Study On Strength And Deformability Of Hard Brittle Sandstone

In recent years,there have been a growing number of rock mass projects,in particular the underground constructions,including underground mining,hydrocarbon production,geothermal exploitation,deep and long tunnel and CO2 sequestration.In these rock mass engineerings,hard brittle rock mass is prone to brittle failure under the influence of excavation disturbance,displaying entirely different mechanical behavior and mechanism of deformation and failure.Understanding of the failure mode and mechanism of rock under differenct stress states is thus crucial to the project design,construction security and long-term operation stability.Sandstone formations are widespread in the Earth's shallow crust.They can form oil and gas reservoirs and thus are important engineering materials.In addition,sandstone can be considered to be ideal medium for geological storage of greenhouse gases and waste for its porosity.Therefore,the strength and deformation behavior of sandstone under real crustal stress states is a key scientific problem in the rock mass projects in deep.A large number of in-situ and laboratory tests show that,the brittle failure process of rock is intimately related to the initiation,propagation and coalescence of microcracks,accompanied by the rapid release of a huge amount energy.In this study,laboratory tests,including Brazilian test,uniaxial compression test,conventional triaxial compression test and true triaxial compression test,are performed on a low-porosity sandstone,which is referred to as Zigong sandstone.The strength and deformability of the Zigong sandstone are investigated systematically by means of acoustic emission(AE)monitoring technique and scanning electron microscope(SEM)inspection,revealing the macro-and micro-scopic mechanisms of brittle failure in this hard brittle sandstone.This research provides important reference for the understanding of strength and deformation properties of hard brittle sandstone under complex stress states.The main findings of this study are summarized as follows:(1)In the Brazilian tests,two kinds of sensor array configurations are proposed for the AE monitoring.For each configuration,three-dimentional evolution of damage in the Zigong sandstone Brazilian disc is investigated by the concurrent use of active and passive ultrasonic techniques.During each test,P-wave velocities along various raypaths are measured,based on which the time-dependent transversely isotropic velocity model can be further constructed.Then the collapsing grid search algorithm is adopted together with the velocity model,facilitating the accurate localization of AE events and thus the quantitative analysis of the spatiotemporal evolution of damage.Finally,the determined crack initiation positions and source mechanism are used to evaluate the indirect tensile strength.(2)Two sets of uniaxial compression tests are conducted on the Zigong sandstone specimens under a constant circumferential strain rate and a constant axial strain rate,respectively.It is found that the post-peak behavior of the Zigong sandstone is a combination of Class I and Class II behavior,the latter of which can be realized only by the circumferential strain control method.Similar to the Brazilian tests,active and passive ultrasonic techniques are used concurrently during each uniaxial compression test.P-wave velocity has been found to be strongly anisotropic and is used for the construction of the time-dependent transversely isotropic velocity model for each specimen.Discrete AE events are harvested from the continuous waveforms and are then used for source location analysis based on the constructed velocity model and the collapsing grid search algorithm.Finally,the effects of control methods on the P-wave velocity,AE characteristics,strain localization,and attenuation properties are analyzed.(3)In the conventional triaxial compression tests,two control methods are also adopted to conduct a comparative study on the post-peak behavior of Zigong sandstone under confinement.After investigating the effects of confining pressure on the stress thresholds and failure plane angle,emphasis is given to the quantitative analysis on the post-peak energy balance and the corresponding effect of confinement.Finally,the brittle-ductile transition pressure is predicted successfully based on the Hoek-Brown strength criterion and the frictional hypothesis proposed by Orowan.(4)In the true triaxial compression tests,the failure process of the Zigong sandstone under true triaxial stress states is investigated based on a "Mogi-type"true triaxial apparatus.Based on the complete stress-strain curves,the effect of the intermediate principal stress on the mechanical deformation,stress thresholds,and failure plane angle.In addition,a detailed microscopic inspection on micromechanical behavior and microcracks is conducted by means of SEM,which provides the microscopic explanation for the anisotropic dilatancy,and reveals the microscopic features of the process zone of the left-lateral mode ? crack under the true triaxial stress states.(5)Given that the effect of the intermediate principal stress is overlooked in several classic strength criteria,a generalized three-dimentional nonlinear strength criterion is proposed by combining a new deviatoric function and a power function as a meridian function,which is a generalization of several classic criteria,including the Tresca,Drucker—Prager,Mohr-Coulomb,Lade-Duncan and Matsuoka-Nakai failure criterion.Its strength parameters have clear physical meaning and their determination procedures are convenient.Comparisons between the failure criterion and experimental results are presented for uncemented/cemented Monterey sand,normally consolidated Fujinomori clay,rockfill,concrete,Mu-San sandstone and granite,which reveal that the proposed failure criterion captures experimental trend quite well.(6)Combining the merits of the generalized three-dimentional nonlinear strength criterion and Hoek-Brown strength criterion,a modified three-dimentional Hoek-Brown strength criterion is further proposed.The strength parameters can be determined only based on the conventional compression test data,which greatly facilitates the application of the modified Hoek-Brown strength criterion in rock engineering.