Showing papers by "Jian Zhao published in 2018"

Grain-Based Discrete Element Method (GB-DEM) Modelling of Multi-scale Fracturing in Rocks Under Dynamic Loading

Abstract: This study aims to explore dynamic behaviours of fracturing and damage evolution of rock materials at the grain scale. A grain-based discrete element method (GB-DEM) is proposed to reveal microscale characterisation and mineral grain compositions of rock materials realistically. Micro-parameters of GB-DEM are obtained by calibrating quasi-static strengths, elastic modulus, stress–strain curves, and fracture characteristics of igneous rocks. Comprehensive numerical simulations are conducted to compare with dynamic experimental results obtained by the split Hopkinson pressure bar (SHPB). The reasonability of using the GB-DEM is presented to validate fundamental pre-requisites of the SHPB technique. Combined with crack strain and acoustic emissions, the rate dependency of crack initiation stress threshold and crack damage stress threshold is investigated. The dynamic damage evolution in the form of Weibull distribution is distinctively different from that in static tests and the shape/scale parameters are presented as functions of strain rate. Moreover, microcharacteristics of crack fracturing transition and fracturing patterns formation are discussed in detail. It is found that there exist two classes of mechanical behaviour (i.e., Class I and Class II) observed from stress–strain responses of dynamic tests. Main fracturing surfaces induced by intergranular fractures split the specimen along the direction of stress wave propagation in the type of Class I behaviour. Branching cracks derive the cracks’ nucleation and in turn increases the fragment degree. A shearing band formed near the fracture surface is caused by grain pulverisations, which eventually enhances the sustainability of rocks under dynamic loading. At last, we propose a generalised equation of dynamic increase factor in the range from 10− 5 to 500/s, and also discuss the characteristic strain rate.

Brazilian Tensile Strength of Anisotropic Rocks: Review and New Insights

Abstract: Strength anisotropy is one of the most distinct features of anisotropic rocks, and it also normally reveals strong anisotropy in Brazilian test Strength (“BtS”). Theoretical research on the “BtS” of anisotropic rocks is seldom performed, and in particular some significant factors, such as the anisotropic tensile strength of anisotropic rocks, the initial Brazilian disc fracture points, and the stress distribution on the Brazilian disc, are often ignored. The aim of the present paper is to review the state of the art in the experimental studies on the “BtS” of anisotropic rocks since the pioneering work was introduced in 1964, and to propose a novel theoretical method to underpin the failure mechanisms and predict the “BtS” of anisotropic rocks under Brazilian test conditions. The experimental data of Longmaxi Shale-I and Jixi Coal were utilized to verify the proposed method. The results show the predicted “BtS” results show strong agreement with experimental data, the maximum error is only ~6.55% for Longmaxi Shale-I and ~7.50% for Jixi Coal, and the simulated failure patterns of the Longmaxi Shale-I are also consistent with the test results. For the Longmaxi Shale-I, the Brazilian disc experiences tensile failure of the intact rock when 0° ≤ βw ≤ 24°, shear failure along the weakness planes when 24° ≤ βw ≤ 76°, and tensile failure along the weakness planes when 76° ≤ βw ≤ 90°. For the Jixi Coal, the Brazilian disc experiences tensile failure when 0° ≤ βw ≤ 23° or 76° ≤ βw ≤ 90°, shear failure along the butt cleats when 23° ≤ βw ≤ 32°, and shear failure along the face cleats when 32° ≤ βw ≤ 76°. The proposed method can not only be used to predict the “BtS” and underpin the failure mechanisms of anisotropic rocks containing a single group of weakness planes, but can also be generalized for fractured rocks containing multi-groups of weakness planes.

Stress Thresholds of Crack Development and Poisson’s Ratio of Rock Material at High Strain Rate

Abstract: The crack initiation and propagation induced by loading finally lead to the failure of rocks. An identification of crack evolution has been widely investigated in quasi-static tests (Hoek and Martin 2014), which is essential to prevent the catastrophic rock engineering disaster. A consensus in those studies is that the crack process has several stages corresponding to several stress thresholds. The validity of these thresholds was majorly verified by the interpretation of acoustic emission (AE) method (Eberhardt et al. 1998; Moradian et al. 2016; Zhao et al. 2015). Xue et al. (2014) summarised the studies of uniaxial compression tests to evaluate similarities and differences of stress thresholds among igneous, metamorphic and sedimentary rocks. Using the volumetric strain–stress and petrographic technology, Nicksiar and Martin (2013) discussed the relationship between the mineral composition and crack initiation stress. The formation and propagation of microcracks, and fracturing mechanisms are also dependent on Poisson’s ratio of geomaterials (Gercek 2007). With the increased accuracy in computational input for numerical analyses, a precise knowledge of the Poisson’s ratio is of importance. In quasi-static tests, however, the lateral strain is substantially challenging to determine (Swamy 1971). The strain gauge is widely used, but large strain will generate when the stress is above its peak value. Due to the influences of localised failure, it is desirable to measure the lateral strain as an average over a part instead of one point of the specimen. Such measurements include chain extensometer or fluid-field vessel by the fluid level change, but these contact methods to some extent lead to a confinement. Therefore, non-contact optical techniques have substantial advantages. The digital image correlation (DIC) has been proved robust in dealing with multiple deformation conditions (Xing et al. 2017). Cui et al. (2016), Pan et al. (2015) and Pritchard et al. (2013) used 2D-DIC to investigate the axial and transverse strain of composite material or alloy with a thin-section shape in static tests. Munoz et al. (2016) applied the 3D-DIC to determine the crack evolution stages of sandstone in quasi-static uniaxial compression tests. In addition to the static condition, rock engineering may encounter high-strain-rate loadings from explosion, impact or seismic activity (Barla and Zhao 2010; Zhou and Zhao 2011). To the best of our knowledge, there is no report on the crack thresholds or direct measurement of dynamic Poisson’s ratio of rock under high-strain-rate conditions. The primary challenge is that the lateral strain cannot be accurately acquired by traditional methods such as the strain gauge and extensometer under dynamic loading conditions. In this study, stress and strain thresholds of crack development of sandstone under different high strain rates with a split Hopkinson pressure bar (SHPB) were characterised by the high-speed 3D-DIC technique. The real-time evolution of Poisson’s ratio was extracted together with its strain-rate effect according to strain fields in 3D-DIC.

