ABSTRACT: Mining engineers must certify that the capacity of excavations in their mine exceeds the expected demand. This requires the engineer to satisfactorily estimate the expected future loads that an excavation will be subjected to, as well as the capacity of the rock mass—ground support system. In seismically active mines, this essential task is more difficult as high rates of deformation and dynamic loading are still comparatively infrequent events and the simulation of support and reinforcement for dynamic loading is more complex. Empirical methods of estimating excavation capacity for dynamic rock related hazards are also very poorly evolved, and have limited efficacy. In this paper, computational forecasting of potential for dynamic deformation in rock masses surrounding mines as well as the simulation of potential large events themselves is discussed. 1 INTRODUCTION When a hard working pillar, or rock bridge in the mine fails, or a highly stressed fault slips, load is redistributed from that yielded ‘structural element’ of the rock mass. Other parts of the mine must do that ‘work’. Whether other components elsewhere in the mine subsequently fail is a function of their capacity and the load path and ultimate loading which evolves as the system converges to a new equilibrium. Subsequent failures are rarely due to impulsive loads or vibrations from the seismic wave emanating from the original failure. Though seismic waves do damage excavations in mines, the magnitude and occurrence of the hazard is less than the exposure of personnel to increasing rock mass damage and deformation due simply to load redistribution caused by mining and under-supporting of excavations.

ABSTRACT: Plate impact experiments have been conducted on one fine-grained metamorphic rock, Carrara marble and one coarse-grained igneous rock, South Africa Gabbro. Embedded manganin stress gauges (i.e. longitudinal and lateral gauges) are used for the measurements of shock stresses. Shear strength are obtained by the difference of the longitudinal stress and lateral stress. Results suggest that rock materials are deforming in the inelastic manner and Hugoniot Elastic Limit (HEL) is deduced. Experimental results are compared and contrasted to the existing data for other geological materials. The detailed experimental procedures used to make measurements are to great extent standard methods when determining mechanical properties and deformation fields at high pressure.

ABSTRACT: The stress wave caused by underground explosion is usually assumed to propagate across rock masses in cylindrical form. The cylindrical wave propagation is a great concern of underground structure safety. This paper presents a study on cylindrical P-wave propagation across a single joint with linear elastic behavior using UDEC software. A semicircular cylindrical cavity is established at the bottom of the model to simulate the cylindrical wave loading. By calculation, UDEC is proved to be efficient to simulate cylindrical wave propagation. For the simplified UDEC models, the corresponding transmission and reflection coefficients are obtained. Parametric studies are carried out to investigate the influence of joint stiffness, the frequency of incident wave and the position of monitoring point on the transmission and reflection coefficients.

ABSTRACT: Both the positive and inverse problem of spherical waves propagating in concrete are studied. Using characteristics method, the nonlinear spherical waves in concrete, including the radial stress σ r ( r , t ), circumferential stress σ t ( r , t ), radial strain ε r ( r , t ) and the circumferential strain ε t ( r , t ) are calculated based on the ZWT constitutive relation. Numerical results illustrate that the viscosity effects and nonlinear effects in concrete cannot be ignored. Based on universal spherical wave conservation equations and combined with the rate-independent law of elastic volume deformation, the inverse analysis method for spherical waves (called ‘nv + T0/SW’) are developed to determine constitutive behavior of concrete when n particle velocity profiles are measured. The satisfactory agreement between the ‘characteristics solutions’ and the ‘nv + T0/SW’ inverse solutions confirms the validity of the proposed new method, which may be more suitable for studying the dynamic behavior of heterogeneous materials with large aggregates such as concretes and rocks et al. 1 INTRODUCTION In practical engineering problems, such as underground explosions and engineering blasts, there are often problems of point explosion, point impact and spherical cavity-walls subjected to explosion loading, where the spherical wave propagation should be dealt with (Wang 2005, Zukas et al. 1982). The dynamic response of concrete structures under explosive/impact loading is intrinsically dependent on the dynamic behavior of concrete materials involved in the structures. Thus the importance of research on the dynamic behavior of concrete materials under high strain rates (called strain rate effect for short) cannot be overemphasized.

