Granular Soils Research Articles

Experimental study on fractal characteristics of seepage-induced failure in granular soil

Experimental study on fractal characteristics of seepage-induced failure in granular soil

Discrete element modelling of thermal penetration test with heating and cooling

A three-dimensional discrete element method (DEM) is employed to simulate thermal penetration tests with heating and cooling. This study investigates the mechanical behavior of soil particles during penetration, and examines the effects of temperature boundary condition, overburden load, and heating duration on the heat transfer mechanisms of the probe-soil system. Simulation results of penetration indicate that larger overburden load causes an increase in tip resistance (qc), sleeve friction (fs), and the isotropy of particles, whereas less influence is found on the distributions of penetration-induced displacement. Despite of the negligible effects of various temperature boundary conditions, the thermal responses of DEM model show that both the peak temperature of the probe heating module and the cooling rate of soil increase with the overburden load, while the probe insulation section shows a decreasing trend. The heat concentrates around the vicinity of the cone tip and probe shaft, and the high temperature area gradually expands with longer heating duration. Additionally, thermo-coupling analysis illustrates that the heat transfer process has a milder effect on the mechanical properties of granular soils. The numerical results are then validated against laboratory model chamber tests under comparable conditions, showing acceptable agreements.

Initiation of flow liquefaction in granular soil slopes: drained versus undrained conditions

Initiation of flow liquefaction in granular soil slopes: drained versus undrained conditions

Particle shape effects on dynamic properties of granular soils: A DEM study

Particle shape is an essential aspect of granular soils that affects their mechanical properties. It is challenging to isolate the effect of particle shape from other mechanical properties such as inter-particle friction coefficient and particle shear modulus in laboratory tests. The discrete element method (DEM) makes it feasible to investigate the effect of macroscopic and mesoscopic particle morphologies including sphericity and roundness. This study provides a systematical DEM investigation of the particle shape effect on the dynamic properties of granular soils by simulating cyclic undrained triaxial tests at various initial confining pressures and void ratios. The results indicate that specimen consisting of angular particles generally exhibit a higher secant shear modulus G than that of rounded particles. Additionally, particle shape has no significant effect on modulus ratio G/G0 but a noticeable impact on damping ratio D at relatively large strains due to dilatancy. A weighted coordination number CN' was proposed to reveal the microscopic effect of particle shape on G0. Contact sliding was found to be the main cause of modulus degradation and damping increase with increasing shear strain. The tangential shear modulus ratio Gtan/G0 is inversely correlated to the weighted contact sliding ratio fs'.

Rainfall-induced flowslides of granular soil slopes: Insights from grain-scale modeling

Rainfall-induced flowslides in soil slopes remain one of the most destructive natural hazards. To prevent and mitigate the hazard requires a good understanding of soil behavior under the constant shear drained (CSD) stress path. This stress path differs from typical triaxial compression stress paths that have been extensively studied in soil laboratories. Here we present an attempt to explore various behaviors of loose granular packings along different stress paths and the associated micromechanical mechanisms using a robust grain-scale modeling technique. Through systematic simulations of the CSD, constant-volume (CDCV), and conventional drained (CD) shearing stress paths, we show that the behavior under the CSD stress path conforms to critical state concepts at both macro- and micro-scale. The evolutions of the stress ratio (q/p'), coordination number (Z), and fabric anisotropy (ac) along the CSD stress path are highly similar to those along the CD stress path. A substantial plastic strain tends to develop once the second-order work becomes nonpositive along the CSD stress path, while a dramatic drop of shear strength will occur at the vanishing of second-order work along the CDCV stress path. In both cases, the second-order work vanishes at an almost identical stress ratio, which demarcates the lower boundary of the potential instability zone in the stress space. We establish an explicit relationship between this stress ratio and the initial state parameter of the packing. The significance of this relationship lies in its applicability for both CSD and CDCV loading paths and its root in the critical state theory.

