Granular Soils Research Articles

Propagation velocity of landslide-induced liquefaction and entrainment of overridden loose, saturated sediments

Loose saturated granular materials are particularly susceptible to instability, resulting in deviatoric strain softening, and static liquefaction. When instability occurs in the context of a landslide, the consequences in terms of the mobility of the debris and risk to life and property can be catastrophic. Physical model landslides initiated in a geotechnical centrifuge under rising groundwater conditions were used to trigger instability and static liquefaction. Four experiments with a loose contractile soil and two experiments with a dense dilative soil were performed. The velocity of propagation of the liquefaction front within the loose granular soils at the base of a landslide was quantified using a dense sensor network of pore water pressure sensors and high-speed imaging. On triggering of a landslide, a localized toe failure was observed to shear and liquefy the soil at the base of the landslide. However, the velocity of this initial failure (0.5 m/s) was an order of magnitude slower than the subsequent 4.2 m/s propagation velocity of the liquefaction front. These experiments demonstrated and quantified how a localized failure onto a liquefiable deposit may propagate liquefaction much farther than simply the runout of the localized failure, and highlight the potential implications and consequences of such an occurrence.

Reference site selection based on state-and-transition models for soil health gap evaluation within cropland reference ecological units

Comparative soil health studies are critical in soil conservation and gauging the success of different management practices in soil health improvement. A primary challenge in these studies is the selection of a consistent natural reference site. Current choices vary widely, from minimally disturbed areas to pristine prairies. This methodological paper underscores the need for deliberate and thoughtful reference site selection for benchmark soil properties. Utilizing the State &Transition models, the study introduces a framework for this selection, drawing upon the ecological site (ES) and reference plant communities detailed in an ecological site description (ESD) within a respective Major Land Resource Area (MLRA). This study advocates for a localized classification within the framework of the Cropland Reference Ecological Unit (CREU), emphasizing the significance of local precipitation and soil data to ensure unbiased comparisons. Soil samples from eastern (MLRA 106) and western (MLRA 67A) Nebraska were collected, representing distinct pedogenetic and climatic differences. Analysis of soil organic matter between MLRAs displayed substantial variations, suggesting potential biases and complexities in soil health gap calculations when using reference sites and croplands not in the same MLRA, soil types (texture class), or precipitation zones. However, refining the comparisons by delineating the MLRA, soil, and precipitation zones within the framework of CREU yielded more consistent and realistic comparative data. Integrating the MLRA and ES, complemented by granular soil and precipitation data, provides a robust method for establishing soil health benchmark data.

Development of dynamic centrifuge models for measurement and visualization of deformation mechanisms in liquefiable soils

Recent advancements in geotechnical physical modeling have enabled visualization and analysis of deformation mechanisms in soil sections through the integration of Particle Image Velocimetry (PIV) in dynamic centrifuge modeling. PIV requires a rigid wall container. In this paper, we summarize the design considerations that enable reliable measurement and visualization of soil deformation mechanisms at the University of Colorado (CU) Boulder’s 400 g-ton, 5.5 m radius centrifuge facility. The primary objective of this system is to investigate the response of complex and stratigraphically variable, saturated granular soil deposits beneath shallow-founded structures, though it can also be used for other configurations that benefit from advanced visualization in the centrifuge. The setup aims to enable quantification and visualization of different deformation mechanisms, including shear, volumetric, and soil ejecta formation, near and away from structures. The paper provides detailed design considerations for the system components, including a rigid container with a transparent Perspex wall and duct seal inclusions, a linear-elastic single-degree of freedom (SDOF) structure, soil texture, a potential ground improvement technique, and a high-speed camera system. Through fully-coupled, three-dimensional (3D) nonlinear finite element analyses, we investigate the influence of container type, domain size, and duct seal geometry on boundary effects, with consideration of nonlinearities within various soil profile configurations with and without the presence of a building model. The findings highlight the critical impact of proximity to lateral boundaries of the container on accelerations, excess pore pressures, foundation settlement, tilt, and shear strains, as well as the benefits of duct seal in reducing those boundary effects. The results also show that the extent of boundary effects on key performance measures depends on the properties of the container, soil layers, and ground motion. The simulations also show that surface ejecta potential within stratigraphic soil profiles is highly sensitive to the depth of the groundwater table and variations in soil permeability, which must be considered in designing experimental programs that investigate the development of soil ejecta.

