Estimating shear wave velocity with the SCPTu and Bender element

The shear wave velocity (vs) is an important soil and rock property that can both be used in several geotechnical problems including for evaluation of dynamic properties of soils as well as in determining the maximum value of soil stiffness at small strain. This property is also seen to give good correlations with other soil parameters used in settlement and stability analyses. The Norwegian Public Roads Administration (NPRA) has recently invested in equipment to measure this fundamental soil property both in the laboratory using bender elements and out in the field using a seismic CPTu (SCPTu). NPRA has also developed internal procedures and techniques to standardize logical interpretations. To assist with this standardization procedure, NPRA has conducted soil investigations at a site in Fredrikstad municipality in the southern part of Norway. The investigations included SCPTu and extraction of high quality mini-block samples. The shear wave velocity is estimated after consolidation in the triaxial apparatus. The laboratory program was done right after the sample extraction and repeated on stored samples a couple of weeks later. Comparison of vs measured in the laboratory was then made with the field measurements with SCPTu. The work gives comparison of the field and laboratory measurements. Correct interpretation approaches are necessary for the laboratory tests as these are found to be more sensitive to small changes in experimental conditions than the SCPTu. Recommendations on how to reduce discrepancies between laboratory and field data are given