the Deltares pile load test database and to the 4 well documented pile load tests where distinctions between residual loads were made, in order to look into the effect of residual load on the accuracy of shaft capacity calculation methods.Additionally, results from the research carried out in this report into the αsfactor used by the Dutch norm, concluded a presence of a strong correlation between the αs of sand layers and friction fatigue terms used in other shaft capacity calculation methods. This can be used in the future to improve the current shaft capacity calculation method used by the Dutch norm.