Given the insufficiency in research on the mechanism of fine particle impact on gravelly soil subgrade deterioration, a series of saturated gravelly soil consolidated drained triaxial shear tests was conducted using the GDS triaxial testing system under varying fines contents and effective confining pressures to investigate the effect of fine particle contamination on the static shear characteristics of gravelly soil. The results indicate that: (1) As the fines content increases, the stress-strain curve development pattern transitions from strain softening to strain hardening, with a critical threshold at a fines content of Fc=15%. (2) The addition of fine particles leads to a decrease in the principal stress ratio, brittleness index, peak strength, cohesion, and internal friction angle of the gravelly soil, while the degradation indices increase. The relationship between the degradation indices of peak strength and cohesion and fines content can be described by quadratic functions, and the degradation index of the internal friction angle by a cubic function. (3) With increasing fines content, critical state parameters decrease. The effective stress path shows retracing behavior, becomes shorter, and shifts to the left. (4) The addition of fine particles results in a decrease in the secant modulus, and the volumetric strain-axial strain curve changes from contractive-dilative to purely contractive.

Evaluating cyclic liquefaction of soil from the perspective of energy dissipation provides a more comprehensive insight into its liquefaction mechanism. This study conducted a series of undrained cyclic triaxial tests using discrete element method to investigate the influence of plastic fines content (FC) on the dynamic characteristics of sand-clay mixtures. A new evaluation index, the Viscous Energy Dissipation Ratio (VEDR), is introduced to assess the energy dissipation performance of sand-clay mixtures. Macroscopically, it is shown that when FC 30 %, the trend reverses. In terms of energy dissipation, as the fines content increases, VEDR gradually transitions from the sand-like to the clay-like mode, exhibiting a unique transitional mode when FC = 50 %. Microscopically, the development of bond breakage is highly similar to that of VEDR. The bond breakage facilitates particle sliding and rolling, which is the fundamental factor causing the differences of energy dissipation between pure sand and sand-clay mixtures. This paper contributes to the mechanistic study of liquefaction criteria based on energy theory by establishing the connection between microscopic particle behavior and macroscopic energy dissipation during the cyclic liquefaction process.

When loose, saturated sands and non-plastic silts are subjected to undrained cyclic loading, they will generate positive pore pressures. This increase in pore pressures leads to a decrease in effective stress with a corresponding decrease in shear strength and increase in liquefaction susceptibility. For combinations of sand and non-plastic silt, the threshold fines content can be defined as the non-plastic silt fines content at which the soil changes from sand-like behavior to silt-like behavior. Soils below the threshold fines content behave like sands and soils above the threshold fines content behave like silts. During cyclic triaxial and cyclic direct simple shear tests performed on specimens of sand and silt prepared to the same relative density but different fines contents, two rates of pore pressure generation were observed. When compared at five cycles of loading, soils with silt contents above the threshold fines content were found to produce pore pressure ratios as much as 50% higher than those observed for soils with silt contents below the threshold fines content. When evaluated in terms of cycles, cycle ratio, and dissipated energy ratio, the rate of pore pressure generation was found to be more rapid for soils above the threshold fines content than for soils below the threshold fines content.

Evaluating the cyclic strength development using energy-based methods is a novel concept in studying the dynamic properties of sand-clay mixtures under cyclic loading. In this study, a series of undrained cyclic triaxial tests were conducted on sand-clay mixtures, and the performance of different fine-grain contents on the dynamic properties and the energy dissipation of sand-clay mixtures was investigated based on the energy-based methods. The results demonstrated a gradual increase in the cyclic strain amplitude and the residual axial strain with increasing fines content (FC) under cyclic loading with a controlled cyclic stress ratio; in contrast, the accumulation of pore-water pressure slowed down. An initial decrease in the cyclic strength of the mixtures was observed with an increase in their fines contents; however, further increasing the FC enlarged the cyclic strength of the sand-clay mixtures. This transition was observed when the threshold fines content reached about 30%. The viscous energy dissipation ratio (VEDR), which is a nondimensional energy ratio based on the relationship between cyclic stress and strain and reflects the characteristics of dynamic properties, was utilized to compare three critical phase transition points, namely, VEDRvalley, VEDRpeak, and VEDR5%strain, in the energy dissipation of the sand-clay mixtures. Based on the VEDR results, the cyclic strength development indexes were established. Furthermore, low-vacuum environmental scanning electron microscopy revealed that as the FC increased, the particle composition of the sand-clay mixtures transitioned from predominantly coarse-grained to fine-grained, resulting in a change in the cyclic behavior of the mixtures from sandlike to claylike. The cyclic strength development indices provided further insights into and quantified the effect of fines contents of the sand-clay mixtures on their cyclic strength development process.

