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
Effect of Gradation on the Strength and Stress-Dilatancy of Coarse-Grained Soils: A Comparison of Monotonic Direct Simple Shear and Triaxial Tests
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
- Award ID(s):
- 1916152
- PAR ID:
- 10335418
- Date Published:
- Journal Name:
- GeoCongress 2022
- Page Range / eLocation ID:
- 226 to 236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
The strength and stress-dilatancy of uniform sands has been studied extensively in geotechnical investigations, and practitioners can draw on a wealth of previously reported data for the estimation of their volumetric response. However, the suitability of accepted stress-dilatancy theory and empiricism has not been evaluated for well-graded gravelly soils. Axisymmetric, isotropically consolidated drained compression, and pure shear, plane strain quasi-K0 consolidated drained tests were performed on well-graded Kanaskat gravel using confining pressures ranging over three orders of magnitude to determine its stiffness, strength, and stress-dilatancy response. The plane strain stiffness, strength, and stress-dilatancy of Kanaskat gravel is observed from tests performed using a large cubical true-triaxial device with flexible bladders. The observed response is interpreted with a view of experimental boundary conditions and their impact on the volumetric response. The observed plane strain shear modulus and friction, and dilation angles of well-graded sandy gravel soils commonly used in practice are significantly higher than those measured in the triaxial compression stress path. Existing empirical and modified stress-dilatancy expressions proposed for low confining pressures underestimate the observed dilation response; however, another common empirical approach appears to adequately capture the dilatancy. The data reported herein should help practitioners estimate plane strain behavior of sandy gravel mixtures.more » « less
-
Inherent particle properties such as size, shape, gradation, surface roughness and mineralogy govern the mechanical behavior of coarse-grained soils. Obtaining a detailed understanding of soil behavior requires parametrization of the individual effects of these properties; however, isolating these effects is a challenge in experimental studies. The advances in 3D printing technology provide the ability to generate artificial sand- and gravel-sized particles with independent control over their size, shape, and gradation. This paper summarizes the strength and stiffness behavior of specimens composed of 3D printed (3DP) particles. Specifically, results of triaxial compression and bender element tests on 3DP sands are provided and compared to corresponding results on the natural sands. The 3DP sands show characteristic behaviors of natural sands, such as dependence on effective stress and stress-dilatancy. However, the 3DP soils are more compressible due to the smaller stiffness of the constituent polymeric material. The results show a decrease in critical state friction angle (φ′cs) and an increase in shear wave velocity (Vs) as the particle roundness and sphericity are increased, in agreement with published trends for natural soils. The agreement in trends highlights the potential for investigations using 3DP soils to extend the understanding of soil behavior.more » « less
-
null (Ed.)Binary granular soil mixtures, as common heterogeneous soils, are ubiquitous in nature and man-made deposits. Fines content and particle size ratio are two important gradation parameters for a binary mixture, which have potential influences on mechanical behaviours. However, experimental studies on drained shear behaviour considering the whole range of fines content and different particle size ratios are scarce in the literature. For this purpose, we performed a series of drained triaxial compression tests on dense binary silica sand mixtures with 4 different particle size ratios to systematically investigate the effects of fines content and particle size ratio on the drained shear behaviours. Based on these tests, the strength-dilation behaviour and critical state behaviour were examined. It was observed that both fines content and particle size ratio have significant influence on the stress-strain response, the critical state void ratio, the critical state friction angle, the maximum dilation angle, the peak friction angle, and the strength–dilatancy relation. The underlying mechanism for the effects of fines content and particle size ratio was discussed from the perspective of the kinematic movements at particle level.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1061/9780784484043.022