Improvement of microstructure of granular soil in the densification process caused by impact loading

Improvement of the mechanical properties of soil, such as density, compressibility and shear strength, is typically due to the variation in its inherent microstructure. This paper presents a microscopic study of the densification mechanism in granular soils subjected to impact loading. Both model test and discrete element simulation were carried out to quantitatively analyze the fabric evolution from a particle-scale perspective. Irregularly shaped particles were used in the simulation, on basis of which realistic packing structural information such as average contact number, contact area and branch vector length could be learned. The results reveal the microscopic densification mechanism that impact loading not only promotes the increase of contact number, but also enhances the contact area around per particle. Increasing of contact area has enriched the distribution of sutured contacts to form more steady mechanical status of soil.

Evolution of anisotropic capillarity in unsaturated granular media within the pendular regime

While the shear behavior of granular soils is directly related to the microstructure of contacts which often leads to the coaxiality between Cauchy stress and Satake fabric tensors, it is generally accepted by the geomechanics and geotechnical engineering community that the capillary effects are isotropic. At low saturation levels, however, the pore fluid tends to form interparticle menisci that can also manifest an anisotropic structure, which may result in the development of anisotropic capillarity in wetted granular media. To study the interplay between the solid grain contacts and the liquid bridges at the micro-scales, this study adopts a coupled discrete element method that utilizes a linear contact model combined with a capillary model, and explores their effects by conducting a series of numerical experiments. The distributions of contact and capillary force orientations during the experiment are further investigated to better understand how their alignments affect the global response of the granular assembly subjected to a deviatoric loading. The results indicate that the global shear stress response is not only affected by the contact fabric but also by the network of liquid bridges, and we also observe that the particles may lose contact while the pendular menisci may not be destroyed during the elastic unloading.

Hydraulic heave in granular soils under hypergravity conditions

A series of discrete element method (DEM) simulations have been conducted to investigate the initiation and development of hydraulic heave for granular soils under hypergravity. During the development of hydraulic heave, local failure can occur when the stress-reduction factor (α) is less than 0.8, in which the particles are reorganized by the fluid and subsequently regain stability as particles settle down. However, if α is greater than 0.8, global failure occurs, causing the mobilization of the entire soil. The triggering mechanism of the hydraulic heave is the force equilibrium between the contact force and critical fluid force, which scales up proportionally with g-level. The non-linear variation of critical hydraulic gradient (icr) with g-levels is attributed to the non-linear relationship between fluid force and hydraulic gradient. Finally, an analytical model along with a fitting formula, is proposed to predict this non-linear relationship between icr and g-level.

Numerical investigation of granular column-improved soils using the direct shear test

The granular columns treatment is widely used in the weak soils. The present paper focuses on the numerical modeling of a direct shear test on granular columns in soils. The finite difference code Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) was used in this research work, to evaluate the equivalent properties of granular column-improved soils with different diameters of the columns, and different plan configurations. The results of these numerical tests are discussed in terms of increases the shear load-strength and decreases the horizontal displacement due to the variation of arrangements, Young's modulus and the friction angle of the granular columns. Different types of failures observed in the granular columns. Bending failure mode is the main in the granular column with large values of Young's modulus and the friction angle when subjected a lateral load.

