•Morphologies of coral sand particles are acquired and characterized via X-ray CT.•Random field spherical harmonics approach for generating particles are presented.•SDF-DEM is introduced for modeling ...coral sand with intraparticle voids.•SDF-DEM simulated column collapse results match well with those of laboratory.
Coral sand is the main geomaterial on tropical reefs and its constituent particles are featured with complex shapes and abundant intraparticle voids. This work presents a discrete element method (DEM)-based numerical modeling of coral sand in consideration of both the irregular shape and intraparticle voids of particles. To develop the DEM model for coral sand, the acquisition and characterization of coral sand particles are first introduced. A signed distance field-based DEM applicable to arbitrarily irregular-shaped particles is then presented, in conjunction with two versatile particle models, namely spherical harmonics (SH) and level set (LS). To generate virtual coral sand particles that conform to target morphology characteristics, the combined random field and SH approach and the remedy to elongated particles are also presented. The developed DEM model is calibrated and validated against laboratory column collapse tests, where the agreements between numerical simulations and laboratory experiments demonstrate the good validity and accuracy of DEM model for coral sand. The performance of SH and LS particle models are also compared, providing a useful reference for the selection of appropriate particle models for coral sand in practice.
This paper presents a machine learning (ML)‐enabled discrete element method (DEM) for the computational mechanics of irregular‐shaped particles. ML‐enabled DEM, as with most conventional DEMs, ...encompasses four main steps in one typical calculation cycle, namely, (1) the detection and resolution of contacts, (2) the evaluation of contact behavior, (3) the calculation of particle motion, and (4) the updating of particle geometric descriptions. Unlike conventional DEMs, the proposed method constructs and employs neural networks to detect particle contacts and resolve contact geometric features. Neural networks take particle geometric descriptors as inputs and output the contact status and contact geometric features. Using two‐dimensional elliptical particles as an example, the performance of the ML‐enabled DEM is investigated through five numerical experiments and compared with analytical solutions or conventional DEM methods. A sixth numerical experiment involving irregular‐shaped particles is also presented to showcase the potential and applicability of the proposed method for other particle shapes. ML‐enabled DEM can accurately capture the trajectory and energy evolution of individual particles, the fabric characteristics of dense packing, and the mechanical behavior of packing under large loads, while demonstrating computational efficiency over conventional methods.
•Shield tunnel lining damage induced by soil-water inrush is studied.•Numerical simulation matches with field results reasonably.•Strength reduction method have been applied for the consideration of ...soil loss.•Lessons learned from this incident have been presented.
This paper reports a case study on shield tunnel lining damage induced by soil-water inrush occurred in Tianjin Metro Line 1, China through both field monitoring and numerical simulation. This incident was triggered by the non-watertight boring work of thru holes adjacent to the cross passage between the twin tunnels. Under high hydraulic gradient, the seepage-prone weak zone was formed and extended, then the outburst of soil-water slurry was occurred. Measures including plugging engineering cotton, injecting quick-setting cement and welding partition plate of steel segments, had been taken but in vain. The outburst of soil-water slurry induced soil movement around the cross passage, thus leading to the damage of tunnel lining and ground surface settlement. After sealing the water ingress holes, stabilization methods including surface grouting and inside tunnel back grouting were applied. The mechanisms of segment lining damage and the effectiveness of stabilization are investigated through both numerical simulation and field monitoring data analysis. Lessons learned from this incident have been discussed, thus providing reference for potential shield tunnelling under similar engineering conditions.
In this note, the non-linear fluid flow through rough fracture is investigated experimentally. The rough surface of fracture is generated synthetically based on the fractal dimension (D) and standard ...deviation (σ). 3D printing technique is used to produce the fracture specimen, in which, different components of the specimen are printed and assembled so that the fracture roughness and aperture can be easily controlled. After that, the self-designed apparatus is used to conduct the fluid flow test on the fractured specimens under various pressure gradients. It is found that, the surface roughness imposes an important impact on the nonlinear characteristics of fluid flow through fracture; and the root-mean-square of the first derivative of profile, Z2, is found effective in characterizing the fracture roughness. Based on the test results and the derived parameters in Forchheimer equation, an empirical equation is proposed to relate the hydraulic aperture (eh) to Z2 and mechanical aperture (em). Furthermore, the correlation among the nonlinear coefficient β, em and Z2 is established.