ABSTRACT: To simulate the in situ stress state of underground rocks, a modified Split Hopkinson Pressure Bar (SHPB) system is utilized to load Brazilian Disc (BD) samples hydrostatically, and then exert dynamic load to the sample by impacting the striker on the incident bar. Five groups of Laurentian granite BD samples (with static BD tensile strength of 12.8 MPa) under the hydrostatic confinement of 0 MPa, 5 MPa, 10 MPa, 15 MPa, and 20 MPa were tested under different loading rates. The result shows that the dynamic tensile strength increases with the hydrostatic confinement stress. It is also observed that under the same hydrostatic pre-stress, the dynamic tensile strength increases with the loading rate, revealing the so called material rate dependency. Furthermore, the increment of the tensile strength decreases with the hydrostatic confinement stress, which resembles the behavior of rock static tensile strength under confining pressure as reported in the literature. 1 INTRODUCTION Tensile failure is a main failure mode of rocks in underground rock engineering projects, in which rocks are subjected to dynamic disturbances while under in situ stresses. As well known, pores and microcracks are potential sources of failure for rock materials because of stress concentration (Aadnoy & Angellolsen 1995, Bordia 1971, Zhang et al. 2014). When disturbed by dynamic loads from blasting, seismicity, or rockbursts, the underground rocks would be vulnerable to tensile failure. The discontinuities in the rock result in tensile stress locally around the opening, so the dynamic tensile failure of rocks under pre-tension has been investigated by researchers (Wu et al. 2015, Zhou et al. 2014). However, the far field rocks under hydrostatic pressure can also fail in tension under dynamic loads. Therefore, it is the purpose of this paper to investigate the dynamic tensile failure of rock materials under hydrostatic pressure state.

ABSTRACT: Based on the block hierarchical structure of practical rock masses, this paper presents a systematic study of the mechanical process that structural blocks at different hierarchical levels undergo a confined motion (rotation and translation) under the explosion seismic wave, thus actuating strain concentrations at the surfaces of weakening. Then some neglected conditions of Kocharyan's work in calculating scales of activated blocks is given, the equivalent strain ε * is redefined, and the calculation formula of irreversible deformation region radius under large-scale underground explosion is re-derived. The existing test data confirms its accuracy.

ABSTRACT: The P-wave propagation through fractured rock mass is simulated using UDEC with a cohesive fracture model. A series of laboratory tests from literature on P-wave propagation through rock mass containing single fracture and two parallel fractures are introduced and the numerical model used to simulate these laboratory tests are described. The simulation results show that the proposed model, particularly the cohesive fracture model can capture very well the wave propagation characteristics in rock mass with un-welded and welded fractures with and without filling materials. In the meantime, in order to identify the significance of fracture on wave propagation, filling material with different particle sizes and the fracture thickness are discussed, and both factors are found to be crucial for wave attenuation. 1 INTRODUCTION Natural rock mass contains continuous intact rocks and discontinuities, such as faults, joints, fissures and beddings. These discontinuous geological structures are most likely filled with weathered residuals, for example sands, clays and organic materials from plants. A better understanding of the wave propagation mechanism of the filled discontinuities in rock mass is theoretically and practically meaningful.

ABSTRACT: The underground facilities are used for a wide range of civil and military applications (railways, highways and material storage). These structures are constructed in different possible ground conditions ranging from soft ground to massive jointed rock. Generally, the loads on tunnels consist of both static (overburden) and dynamic (blast, earthquake and traffic) loads. Therefore, the support system is usually designed to resist all transient loads developed during construction activities as well as the short-term ground loads. Therefore, in the present work an attempt is made to simulate the in-situ condition through the physical modeling, to understand the tunnel deformation behavior by varying overburden thickness in soft rocks. The model material is selected on the basis of engineering properties of the different mix proportions of PoP, sand and clay (UCS, elastic modulus and Poisson's ratio), and the Plaster of Paris (PoP) mixed with sand and some percent of clay (50% PoP, 40% Badarpur sand, and 10% Kaolinite) is selected for physical modeling, as the PoP alone shows brittle behavior. It is interpreted that the height of overburden and thickness of lining are critical factors in correlating degree of fracture and damage to the tunnel—lining. Further, the deformation and degree of damage is determined for lined and unlined material and hence controlling the fracture in the existing conditions.

ABSTRACT: The paper investigates the effect of contact area of joint surface on stress wave propagation and the dynamic property of joints by using Split Hopkinson Pressure Bar (SHPB) apparatus. Artificial joints were made with two blocks, one surface of one block was sawn to shape a number of notches while the surface of the other one was flat and smooth. Each sample has the same depth but different width of notches. Based on the fundamental theory of SHPB test, the transmission coefficients, the deformation of rock specimens and the closure volume of joints were obtained. The experimental results show that the deformation of rockmass is mainly caused by the joint deformation and the closure volume of joints increase when contact area declines. The transmission coefficients increase almost linearly with contact area ratio for a given notch width.