On torsional wave in void type porous layers between viscoelastic and piezoelectric media with parabolic irregularity

The viscoelastic earth model is increasingly utilized by seismologists for studying surface seismic wave propagation through Earth’s porous crustal regions such as granular materials, including rock, soils, and synthetic porous bodies. The present study characterizes torsional wave propagation in an irregular void-type porous layer sandwiched between an initially stressed inhomogeneous viscoelastic layer and a piezoelectric substratum. Displacement fields for the layer and half spaces are obtained separately using a variable separable technique, whereas volume fraction and electric potential field are derived for layer and substratum, respectively using the matrix calculus approach. Dispersion relations have been derived for both electrically open and short circuit conditions and agree with standard results. Particular cases have been deduced to validate the finding of the study with literature results. Effects of change in relevant parameters such as initial stress, heterogeneities, attenuation coefficient, void parameters, piezoelectricity and dissipation factor have been analyzed on the phase velocity of the torsional wave through various plots and summarized in a tabular form for ready reference to researchers. Analysis of the present geophysical problem may be useful in characterizing torsional velocity in porous silicate rocks which are abundantly found in the transition zone between oceanic and continental crust called Conrad discontinuity. Highlights Torsional propagation in a void-type porous layer crammed between an initially stressed inhomogeneous viscoelastic layer and piezoelectric half-space has been investigated. Variable separable technique, the matrix calculus approach is used for computing the displacement fields of respective media. A comparative study has been carried out for both electrically open and short conditions. The impact of pertinent parameters including initial stress, heterogeneities, attenuation coefficient, void parameters, piezoelectricity, and dissipation factor on the phase velocity of the torsional wave has been examined.

Load–Settlement Behavior of Coconut Midrib Strip Geogrid Reinforced Granular Soil

Load–Settlement Behavior of Coconut Midrib Strip Geogrid Reinforced Granular Soil

Effect of particle size ratio and fines content on drained/undrained behavior of binary granular soil according to confining pressure and packing density: a DEM simulation

Effect of particle size ratio and fines content on drained/undrained behavior of binary granular soil according to confining pressure and packing density: a DEM simulation

Role of geofoam inclusions on the liquefaction resilience of transportation geostructures

Though the effectiveness of geofoam buffers for improving the dynamic load response and seismic performance of road embankments and retaining walls is well-established, their role on soil liquefaction is not investigated by many. This study explores the potential of Expanded Polystyrene (EPS) geofoam inclusions for liquefaction mitigation in sands through a series of stress-controlled constant volume cyclic simple shear tests. The study quantifies the increase in liquefaction resistance of sand with geofoam inclusions and reduction in modulus degradation and shear deformations and increase in energy dissipation with loading cycles. Effects of geofoam density and thickness of the layer on the liquefaction response were also investigated through systematic series of experiments with geofoam of three different densities and two different thicknesses. Findings from this experimental study suggest that providing a thick low-density geofoam layer provided the maximum benefit among all the inclusions. Fundamental mechanisms governing this improved response are explored through a series of bender element tests. Results from this study confirm the effectiveness of geofoam buffers in mitigating liquefaction in granular soils and recommend their use in road embankments and retaining walls being constructed in zones of high seismicity to avoid liquefaction and associated lateral spreading failures in these structures.

Influence of hydraulic and geometric conditions on the early stages of the filtration process in idealised granular soils comprising spherical particles

AbstractThe early stages of particle‐scale filtration in idealised granular soils were investigated using coupled Computational Fluid Dynamics and Discrete Element Method. Filtration simulations were conducted for assemblies of finer base particles underlying coarser filter particles with upward seepage flow perpendicular to the base‐filter interface. A wide range of size ratios that covered the complete spectrum of filtration behaviours were considered, along with different ramping accelerations for seepage flow to investigate the combined influence of geometric characteristics and hydraulic loading. The influence of flow conditions and size ratios on the time‐dependent infiltration depth of transported base particles was observed to be universal with respect to a newly proposed dimensionless time parameter. The susceptibility to filtration during the early stages was quantitatively investigated with the number of infiltrating base particles and the filling degree of pores within the filter. The susceptibility during the early stages was shown to have a strong correlation with pore space geometry at the filter boundary, which was also reflected by the maximum pore filling degree of the filter. While filter susceptibility has been conventionally evaluated by the size ratio of the assembly, the consideration of the pore space geometry at the filter boundary provided an alternate approach of evaluating local filter susceptibility.

