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: 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 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: 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: The development of a successful Enhanced/Engineered Geothermal System (EGS) reservoir at commercial scale has proved an elusive goal—despite the prospects of an immense resource and a near-limitless supply of sustainable baseload power. Key challenges relate to both creating and sustaining a high-throughput reservoir from an initially very low permeability deep reservoir and in achieving this with minimal induced seismicity. In the following we explore the nature of early- and late-time induced seismicity anticipated in such projects, observed at contemporary and historic sites, and explore how this may be linked to permeability evolution through permeability-seismicity scaling and in coupling with the evolving rate and state properties of the reservoir.

ABSTRACT: The mechanical properties of Longyou Sandstone (LS) (a material for Longyou rock caverns) under the combination of high loading rate and high temperature are important parameters for the protection of the caverns. Thus, the LS sample were first heat treated by various temperatures (150°C, 250°C, 350°C, 450°C, 600°C and 850°C) and the thermal damage was then characterized by the 3D X-ray Micro-Computed Tomography (CT) method. Meanwhile, the density and P-wave velocity of heat-treated sample and reference sample were measured. The values of CT value, density, and P-wave velocity decrease with the elevated heat-treatment temperature except for 450°C. Thereafter, according to the suggested method for determining dynamic tensile strength and dynamic fracture toughness of rock by International Society of Rock Mechanics (ISRM), Brazilian Disc (BD) and Notch Semi-Circular Bend (NSCB) samples were employed in the Split Hopkinson Pressure Bar (SHPB) system to measure the dynamic tensile strengths and the Mode-I fracture toughness of these LS samples under different dynamic loading rates. The dynamic tensile strength and fracture toughness of LS increase with the loading rate at a given heat-treatment temperature, and both the tensile strength and fracture toughness at the same loading rate decreases with the heat-treatment temperature except for 450°C. This phenomenon can be explained by the variation of the numbers of micro-cracking in LS sample for the different heat-treatment temperatures. With the thermal damage variable derived from the average CT value, empirical formulas were established to relate the dynamic tensile strength and fracture toughness of LS to the loading rate and the heat-treatment temperature. The results demonstrate that the dynamic tensile strength and Mode-I fracture toughness of LS are significantly influenced by the heat-treatment, especially under higher loading rate.

ABSTRACT: Rock dynamics attracts great attention in rock mechanics. However, specimen preparation in experimental study is not easy. 3D printing started to be involved to improve this process, Including Polylactic Acid (PLA) and resin. However, their plastic behavior is much unlike brittle rock. In this work, a gypsum-like power material using inkjet 3D printing (3DP) technique was applied to produce pre-cracked specimens for dynamic Split Hopkinson Pressure Bar (SHPB) test. High speed camera observed that the failure pattern of this material is similar to brittle rock-like material such as Portland Cement Concrete (PCC). Our results indicate that sample preparation by 3D printing will make far more contributions to rock dynamics because it is highly competitive due to its quickness in prototyping, precision and flexibility on the geometry, and high material homogeneity.