ABSTRACT: Earthquakes occurred frequently this century. The earthquake created an inestimable damage to tunnels and underground structures. Also, cracked lining widely exist in practical projects. Based on model test, the preliminary study to the seismic response rule of cracked lining tunnel had been conducted. The following conclusions can be obtained. Under the same rock pressure, the additional moment of hard rock is larger than for soft rock. The bigger the rock pressure is, the larger additional moment of structures will be. The additional moment of intact lining and cracked lining are relatively large in the direction of 45 degrees to the horizontal direction. When there is only one crack, the moment of the lining is reduced near the crack. When there are more cracks in the lining, the bending moment has stress concentration near the crack. The conclusion of these experiments can provide a reference for the designer. 1 INTRODUCTION Many tunnels have been damaged noticeably in earthquakes in last two decades including 1999 Chi-Chi, Taiwan earthquake (Wang 2001) and 2008 Wenchuan earthquake (Wang 2013). The response of tunnel lining was studied by shaking table (Yakovlevich 1987). Experiments were carried out on the underwater tunnel by means of the mechanical vibration (Shunzo 1984). The seismic response of two parallel shield tunnels were tested, and the influence of the parallel distance on the seismic response of the structures was studied (Goto 1988). Based on the investigation of earth-quake damage and seismic analysis, the characteristics of dynamic response of underground structure were summarized (Kuesel 1969). The basic rules of seismic damage of underground structures were summarized (Hashash 2001). In 1980s the flexibility ratio concept was introduced into the seismic research (St. John 1980). An analytical method for the interaction between structure and ground was proposed by scholars (Yousser 2001). However, study on seismic response of cracked lining is rare. In this paper, we analyze the seismic response of cracked lining by model experiment.

ABSTRACT: The paper gives a briefly introduce about the recently experimental studies for the dynamic property of rock joints based on SHPB tests. The joints include artificial joints filled with sand and clay and artificial non-filled joints cut on granite and aluminum alloy blocks. The experimental method for wave separation is explained for the modified SHPB test with short rock bars. The influence factors on the dynamic property of rock joints are investigated, such as the water content, the thickness of filling layer, the pulse loading intensity, the loading rate, the joint matching coefficient and the spatial geometry of joint surface. 1 INTRODUCTION The mechanical behavior of rock mass is significantly affected by the vastly existing discontinuities, primarily joints. One of the main tasks in the fields of rock mechanics and engineering is to well understand the mechanical properties of the discontinuities and their effects on rock mass behavior. The discontinuities are generally called as joints of rock masses. In nature, there are many kinds of rock joints. As for the filling material, the rock joints are divided into filled joints with filling material or non-filled joint. The physical properties of rock joints affect the normal and shear strength, deformability, permeability, etc. The test results of Pyrak-Nolte et al. (1990) showed that the dynamic stiffness of rock joints was always higher than the quasistatic stiffness, and the dynamic stiffness was related to the stress and saturated conditions.

ABSTRACT: The Representative Elementary Volume (REV) of fractured rock masses plays an important role on determining the rock mechanical characteristics. Firstly studying the general methodology of rock REV are summarized, and the modelling of Discrete Fracture Networks (DFN) is proposed as the investigation foundation of the REV of fractured rock masses, and then the numerical method to determine the REV is analysed and used in the case study. Combined modelling technique of random fractured network with discrete numerical method, the size effect of rock parameters has been analysed under single fluid flow condition and hydro-mechanical coupling condition. The calculation results of examples mentioned above indicate that the permeability tensor will decrease with the increasing stress and gradually tend to smooth while the studying scale increasing. The above achievements show that the proposed numerical method is an efficient approach to determine the rock mechanical parameters and REV, and also should be extended to complex three-dimensional problem in the geotechnical field in future. 1 INTRODUCTION The natural rock masses has experienced a long period of complex and unbalanced geological process, the rock masses has been cut by the fractures, and the randomness of the fractures distribution has induced the rock masses to present obvious discontinuous, heterogeneous and anisotropic characteristics (Harrison et al. 2000, Jing 2003). And the geological investigation has stated that fractures in disorder are obeying some probability distribution, and the geometric characteristics of above factures such as fracture width, dip, length and density determine the formation of the fractured network, so the mechanical property of rock masses implies some uncertainties. According to current research investigations, the value of rock mechanical parameters obtained by the field tests or laboratory tests appear obviously different due to different rock sample size, so the rock size effect is widely used to research on the rock strength, deformation, permeability and stress measure.

