Interpretation of Cone Penetration Tests in Gravelly Soil
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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 encounter increases in penetration resistance due to their small diameters relative to gravel particles, some looser sandy gravels have shown reasonable success predicting gravel liquefaction with the CPT. 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 CPT did not consistently indicate a sandy gravel profile but was often classified as 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.