Update of Correlations between Cone Penetration and Boring Log Data (2008)

The cone penetration test (CPT) has been widely used in Louisiana in the last two decades as an in situ tool to characterize engineering\nproperties of soils. In addition, conventional drilling and sample retrieval using Shelby tube followed by laboratory testing is still the\nacceptable practice in identifying soils engineering properties. The main objective of this project is to update the correlations that are\ncurrently used by Louisiana Department of Transportation and Development (LADOTD) to interpret CPT data for engineering design\npurposes and to assess the reliability of using CPT data to predict soil shear strength in both the magnitude and spatial variations in the\nfield with respect to the Load and Resistance Factor Design (LRFD) methodology. The results of laboratory soil testing were retrieved\nfrom borehole logs and were used as reference measurements in this study.\nThe research team collected project data files in paper printout format from LADOTD and soil testing engineers. Most project files did\nnot have spatial coordinates; therefore, aerial images were used to identify latitude and longitude coordinates of CPT and borehole\nlocations. The borehole data was not available for all the located CPT soundings. Efforts were made to obtain any available data from\nLADOTD electronic archive as well as paper project documents. A total of 752 CPT tests were documented in which 503 were matched\nwith adjacent boreholes and 249 did not have adjacent borehole data available. The CPT data was used to predict soil undrained shear\nstrength, bulk density and classification according to Robertson and Zhang and Tumay methods [1], [2].\nThe CPT data was then used to develop a database of undrained shear strength estimates with corresponding results from boreholes. The\nresults in the database were preprocessed to apply some constraints on data points included in the calibration study, such as setting a\nmaximum threshold on the distance between CPT and borehole locations; a minimum and maximum threshold on undrained shear\nstrength values were used to represent realistic soil properties. The resulting database included results from 251 CPT soundings with\nborehole results in their vicinity that meet the aforementioned constraints. From these CPT soundings, 862 unique undrained shear\nstrength data points were obtained at various depths. The dataset was analyzed for general as well as specific trends in order to identify\nappropriate parameters to be included in the study. Soil classification was clearly the most plausible parameter based on which the CPT\nundrained shear strength estimates should be calibrated.\nThe calibration of the CPT expression for undrained shear strength was conducted using two approaches. The first approach is a direct\ncorrelation based of the transformation model currently used by LADOTD for estimating the shear strength. The First Order Reliability\nMethod (FORM) forms the basis for the second approach, which is more detailed and accounts for all sources of uncertainty. Optimum\nCPT coefficient values were computed for various target reliability values. The results were summarized and implementation procedures\nwere recommended for future research.\n