ABSTRACT: Oil shale is a compact laminated rock of sedimentary origin containing organic matter known as kerogen. The steps involved for in-situ extraction process are hydrofracturing, injection to achieve communication, and fluid migration at the underground location. Therefore, it is very important to understand the physico-mechanical behaviour of such anisotropic rocks. The favourable characteristics of Assam coal for conversion to liquid fuels have been known for a long time. Studies have indicated that these coals and carbonaceous shale constitute the principal source rocks that have generated the hydrocarbons produced from the region. In the present work an attempt is made to understand the engineering behavior of Indian oil shales experimentally. The in situ coring is performed to get the samples for testing purposes, as coring in laboratory is very difficult due to its highly anisotropic nature. Different tests are performed to understand the petrology of these samples, further the chemical analyses are also done to exactly quantify the organic and inorganic contents in these rocks. The physical and mechanical properties of these rocks are investigated by considering different anisotropic angles. Further, the petrophysical test results are correlated with the mechanical properties. These properties and correlations will further help in increasing the yield of these rocks.

ABSTRACT: The adaptive automatic triaxial testing machine test study on mechanical properties at different temperatures of the role of gneissic granite conduct, failure mechanism analysis gneissic granite peak stress, peak strain, elastic modulus variation with temperature. The results showed that: the temperature of the mechanical characteristics of gneissic granite greater impact, as the temperature rises, the mechanical properties of gneissic granite obvious damage, peak stress and elastic modulus decreased peak strain increases. In gneissic granite elastic modulus variation with temperature start, assuming that rock infinitesimal strength to obey two Weibull distribution parameters derived considering thermal effect of damage evolution equation and damage constitutive equation. 1 INTRODUCTION Rock is a different cement and mineral particles and contains a lot of micro-cracks and micro-holes and other defects in natural materials, their physical and mechanical properties by severe temperature. Processing railways, highways, utilities, inter-basin water transfer, and other mineral resources in the field of underground rock engineering, rock mechanics often need to consider the effect of temperature characteristics. Thus, fully aware of the influence of temperature on mechanical properties of rock, for ensuring the long-term temperature influence of rock engineering stability and security of the key.

ABSTRACT: Experiments of granite under dynamic compression at strain rate 42/s, 65/s and 84/s with a Split Hopkinson Pressure Bar (SHPB) system are conducted in the present paper. Quasi-static test at strain rate 3 × 10 −5 /s is performed with Material Testing System (MTS) for reference of specimen at low strain rate. A high speed camera and a Charge Coupled Device (CCD) are employed in dynamic and quasi-static tests respectively for images acquirement of specimen surfaces. Digital Image Correlation (DIC) method is applied to the images shot in the compression process for the quantitative full-field transverse normal strain measurement on surface of specimen. Color maps of strain field show the strain localized on regions where cracks concentrate. Statistics on strain filed exhibit a typical Weibull distribution. According to different features in strain distribution, the compression process is divided into three stages which coincide with the stages of cracks nucleation, interaction and breaking through specimen leading to failure, respectively. Evolution of Weibull parameters for strain field indicates the strain rate effects on the crack propagation and failure mechanism of granite. 1 INTRODUCTION Brittle rocks such as granite, marble and sandstone consist of mineral grains with multiple species and sizes, and contain microscopic pores and cracks. Therefore, great heterogeneity of internal structure exists in brittle rocks, and the deformation is heterogeneous for brittle rocks. Tensile microscopic cracks nucleation, interaction and coalesce to macroscopic cracks is the main failure mechanism for brittle rocks subjected to compression (Tapponier & Brace 1976). The mechanical behaviors of brittle rocks have been substantially investigated at different strain rates (Frew et al. 2001, Zhang et al. 2000), varying from quasi-static to dynamic conditions. Significant strain rate effects are shown in brittle rocks. However, deformation of specimen is acquired indirectly from traditional methods such as strain gauges and mechanical extensometers. These measurements are not sufficiently accurate for the slight strain of brittle rocks. Moreover, the mesoscopic local strain is impossible to observe since only global deformation can be measured by traditional methods. Thus, investigations on strain rate effects and failure mechanism of brittle rocks are far from plenty.

ABSTRACT: Hopkinson bundle bars device is developed on the Split Hopkinson Pressure Bar (SHPB) system to investigate the dynamic slip characteristics of cement joint under compression-shear combined loading. The experimental configuration of one-bar/three-bar incidence and three-bar transmission is employed to measure signals of different locations in specimen, which used to determine the slipping state of joint surfaces. And the overall slipping states of joint surfaces are then obtained. The study is helpful to reveal local slipping and whole slipping state of joint surface. 1 INTRODUCTION Rock is one of the most widely used materials in geotechnical engineering, civil engineering, hydraulic engineering, etc. Natural rocks are complex medium with such defects as cracks and pores at the meso and micro scale, and joints at the macro scale. Quasi-static and dynamic strength properties of rock mass in geotechnical engineering are mainly determined by these complicated meso and macro geometry structures. Rock materials are both load-path dependent and strain-rate dependent. Specific efforts had been made on the improvement of experimental techniques to investigate the compression properties of rock joints.

