Department of Soil and Foundation (1993 - Present)
Civil Engineering, Geotechnical Engineering
Civil and Mineral Engineering, University of Minnesota, Minneapolis, United States
Civil Engineering, Geotechnical Engineering
Civil Engineering, Shiraz, Shiraz, Iran
Civil Engineering
Civil Engineering, Iran University of Science and Technology, Tehran, Iran
Quasi-brittle materials such as rock are rate sensitive materials and their behaviour under dynamic loading is not identical with that under static loading. In this study, numerical Brazilian tensile tests are conducted using a Split Hopkinson Pressure Bar system in an attempt to reproduce the dynamic increase factors (DIF) of the experimental tests. The rock is modelled by a bonded particle system made of spherical particles which interact at the contact points. The numerical results indicate that while the bonded particle system with a simple contact bond model can closely mimic the static behaviour of the sandstone specimens, it lacks what is needed for a rate dependent material. Therefore, a micromechanical model in which the contact bo
Loose specimens with a void ratio of 0.86, corresponding to a relative density of 30% for the pure sand, and normal stresses of 50, 100 and 150 kPa were used in this work to study strength and deformation behaviour of sand rubber mixtures. Three types of rubber particles with D-rubber/D-sand?=?0.25, 1, and 4, and different rubber to sand ratio of 5, 10, 15, 20, 25, 30, 40 and 50% were investigated by performing more than 300 direct shear tests. The shear strength and deformation characteristics of sand-rubber mixtures were dependent on rubber proportions of the mixtures and size ratio. Angle of friction and cohesion intercept increased and reduced when up to 20% rubber fraction was used, but reduced and increased afterwards. Di
Rock is a rate dependent material and this rate dependency needs to be considered when dynamic loading such as rock blasting is of interest. In this study, CA3 computer program which is a hybrid bonded particle-finite element program is used to numerically simulate the Brazilian tensile test in the Split Hopkinson Pressure Bar (SHPB) testing. The rock is idealized using the Bonded Particle Model (BPM) while the incident and transmission bars are simulated by a finite element system. It is shown that the induced damage and brittle failure in the Brazilian specimen is consistent with the physical observation. On the other hand, the dynamic tensile strength of the simulated rock is much smaller than the physical result suggesting that inertia
Investigations of the growth, interaction, and coalescence of cracks are important because they help to provide tools for the more realistic modeling of rock masses containing low persistence discontinuities and better estimations of the strength and stiffness of a rock material. Understanding the coalescence mechanism is useful for justifying the mechanism of continental crustal deformation, evaluating the structural failure of slopes with rock bridges, and analyzing the stability of tunnels when a mode I or mix mode failure mechanism is involved. The evaluation of crack growth can provide valuable information about the mechanism for the formation of new geological structures, and the formation, evolution, and growth of faults. This paper
Crack initiation and propagation in rock involve the development of a nonlinear zone around the crack tip called fracture process zone. The existence of fracture process zone influences the fracture behavior and therefore it must be carefully investigated. This zone is normally idealized as a cohesive zone whose mechanical properties are difficult to measure. In this study, a relatively simple approach is proposed to identify the properties of the cohesive zone and the length of the cohesionless crack in a sandstone specimen under three-point bending. Two types of specimens were tested: beams with center notch and with smooth boundary. The digital image correlation (DIC) was utilized to obtain the opening displacement around the position wh
A model that allows micromechanical parameters to soften as a measure of plastic deformation is discussed. In particular, a microdilation angle is involved to help for calibration of macroscopic volumetric deformation. Through biaxial and shear tests numerical simulations, it is shown that macrodilation angle of bonded particle system can be controlled only when small particles are used. The genesis pressure that causes small overlap of particles has an impact on dilation angle as well and can be utilized as a controlling factor to calibrate a bonded particle material for dilation angle and post-peak behavior.
The precision and speed of numerical simulations of physical phenomena has led to their increasing use in designing and research applications. These precision and speed are owed to the improvements in numerical methods and significant advancements in computing power of CPUs and GPUs.Particle-based methods are some of the most recently developed numerical simulation methods. Development of these methods has been long delayed due to the need for a relatively high computational effort.
Split Hopkinson Pressure Bar (SHPB) testing was used to evaluate the strength characteristics of sandstone under uniaxial compressive loading. The physical results suggest that rock strength increases under dynamic loading. A hybrid bonded particle-finite element model was used for numerical simulation of SHPB tests. A parameter called rock strength enhancement coefficient was introduced which is multiplied by the relative velocity of particles at the contact points to increase the bond strength between the particles. It is shown that a much better match between the physical and numerical results is realized if this enhancement coefficient is applied in the numerical simulation.
