Reliability Analysis of CPT Measurements for Calculating Undrained Shear Strength

The cone penetration test (CPT) has been widely used in Louisiana to classify soils, measure undrained shear strength (Su), and identify bearing stratum for driven piles. This paper compares the values of Su based on CPT measurement with Su of the unconfined compression test. A total of 752 CPT soundings were collected and archived using ArcGIS software in which 503 were matched with adjacent boreholes and 249 did not have adjacent borehole data available. The dataset was analyzed for general as well as specific trends in order to identify appropriate parameters to be included in the investigation. The calibration of the CPT expression for Su was conducted using the first order reliability method (FORM) and accounting for all sources of uncertainty. Optimum CPT coefficient ( N kt) values to calculate Su were computed for various target reliability values. It was determined that the soil classification is the only parameter showing clear trends that affect CPT estimates of the undrained shear strength. Values of N kt for each soil type based on the Robertson (1990) classification and the Zhang and Tumay (1999) classification were determined for three target reliability levels. It is obvious that the N kt coefficient for soils with higher clay content is lower than those with less clay content. A single Nkt value that is valid for all soil types is unwarranted as will lead to acceptable results for some soil conditions and unacceptable results for others, which can be unconservative.