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Gravelly soils have liquefied at multiple sites in at least 27 earthquakes over the past 130 years. These gravels typically contain more than 25% sand which lowers the permeability and makes them susceptible to liquefaction. Developing a reliable, cost-effective liquefaction triggering procedure for gravelly soils has been a challenge for geotechnical engineers. Typical SPT- or CPT-based correlations can be affected by large-size gravel particles and can lead to erroneous results. To deal with these problems, we have developed liquefaction triggering curves for gravelly soils based on (1) shear wave velocity (Vs) and (2) a large diameter cone penetrometer. With a cone diameter of 74 mm, the Chinese Dynamic Cone Penetration Test (DPT) is superior to smaller penetrometers and can be economically performed with conventional drilling equipment. Using logistic regression analysis, the DPT has been directly correlated to liquefaction resistance at sites where gravels did and did not liquefy in past earthquakes. Probabilistic liquefaction resistance curves were developed based on 137 data points from 10 different earthquakes in seven countries. Using a similar data set, probabilistic liquefaction triggering curves were also developed based on Vs measurements in gravelly soils. The Vs-based liquefaction triggering curves for gravels shift to the right relative to similar curves based on sands. New magnitude scaling factor (MSF) curves have also been developed specifically for gravel liquefaction which were found to be reasonably consistent with previous curves for sand.

 

Investigating the role of sand and fines content and in situ drainage conditions in governing the hydraulic conductivity of gravelly deposits is highly important to characterize the liquefaction potential of gravelly soil. In this study, a variation of hydraulic conductivity with sand content has been empirically obtained based on the existing gravel liquefaction case histories. It is found that the hydraulic conductivity of a soil matrix with more than 20%–30% sand content by mass is low enough to cause liquefaction without any impervious confining layer. In addition, a numerical study has been performed using the commercial software FEQDrain to study pore pressure generation in gravelly soil at various relative densities and hydraulic conductivities with and without an impermeable cap layer when subjected to various earthquake loadings. For both unconfined and confined condition, excess pore pressure ratios consistently increase with a decrease in hydraulic conductivity ( k) and relative density ( D_r). The excess pore pressure ratio is correlated with hydraulic conductivity, soil compressibility, and cyclic stress ratio (CSR). For the confined condition, pore pressure in the gravel layer is primarily governed by the overlying cap layer and even a sandy cap layer instead of a highly impervious clay layer can cause liquefaction.