Quantitative Investigation On The Mechanical Characteristics And Cracking Behavior Of Rock Masses Containing An Opening And Joints

Karst landforms are widely distributed in western China.Under the long-term karst action,many karst caves are formed in rock masses,which control the mechanical characteristics and failure behavior of rock masses.Similarly,tunnel,roadway,and goaf can be regarded as artificial caves.The stability of surrounding rock around these natural and artificial openings is a concern of engineering and academic fields.Under external loading,the surrounding rock is prone to cracking,subsidence,and collapse,which will further cause major safety problems to the engineering construction and operation.Engineering practice indicates that many natural discontinuities with various scales and properties are generated inside rock masses.These discontinuous deteriorate the strength and failure characteristics of rock masses and result in complex anisotropy,heterogeneity,and discontinuity of rock masses.Therefore,the stability problem of surrounding rock around caves can be attributed to the mechanical problem of brittle materials containing openings and joints.Grouting technology is commonly used in underground engineering to treat opening problems.After grouting material fills the opening,there is an interaction between the grouting body and fractured rock mass,whereas less research is being done to consider this mechanical interaction.Based on the rock mass structure characteristics,various rock mass models containing joints and an opening,considering multiple factors involving the surrounding rock,opening,joint,and grouting body were proposed.The mechanical characteristics and cracking behavior of model specimens in uniaxial compression were quantitatively studied by using laboratory tests,Rock Failure Process Analysis(RFPA)simulations,fractal geometry theory,and digital image correlation(DIC)method.The reinforcement mechanism of the grouting body was quantitatively summarized by comparing the results of specimens without and with the grouting body.(1)Uniaxial compression tests were conducted on the rock-like specimens containing an opening and a joint.The failure process of the specimens was numerically simulated by RFPA.The influence of joint inclination and grouting body on the stress distribution,mechanical properties,and failure mode was analyzed by comparing the experimental and numerical results of specimens without and with the grouting body.The grouting body reduces the stress concentration,the maximum compressive stress and tensile stress of specimens decrease.The failure modes,mechanical parameters and their variation amplitudes after filling are closely related to the joint inclination.A horizontal/vertical inclined joint leads the specimens without and with the grouting body to fail in a mode of cutting through the rock bridge.The peak and residual strength are the smallest and their amplifications are the highest in the case of an inclined flaw where the failure mode is controlled by the presence of such a joint.(2)The rock-like specimens containing an opening and two joints were prepared by 3D sand printing technology.Digital image correlation method was used to non-constantly monitor the full field during the compression process.The covariance matrices of three strain components containing horizontal,vertical,and shear strain were calculated.The effective variance of the strain field was introduced to quantify and identify the failure behavior of these specimens.The mechanical properties of 3D sand printing standard specimens are similar to natural sandstone,which can be grouped as a rock-like material.Based on the results of DIC,three basic types of crack are successfully identified.The failure patterns of specimens containing an opening and two joints are all tensile and shear mixed failure.The coalescence patterns between the opening and the joint are influenced by their horizontal distance and can be classified into tensile coalescence,rotation coalescence,and shear coalescence.The strain field evolution can be described in detail by the effective variance of the strain field.The effective variance of the strain field is close to zero at the original crack closure stage and elastic deformation stage and behaves in different evolution laws after crack initiation.Based on the effective variance of the strain field,a quantitative identification method for the crack propagation type was proposed.(3)The rock model containing an opening and persistent multiple joints was taken as the research object.The matrix material of model specimens was made by cement mortar.The uniaxial compression tests and RFPA simulations were conducted on these model specimens.Box-counting dimension method was introduced to quantify newly formed cracks geometric distribution,and its correlation with cracking characteristics was investigated.The failure modes of the specimens can be classified into two types,namely,cutting through the joint and sliding along the joint.The grouting body exerts little influence on the failure pattern but has an appreciable reduction in the stress values and an appreciable increment in the mechanical properties.The fractal dimension of newly formed cracks geometric distribution of failure specimens is closely related to the joint inclination and grouting body.With the increase of joint inclination,the fractal dimension decreases first and then increases.For the same value of joint inclination,the fractal dimension of specimens with grouting body is larger than that of specimens without grouting body.Further data fitting of fractal dimension and mechanical properties revealed that the relationship between the two could be expressed be an exponential growth model.(4)The sandstone specimens containing an opening and non-persistent multiple joints were prepared by natural sandstone.Digital image correlation method was applied to capture the deformation fields of the specimen surface during compression.The mechanical mechanism of crack evolution of the specimens and the effect of the grouting body were quantitatively analyzed.The DIC measurements were further refined,including the analysis of the relative movement trend of the measuring points on both sides of the new cracks and the full-field strain dispersion evolution analysis.The grouting body increases the mechanical properties of the specimens in the form of increased peak strength,elastic modulus,and residual strength.By displacement vector analysis,four basic types of crack,nine types of crack coalescence among joints,and six categories between opening and joints are summarized.The grouting body may alter the cracking behavior of the specimens,especially for specimens with joint inclinations of 45° and 60°,whose crack coalescence types are changed.The differentiation rate of effective variance of strain field was calculated by the finite-difference derivative to describe the strain field evolution.Based on the evolutionary characteristics of the differentiation rate of effective variance,a method for recognizing precursory anomaly was proposed to quantitatively describe the influence of the grouting body on the precursory anomaly characteristics.The precursor occurrence stress and time are larger for the specimens with the grouting body than those of the specimens without the grouting body,while the stress ratio and time are smaller than those of the specimens without the grouting body.