Analytical model of bolt shear resistance considering progressive yield of surrounding material

Existing analytical models usually fail to match with the actual conditions due to ignoring the nonlinear behavior of the surrounding material reaction force, which changes progressively with the joint shear displacement from elastic stage to yield stage. To tackle this problem, this study proposes a new analytical model to describe the bolt deformation and bolt contribution from elastic stage to plastic stage. The developed model is verified by available experimental direct shear tests of bolted joints and compared with existing models. Then, based on this model, the effects of the joint dilation angle, the bolt installation angle, the friction angle, and the surrounding material strength on bolt contribution are also analyzed and its implication is further discussed. Our results show that the proposed model can precisely describe the evolution of bolt contribution from elastic stage to plastic stage. Compared with surrounding material strength, the augmentation of the joint dilation angle and friction angle is more beneficial to increase the bolt contribution and the optimal installation angle. The work presented is to attempt to provide a reference for the understanding of bolting mechanism of jointed rock mass, the development of bolting theories and the practice of bolting engineering.

Numerical Simulation for Fracture Propagation in Elastoplastic Formations

Current hydraulic fracture models are mainly based on elastic theories, which fail to give accurate prediction of fracture parameters in plasticity formation. This paper proposes a fluid–solid coupling model for fracture propagation in elastoplastic formations. The rock plastic deformation in the model satisfies the Mohr-Coulomb yield criterion and plastic strain increment theory. The extended finite-element method (XFEM) combined with the cohesive zone method (CZM) is used to solve the coupled model. The accuracy of the model is validated against existing models. The effects of stress difference, friction angle, and dilation angle on fracture shape (length, width), injection pressure, plastic deformation, induced stress, and pore pressure are investigated through the model. The results indicate that compared with elastic formation, fracture propagation in elastoplastic formation is more difficult, the breakdown pressure and extending pressure are greater, and fracture shape is wider and shorter. The plastic deformation causes the fracture tip to become blunt. Under the condition of high stress difference or low friction angle formation, it is prone to occur large plastic deformation zones and form wide and short fracture. Compared with friction angle, dilation angle is less sensitive to plastic deformation, fracture parameters, and fracture geometry. For the formation with high stress difference and friction angle, the effect of plasticity deformation on fracture propagation should not be ignored.

Sensitivity of the Seismic Moment Released During Fluid Injection to Fault Hydromechanical Properties and Background Stress

Fluid pressure perturbations in subsurface rocks affect the fault stability and can induce both seismicity and aseismic slip. Nonetheless, observations show that the partitioning between aseismic and seismic fault slip during fluid injection may strongly vary among reservoirs. The processes and the main fault properties controlling this partitioning are poorly constrained. Here we examine, through 3D hydromechanical modeling, the influence of fault physical properties on the seismic and aseismic response of a permeable fault governed by a slip-weakening friction law. We perform a series of high-rate, short-duration injection simulations to evaluate the influence of five fault parameters, namely the initial permeability, the dilation angle, the friction drop, the critical slip distance, and the initial proximity of stress to failure. For sake of comparison between tests, all the simulations are stopped for a fixed rupture distance relative to the injection point. We find that while the fault hydraulic behavior is mainly affected by the change in initial permeability and the dilation angle, the mechanical and seismic response of the fault strongly depends on the friction drop and the initial proximity of stress to failure. Additionally, both parameters, and to a lesser extent the initial fault permeability and the critical slip distance, impact the spatiotemporal evolution of seismic events and the partitioning between seismic and aseismic moment. Moreover, this study shows that a modification of such parameters does not lead to a usual seismic moment-injected fluid volume relationship, and provides insights into why the fault hydromechanical properties and background stress should be carefully taken into account to better anticipate the seismic moment from the injected fluid volume.

Research on Peak Shear Strength Criterion of Rock Joints Based on the Evolution of Dilation Angle

The surface morphology of joints directly determines the contact area and peak shear strength of rock in shearing. This research gives a detailed description of the three-dimensional roughness parameters proposed by Grasselli, and discusses the morphology characteristics of the roughness parameter C in different ranges. Quoting Xia's direct shear test data, the relationship of peak shear strength, peak shear strength/normal stress, residual strength, dilation with roughness and normal stress are studied. The deficiencies in the previous classical criteria are explained in detail. A new peak shear strength criterion is established based on the boundary conditions of the dilation angle. The nonlinear characteristic of peak shear strength is included in the peak dilation angel, so that the new criterion satisfies the Mohr-Coulomb law of nonlinear form. In the expression of the initial dilation angle, the new criterion satisfies the fact that the peak shear strength decreases after once sheared. Finally, the test data of Grasselli and Yang are used to compare the prediction accuracy of Grasselli’s criterion, Yang’s criterion, Xia’s criterion and the new criterion. In general, the new criterion has the highest degree of agreement with the test results.

Shear Failure Mechanism and Acoustic Emission Characteristics of Jointed Rock-Like Specimens

Evidence indicate that the stability of rock mass is highly associated with the shear behaviours of jointed surfaces under the effect of in situ stress conditions. Understanding the shear failure mechanism of jointed surface has great significance for tunneling and drilling engineering. Direct shear tests were conducted on jointed rock-like specimens to investigate the influence of joint roughness and normal stress on shear failure characteristics. In the present study, regular triangular sawtooth was produced to simulate different asperities. Based on the direct shear test, the specimens exhibited four types of failure modes: damage tend to occur on the sawtooth tips under low normal stress; whereas damage occurred on a large scale under high normal stress; a localized region of the sawtooth was worn when the dilation angle was small; meanwhile the sawtooth tips or base were cut off when the dilation angle was large. In addition, Acoustic Emission (AE) technology was adopted to synchronously monitor the development of cracks during testing. Further attempt has been carried out to simulate the crack initiation, propagation and coalescence using Particle Flow Code (PFC). The numerical model has successfully verified and explained the crack behaviors determined by the shear failure mechanism in the physical test. Additionally, the irregular profile was introduced in the PFC, it was found that the failure behavior in sawtooth profile has established a good conclusion to fully understand the failure mechanism in the irregular profile. This work can provide some reference for evaluating the behavior of underground engineering composed of jointed rock masses during the shearing.

Sensitivity Analysis of Composite Cellular Beams to Constitutive Material Models and Concrete Fracture

Composite cellular beams are an advantageous solution that can be used to reduce floor height by solving service ducts problems. In the previous literature, there is little information on numerical modeling that considers sensitivity analysis in composite cellular beams, varying the constitutive models of steel and concrete materials. The concrete, when submitted by external loading, undergoes volume variations caused by inelastic deformations. The parameter that measures dilatancy is known as the dilation angle. This work aims to analyze the sensitivity of the computed response of composite cellular beams to the constitutive models of steel and concrete materials, and the parameters that constitute concrete damage plasticity (CDP). Geometrical nonlinear analyses are performed based on tests, considering solid elements for the composite slab and shear connectors, and shell elements for the cellular beam. It was concluded that the flexural behavior was not sensitive to dilation angles, unlike structures in which the resistance is governed by shear forces. For a dilation angle equal to [Formula: see text], a better post-peak behavior was observed in the load-displacement relationship. It was found that by varying the viscosity parameter (or relaxation time), the load-displacement behavior relationship is not affected.