On Shear Wave Velocity Testing in Clay
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The soil stiffness at small strains is an important parameter for settlement predictions and for understanding soil behaviour under seismic/dynamic loading. The small strain stiffness is usually found from the shear wave velocity, Vs, obtained by wave propagation tests in field and laboratory. Results from laboratory testing have proved to be different from results obtained by in situ testing. The results from invasive and non-invasive tests in situ also tend to differ.In this study, various wave propagation tests are carried out to obtain Vs in clay. The results from the various tests are compared and discussed. The assessed field tests are Seismic Cone Penetration Test (SCPT), Crosshole Test (CHT) and Multichannel Analysis of Surface Waves (MASW). Bender element testing on 54 mm samples is carried out in the laboratory. Testing is performed at various sites, but only the results from the Tiller site are presented in this thesis. The Tiller site is a NTNU research site consisting of low plasticity clay. Quick clay is expected below about 8 m depth. Two profiles, 50 m apart, are investigated at Tiller. Full Vs-profiles to about 20 m depth are found using SCPT and MASW. The SCPT data are processed using both cross-correlation and crossover method. The crossover method, with a pseudo-interval spacing of 3 m or more, provide the best results. These results are considered to be the most accurate of all the assessed tests. MASW surveys were carried out in two rounds. The original survey provides significantly higher Vs-results than expected at depths below 8 m. Lower frequencies are produced in the repeated survey. The repeated survey provides a Vs-profile similar to the result obtained from SCPT. CHT is only performed at one profile and the equipment measures Vs at just one depth. Most of the received signals obtained at Tiller are unclear. Vs is however estimated from the received signals. A significant uncertainty is not expected. The result from CHT corresponds well to results from SCPT and MASW.Eight samples are tested in a triaxial cell with bender elements. The results show significantly lower Vs compared to the field tests. Five samples from the same depth with various height are tested. No difference in Vs related to sample height are found. That suggest that there is no system time delay in the system. Two samples are consolidated past the primary consolidation to investigate the aging effect. The bender element results are found to correspond fairly well with in situ measurements when they are adjusted for the aging effect. The aging effect does however seem to be different in sensitive and non-sensitive clay samples. A brief study on the consequences of variation in Vs in practical engineering is conducted with respect to settlement and earthquake engineering. Both the settlement and earthquake analyses show significant influence of variation in Vs.