skip to main content

Explore Research Products in the NSF-PAR

Title: The effect of gradation on the response of saturated sands when subjected to seismic loading: A centrifuge test
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.  more » « less
Award ID(s):
1916152
NSF-PAR ID:
10335413
Author(s) / Creator(s):
; ; ; ;
Date Published:
Journal Name:
GeoNiagara 2021
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ( , Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering)
    Rahman and Jaksa (Ed.)
    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. 
    more » « less
  2. ; ( , GeoCongress 2020)
    Recent studies have focused on how the dynamic response of a clean sand changes with increasing fines content; however, there remains a limited understanding regarding the effects of increasing coarse content. This study aims to elucidate these effects at a system level via centrifuge testing of two uniformly-graded and one well-graded soil mixture which range in mean grain diameter (D50) from 0.18 to 2.58 mm and in coefficient of uniformity (CU) from 1.53 to 7.44. Models of each soil mixture were prepared to approximately 50% relative density (DR) and subjected to uniform cycles of sinusoidal acceleration at various Arias intensities (Ia). The high hydraulic conductivity (k) of the coarsest, uniformly-graded mixture prevented significant excess pore pressure generation; however, liquefaction was induced in the other two mixtures. Furthermore, the well-graded mixture exhibited a stronger dilative tendency than the clean sand. The centrifuge results were compared to cyclic direct simple shear (DSS) results in order to consider the complementary perspectives that centrifuge and element testing can provide. 
    more » « less
  3. ( , Geo-Congress 2023)
    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. 
    more » « less
  4. ; ; ( , International Journal for Numerical and Analytical Methods in Geomechanics)
    Summary

    Liquefaction is associated with the loss of mean effective stress and increase of the pore water pressure in saturated granular materials due to their contractive tendency under cyclic shear loading. The loss of mean effective stress is linked to loss of grain contacts, bringing the granular material to a “semifluidized state” and leading to development and accumulation of large cyclic shear strains. Constitutive modeling of the cyclic stress‐strain response in earthquake‐induced liquefaction and post‐liquefaction is complex and yet very important for stress‐deformation and performance‐based analysis of sand deposits. A new state internal variable named strain liquefaction factor is introduced that evolves at low mean effective stresses, and its constitutive role is to reduce the plastic shear stiffness and dilatancy while maintaining the same plastic volumetric strain rate in the semifluidized state. This new constitutive ingredient is added to an existing critical state compatible, bounding surface plasticity reference model, that is well established for constitutive modeling of cyclic response of sands in the pre‐liquefaction state. The roles of the key components of the proposed formulation are examined in a series of sensitivity analyses. Their combined effects in improving the performance of the reference model are examined by simulating undrained cyclic simple shear tests on Ottawa sand, with focus on reproducing the increasing shear strain amplitude as well as its saturation in the post‐liquefaction response.

     
    more » « less
  5. ; ; ; ; ; ( , GeoCongress 2022)
    A broad spectrum of well-graded, coarse-grained soils are commonly present in natural deposits, though characterization of these materials has been approximated using sand-based engineering methods in liquefaction evaluations. Through combined results of 31 constant stress direct simple shear and drained triaxial compression tests, this study experimentally investigates the effect of mean grain size (D50) and gradation (Cu) on the drained monotonic strength and stress-dilatancy of poorly- to well-graded, coarse-grained soils. Coarse-grained mixtures of varying D50 and gradations were prepared to relative densities of 20%–75% and tested under a range of overburden stresses. Results are analyzed in terms of the frictional resistance and dilative contributions to the shear strength of soils with varying gradations, as compared to clean sands, using different shearing modes. It is shown that (1) increased gradation of soils increases the peak shear strength and frictional resistance due to a greater initial rate of dilation exhibited in well-graded, coarse-grained soils; and (2) current stress-dilatancy relationships underestimate the dilative behavior of well-graded test materials. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email