Development of a Unified Rock Bolt Model in Discontinuous Deformation Analysis

Abstract: In this paper, a unified rock bolt model is proposed and incorporated into the two-dimensional discontinuous deformation analysis. In the model, the bolt shank is discretized into a finite number of (modified) Euler–Bernoulli beam elements with the degrees of freedom represented at the end nodes, while the face plate is treated as solid blocks. The rock mass and the bolt shank deform independently, but interact with each other through a few anchored points. The interactions between the rock mass and the face plate are handled via general contact algorithm. Different types of rock bolts (e.g., Expansion Shell, fully grouted rebar, Split Set, cone bolt, Roofex, Garford and D-bolt) can be realized by specifying the corresponding constitutive model for the tangential behavior of the anchored points. Four failure modes, namely tensile failure and shear failure of the bolt shank, debonding along the bolt/rock interface and loss of the face plate, are available in the analysis procedure. The performance of a typical conventional rock bolt (fully grouted rebar) and a typical energy-absorbing rock bolt (D-bolt) under the scenarios of suspending loosened blocks and rock dilation is investigated using the proposed model. The reliability of the proposed model is verified by comparing the simulation results with theoretical predictions and experimental observations. The proposed model could be used to reveal the mechanism of each type of rock bolt in realistic scenarios and to provide a numerical way for presenting the detailed profile about the behavior of bolts, in particular at intermediate loading stages.

Dynamic increase factors of rock strength

Abstract: Understanding dynamic mechanical response of rocks subjected to extreme loadings is necessary in rock engineering analysis and design. The dynamic increase factor (DIF), is usually defined as the ratio of the dynamic strength to the quasi-static strength in uniaxial compression or tension, which has been widely accepted as an important parameter to measure rate sensitivity of brittle materials. This paper summarizes the features of rate dependency of rocks and systematically reviews the expressions for the relationship between strain rates and mechanical strength of geo-materials. In addition, based on a number of experimental results, an aggregated DIF for compressive and tensile strength of rocks is proposed, which can provide a good prediction of rock strength over a wide range of strain rates.

The mechanism of hysteretic ground settlement caused by shield tunneling in mixed-face conditions

Abstract: Shield is used more and more widely, such as coal mine roadway, hydropower tunnel, traffic tunnel and so on. But Tunneling with a tunnel boring machine (TBM) may cause inevitable ground subsidence. Though over the world numerous researches have been conducted for surface settlement induced by TBM in soft ground, the researches of surface settlement induced by TBM in a sandy cobble stratum are limited and a comprehensive study of the mechanism of delayed settlement induced by TBM in a sandy cobble stratum is unavailable. A ground stable state or surface settlement can be determined based on real-time monitoring data for surface dynamic subsidence. Subsequently, TBM tunneling parameters can be adjusted to accommodate various geological conditions. A sand-pebble-soil matrix is a typical heterogeneous material. The macro-mechanical performance of this matrix significantly differs from any material. In a general situation with a low water level and minimal disturbance, a stratum can stabilize by itself for a long period of time. Considering the characteristics of the stratum, ground loss can be divided into two phases: immediate settlement, which tends to stabilize, and delayed settlement, which tends to occur in sand-cobble strata, where settlement develops at a much slower rate than in single-medium strata. Monitoring data is not sufficient to guide the construction in the case of delayed settlement. Cobble-soil matrix can be treated as a spatial structural system that is constituted by single granular soil, aggregates of granular soil and pebble grains. Based on Particle Flow Code in 2 Dimensions (PFC2D), the mechanical characteristics of the matrix and the TBM tunneling process were numerically simulated. Movements of the pebble grains were traced and recorded in real time. The model addressed the mechanism of surface collapse from the perspective of mesomechanics. According to the model, a matrix formed self-stabilizing arch that overlies an underground cavity seems gradually wear out with expanding the cavity and eventually penetrating to the ground surface. The law of ground movement and the formation mechanism of ground subsidence in TBM advancement were investigated. The main factors that affect surface subsidence are the speed of advancement, the underground water level and the supporting period. In the numerical analysis, the surface-loss lag was reproduced and the field monitoring data were verified. The findings of this study provide a new method for investigating ground subsidence in similar strata.

Effect of joint thickness on seismic response across a filled rock fracture

Abstract: This study aims to investigate the influence of joint thickness on seismic response across a filled fracture with strong nonlinear deformability. To simulate seismic attenuation of thicker joints s...