The 2008 M _w 7.9 Wenchuan, earthquake caused naturally deposited ravelly soils to liquefy over a wide area. Although liquefaction of gravely soils is recognized by the geotechnical profession, observations of liquefaction and nonlique‐faction case histories within the literature are few. Through several years of systematic study following the 2008 Wenchuan earthquake (M _w 7.9), 92 locations of gravel liquefaction were identified, described, and mapped. These locations lie within an approximately 3,000 km ² area of the Chengdu Plain. Peak ground accelerations estimated at the sites range from 0.15 g to 0.49 g. Taken collectively, these studies reveal the necessary conditions for liquefaction triggering in gravelly materials. Grain size analyses indicates that the ejecta was much finer than the gravels that liquefied. Gravel contents of liquefied soils ranged from 5% to more than 85%. The liquefied gravelly soils were loose, but their measured shear wave velocities range from 133 m/s to 267 m/s, with corrected values ranging from 154 m/s to 31 4m/s. The unique depositional conditions under Chengdu Plain provide favorable conditions for extensive liquefaction of gravelly soils. The shallow soil profiles consist of a 0.5 m to 5.5 m impermeable soil (i.e., the capping layer) overlying gravels ranging in thickness from a few meters to hundreds of meters.

Coral sand is characterized by unique particle morphology and pore structure, which result in pronounced dilatancy and a high internal friction angle during shear. The dilatancy angle is a critical parameter for finite element analyses of sand foundation bearing capacity; the inappropriate selection of this parameter can lead to significant computational errors. In this research, a series of consolidated drained triaxial tests were conducted on coral sand samples from the South China Sea to investigate the dilatancy behavior and its effect on shear strength parameters. A dilatancy equation for coral sand was proposed, incorporating the dilatancy index, relative density, and mean effective stress. The results indicate the following: (1) Within the confining pressure range of 25–400 kPa, the stress–strain curves exhibit varying degrees of strain softening. When the effective confining pressure reaches 400 kPa, the dilatant behavior is nearly suppressed, resulting in a transition from dilatancy to contraction; (2) The peak internal friction angle decreases significantly with increasing effective confining pressure. However, the sensitivity to confining pressure varies for samples with different relative densities (Dr = 30–90%), with denser samples showing a more rapid reduction in peak friction angle; (3) At a confining pressure of 25 kPa, the maximum dilatancy angle of coral sand samples reaches 44.2°, significantly exceeding the typical range observed in terrestrial quartz sands. This difference may be attributed to the irregular and angular characteristics of the coral sand particles; (4) Based on Bolton’s dilatancy theory, a dilatancy equation applicable to coral sand was developed, demonstrating a strong linear relationship among the dilatancy index (I_R), relative density (Dr), and peak mean effective stress (p′_f). These findings provide valuable guidance for the selection of strength parameters for engineering applications involving coral sand.

The 2008 Wenchuan M_s 8.0 earthquake caused extensive naturally deposited gravelly soils liquefied. The merit of the observation overwhelms the engineering point of view of gravel as non-liquefiable soil, and greatly enriches archiving field cases of gravel liquefaction worldwide. Though years of systematical studies, we mapped the 92 identified gravelly soils liquefied zones (sites) as sporadically lay in an area of more than 3000 km² in Chengdu Plain. The gravel contents (G_c-values) of liquefied gravelly soils range from 5% to 85% or even larger. The liquefied gravelly soils are loose, but their measured shear-wave velocities (V_s) start from 133 m/s to 267 m/s, and are corrected to 154 m/s to 314 m/s. The PGA-values are predicted from 0.15 g to 0.49 g at liquefied gravelly sites. The shallow soil structure under Chengdu Plain basically consists of an impermeable capped layer and underlain gravelly soils ranging from meters to hundred meters, which provides a fundamental interpretation for gravelly soils liquefaction. A minimum ground shaking of PGA 0.15 g is essential to trigger natural gravelly soils liquefaction, yet massive gravelly soils liquefaction requires 0.20 g to 0.40 g of ground shaking intensity.