Mingjing Jiang

An efficient technique for generating homogeneous specimens for DEM studies

Abstract A new technique, designated as the Multi-layer with Undercompaction Method (UCM), capable of generating homogeneous specimens for DEM studies is presented herein. The specimen are compacted in several layers using undercompaction criteria based on average planar void ratio. The proposed approach was compared to other available methods and was proven to generate very homogeneous specimens provided a nonlinear average undercompaction criterion was used. Furthermore, the UCM method was efficient for a variety of density conditions ranging from very loose to dense states.

An Experimental Investigation on the Mechanical Behavior Between Cemented Granules

An experimental investigation of the mechanical behavior of cemented granules is presented in order to verify and further clarify the bond contact model used in numerical simulations of cemented sands. The cemented granules were idealized by a pair of aluminum rods glued together by means of calcium aluminate cement. A series of cemented rods was prepared using a specially designed sample preparation device. Then, the mechanical relationships between the cemented rods (i.e., force-displacement relationships and failure conditions) were examined in both simple loading and complex loading tests using newly developed auxiliary loading devices. The results show that the tensile force increases linearly up to its peak strength and then drops suddenly to zero, whereas the compressive force increases bilinearly up to its peak strength and then decreases to the residual strength gradually. Similarly, the shear force increases almost linearly up to its peak strength and then drops to the residual strength rapidly, whereas the torque increases up to its peak strength and then decreases to the residual strength gradually. In addition, both the peak shear strength and the peak torsional strength increase at first and then decrease with increasing normal force. The strength envelope of the cemented rods is observed to be an olive-shaped shell in the shear force-normal force-torque space.

DEM-Aided Discovery of the Relationship between Energy Dissipation and Shear Band Formation Considering the Effects of Particle Rolling Resistance

AbstractThe importance of particle rolling resistance to the mechanical behavior of granular materials is well recognized and has been a topic subject to intensive discrete element method (DEM) investigation over the last two decades. However, little effort has been made to explore the energy input and dissipation behavior under the influence of varying degrees of interparticle rolling resistance, especially in relation to the development of shear band. This paper aims to eliminate this deficiency through a comprehensive two-dimensional DEM study on the relationship between the particle-scale energy dissipation and shear band development. Novel insights into the energy allocation at the small- and large-strain stages, and the development of localized bands of sliding and rolling dissipations, as well as the anisotropy of accumulated sliding and rolling dissipations within the shear band are presented for the first time.

Properties of TJ-1 Lunar Soil Simulant

Tongji-1 lunar soil simulant (TJ-1 for short) was recently invented by Tongji University, China. It could be used to construct testing ground in the preparatory impact tests of lunar landers under the extremely adverse condition in the Earth environment, where the interaction pressure between lunar lander and lunar soil has the important effect on the safety of lunar lander. The TJ-1 is originated from red volcanic ash deposits collected from northern China. The raw materials are firstly dried, crushed, and sieved to obtain materials of seven particle sizes. Gradation is then used as the controlling parameter to generate TJ-1 with desired mechanical properties. The TJ-1 demonstrates certain similarity to lunar regolith in physical and mechanical properties, and it is an ideal substitute of lunar regolith located from 0 to 30 cm below lunar surface in the mare region on the Moon. Compared with other simulants introduced in the literature, TJ-1 has an internal frictional angle as high as 45° at e ¼ 1:0 and hence is advantageous over other simulants when high shear strength is desired. The TJ-1 will be a good choice as a ground material in the preparatory impact tests of lunar landers in the Chinese Lunar Exploration Program. DOI: 10.1061/(ASCE) AS.1943-5525.0000129.

Analytical Solutions for the Construction of Deeply Buried Circular Tunnels with Two Liners in Rheological Rock

The construction of underground tunnels is a time-dependent process. The states of stress and strain in the ground vary with time due to the construction process. Stress and strain variations are heavily dependent on the rheological behavior of the hosting rock mass. In this paper, analytical closed-form solutions are developed for the excavation of a circular tunnel supported by the construction of two elastic liners in a viscoelastic surrounding rock under a hydrostatic stress field. In the solutions, the stiffness and installation times of the liners are accounted for. To simulate realistically the process of tunnel excavation, a time-dependent excavation process is considered in the development of the solutions, assuming that the radius of the tunnel grows from zero until its final value according to a time-dependent function to be specified by the designers. The integral equations for the supporting pressures between rock and first liner are derived according to the boundary conditions for linear viscoelastic rocks (unified model). Then, explicit analytical expressions are obtained by considering either the Maxwell or the Boltzmann viscoelastic model for the rheology of the rock mass. Applications of the obtained solutions are illustrated using two examples, where the response in terms of displacements and stresses caused by various combinations of excavation rate, first and second liner installation times, and the rheological properties of the rock is illustrated.

