Abstract

The definition of the shear wave velocity profile is a fundamental step for the seismic characterization of a site in the context of Eurocode 8 and for the conduction of earthquake geotechnical engineering efforts such as site response analysis. Shear wave velocity profiles can be obtained: (1) directly from seismic geophysical and seismic geotechnical tests; or (2) indirectly, from “static” in-situ geotechnical tests such as dilatometer tests (DMT) and cone penetration tests (CPT). In the latter approach, shear wave velocity is estimated by using transformation models which are typically derived from data collected at other sites. This paper illustrates the procedures and main results of the comparative assessment of the performance of existing DMT-based and CPT-based transformation models to estimate shear wave velocity at two adjacent spatial locations in a rural site in the region of Tuscany in central Italy. Model-predicted shear wave velocity profiles were compared with direct measurements obtained by geophysical seismic dilatometer (SDMT) testing. The comparative assessment involved the definition, calculation, and assessment of quantitative performance statistics. The paper provides a critical analysis and a discussion of the outcomes with respect to soil type.

Abstract

Although gravelly soils have been observed to liquefy in 27 earthquakes in the past 120 years, many engineers believe that gravel cannot liquefy due to its high hydraulic conductivity. Gradations from gravel liquefaction case histories have shown these deposits typically contain 25 to 40% sand, reducing the hydraulic conductivity and enabling excess pore pressures to cause liquefaction. While cone penetrometers (CPT), typically used to evaluate liquefaction resistance in sand, may show increases in penetration resistance due to their small diameter relative to gravel particles, the CPT has successfully predicted gravel liquefaction for looser sandy gravels. Case histories in Wellington, New Zealand demonstrate the successful identification of gravel liquefaction hazards using CPT. Although some layers in the profile indicated high penetration resistance, most of the profile was correctly predicted to liquefy. The Soil Behavior Type (SBT) from the CPT did not consistently indicate a sandy gravel profile but was often classified as behaving like a sand or silty sand; likely influenced by higher sand percentages between gravel particles. To evaluate the ability of the CPT to characterize gravelly soils and their liquefaction potential, additional field case histories are desirable. This paper presents test results from two case histories, one in Wellington, New Zealand, and one in Petrinja, Croatia, where gravels have liquefied. In both cases, the CPT occasionally overestimated liquefaction resistance in gravel layers. The advantages of using a 74 mm diameter Dynamic Cone Penetrometer (DPT) are also highlighted with companion testing.