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INTERPRETATION AND CORRELATION OF CPTU AND VANE SHEAR TESTS FOR VERY SOFT VARVED SILTY CLAYS – A CASE STUDY
Session: In Situ Testing and Instrumentation I / Essais in situ et instrumentation I
Lincar Pedroni, SNC-Lavalin (Canada)
Denise Leahy, SNC-Lavalin (Canada)
A geotechnical testing program has been carried out for site characterization for the design of an earthfill dyke at the south limit of the Northern Quebec region. The preliminary program involved piezocone testing (CPTu) and field vane shear testing (VST) associated with sampling of the very soft varved silty clay foundation. The undrained shear strength profiles estimated from the VST tests were much lower than expected, based on the CPTu tests results. The Nkt coefficient used to compare the VST and CPTu results were in the typical range for the clay present on site. Additional VST tests were carried out with a manual vane apparatus in several locations down to a depth of about 6 m. The results of the second field test program did not correlate with the CPTu results, even if the additional vane tests seemed to be more representative. As a result, a new testing program was recommended in order to obtain high quality measurements along the dyke alignment. This third testing program has been made taking into account the difficulties and uncertainties of the previous testing programs: therefore vane tests were carried out with care in order to eliminate any remoulding of the soft clay and CPTu tests were performed using a more accurate probe. This paper presents the site background, the testing programs and results, and highlights the difficulties encountered in the testing of very soft varved silty clays and in the calibration of CPTu profiles using the VST data.
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