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Natural soil deposits can consist of particles with a wide range of sizes. In current practice, the assessment of shear strength and stress-dilatancy behavior of coarse-grained soils is based on methods developed for poorly graded sands, without explicit consideration for differences in gradation. This paper investigates the influence of the range of particle sizes on the monotonic shear strength and the stress-dilatancy response of poorly- to well-graded soils. Using the 3D discrete element method (DEM), the applicability of commonly used sand-based stress-dilatancy frameworks is assessed for a range of gradations. This DEM investigation employs clumps of spheres to accurately simulate the particle shapes on specimens with coefficients of uniformity (CU) varying between 1.9 and 6.9. These specimens were subjected to isotropically consolidated drained triaxial compression at various relative densities and confining stresses with the objective of isolating the effects of particle size distribution from those of particle shape. The peak and critical state shear strengths and the dilatancy responses of the specimens with different gradations are evaluated. For the same state parameter, the results indicate an increase in the shear strength and rate of dilation as the range of particle sizes increases. However, the critical state line shifts downward, and its slope decreases as CU is increased. The DEM results are compared to Bolton’s stress-dilatancy relationship to highlight the inadequacies of using clean sand-based frameworks in capturing the behavior of well-graded soils. 

Well-graded soils can be found in nature and in engineered structures, such as dams and embankments. Prediction of their behavior is still an engineering challenge in part due to the lack of data in the literature, arguably due to difficulties associated in testing these soils in the laboratory and in situ. Particularly, there is still debate over the effect of the increased range of particle sizes (i.e., widening gradation) on the shear strength and dilatancy of coarse-grained soils. This paper presents the results of drained and undrained isotropically-consolidated triaxial compression tests on six soil mixes of varying gradation. These soils were sourced from a single natural deposit and selectively sieved and mixed to isolate the effects of gradation from those of particle shape and mineralogy. The results indicate that the critical state lines in void ratio – mean effective stress space move downward as the gradation becomes wider. For the same state parameter, the soils with a wider gradation exhibit greater dilatancy and generate negative excess pore pressures with greater magnitudes than the poorly-graded soils. In drained conditions, the greater dilatancy of the well-graded soils leads to greater peak friction angles, while in undrained conditions it leads to greater undrained shear strengths. The results show that these differences in behavior can only be captured when interpreting the results in terms of the state parameter and normalized state parameter, while comparing the results in terms of the void ratio or relative density obscures the effect of differences in gradation. 

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. 

A reliable prediction of liquefaction-induced damage typically requires nonlinear deformation analyses with an advanced constitutive soil model calibrated to the site conditions. The calibration of constitutive models can be performed by relying primarily on a combination of commonly available properties and empirical or semi-empirical relationships, on laboratory tests on site-specific soils, on in-situ penetration tests, or a combination thereof. Chiaradonna et al. (2022) described a laboratory-based calibration approach of the PM4Sand constitutive model and evaluated the prediction accuracy against the response of a centrifuge experiment of a submerged slope. This paper addresses an alternate calibration approach in which the PM4Sand model is calibrated using centrifuge in-situ CPT data. The model performance for the resulting calibration is evaluated against the centrifuge experimental data and prior simulations from Chiaradonna et al. (2022). In this case, the CPT-based calibration resulted in more accurate estimations of the dynamic response and permanent displacements.