Update of correlations between cone penetration and boring log data.
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Update of correlations between cone penetration and boring log data.

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      Final report; Feb. 2007-March 2008.
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    • Abstract:
      The cone penetration test (CPT) has been widely used in Louisiana in the last two decades as an in situ tool to characterize engineering properties of soils. In addition, conventional drilling and sample retrieval using Shelby tube followed by laboratory testing is still the acceptable practice in identifying soils engineering properties. The main objective of this project is to update the correlations that are currently used by Louisiana Department of Transportation and Development (LADOTD) to interpret CPT data for engineering design purposes and to assess the reliability of using CPT data to predict soil shear strength in both the magnitude and spatial variations in the field with respect to the Load and Resistance Factor Design (LRFD) methodology. The results of laboratory soil testing were retrieved from borehole logs and were used as reference measurements in this study. The research team collected project data files in paper printout format from LADOTD and soil testing engineers. Most project files did not have spatial coordinates; therefore, aerial images were used to identify latitude and longitude coordinates of CPT and borehole locations. The borehole data was not available for all the located CPT soundings. Efforts were made to obtain any available data from LADOTD electronic archive as well as paper project documents. A total of 752 CPT tests were documented in which 503 were matched with adjacent boreholes and 249 did not have adjacent borehole data available. The CPT data was used to predict soil undrained shear strength, bulk density and classification according to Robertson and Zhang and Tumay methods [1], [2]. The CPT data was then used to develop a database of undrained shear strength estimates with corresponding results from boreholes. The results in the database were preprocessed to apply some constraints on data points included in the calibration study, such as setting a maximum threshold on the distance between CPT and borehole locations; a minimum and maximum threshold on undrained shear strength values were used to represent realistic soil properties. The resulting database included results from 251 CPT soundings with borehole results in their vicinity that meet the aforementioned constraints. From these CPT soundings, 862 unique undrained shear strength data points were obtained at various depths. The dataset was analyzed for general as well as specific trends in order to identify appropriate parameters to be included in the study. Soil classification was clearly the most plausible parameter based on which the CPT undrained shear strength estimates should be calibrated. The calibration of the CPT expression for undrained shear strength was conducted using two approaches. The first approach is a direct correlation based of the transformation model currently used by LADOTD for estimating the shear strength. The First Order Reliability Method (FORM) forms the basis for the second approach, which is more detailed and accounts for all sources of uncertainty. Optimum CPT coefficient values were computed for various target reliability values. The results were summarized and implementation procedures were recommended for future research.
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