ABSTRACT: A three dimensional (3D) Voronoi tessellations based on Distinct Element Method (DEM) is proposed for simulation of crack initiation and propagation in polycrystalline-based brittle rocks. Damage evolution equations for brittle rocks have been extensively studied in the past. However, the damage model with respect to Mesoscopic Method has not been widely developed due to difficulty of direct observation for microcracks in laboratory tests. The Voronoi tessellation approach which can measure the invisible cracks was firstly utilized to simulate the Uniaxial Tension Test (UTT) in this study. A series of simulated Uniaxial Tension Tests were conducted to investigate the relationships between micro-parameters and macro-response and validation study with laboratory test was also carried out. Moreover, a novel damage model relying on crack initiation and propagation was defined and comparisons of evolution equation between existed models were also conducted to validate the accuracy and robust of the model. A good agreement was found between the simulated results and laboratory observations (e.g. failure modes, peak strength and cracks location). Different methods are proposed to detect the damage thresholds such as crack initiation value (CI) and crack interaction value (CD). Comparisons are also discussed to accurately measure the thresholds values.

ABSTRACT: Fractures with weakly and strongly correlated aperture distributions were used to explore a fundamental scaling relationship between fracture stiffness and fluid flow in a fracture. Computational models were used to analyze fluid flow through a fracture undergoing deformation and chemical alteration. Fracture stiffness captures the deformation of the fracture void geometry and enables collapse of the numerical flow-stiffness data, from multiple length scales, to a single scaling function. Simulations of wave propagation across fractures with varying contact area but with fixed asperity spacing show that elastic wave transmission is also sensitive to the fracture geometry. The hydro-mechanical scaling relationship provides a path forward for the remote characterization of fluid flow through fractured rock. 1 INTRODUCTION Long term safety and optimization of subsurface engineering activities require geophysical monitoring methods that can delineate and characterize evolving fractures and fracture systems. Fractures and other discontinuities pose serious challenges for seismic detection because such mechanical discontinuities span many orders of magnitude in size, are structurally complex and are subjected to complicated mechanical, hydraulic and geochemical processes that alter fractures over time. Thus, both the interrelationship among fracture properties (Fig. 1) and the scaling behavior of these relationships are needed for the optimal design of geophysical monitoring systems and for simulations used to interpret geophysical data to predict subsurface performance.

ABSTRACT: In rock crushing process, a key goal is understanding of fragmentation behaviour, which could be used to improve productivity and efficiency in mining operations. In this study, the input parameters, such as feed size, output particle size, and the energy expended in the process are carefully monitored. The test was based on a modified Hopkinson pressure bar system and a Piston-holder system. The energy consumed by the sample during fragmentation was determined by considering the energy carried by the incident wave and the reflected wave. A granite with the feed size ranging from 9.5 to 12.7 mm was employed in this study. The samples were impacted sequentially from one to ten times, with fragment size reduction determined after each set of impacts by sieve analysis. The link between fragment size distribution and the energy absorbed by the samples was examined.

ABSTRACT: Low Rate Of Penetration (ROP) is one of the key technical difficulties in the anisotropic shales reservoir. Rock fragmentation efficiency is improved under coupled dynamic-static load. A new model for estimating the indentation fragmentation process of shale is developed. The damage and failure behaviors of rocks are simulated by using the explicit contact finite element method. According to the macro indentation experiment, a simulation model of rock fragmentation is verified. Rock fragmentation process is studied under three types of load conditions, including static load, dynamic load and coupled dynamic-static load. The results show that rock failure occurs along with the direction of maximum principal stress, mainly depending on tensile and shear stress, followed by the compressive stress. With the increase of static load, indentation depth increases, and the volume of fractured rock is enlarged as well, but there is a non-linear relationship between the static pressure and fragmentation volume. With the increase of dynamic load velocity, the transverse transmission range of stress wave get smaller, the longitudinal transmission depth of stress wave increases, and the transmission depth of the impact energy is improved. When the peak value of load is the same, energy density under dynamic-static combination load is higher than that under static load condition, and rock fragmentation efficiency is also improved significantly, which further explains the indentation fragmentation mechanism of shale. The mechanical behaviors of indentation fragmentation under complex dynamic-static loads conditions should be considered in the design of ROP optimization and drilling tools.