Progressive internal erosion during triaxial shearing: an X-ray micro-computed tomography study

Fines removal during internal erosion results in the permeability increase and strength reduction of granular soils. This study examines internal erosion during shearing of granular soils under triaxial loading conditions using a specially designed apparatus and X-ray computed tomography scans. Two gap-graded soil samples with different fines content are tested and scanned multiple times to characterise sample- and pore-scale volume change and fines content using image-processing techniques. Results show that internal erosion increases gradually with axial strain and has a complex impact on permeability increase due to the spatial variation of fines content. Triaxial shearing induces both pore contraction and dilation, destabilises the clogging of fine particles and promotes internal erosion. Also, the erosion of force-supporting fine particles at a high fines content facilitates pore contraction. The progressive internal erosion due to mechanical disturbance leads to the formation of a flow channel from the upstream loading platen throughout the sample, resulting in a dramatic increase in permeability. Overall, this study provides novel insights regarding the interplay between the hydro-mechanical processes during shearing under constant seepage.

High-cycle shakedown, ratcheting and liquefaction behavior of anisotropic granular material with fabric evolution: Experiments and constitutive modelling

Although the mechanical response of granular materials strongly depends on the interplay between their anisotropic internal structure (fabric) and loading direction, such coupling is not explicitly considered in existing high-cycle experimental datasets and models. High-cycle experiments on granular specimens specifically prepared with various fabric orientations are presented. It is found that the high-cycle strain accumulation behavior can change remarkably, from shakedown to ratcheting, when the fabric orientation deviates more from the loading direction. Inspired by the experimental observations, a fabric-dependent anisotropic high-cycle model is proposed, by proper recasting of an existing model formulated within Critical State Theory, into the framework of Anisotropic Critical State Theory. The model explicitly accounts for the fabric evolution, which is linked to plastic modulus, dilatancy and kinematic hardening rules. The model can quantitatively reproduce the high-cycle strain accumulation (i.e., shakedown and ratcheting) under drained conditions, as well as pre-liquefaction and post-liquefaction responses granular materials having widely ranged fabric anisotropy, densities and cyclic loading types using a unified set of constants. It exhibits a unique feature of simulating the distinct high-cycle strain accumulation and liquefaction of granular material with various fabric anisotropy, while the existing high-cycle models treat them equally. The successful reproduction of the anisotropic sand element response under high-cycle drained and undrained conditions makes it possible to perform whole life analysis of various foundations on granular soil subjected to high-cycle loading events.

Improving the efficiency of isolated-footing resting on loose sand soil using grout diaphragm walls: an experimental and numerical study

In light of rising loads from several sources, including additional stories, eccentric loads, and increased live loads, foundations often face increased demands. To address this, horizontal reinforcements are now commonly positioned beneath footings to enhance the bearing capacity of the loose-dense sand subgrade. By grouting on both sides of the footing, not only can vertical settlement be minimized, but also the soil movement in the horizontal direction under the chosen loaded footing can be reduced. The objective of this study is to conduct extensive experimental work on twenty-one (21) soil models to assess the efficiency of a circular footing resting on granular soil injected with grout diaphragm walls. Specifically, this study investigated the bearing capacity of granular soil in relation to the breadth (b) and length (L) of grouted walls. The results showed that, installing grouted wall injection on both sides of the existing footing is an excellent method to improve the bearing capacity of the subgrade layer. To check the validity of the chosen computational processes, both PLAXIS (3D) software and a 2D Finite Element Program GeoStudio 2018 were used. The findings indicate a direct correlation with the experimental observations in that the reinforcement has a considerable effect on the bearing capacity of a circular-footing resting on granular soil.