Groutability classification of granular soils with cement grouts

This research aims to develop a methodology for applying the geostatistical method to generate a groutability classification for granular soils. To ensure the precision of the suggested technique, a total of 103 data samples were used. Predicting the groutability of granular soils has always been difficult because of many soil characteristics. As a result, a new two-dimensional graph, the groutability classification of granular soil (GCS) chart, was developed. GCS establishment was based on data analysis of the grain size of soil and cement-based grouts (N1 and N2), relative density (Dr) and fines content of the soil (FC), water/cement ratio of grout mixture (w/c), and grouting pressure (P), all of which have a direct impact on the groutability of soil media. The geostatistical method was used to develop and compile the GCS graph based on the aforementioned parameters with the use of coefficient S, which is a coefficient of the scoring set of parameters including P, w/c, Dr, and FC. The validation process was carried out hierarchically, with an additional set of 30 data. The proposed method has a prediction accuracy of roughly 96.7%, demonstrating a helpful tool. The proposed approach can be easily implemented in practical engineering situations because it has a comparable syntax to commonly used formulae. It should be noted that the proposed formula was only tested using the data samples collected, and the applicability of the produced procedure to other situations requires more examination.

Seismic Bearing Capacity of Eccentrically and Obliquely Loaded Strip Footings on Geosynthetic-Reinforced Soil

The paper aims to evaluate the pseudostatic seismic bearing capacity of shallow foundations on geosynthetic-reinforced granular soil subjected to inclined/eccentric combined loading. This is achieved through a comprehensive set of lower-bound finite-element limit analysis (LB FELA) simulations adopting the second-order cone programming. The pseudostatic seismic loading is incorporated into the element equilibrium equations as body forces. Both sliding and structural modes of geosynthetic failure are simulated within the FELA framework. The influences of pseudostatic acceleration, embedment depth (u), and the ultimate tensile strength (Tu) of the reinforcement layer on the failure mechanism, bearing capacity ratio, and failure envelope of the overlying obliquely/eccentrically loaded shallow foundation are thoroughly investigated and discussed. It is generally concluded that at a given reinforcement embedment depth, failure envelopes shrink substantially with the increase of the seismic horizontal acceleration and the reduction of the geosynthetic ultimate tensile strength.

A Methodology to Investigate the Design Requirements of Plant Root-Inspired Robots for Soil Exploration

This paper proposes a methodology to investigate soil penetration requirements to extract specifications for designing robotic soil excavators and explorers. To this purpose, a three-dimensional (3-D) numerical model based on the Discrete Element Method (DEM) is proposed to simulate an intruder penetration and its interaction with the environment. In this study, the penetration process was analyzed as a function of the intruder diameter to median particle size ratio (Droot/D50), highlighting important differences for small and big ratios conditions (i.e., Droot/D50<<1 and Droot/D50>>1). In particular, the soil resistance force that autonomous penetration systems must overcome when moving into cohesionless granular soil was estimated based on the intruder size (diameter) and the median granularity (D50). This study estimates how the penetration requirements vary according to different penetration strategies. Specifically, a plant root-inspired axial movement emulating the growth from the tip adopted by plants is compared with a penetration obtained by pushing a system from the top. Our findings provide important guidelines about the design requirements (system size, penetration strategy, and actuation power) for artificial penetrating systems, like autonomous explorative robots, to improve their performances during underground exploration.