The presence of fines can significantly influence the mechanical behavior of soils. In this study, a hypoplastic model is extended to simulate the stress-strain relationship of sand-fines mixtures. Firstly, three modifications are incorporated into the model to accurately simulate the effective stress path, hardening rate, and limited flow type response of sand during undrained loading. Additionally, a novel formulation is proposed to capture the critical state line of soil mixtures across a wide range of fines content. This formulation is then integrated into the characteristic void ratios of the hypoplastic model, enabling it to effectively consider the combined influence of void ratio, confining pressure, and fines content on the density state of the sand-fines mixtures. The predictive capability of the model is demonstrated through a comparison of simulation results and experimental data for undrained triaxial tests conducted under various conditions.

This study explores the impact of non-plastic fines content, initial confining pressure, and grading characteristics on the undrained shear strength and excess pore pressure of sand-silt mixtures; a series of undrained compression triaxial tests were carried out on reconstituted Chlef sand (Algeria) samples with different percentages of silt content (Fc = 0, 5, 10, 15, and 20%), at an initial relative density (RD = 50%) subjected under three different confining pressures (p ' c = 20, 50, and 100 kPa). Observations from these tests unveiled intriguing insights. Notably, it was discovered that soil specimens with lower fines content and higher initial confining pressures showed increased resistance to liquefaction. Conversely, liquefaction resistance diminished under conditions of higher fines content and lower initial confining pressures. Moreover, the analysis of test results underscored the substantial influence of gradation on the peak shear strength and maximum excess pore pressure of sand-silt mixtures. This suggests that the distribution of particle sizes within the mixture plays a pivotal role in its mechanical behavior and susceptibility to liquefaction. Furthermore, the study's findings revealed the presence of straightforward correlations between various parameters. These correlations include those between peak shear strength (qpeak), maximum excess pore pressure (Delta umax), fines content (Fc), initial confining pressure (p ' c), and specific grading characteristics such as D10, D30, D50, D60, Cu, D10R, D50R, and CuR. These correlations offer valuable insights into the interplay of factors affecting the mechanical properties of sand-silt mixtures, aiding in the development of predictive models and engineering solutions for infrastructure projects.

Seismic events remain a significant threat, causing loss of life and extensive damage in vulnerable regions. Soil liquefaction, a complex phenomenon where soil particles lose confinement, poses a substantial risk. The existing conventional simplified procedures, and some current machine learning techniques, for liquefaction assessment reveal limitations and disadvantages. Utilizing the publicly available liquefaction case history database, this study aimed to produce a rule-based liquefaction triggering classification model using rough set-based machine learning, which is an interpretable machine learning tool. Following a series of procedures, a set of 32 rules in the form of IF-THEN statements were chosen as the best rule set. While some rules showed the expected outputs, there are several rules that presented attribute threshold values for triggering liquefaction. Rules that govern fine-grained soils emerged and challenged some of the common understandings of soil liquefaction. Additionally, this study also offered a clear flowchart for utilizing the rule-based model, demonstrated through practical examples using a borehole log. Results from the state-of-practice simplified procedures for liquefaction triggering align well with the proposed rule-based model. Recommendations for further evaluations of some rules and the expansion of the liquefaction database are warranted.

Partially saturated soils below the groundwater table usually exist because of decomposition of organics within the soil deposit or soil desaturation technologies for liquefaction mitigation. Therefore, the cyclic responses of partially saturated soils should be paid more attention to and it is necessary to establish a practical model to evaluate the liquefaction resistance improvement LRR20 (the ratio of liquefaction resistance of partially saturated soil to that of fully saturated soil) due to the decrease of saturation. Some researchers have tried to quantify the liquefaction resistance of partially saturated soil using the P-wave velocity Vp. However, whether the LRR20-Vp relationship is influenced by initial states such as relative density, structure or fabric of soil, and fines content, is not clear yet. For this purpose, an experimental programme comprising undrained cyclic triaxial and bender elements tests was performed on partially saturated soils under various conditions. Results show that the cyclic shear behaviors of partially saturated sands with different B-values are significantly different from those of fully saturated sands. For partially saturated sands, as excess pore water pressure (EPWP) accumulates to a certain proportion of initial effective confining pressure, it suddenly develops with a significant increase followed by the occurrence of more than 5 % of axial strain. Based on the test results including those from independent literature, a unique experimental LRR20-Vp relationship is proposed for different relative densities, sample preparation methods, initial effective confining pressures, and fines contents.