Prediction of constrained modulus for granular soil using 3D discrete element method and convolutional neural networks

To efficiently predict the mechanical parameters of granular soil based on its random micro-structure, this study proposed a novel approach combining numerical simulation and machine learning algorithms. Initially, 3500 simulations of one-dimensional compression tests on coarse-grained sand using the three-dimensional (3D) discrete element method (DEM) were conducted to construct a database. In this process, the positions of the particles were randomly altered, and the particle assemblages changed. Interestingly, besides confirming the influence of particle size distribution parameters, the stress-strain curves differed despite an identical gradation size statistic when the particle position varied. Subsequently, the obtained data were partitioned into training, validation, and testing datasets at a 7:2:1 ratio. To convert the DEM model into a multi-dimensional matrix that computers can recognize, the 3D DEM models were first sliced to extract multi-layer two-dimensional (2D) cross-sectional data. Redundant information was then eliminated via gray processing, and the data were stacked to form a new 3D matrix representing the granular soil's fabric. Subsequently, utilizing the Python language and Pytorch framework, a 3D convolutional neural networks (CNNs) model was developed to establish the relationship between the constrained modulus obtained from DEM simulations and the soil's fabric. The mean squared error (MSE) function was utilized to assess the loss value during the training process. When the learning rate (LR) fell within the range of 10-5–10-1, and the batch sizes (BSs) were 4, 8, 16, 32, and 64, the loss value stabilized after 100 training epochs in the training and validation dataset. For BS = 32 and LR = 10-3, the loss reached a minimum. In the testing set, a comparative evaluation of the predicted constrained modulus from the 3D CNNs versus the simulated modulus obtained via DEM reveals a minimum mean absolute percentage error (MAPE) of 4.43% under the optimized condition, demonstrating the accuracy of this approach. Thus, by combining DEM and CNNs, the variation of soil's mechanical characteristics related to its random fabric would be efficiently evaluated by directly tracking the particle assemblages.

Resolved CFD-DEM Modeling of Suffusion in Gap-Graded Shaped Granular Soils

Resolved CFD-DEM Modeling of Suffusion in Gap-Graded Shaped Granular Soils

Safety Factor and Settlement Analysis of Borepile Foundation in Warmadewa Gianyar Hospital Building

Building structure planning must be distinct from foundation structure planning. The foundation plays an important role in transferring building loads to the ground. The Warmadewa Gianyar Hospital building consists of 5 floors with a building area of 8323. 471 m2 on a granular soil location where there are three layers of soil types, namely sandy silt soil, silty sand, and sand mixed with rock grains. The Borepile foundation is installed at a depth of 12 m with an N SPT value of 60. This building carries dead loads, live loads, rain loads, wind loads, and earthquake loads. This building will use a bore pile foundation by modeling bore pile piles in GEO5 software to obtain safety factor values and installed reinforcement. Based on the analysis results, the pile diameter used is 450 mm and has a length of 12 m. The analysis obtained the carrying capacity with a safety factor of 2.1> 2 and a maximum settlement of 4.77 mm.

Study on the depth of cantilever sheet pile wall based on the type of soil where sheet pile embedded

The northern part of Semarang is known as a region having a thick depth of soft clay. In the construction of sheet pile wall for Terboyo Retention Pond dike Semarang must take into account stability aspect and financial aspect. The characteristic of granular soil is different from that of cohesive soil, and the stability aspect relies on the type of soil. This study aims to investigate the influence of the type of soils where sheet pile embedded to the theoretical depth of cantilever sheet pile wall. This research used the soil data from Terboyo Retention Pond of Semarang. Modelling of wall was differentiated based on the type of soil where sheet pile embedded whereas the type of soil above the dredge line was the same. Two type of soils where sheet pile embedded, clay and sand were modelled. The variation of soil parameters of both soils was also modelled. Safety factor for clay soil was taken 1.5. The result shows that different soil in which wall embedded give significant difference of length of sheet pile. Based on N-SPT, with the same N-SPT value theoretical depth of cantilever sheet pile wall in sand soil is deeper than that of in clay soil.

