Numerical Simulations of Spread Foundations Supported on Stone Columns Using the Discrete Element Method

The implementation of stone columns as a ground improvement technique has become more popular in geotechnical construction practice as a result of their ability to improve strength, stiffness and permeability characteristics of weak clayey soil deposits. There are several analytical and empirical approaches to estimate the bearing capacity of stone column foundation systems; however, there is notable variation in the performance of these existing methods when compared with full-scale experimental results. For very weak cohesive soils (i.e., undrained shear strength less than 15 kPa), the use of conventional stone columns becomes restricted because of the insufficient confinement that these types of soils can provide to the columns. Hence, the inclusion of cement-coated aggregate has been developed as an alternative method to improve the efficacy of stone columns in soft soils. Limited information is available regarding the global performance, load-transfer mechanism, and design of these types of cemented stone columns under various field conditions. Efforts to refine the accuracy of current design methods and reinforcement techniques for conventional stone columns naturally point to the need for improving the understanding of the fundamental load-transfer mechanisms of stone columns. Three-dimensional discrete element method (DEM) simulations of small- and full-scale footing loading tests were developed to investigate the effects of aggregate strength, pier length, aggregate Young’s modulus, area replacement ratio, cement content, and undrained shear strength of the matrix soil on the bearing pressure-displacement responses of isolated foundations supported on stone columns. The elemental responses of the aggregate and plastic matrix soil were calibrated against laboratory and in-situ test data from a well-characterized site and compared against the results of small- and full-scale footing loading tests. The column aggregate material was represented by discrete-deformable tetrahedrons in conjunction with strain-softening and strain-hardening models in order to improve the simulation of the nonlinear response of the cemented aggregate. Joined deformable blocks were employed to represent the continuous mechanical behavior of the surrounding clayey soil. The numerical results are in excellent agreement with the experimental laboratory and field data and provide improved estimates of the bearing pressure-displacement curves of the column-foundation systems investigated in this study. The Young’s modulus of the aggregate column and the area replacement ratio were found to have the greatest influence on the bearing pressure-displacement response. The DEM results also improve the understanding of the effects of granular material-cementation on the performance of stone columns. At low cement contents the stone column exhibits a type of bulging failure mechanism similar to uncemented stone columns, but at higher cement contents (10 % in this study), bulging is not observed, and the behavior resembles more like that of a concrete pile. These types of behavioral differences also have different implications for single isolated stone columns and group column behavior

A study of the influence of particle gradation in bonded assemblies

The discrete element method (DEM) has been used extensively to study soil, rock, and masonry behavior because of its ability to model the materials as individual particles or bonded clumps of particles. DEM allows for examination of the macro- and micro-scale response and provides a means to study the fundamental material behavior, but it is still considered computationally expensive in relation to other methods. To lower computational costs, the smallest particle sizes are often considered negligible and are left out of the model. Additionally, rock or intact materials are often modeled as a bonded assembly of uniform spheres. To date, few research studies have considered the influence of particle size and gradation on the strength and fracture behavior of bonded assemblies. This research aims to examine the influence of particle gradation in bonded assemblies through laboratory calibrated DEM simulations. Additionally, the role of the cement-sized particles will also be investigated. While the overall motivation for this study is related to the behavior of mortar in historic preservation applications, the preliminary studies can be directly applied to other geo-related materials such as cemented sands and rock specimens. This study addresses two critical questions associated with the computational efficiency of bonded assembly models (1) Does particle gradation influence the overall strength and fracture behavior, and (2) Do the smallest size particles influence the overall results enough to justify the additional computational cost? In this study two mortar materials, varying only in sand particle gradation, are subjected to physical laboratory compression strength tests to assess whether or not the influence is observed in physical experiments. Additionally, the compression test results act as a means of calibrating the simulations in DEM. These simulations will examine the macro- and micro-scale influence of particle gradation on the strength of bonded assemblies. Additional simulations are used to examine the effects of modeling the cement-sized particles in the bonded assembly. The results of the physical experiments and the development of the DEM simulations are discussed herein

Multiscale Analysis of Soil-Strap Interactions in Mechanically Stabilized Earth Retaining Walls