Analytical Solutions for Tunnels of Elliptical Cross-Section in Rheological Rock Accounting for Sequential Excavation

Time dependency in tunnel excavation is mainly due to the rheological properties of rock and sequential excavation. In this paper, analytical solutions for deeply buried tunnels with elliptical cross-section excavated in linear viscoelastic media are derived accounting for the process of sequential excavation. For this purpose, an extension of the principle of correspondence to solid media with time varying boundaries is formulated for the first time. An initial anisotropic stress field is assumed. To simulate realistically the process of tunnel excavation, solutions are developed for a time-dependent excavation process with the major and minor axes of the elliptical tunnel changing from zero until a final value according to time-dependent functions specified by the designers. In the paper, analytical expressions in integral form are obtained assuming the incompressible generalized Kelvin viscoelastic model for the rheology of the rock mass, with Maxwell and Kelvin models solved as particular cases. An extensive parametric analysis is then performed to investigate the effects of various excavation methods and excavation rates. Also the distribution of displacements and stresses in space at different times is illustrated. Several dimensionless charts for ease of use of practitioners are provided.

Application of Discrete Element Method to Geomechanics

This chapter introduces recent application of the Distinct Element Method (DEM) to geomechanics. Different contact laws were introduced and used to investigate the noncoaxiality of granular materials, effective stress in unsaturated soils, bonding effect in natural soils and penetration mechanism in granular ground. The study shows that DEM is a promising tool to solve some difficult problems not only in fundamental geomechanics but also in complex boundary value problems in geotechnical engineering.

DEM analyses of shear band in granular materials

Purpose – Shear-induced strain localization in granular materials has been a hot topic under intensive research during the last four decades. However, the micromechanical process and mechanisms underlying the initiation and development of shear bands are still not fully understood. The purpose of this paper is to eliminate this deficiency. Design/methodology/approach – The paper carries out several two-dimensional distinct element method simulations to examine various global and local micromechanical quantities particular the energy dissipation and local stress and strain invariants with a special emphasis on the initiation and propagation of shear bands. Moreover, the effects of various influential variables including initial void ratio, confining stress, inter-particle friction coefficient, rolling resistance coefficient, specimen slenderness and strain rate on the pattern, scope and degree of shear bands are investigated. Findings – Novel findings of the relationship between sliding and rolling dissipa...

DEM analysis of geomechanical properties of cemented methane hydrate-bearing soils at different temperatures and pressures

AbstractMechanical properties of methane hydrate–bearing soils are influenced by the surrounding temperature and pore pressure. Studies of such influences are of great significance for the safe exploration of methane hydrate. First, a thermo-hydro-mechanical bond contact model is introduced to elucidate the microscopic contact scale behavior of grains with methane hydrate bonds. Second, such a model is incorporated into the distinct-element method (DEM), and a dimensionless temperature-pressure distance parameter is introduced. The influences of temperature and pore pressure along heating and depressurization paths are analyzed by conducting DEM biaxial compression tests, which are also compared with the results of laboratory triaxial compression tests conducted by others. The variation rules of the macromechanical properties (i.e., strength, elastic modulus, peak friction angle, cohesion, and dilatancy angle) with changing temperatures and pore pressures are then analyzed for different effective confinin...

A novel specimen preparation method for TJ-1 lunar soil simulant in hollow cylinder apparatus

Abstract Conventional methods for hollow cylinder apparatus (HCA) specimen preparation are not applicable for TJ-1 lunar soil simulant due to its wide particle size distribution. A novel method to prepare uniform TJ-1 specimen for HCA tests is put forward. The method is a combination of the multi-layering dry-rodding method and a new under-compaction criterion in the multi-layer with under-compaction method (UCM). In the novel method, the specimen is prepared with 5 layers by dry-rodding and the UCM is used to determine the height after each layer is compacted. The density uniformity of specimen is evaluated by the freezing method to find out the best under-compaction criterion. Two HCA specimens with the same target density are prepared by the novel method and examined in the tests of pure rotation of the principal stresses. Their conformable mechanical behaviors ascertain the effectiveness of the method to produce uniform and reproducible HCA specimens. Four groups of HCA tests are carried out to investigate the anisotropic and non-coaxial behaviors of TJ-1 lunar soil simulant. The results indicate that the principal stress direction, the deviator stress ratio, the stress level and the coefficient of the intermediate principal stress significantly influence the strength and deformation properties of TJ-1 lunar soil simulant.