The soil fabric varies significantly depending on the deposition process that forms the grain skeleton. Each deposition method produces a specific type of soil fabric, which can be linked to a particular soil density. When represented as relative density, determined using limit densities from standard index tests, a wide range of relative densities can be observed for different sands produced by the same deposition method. The influence of this variation in relative density, resulting from a single deposition method, on the development of the excess pore water pressure (PWP) should be further investigated. A fast testing of the excess PWP accumulation in sandy soils during undrained cyclic shearing can be easily performed using the newly developed PWP Tester. In the PWP Tester, specimens are prepared through sedimentation in water, which yields a comparable fabric in different sands but significantly different relative densities. Despite these relative density differences, the rate of the excess PWP evolution during undrained shearing is remarkably similar among different sands. This indicates that relative density should not be regarded as a primary factor influencing the development of the excess PWP and that the soil fabric plays equal or even a greater role.

This study investigates the seismic response of a reinforced concrete (RC) tunnel using two-dimensional plane strain finite element models calibrated and validated against experimental results. A comprehensive parametric study is then conducted to explore the influence of tunnel-soil flexibility ratio, soil relative density, Arias intensity of the input motion, and ground motion components on the seismic soil-structure interaction (SSI). The results demonstrated that the flexibility ratio and racking coefficient increase with overburden height, while soil deformations decrease. Acceleration amplification factors rise from the bottom soil to the ground surface, with dense soil showing higher amplification especially in the regions at and near the tunnel field. The horizontal amplification factor exhibits greater variability with increasing seismic energy intensity, and the effect of the vertical motion becomes more pronounced near the structure. The vertical amplification factor is lowest for the horizontal component, while the vertical and combined components exhibit higher values influenced by the presence of the tunnel with lower earthquake intensity. Soil relative density significantly influences the vertical and lateral pressures on the tunnel, with dense sand causing maximum vertical pressures on the top slab and walls. The vertical earthquake component has a greater impact on the tunnel's top slab pressure distribution than the horizontal component. Seismic bending moments are influenced by earthquake components, with the vertical component leading to the greatest positive bending moment values in the middle of the roof slab. Vertical soil deformation is significantly affected by the horizontal input motion component, whereas the vertical component minimally affects lateral soil deformation. These findings underscore the importance of capturing stress-strain response under cyclic loading, particularly near the tunnel crown, where complex stress interactions lead to increased variability in behavior.

Soil liquefaction poses a significant risk to both human lives and property security. Recent in-situ cases have shown that clayey sand can experience multiple liquefaction events during mainshock-aftershock sequences, known as repeated liquefaction. While existing studies have focused on the cyclic behavior of initial liquefaction events, there is a lack of research on the mechanisms and cyclic response of repeated liquefaction in clayey sand. The factors that control repeated liquefaction in clayey sand are still not fully understood. In this study, a series of cyclic triaxial tests were conducted on sand with varying clay content (0 %, 5 %, 10 %, 15 %, and 20 %) under earthquake sequences. The test results showed that the liquefaction resistance initially decreased significantly and then increased with the number of liquefaction events. Sands with higher clay content exhibited earlier recovery of resistance during continuous liquefaction events. The analysis of the test results revealed that the repeated liquefaction resistance of clayey sand was quite intricate. Sands with a relative density (after reconsolidation) below 80 % were primarily influenced by the degree of stress-induced anisotropy, while sands with a relative density above 80 % were mainly affected by relative density.

Current research on soil-structure interface properties mainly focuses on sand, clay, and silt, with little attention given to sandy gravel. In order to study the effects of relative density and interface materials on the shear behavior of the sandy gravel-structure interface, a series of large-scale direct shear tests on sandy gravel were carried out, and stress-strain relationships, volume change curves, and shear strengths were investigated. The results show that the angle of internal friction of sandy gravel increases linearly with relative density (R2 is 0.998), from 43.0 degrees to 48.0 degrees when the relative density increases from 0.3 to 0.9. The growth trend of cohesion increases, the shear behavior transitions from strain hardening to strain softening, and the shear strength increases linearly with the increase in relative density. The interfacial shear strengths and interface adhesion of sandy gravel with steel and concrete interfaces increase linearly with relative density, and the shear curves are strain hardening. Furthermore, the interface friction angle of concrete increases linearly with relative density (R2 is 0.985), from 30.2 degrees to 34.2 degrees, while the interface friction angle of the steel interface remains relatively constant around 28.9 degrees. Finally, relative density was introduced into the Mohr-Coulomb shear strength formula, and the relationship equations of relative density and normal pressure with the shear strength and interfacial shear strength of sandy gravel were established. The validation results show that the error margin of the formula is within 4%. This formula can be used to evaluate changes in the mechanical properties of sandy gravel formations and the bearing capacity of pile foundations after they have been disturbed by factors such as construction.

At least 32 case histories have shown that liquefaction can occur in gravelly soils (both natural deposits and manmade reclamations) during severe earthquakes, causing large ground deformations and severe damage to civil infrastructures. Gravelly soils, however, pose major challenges in geotechnical earthquake engineering in terms of assessing their deformation characteristics and potential for liquefaction. In this study, aimed at providing valuable insights into this important topic, a series of isotropically consolidated undrained cyclic triaxial tests were carried out on selected sand-gravel mixtures (SGMs) with varying degrees of gravel content (Gc) and relative density (Dr). The pore water pressure generation and liquefaction resistance were examined and then further scrutinized using an energy-based method (EBM) for liquefaction assessment. It is shown that the rate of pore water pressure development is influenced by the cyclic resistance ratio (CSR), Gc and Dr of SGMs. However, a unique correlation exists between the pore water pressure ratio and cumulative normalized dissipated energy during liquefaction. Furthermore, the cumulative normalized energy is a promising parameter to describe the cyclic resistance ratio (CRR) of gravelly soils at various post-liquefaction axial strain levels, considering the combined effects of Gc and Dr on the liquefaction resistance.