A numerical pullout test was built using the discrete element method (DEM) to model and capture the pullout response of steel reinforcements and soil in mechanically stabilized earth (MSE) walls. Through numerical modeling, microscale phenomena showing aggregate behavior in response to the reinforcement can be used to gain insight into the macroscale structure. The general setup of the simulation is a steel specimen encased in a rectangular apparatus filled with particles. A normal pressure is applied to the top layer of particles while the strap is slowly removed from the box until it reaches a prescribed displacement.The simulation was created using YADE, an open-source DEM software, which allows for rapid scene construction via scripting. The numerical model uses an iterative approach to step through time while resolving contacts at each step and translating those contacts into forces to ultimately provide updated positions for each body at every time step. For this research, a non-cohesive, elastic-frictional Cundall-Strack contact model was employed to resolve interactions on an individual body basis. Test parameters were largely based on the experimental setup of pullout tests performed by Weldu. Particle packings for the pullout simulation were calibrated to the aggregate used in Weldu’s experiments by setting up a simple triaxial compression simulation within YADE to derive the correct microscale particle friction angle such that it produced the proper macroscale behavior.Using the numerical model, three sets of experiments from Weldu’s research were reproduced with particle uniformity coefficients of 1, 2, and 3. Simulations sets were run at various normal pressures and included 400,860 particles at the upper end. The numerical tests resulted in an encouraging degree of correlation to the laboratory experiments, with pullout residuals being as close as 2% different and an average of 14% different. In addition, this thesis discusses some of the microscale data extracted from the simulations, such as force chains and rolling characteristics, and how numerical simulations could be used in the future to help guide pullout testing and MSE wall design

Investigation of the Permeability of Soil-rock Mixtures Using Lattice Boltzmann Simulations

Based on the discrete element method and the proposed virtual slicing technique for three-dimensional discrete element model, random pore-structural models of soil-rock mixtures are constructed and voxelized. Then, the three-dimensional lattice Boltzmann method is introduced to simulate the seepage flow in soil-rock mixtures on the pore scale. Finally, the influences of rock content, rock size, rock shape and rock orientation on the simulated permeability of soil-rock mixtures are comprehensively investigated. The results show that the permeability of soil-rock mixtures remarkably decreases with the increase of rock content. When the other conditions remain unchanged, the permeability of soil-rock mixtures increases with the increase of rock size. The permeability of soil-rock mixtures with bar-shaped rocks is smaller than that of soil-rock mixtures with block-shaped rocks, but larger than that of soil-rock mixtures with slab-shaped rocks. The rock orientation has a certain influence on the permeability of SRMs, and the amount of variation changes with the rock shape: when the rocks are bar-shaped, the permeability is slightly decreased as the major axes of these rocks change from parallel to perpendicular with respect to the direction of main flow; when the rocks are slab-shaped, the permeability decreases more significantly as the slab planes of these rocks change from parallel to perpendicular with respect to the direction of main flow

A Study on the Triaxial Shear Behavior of Geosynthetic Reinforced Soil by Discrete Element Method

A 3D DEM model using Particle Flow Code (PFC3D) software was developed utilizing a bonded-ball flexible membrane approach to study cohesionless soil as a discontinuous discrete material. The 3D model was calibrated and verified with experimental data, and a sensitivity analysis was carried out for the microparameters. Triaxial tests were simulated to observe the stress-strain curves and volumetric changes, as well as the strength parameters of soils consisting of spherical particles with different gradations but the same porosity. One important finding is that the relationships between particle size and deviatoric stress, internal friction angle, and dilatancy angle were found to be linear. Geosynthetics were added to the developed model to study the stress-strain behaviors of reinforced soil in a geosynthetic reinforced soil (GRS) mass, which have important applications that can improve the design of the structures. Results indicate that geosynthetics improve the cohesion to the granular soil

Discrete Element Modeling of Soil-Geogrid Interactions

Geogrids are the geosynthetics of choice for soil reinforcement applications. To evaluate the efficiency of geogrid reinforcement, several methods are used including field tests, laboratory tests, and numerical modeling. Field studies consume a long period of time and conducting these investigations may become highly expensive because of the need for real-size structures. Laboratory studies present also significant difficulties: large-size testing machines are required to accommodate realistic geogrid designs. The discrete element method (DEM) may be used as a complementary tool to extend physical testing databases at a lower cost. Discrete element models do not require complex constitutive formulations and may be fed with particle scale data (size, strength, shape) thus reducing the number of free calibration parameters. The thesis reviews the different approaches followed to model soil-geogrid interaction in DEM and presents preliminary results from pull-out and triaxial conditions. Moreover, a numerical model of triaxial test with or without geogrid was developed and validated by laboratory test values that were provided by other researchers

Relationship of rock microscopic parameters with the elastic modulus and strength

The microscopic damage of materials will induce changes in the macroscopic mechanical characteristics of rock material. When simulating engineering problems using the discrete element method, to explore the macroscopic mechanical response of rock material, the microscopic parameters that match the macro material characteristics must be obtained. In this paper, the influence of macroscopic mechanical properties of rock materials is studied through the variation of microscopic parameters, and the quantitative relation between macroscopic parameters of rock material is discussed. The results show that, (1) In accordance with the order of influencing factors, the parameters affecting the elastic modulus of the specimen are parallel bond elastic modulus, particle contact modulus, and parallel bond stiffness ratio. (2) The Poisson’s ratio of the specimen was most influenced by the parallel bond stiffness ratio, and their relation was nonlinear. The influence of parallel bond modulus and friction factor on the Poisson’s ratio was negatively correlated. (3) The effect of particle contact stiffness ratio, parallel bond stiffness ratio, and particle contact modulus on the uniaxial compressive strength was less than that of the particle friction factor