The stability of a soil slope, reinforced by a concrete pile, is studied both experimentally and numerically in this work. Our study suggests that when the concrete pile is located in the middle of the slope (at x/r = 0.5), the soil structure collapses under a pressure of 10.9?kPa that is the highest overburden pressure to cause instability of the tested reinforced sandy slope. However, when the pile is located in the upslope (at x/r = 0.75) or downslope (at x/r = 0.25), the slope failure occurs under a pressure of 7.8 or 3.12?kPa, respectively. Therefore, our experimental work suggests that a pile located at the middle of the slope can provide the optimum reinforcement of the soil structure studied in this work.
Dynamic uniaxial compressive strength of Pennsylvania blue sandstone was investigated using split Hopkinson pressure bar both physically and numerically. A hybrid finite-discrete element code called CA3 was employed to simulate the physical tests. The incident and transmitted bars were modeled using finite elements while the rock specimen was represented by a bonded particle discrete system. The incident stress pulse measured in the physical test was utilized as the input for the numerical simulation and was applied to the free end of the incident bar. Analysis of the numerical results suggests an underestimation of the dynamic rock strength; the effect of axial and circumferential inertia of the specimen didn’t manifest the strength valu
Rock fracture and the induced displacement field are investigated through physical and numerical experiments. A Berea sandstone specimen with a central notch was tested in three-point bending. Digital image correlation (DIC) was used to obtain the displacement patterns as the fracture initiated and propagated. The DIC results clearly show the development of a damage zone and displacement discontinuity along the center line of the beam at peak load. It is suggested that a traction-free or cohesionless crack may exist before the unstable growth, a condition normally ruled out in the literature based on numerical modeling within the frame work of a cohesive zone finite element analysis. In this note, the displacement profiles determined from D
Determination of soil engineering properties such as shear strength is essential to analysis many geotechnical problems. Therefore, determination of the reliable values for this parameter is very important. For this purpose, direct shear test as one of the oldest test to examine the shear strength of soils, is conducted on soil samples. There are too many factors which could affect results of direct shear test. Laboratory tests are expensive, difficult and time consuming, hence using numerical method to simulate experimental test and study effective factors can be useful. In this paper direct shear test was numerically modeled using CA2 hybrid finite element-discrete element method code. CA2 solves explicitly equations of motion together wi
The bonded particle model is a powerful tool in studying the mechanical behavior of rock. The common practice in simulation of rock failure using this model is to allow brittle fracture of the contacts between particles or at most tensile softening at the contact points ignoring the shear softening of the material. To overcome this shortcoming, a plasticity model that allows both tensile and shear softening of the filling material at the contact points of the particles was implemented in the CA2 computer program. The model was calibrated to mimic the elastic behavior of the Pennsylvania blue sandstone. It is shown that for a more ductile material, there is less scatter of micro-cracking at the peak load. Furthermore, the ductility parameter
A three point bending fracture test was performed on a typical quasi-brittle material (Berea sandstone). The loading was continued into the post-peak region where crack growth was visible along the center line of the beam. Subsequent inspection of a portion of the specimen showed that part of the fracture offered no resistance to loading – a cohesionless crack existed. Digital image correlation was used to study the nature of the displacement discontinuity associated with the cohesionless (traction free) crack. The pattern identified in the post-peak region was used as a guide to study the displacement discontinuity at peak load. Two possibilities are offered: (1) The critical opening was developed at peak load. (2) A cohesionless crack,
The zone of microcracks surrounding a notch tip—the process zone—is a phenomenon observed in fracture of quasi-brittle materials, and the characterization of the process zone is the topic of the paper. Specimens of different sizes with a center notch fabricated from a granite of large grain (Rockville granite, average grain size of 10 mm), were tested in three-point bending. Acoustic emissions were recorded and locations of microcracks were determined up to peak load. The results show that both the length and width of the process zone increase with the increase of the specimen size. Furthermore, the suitability of a proposed theoretical relationship between the length and width of the process zone and specimen size was st
Cavity expansion theory is a useful theory that has found some utilizations in geotechnical engineering. Specifically, it has been used widely to analyze problems related to deep foundations (V?sic, 1972), stability analysis of underground excavation (Ladanyi, 1967), insitu testing (Mayne, 2001), and penetration tests (Salgado and Prezzi, 2007). Due to the usefulness and applications of this theory, there are several reports in the literature about the numerical analyses of the cavity expansion tests (eg Vu et al. 2005; Vrakas 2016; Tarokh et al. 2016; Molaei et al. 2016; Menendez et al. 2017).Borehole breakout is a direct consequence of the in-situ stresses and stress concentration in rock resulting in preferential rock failure and has bee
no record found