•The characteristics of various forms of energy are studied using finite difference-based strength reduction method.•Both the spatial distribution and evolution of the energy with increasing strength ...reduction factors are analyzed.•The effects of cross-correlated random field soil properties on the energy characteristics are investigated.•The sudden change in gravitational potential energy, dissipated energy, and kinetic energy serves as a good sign of slope failure.•Two distinct mechanisms in the effects of strength reduction on the elastic strain energy are identified and discussed.
The definition of slope failure is a critical issue for strength reduction method-based slope stability analysis. Conventional slope failure criteria, which rely heavily on the distributions of stress, strain and displacement throughout the slope, are relatively complicated and create ambiguity in practical applications. Recognizing such shortcomings, a new energy-based criterion for defining slope failure was recently developed in the literature. In this work, further analyses are performed to investigate the validity of this energy-based criterion for slope stability analysis in which spatially varying soil properties are considered. With the slope being modeled by the finite difference method, random field theory is adopted to generate cross-correlated soil strength properties for the slope. Then, the slope system is solved and analyzed with a consecutive series of strength reduction factors (SRFs). In particular, various forms of energy in the slope system are calculated with different SRFs, and the characteristics of the energy distribution and evolution with increasing SRFs are analyzed. The results indicate that the slope system exhibits a significant energy change when the SRF increases to a critical value, which is a good indicator of slope failure. The areas in the slope characterized by considerable energy dissipation present a profile that is very close to the profile of the critical slip surface of the slope. These findings regarding the characteristics of the energy distribution and energy evolution could be further utilized to develop more efficient approaches to determine factors of safety and critical slip surfaces.
Understanding the frictional characteristics of granular materials (e.g., fault gouge and sand foundations) exposed to dynamic normal load are important for investigations of engineering geology ...disasters and safety assessment. Direct shear tests on granular materials under dynamic normal load were performed in the laboratory using a dynamic shear box device. Experimental results showed that the shear strength and normal displacement varied with variations of normal load, identified by a phase shift and normal stress loading peak. The peak shear stress decreased with increasing normal load frequency and shear velocity, in particular, the peak shear stress changed nonlinearly with the increase of normal load amplitude, characterized by a critical point. The influence of normal load amplitude on the shear strength and normal displacement was analyzed in detail. Furthermore, the results showed that dynamic normal load can both enhance and reduce the shear strength on granular material, and the frictional behaviors were velocity-amplitude-frequency dependent, which can be well interpreted by the Rate and State Friction constitutive law. These findings provide guidance in the design of geotechnical engineering projects when dynamic normal loads are encountered (e.g., blasting, earthquakes, earth tides and reservoir effects).
•The frictional behavior of granular materials under the effects of shear velocity, dynamic normal load frequency, dynamic normal load amplitude and normal load level are studied in laboratory.•Dynamic normal load can both enhance and reduce the shear strength on granular materials.•The frictional strengthening or weakening are determined by a critical velocity-amplitude-frequency value.•Small shear velocity, large dynamic normal load frequency, and very large or small dynamic normal load amplitudes can reduce the shear strength.
•Zonal disintegration has been explicitly modelled.•Numerical simulation matches with experimental results reasonably.•Effects of tunnel shapes, sizes and rock heterogeneity have been ...investigated.•Mechanism and occurrence conditions of zonal disintegration have been studied.