Tunnel construction in urban areas may result in ground deformations that pose a risk to existing buildings and infrastructure; thus, accurate prediction of these induced ground deformations during the design phase is crucial. The paper focuses on the effects of sandy soil relative density on the ground deformations induced by tunnels excavated with Tunnel Boring Machines (TBMs). The study utilizes the Finite Element Method (FEM) and the NorSand model to simulate the behavior of a sandy ground. The validity of the FEM modeling approach is established by comparing predictions with results from six centrifuge tunnel tests from the literature. The centrifuge tests were performed on sand at different relative densities, tunnel diameters, and tunnel depths. The parameters for the NorSand model were determined based on laboratory tests. Only the state parameter was modified to achieve the desired relative density in the numerical simulations. The effects of relative density observed in centrifuge tests (Franza et al., 2019) have been numerically reproduced with no further adjustments of the model parameters. The rich outputs from the numerical models enabled an in-depth investigation of tunnel behavior, yielding new insights into how tunnels respond under varying relative densities, depths, and diameters. A comprehensive analysis of the induced ground deformations caused by shallow tunnels in sandy ground and the potential to damage buildings is included.

This paper employs Discrete Element Method (DEM) simulations to investigate the influence of relative density on soil arching within a plane -strain active trapdoor scenario. For varying relative trapdoor depths, DEM simulations illustrate the key influence of dilatancy on displacement and strain fields and on stress rotation and trapdoor pressure, confirming that shear bands develop at the trapdoor depending on the soil's dilation angle. The interplay between dilatancy and soil cover governs the arching phenomenon and the ground deformation mode; the significance of relative density is also highlighted by its effects on the principal stress rotation and ground reaction curves. To predict the minimum trapdoor pressure, we propose a Limit Equilibrium Method (LEM) solution that considers the type of failure mechanism (trapezoidal or triangular) and the lateral earth pressure as a function of the soil's dilatancy and stress arching shape; this approach coincides with Terzaghi's soil pressure concept at the critical state. LEM predictions of minimum and ultimate (or terminal) trapdoor pressure, and of shear deformation modes, are validated with our DEM results and with literature results. Finally, the impact of effective stresses and relative density on deformation patterns and design charts that quantify the minimum trapdoor pressure is discussed.

Liquefaction phenomenon refers to a phenomenon in which excess pore water pressure occurs when a dynamic load such as an earthquake is rapidly applied to a loose sandy soil ground where the ground is saturated, and the ground loses effective stress and becomes liquid. The laboratory repetition test for liquefaction evaluation can be performed through a repeated triaxial compression test and a repeated shear test. In this regard, this study attempted to evaluate the effects of the relative density of sand on the liquefaction resistance strength according to particle size distribution using repeated triaxial compression tests, and additional experimental verification using numerical analysis was conducted to overcome the limitations of experimental equipment. As a result of the experiment, it was confirmed that the liquefaction resistance strength increased as the relative density increased regardless of the classification of soil, and the liquefaction resistance strength of the SP sample close to SW was quite high. As a result of numerical analysis, it was confirmed that the liquefaction resistance strength increased as the confining pressure increased under the same relative density, and the liquefaction resistance strength did not decrease below a certain limit even though the confining pressure was significantly reduced at a relatively low relative density. This is judged to be due to a change in confining pressure according to the depth of the ground. As a result of analyzing the liquefaction resistance strength according to the frequency range, it was confirmed that there was no significant difference from the laboratory experiment results in the basic range of 0.1 to 1.0 Hz.

In this study, the effects of relative density and confining pressure on the shear characteristics of round gravel are investigated using a large-scale triaxial apparatus and the discrete element method. A simple and efficient numerical method for simulating flexible membranes is introduced. The results show that the stress-strain curves develop from hardened to softened type with increasing relative density, while the stress-strain curves develop from softened to hardened type with increasing confining pressure. As the axial strain increases, the strong contact force chains are vertically distributed, and the larger the relative density and confining pressure, the greater the number and thickness of the strong contact force chains. In the shear process, the distribution of average normal and tangential contact forces show peanut-shaped and petal-shaped, respectively. The increase in relative density increases the anisotropy of the specimen, while the increase in confining pressure results in a decrease. A linear relationship exists between the macroscopic stress ratio and the anisotropy coefficient. The anisotropy coefficient of the normal contact force provides the greatest contribution to the macroscopic shear strength (about 55%), followed by the anisotropy coefficient of the contact normal (about 26%) and that of the tangential contact force (about 19%).

In this study, a series of cyclic triaxial tests were conducted on two poorly graded sands with different particle shapes. The experimental results were integrated with test results reported in the literature to construct a comprehensive database for investigating the effects of particle shape and relative density on excess pore water pressure (EPWP) generation. The sand types in the database were divided into three groups based on the particle shape: subrounded, subangular, and angular. The test results showed that the particle shape and relative density influenced the axial strain development and EPWP generation during undrained cyclic loading. As the relative density increased, the effect of the particle angularity on the shape of the EPWP generation curves decreased. The normalized EPWP generation curves of the subangular particles (C306 sand) exhibited a strong dependence on the relative density. However, the angular and subrounded particles exhibited a weaker influence than the subangular particles on EPWP change for different relative densities. A model was established to predict the EPWP generation with variations in the particle shape and relative density.