Evolution of Void Fabrics and Their Effects on Liquefaction Behaviors of Granular Soils: Insight from DEM-Clump Simulation

Previous studies have demonstrated that macroscopic liquefaction behavior is closely related to the evolution of the microstructure (fabric) of soils. Here we perform three-dimensional (3D) discrete-element method (DEM) simulations of soil liquefaction considering irregular shapes of Toyoura sand particles, which are approximated by 3D clumps. We introduce the Minkowski tensor to quantify the anisotropy of the particle-void cells around each clumped particle, and propose two void fabric proxies, Ed and Ad. The evolution of Ed and Ad during the entire liquefaction process and their effects on pre- and postliquefaction behaviors are investigated. Specifically, Ed quantifies the shape elongation of particle-void distribution, while Ad represents anisotropy of the particle-void orientation, which is closely correlated with the flow strain accumulation. We further quantify irreversible changes in fabrics and anisotropic load-bearing structures developed in liquefaction processes, and propose fabric-based criteria for jamming transition in flow deformation. The volumetric strain during reconsolidation is well correlated with Ed and Ad, whereas the jamming transition during liquefaction can only occur if Ad becomes sufficiently large. The interplay between strain, stress, and fabric evolution during pre- and postliquefaction will improve fundamental understanding and modeling of liquefied soils.

Numerical Analysis of Single Stepped Axially Loaded Pile

Abstract: This study investigates the behavior of a single pile under axial loading in granular soil layers. The study aims to determine the impact of stepping on load settlement behavior and load transformation along the pile, as it relates to depth. The pile has a diameter (D) of 1m and a length (L) of 20m. For the stepped pile, the upper portion is half the length of the pile and the lower portion is the remaining half. The total length and volume of the pile are kept constant for both the single conventional pile and single stepped pile. In the analysis, the diameter of the upper portion of the stepped pile is reduced (stepped pile) by 0.1D to 0.3D, and the diameter of the lower portion is calculated accordingly. Also the load transformation with respect to depth of pile is studied. Based on numerical analysis, the single stepped pile outperforms the single conventional pile in load settlement behavior.

Discrete-element method simulations of the seismic response of tunnels in deep granular deposit

The seismic response of tunnels constructed in a deposit of granular soil of depth greater than 20 m is examined in this paper. Three-dimensional discrete-element method (DEM) simulations were used to model the soil assembly and the tunnel lining. Soil particles were simulated as rigid spherical particles that are allowed to overlap at the contact points. The tunnel lining was modeled as a flexible system using a collection of particles that were glued together using cementitious bonds to mimic the physical properties of an actual tunnel. Free-field boundaries were utilized at the model’s lateral sides to prevent the propagating wave’s reflections back to the domain and enforce the free-field motion. The DEM computational framework was validated using published centrifuge test results. DEM simulation results reveal good agreement with the experimental centrifuge test results when subjected to the same input motion. Further simulations were carried out to investigate the seismic response of the system under different input motion characteristics. The dynamic earth pressure was monitored at certain points on the tunnel lining. It was found that the dynamic earth pressure and tunnel lining forces increased during seismic excitation and the final residual values were, in most cases, considerably larger than the initial static values. The results also showed that the maximum amplification factor, ground settlement and tunnel lining internal forces occurred close to the resonant frequency of the system.

Examination of interface roughness and particle morphology on granular soil–structure shearing behavior using DEM and 3D printing