Zonal disintegration has been frequently encountered in many tunnelling projects such as deep mining and water tunnels in hydropower stations. The mechanisms of this phenomenon cannot be explained reasonably through conventional mesh-based numerical approaches. Thus, in this study, a dual coupled Micro-Macro Continuum-Discontinuum approach named as Distinct Lattice Spring Model (DLSM) has been applied to investigate the mechanisms of zonal disintegration within deep rock masses. Firstly, the 3D numerical modes are built up, with fixed boundaries being set for far fields and displacement loading being applied along the tunnel axis. This numerical mode is then validated through comparing model simulation with laboratory model tests, where reasonable agreement has been achieved for all cases considered (normal rock mass and layered rock mass with different joint spaces). To cater for real tunnels within various rock masses, tunnels excavated in deep rock masses with different sizes, shapes and material heterogeneities are investigated. Numerical study demonstrates that, the DLSM is capable to reproduce the process of zonal disintegration explicitly, along with which the mechanical responses have been captured reasonably. It shows that, the occurrence of zonal disintegration mainly depends on the material heterogeneities and the in-suite stress level. The fracture patterns formed during zonal disintegration rely on tunnels’ shape, size and the distribution of local weakness in surrounding rock masses.
Changes in shear velocity can strengthen or weaken the frictional resistance of joints/faults in natural systems, but the mechanism remains unclear. We investigated the shear behavior of a rough ...basalt fracture in well-controlled, repeatable shear tests under constant and dynamic normal load conditions at different shear velocities. Normal load vibrations, simulating a dynamic normal load, were applied to the upper block of a fractured basalt sample. Simultaneously, a shear load was applied to the bottom block, providing a constant shear velocity. The peak shear strength increased with increasing shear velocity under constant normal load conditions. The peak shear strength decreased at a lower shear velocity under normal load vibrations. When the shear velocity exceeded the critical value,
v
c
, the peak shear strength increased. The apparent coefficient of friction reduced under normal load vibrations. The reduction in the dynamic coefficient of friction increased with increasing shear velocity. We identified a phase shift between the peak normal load and peak shear load with peak shear load delay (D1) and a phase shift between peak normal load and the peak coefficient of friction with the peak coefficient of friction delay (D2). D1 and D2 were dependent on the quasi-static coefficient of friction and shear velocity, and both decreased with increasing shear velocity. D1 decreased with the increasing quasi-static coefficient of friction, while D2 was almost constant with changes in the quasi-static coefficient of friction. A new shear strength criterion was proposed for a rough joint under a constant shear velocity and normal load vibrations.
In rock engineering, damage evolution upon loading imposes significant impacts on the stiffness of rock mass and its deformation characteristics. In order to investigate the influence of both damage ...and plasticity on cavity expansion, a plastic damage solution is derived for undrained spherical cavity expansion in rock medium. For the consideration of plasticity‐damage, Modified Cam‐Clay (MCC) model is selected as the plasticity driver, a damage evolution criterion is adopted and coupled with MCC. The coupled damage MCC model is validated against experimental data in the literature. The proposed cavity expansion solution with the consideration of plasticity damage is verified through a classic solution in literature. The role of damage in undrained spherical cavity expansion is investigated by studying the spatial variations of effective stress, pore pressure and damage for cases with different stress ratios. Distribution of cavity expansion induced plastic and damage zones for cases with different stress ratios are also reproduced and discussed. Cavity expansion results show that, the damage zone should be considered for engineering application as the plastic zone is affected due to the damage evolution. In addition, the stability (e.g., radial stress) of rock mass is overestimated in classical solution without the consideration of damage zone.
Fourier series (FS) is an efficient tool for describing irregular geometries and has been employed to develop the FS-based particle model in the discrete element method (DEM). This work is devoted to ...extending the previous FS-based particle model to the Minkowski and Gilbert–Johnson–Keerthi (GJK)-based contact detection and resolution framework for DEM, and thus to improving its computational efficiency and compatibility with other conventional particle models. In the new FS-based particle model, instead of representing particle surface, the FS is proposed to represent the support function of particle surface. Particle surface and support points are then formulated based on the FS support function. As the Minkowski- and GJK-based detection and resolution framework relies heavily on the convexity of particles, the convexity constraint and the approach to generate convexity preserving FS-based particles are also presented. The accuracy of the new FS-based particle model for shape representation is analyzed using a set of irregular shape templates. DEM simulation of random packing and biaxial compression test with various particle models is also performed to demonstrate the computational performance and numerical stability of the new FS-based particle model.
