INTERPRETATION OF STRENGTH AND STRESS HISTORY OF GLACIO-LACUSTRINE CLAY DEPOSITS BY CPTU

Piezocone penetration tests (CPTu) are gaining acceptance in North America as a fast, economical, and repeatable method for characterizing soil conditions. Because soil classification and geotechnical engineering properties are not determined directly during the testing, correlations are required, based on the CPTu measured values of tip stress, q_c, sleeve friction, f_s, and penetration pore pressure, u₂. For this study, a CPTu sounding was performed within Lake Superior near Duluth, Minnesota at a site overlain by glacio-lacustrine clay deposits formed by Glacial Lake Duluth. To obtain site specific correlations, a companion boring was also performed, in which field vane shear tests were conducted to evaluate the in-situ undrained shear strength, s_u, of the clay, and thin-wall tube samples were obtained for geotechnical laboratory testing. The stress history of the clay soils, in terms of overconsolidation ratio (OCR), was determined directly from laboratory IL consolidation tests, and was also calculated based on the field vane shear test results.

The boring and sounding were both performed off-shore. The lake bottom sediments were underlain by two major clay units that formed the basis of this study; an upper, relatively soft, normally to slightly overconsolidated, lean to fat clay layer, and a lower, stiffer, overconsolidated, lean to fat clay stratum. A plot of the CPTu corrected tip stress, Q_t, f_s, and friction ratio, R_f, shows a distinct boundary between the two clay units. The corrected field vane s_u was compared to the CPTu net tip stress. Simple linear regression of the data produced a composite cone factor, N_kt, for both strata of 14, with a coefficient of variation (COV) of 13.6%. An OCR relationship was obtained by comparing the normalized cone resistance to the laboratory OCR results and the uncorrected field vane derived OCR values. This comparison resulted in a composite correlation factor, k, for both strata of 0.35, with a COV of 18.9%. Plots of continuous interpreted OCR and s_u with depth were then produced by applying the derived N_kt and k factors to the remaining CPTu data for the full sounding depth. The results of this study indicate good agreement in the use of CPTu in predicting stress history and field vane undrained shear strength within the glacio-lacustrine clays in this area.