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The standard of practice when assessing the seismic performance of well graded sands, is to assume the response is similar to poorly graded clean sands, which comprise the majority of the liquefaction case history database. Using the 9-m radius centrifuge at UC Davis, an experiment was designed to elucidate the system-level liquefaction triggering response for a poorly graded and well graded sand. The experiment consisted of two identical 10-degree slopes positioned side-by-side in the same model container, with one slope constructed with a well graded sand and the other with a poorly graded sand. The D10 grain size was the similar for both gradations and therefore the permeability was comparable. The slopes were dry pluviated to the same relative density of Dr=63%, while the absolute densities were different. The dynamic response of both slopes was similar up until liquefaction triggering, with both sands reaching excess pore pressure ratios close to unity within 1-2 cycles of loading. Following the onset of liquefaction, the well graded sand exhibited strong dilative tendencies and embankment deformations attenuated rapidly during successive loading cycles, while the poorly graded sand embankment continued to deform. This study demonstrates that the posttriggering response of well graded and poorly graded sands differ due to their different absolute densities and dilatancies for the same relative density. It is expected that findings from this research will lead to a more rational accounting of gradation properties in the evaluation of and design for liquefaction effects, as well as the interpretation of case histories. 

The standard of practice when assessing the liquefaction susceptibility of geosystems uses an empirical case history database that was primarily developed for clean, poorly graded sands. However, many geosystems in the built environment are either constructed with or founded on well graded soils, creating a disconnect between the sand encountered in practice and the sand used as the basis of knowledge. Using the 9-m centrifuge at the University of California Davis’s Center for Geotechnical Modeling a centrifuge experiment was designed to test the dynamic response of embankments constructed poorly graded and well graded sands at the system level scale. The experiment consisted of two 10-degree slopes, one constructed with a poorly graded sand and the other with a well graded sand positioned side by side in the same model container. Each slope was dry pluviated to the same relative density of Dr=63%, while the absolute densities were different. The slopes were instrumented with dense arrays of pore pressure transducers and accelerometers in the level ground at the head of the slope. The stress-strain behavior between accelerometers was calculated using inverse analysis techniques, providing a 1-D shear-beam soil response at the sensor array location. Liquefaction was triggered, as defined by an excess porewater pressure ratio (ru) of 1.0, but the shear strains at triggering in the well graded sand were significantly less than the strains in the poorly graded sand. During cyclic mobility, strain accumulation in the well graded sand occurred at a slower rate. This study demonstrates that liquefaction triggering and the post-triggering response for saturated sands needs to consider gradation characteristics and clean poorly graded sands cannot act as a single predictor of dynamic response for all sand gradations. 

Ottawa F-65 sand (supplied by US Silica, Ottawa, Illinois) was selected as the standard sand for LEAP-UCD-2017. Between December 2017 and February 2018, each LEAP research team sent 500 g samples of sand to UC Davis for grain size analysis and minimum and maximum dry density testing. The purpose of this testing was to confirm the consistency of the sand used at various test sites and to provide updated minimum and maximum density index values. The variation of measured properties among the different samples is similar to the variation measured during repeat testing of the same sample. Modified LEAP procedures to measure index densities are used to confirm consistency of the sands, and the results from these procedures are compared to results from ASTM procedures. The LEAP procedures give repeatable results with median index densities of ρmin ¼ 1457 kg/m3, ρmax ¼ 1754 kg/m3. Relative densities calculated with facility-specific index densities varied by less than 4%, so we conclude that average index densities from all the sites may be used for analysis of the results. The LEAP procedures are easier to perform than the ASTM procedures and do not require specialized equipment; therefore, continued use of the LEAP procedure for frequent quality control purposes is recommended. However, the values from ASTM procedures are expected to be more consistent with values adopted in liquefaction literature in the past; therefore, we recommend using the median ASTM values for analysis of LEAP data. Index densities from ASTM procedures (ρmin ¼ 1490.5 kg/m3, ρmax ¼ 1757.0 kg/m3) produce relative densities that are 4 –10% smaller than the index densities from the LEAP procedures.