Understanding the mechanical behavior of soil-structure interaction (SSI) is crucial for the safety and stability of infrastructures. From mesoscale to macroscale, the coupled effects of particle morphology and structural surface roughness on SSI has received little attention. Herein, a series of interface shear tests and simulations were carried out by combining 3D printing and discrete element method (DEM). To quantitatively characterize particle shape and structural surface roughness, modified particle shape parameter Sh and interface morphology factor λ are introduced. The results indicate that increasing the inclination of asperities 1/λ promotes lateral slip displacement of particles and results in significant delamination at the interface. In the vertical shear direction, particle displacement exhibits two patterns: backward falling and maximum displacement at the top of the shear zone, and continuous forward displacement decrease along the height of the specimen. Thus, a new index (i.e., maximum backward falling displacement of particles) is proposed to define the shear zone thickness. Particle shapes and interface roughness have different effects on the particle displacement in the vertical shear direction. An increase in Sh reduces relative displacement, while an increase in 1/λ can increase the magnitude of the relative displacement of particle lateral sliding. The overall strength of particles depends on the coupling effect of particle shapes and interface roughness, both of which have corresponding thresholds. As the interface roughness or Sh increases, three parameters used to describe the contact state of clusters first increase until reaching a turning point and then remain stable or slightly decrease. This paper aims to broaden the understanding of the coupling effects of interfaces with various roughness and particles with different shapes on SSI, with the ultimate goal of contributing to the design of structural interfaces more effective and safety in sandy soil area.

One-dimensional compression behavior of granular soils around virgin compression line (VCL)

One-dimensional compression behavior of granular soils around virgin compression line (VCL)

Bearing capacity of shallow strip foundations on reinforced soil subjected to combined loading using upper bound theorem of finite element limit analysis and second-order cone programming

Bearing capacity of shallow strip foundations placed over both unreinforced and reinforced granular soils and subjected to combined loading is calculated by implementation of upper bound finite element limit analysis in conjunction with second-order cone programming technique. The soil mass is considered to be a granular medium reinforced with the inclusion of horizontal single and double layers of geogrid elements. To represent the impact of tensile resistance of reinforcement elements, an axillary variable is utilized to simulate the plastic energy dissipation of reinforcing elements without the application of any stress variables. General loading conditions are considered to profile the expansion of the failure envelopes in the V-H and V-M spaces for shallow strip foundations resting on reinforced soils. Results of this study are compared and validated against three different cases including the bearing capacity of shallow strip foundations subjected to vertical loading in the case of perfectly smooth and fully rough interfaces, limit load of shallow strip foundations subjected to combined loading, and vertical bearing capacity of shallow strip footings over a soil deposit reinforced with a single-layer of geosynthetic. Consistency of the results with those reported in the literature demonstrates the efficiency of proposed extended finite element upper bound formulations. The results are reported in the form of an efficiency factor which can be multiplied by the vertical bearing capacity of the shallow strip footing over unreinforced soil to find the ultimate bearing capacity of the reinforced foundation. The results show that the load inclination angle and eccentricity have significant impacts on the limit load and the optimum embedment depths of the reinforcement layer. It is further noted that the effect of reinforcement layer(s) in the improvement of bearing capacity in the V-M space is more pronounced than in the V-H space. The efficiency of reinforcement elements inclusion in improving the expanse of safe loading domain in the V-H and V-M spaces enhances with an increase in the soil internal friction angle.

A coupled FEM-DEM study on mechanical behaviors of granular soils considering particle breakage

Particle breakage often occurs in the fields of geological, geotechnical and hydraulic engineering like rock avalanche, landslides and high earth-rock dam, which affects the design and countermeasures of relevant engineering structures. Investigation of particle breakage’s effect on mechanical behaviors of granular soils has always been the hotspot within the community of particulate materials. In this study, a coupled finite element method (FEM) and discrete element method (DEM) modeling has been developed to tackle boundary value problems (BVPs) related with particle breakage. In the method, the particle breakage simulation is implemented in DEM through the conventional bonded particle modeling (BPM), while the stress–strain characteristics of representative volume element (RVE) captured by DEM is directly transmitted into the FEM solver to avoid the complicated and phenomenological constitutive equations incorporating into particle breakage. As an illustration, we apply the proposed multiscale modeling to investigating mechanical behaviors of quartz sands subjected to particle breakage through the high confining pressure biaxial compression simulation. And effects of particle strength, particle shape and confining pressure on particle breakage characteristics and further mechanical behaviors of quartz sands are also discussed accompanied with the underlying micromechanisms. Results demonstrate that the coupled FEM-DEM modeling can be able to deal with the complex geomechanics problems involving particle breakage.