Geotechnical model development for a very soft estuarine clay with MASW geophysics, in-situ and laboratory testing

摘要

A geotechnical model was developed for a site in eastern Australia where a very soft estuarine soil deposit was subsequently improved using dry soil mix, stone columns and surcharge with wick drains. The subsurface S-wave velocity distribution provided by MASW geophysics allowed a previously unknown palaeochannel to be identified and characterised the extent of an organic layer found during excavation. In-situ testing with SDMT, T-Bar, CPTu and vane shear methods together with ID oedometer and CRSC laboratory testing permitted more detailed characterisation of the very soft estuarine clay and, to a depth of 8 m, the small strain shear stiffness measurements from MASW and SDMT agreed well. Correlations between T-Bar, shear vane and SDMT data for undrained shear strength indicated that the empirical N_(tbar) factor was about 15.5 and N_(kt) was 18, nearthe upper end of reported values. Comparisons of in-situ and laboratory test data showed the T-bar factor was about 4.3 for assessment of soil yield pressure which is also greater than published values. The CPTu k factor was 0.2 which has been reported for low penetration resistance clays. Yield pressures inferred from CRSC tests were greater than inferred from incremental load oedometer tests and the CRSC interpretations were closer to in-situ test data. Liquid limit measurements were found to be incorrect because the soil contained organics and was sensitive to oven temperature. It is concluded that the integrated applications of MASW geophysics with appropriate in situ and laboratory testing are an effective means for characterising very soft estuarine clays and for locating material property changes related to palaeodrainage processes in these environments in Australia. Empirical factors for assessment of undrained shear strength and yield pressure appear to lie at the extremes of published values.