Predicting the precipitated calcium carbonate and unconfined compressive strength of bio-mediated sands through robust hybrid optimization algorithms

Biological soil stabilization through Microbial-Induced Calcite Precipitation (MICP) demonstrates the potential to be an eco-friendly and sustainable approach to enhance the strength properties of cohesionless granular soils. In this study, based on a comprehensive set of experimental data compiled from literature, the amounts of Precipitated Calcium Carbonate (PCC) and Unconfined Compressive Strength (UCS) of MICP-treated sands are estimated through hybridizing Artificial Neural Network (ANN) and Adaptive Neuro-Fuzzy Inference System (ANFIS) models with metaheuristic algorithms. During the data extraction procedure, a total of 70 and 115 datasets are used to predict the PCC and UCS, respectively. Accordingly, five key features of urea concentration (Mu), calcium chloride (Mcc), optical density of bacterial suspension (OD600), pH and total volume of injection per volume of sample (Vt), are first selected as the inputs of the hybrid models to estimate the amount of calcium carbonate precipitation within the porous structure of MICP-treated granular medium. The experimental data on the PCC along with the basic characteristics of the host sand including mean grain size (D50), uniformity coefficient (Cu) and void ratio (e), are then adopted as the inputs of the numerical models in the second phase whilst the UCS of bio-mediated sands is set as the output. The performance and accuracy of implemented models are rigorously assessed through analyzing various statistical indices and performance criteria. According to the numerical analysis, the ANN-PSO and ANFIS-PSO hybrid models show the best performance in predicting PCC and UCS of bio-mediated sands, respectively. Using the well-established Gene Expression Programming (GEP) algorithm, practical correlations are developed to predict the PCC and UCS values of MICP-mediated soils based on the characteristics of biological binding agents as well as the host material. The Machine Learning (ML)-based models presented in this study provide engineers with a fruitful framework for the rough and preliminary estimation of the amount of calcite precipitation as well as the UCS of bio-mediated sands in the field prior to stabilization and performing complementary tests.

Effective thermal conductivity of granular soils: a review of influencing factors and prediction models towards an investigation framework through multiscale characters

The effective thermal conductivity of soil is important to geo-engineering applications, and it is controlled by factors across different length scales. Through a comprehensive review of these factors, we found that while other more traditional factors have been well studied, there is still a lack of characterisation of soil microscale and mesoscale structures and their influence on effective thermal conductivity. In addition, after reviewing the models available in the literature for soil effective thermal conductivity prediction, it was found that compared with empirical and theoretical models, machine learning models can account for the influence of multi-scale factors; however, research into them is scarce. To overcome the limitations of previous research, we proposed a framework that can investigate the factors influencing soil effective thermal conductivity at multiple scales. It includes the impact of soil structural factors at micro to mesoscale, and this impact is integrated with the influence from other factors for accurate thermal conductivity prediction.

Dynamic response of piles embedded in granular soil to lateral impacts

Soil-embedded vehicle barriers, such as W-beam guardrail systems, play a pivotal role in transportation safety, mitigating the risks associated with vehicular collisions with roadside hazards. The efficacy of these barriers greatly depends on the pile-soil system's kinetic energy dissipation capability during vehicular impacts. However, a comprehensive understanding of how soil strength, embedment depth, and impact velocity collectively govern the dynamic behavior of the pile-soil system remains a gap in current research. This study explores the dynamics of lateral impacts on piles embedded in various granular soils. The process of dynamic lateral impact and interaction between the pile and the soil was modeled using the Updated Lagrangian Finite Element Method (UL-FEM). A damage-based element erosion algorithm was incorporated into the model to accommodate severe mesh distortions and element entanglements of the soil material brought by the pile impact. Validation against well-documented large-scale physical impact tests ascertained the model's fidelity. Our findings elucidate the significant differences in resistive forces between piles in strong versus weak granular soils – notably, the former exhibited resistive forces roughly double their weaker counterparts under equivalent embedment depths and varied impact velocities. Intriguingly, a stiff pile in weak soil necessitates nearly double the embedment depth to match the energy dissipation of its strong-soil counterpart. Furthermore, the study discerned consistent depth of rotation point ranges for piles embedded in distinct soil strengths, regardless of embedment depth and impact velocity.

Mole‐Inspired Robot Burrowing with Forelimbs for Planetary Soil Exploration

The biological world serves as a vast reservoir of inspiration for human innovation, particularly in the realm of robotics designed to navigate intricate granular environments. Among these creatures adept at burrowing, the mole stands out due to its exceptional body structure and unparalleled efficiency, surpassing most engineering burrowing systems. Consequently, the mole serves as an ideal model for designing highly efficient burrowing robots. This study introduces a mole‐inspired robot burrowing with forelimbs, specifically designed for planetary soil exploration. By closely examining the distinctive body morphology of moles, a forelimb burrowing mechanism is devised through biomimetic mapping, and its kinematics is thoroughly analyzed. A robot prototype is developed and an experimental setup is constructed to evaluate its burrowing performance. A series of tests are conducted to assess the capabilities of the robot, including forelimb burrowing, robot crawling, and robot burrowing. The results demonstrate that the proposed mole‐inspired burrowing robot is capable of crawling and burrowing to a certain depth using its forelimbs. Although it exhibits some upward displacement, this issue can be mitigated by modifying the head configuration and adjusting the forelimb posture to effectively overcome the vertical stress and lift force exerted by granular soils due to intensity gradient.

Numerical study on the shear strength of granular materials under the low confining pressure

The relationship between stress and strength in granular soil differs under low confining pressure as compared to high confining pressure, which is of great importance for the geotechnical engineering. Numerical simulations using discrete element method (DEM) are conducted to model consolidated-drained triaxial compression tests, with the confining pressure σc ranging from 1 to1000 kPa. From the analysis of the shear strength of granular soils under various confining pressures and void ratios, it is found that the peak stress ratio ηp under a low σc is significantly lower than that under the high σc, particularly for samples with the same initial void ratio. As the confining pressure increases, ηp for samples with the same initial void ratio increases up to 29 % under the low σc while stabilizing under the high σc. Conversely, ηp for samples with the same void ratio after the consolidation can increase up to 13.5 % under the low σc while declining under the high σc. In addition, the critical confining pressure distinguishing the low and high confining pressures is about 200 kPa for dense samples with initial void ratio of 0.605 used in the study, and it increases as the sample void ratio increases. From the microscopic analysis, the variation of ηp under the low σc can be attributed to the enhanced anisotropy of normal contact force and contact normal under the low confining pressure. Finally, a modified Mohr-Coulomb failure criterion is proposed, which can determine the strength of granular soils under the low confining pressure with different void ratios.

A comprehensive numerical investigation of multi-scale particle shape effects on small-strain stiffness of sands

The effects of multi-scale particle shape characteristics on the small-strain stiffness of granular soils remain controversial. This study revisits this topic using well-calibrated three-dimensional discrete-element simulations incorporating a particle roughness–embedded contact model and particles of realistic shape. Based on the numerical simulation results, the multi-scale particle shape effects on the small-strain stiffness of sands and the magnitude of Hardin's equation parameters are systematically investigated, and the underlying micro-mechanisms are also thoroughly explored. Results indicate that the small-strain stiffness increases with the increase of particles’ overall irregularity due to the increased mechanical coordination number, but decreases with the increase of particle surface roughness because of the decreased contact normal stiffness. In addition, the constant term parameter and void ratio term parameter of Hardin's equation increases and decreases linearly, respectively, with the particles’ overall regularity, but reduces and grows with the particle surface roughness, respectively. Furthermore, the stress exponent is almost unchanged with the particles’ overall regularity, but increases with the particle surface roughness, which determines the relative proportions of contacts under asperity-dominated, transitional and Hertzian stages. The study helps to advance the cross-scale understanding of multi-scale particle shape information in relation to small-strain stiffness of sands.