Cheng-Xiang Yang

Study on the Failure Mechanism of Basalts with Columnar Joints in the Unloading Process on the Basis of an Experimental Cavity

Basalt with columnar joints is a kind of jointed rock body cut by both original joints and aphanitic microcracks. After unloading during excavation, such rock bodies manifest obvious mechanical phenomena such as discontinuities, anisotropy, and liability to fracture. In this work, the failure modes of basalts with columnar joints are studied based on monitoring of test tunnels on the one hand, and also through the mechanical properties of the original joints and those of microfissures of aphanitic microcracks in the unloading process analyzed by a discrete element method using a strain-softening constitutive model. Through comprehensive analysis of the numerical simulation results and the monitored behaviors of the basalts with columnar joints, their failure mechanisms are revealed, which may provide a basis for choosing suitable support plans for this kind of rock mass.

Analysis of EDZ Development of Columnar Jointed Rock Mass in the Baihetan Diversion Tunnel

Due to the time dependency of the crack propagation, columnar jointed rock masses exhibit marked time-dependent behaviour. In this study, in situ measurements, scanning electron microscope (SEM), back-analysis method and numerical simulations are presented to study the time-dependent development of the excavation damaged zone (EDZ) around underground diversion tunnels in a columnar jointed rock mass. Through in situ measurements of crack propagation and EDZ development, their extent is seen to have increased over time, despite the fact that the advancing face has passed. Similar to creep behaviour, the time-dependent EDZ development curve also consists of three stages: a deceleration stage, a stabilization stage, and an acceleration stage. A corresponding constitutive model of columnar jointed rock mass considering time-dependent behaviour is proposed. The time-dependent degradation coefficient of the roughness coefficient and residual friction angle in the Barton–Bandis strength criterion are taken into account. An intelligent back-analysis method is adopted to obtain the unknown time-dependent degradation coefficients for the proposed constitutive model. The numerical modelling results are in good agreement with the measured EDZ. Not only that, the failure pattern simulated by this time-dependent constitutive model is consistent with that observed in the scanning electron microscope (SEM) and in situ observation, indicating that this model could accurately simulate the failure pattern and time-dependent EDZ development of columnar joints. Moreover, the effects of the support system provided and the in situ stress on the time-dependent coefficients are studied. Finally, the long-term stability analysis of diversion tunnels excavated in columnar jointed rock masses is performed.

Evaluation of the Integrity of Deep Rock Masses Using Results of Digital Borehole Televiewers

Rock mass integrity is regarded as an important index to evaluate rock mass quality. Core drilling technology is one of the effective methods used for this. To overcome the problem of core discing and core breakage from the drilling process, a new evaluation method was proposed: a rock mass integrity index (RMIBT) based on high-definition digital borehole televiewer data. The RMIBT values were obtained by measuring the mass proportions of the length of the rock mass without macroscopic fractures on the borehole wall. Their scoring criteria were determined based on rock quality designation. It was applied in multiple deep rock excavations and therefore proved to be useful, especially where core discing occurs, in specific spatial distributions of structural planes, as well as logging errors from core breakages induced by drilling. In addition, the RMIBT can be used to assess dynamically the integrity of macroscopic rock masses and the evolution of fractures in the excavation damaged zone, thus providing a basis for the evaluation of rock masses in deep rock excavations.

In Situ Observation of Rock Spalling in the Deep Tunnels of the China Jinping Underground Laboratory (2400 m Depth)

To study rock spalling in deep tunnels at China Jinping Underground Laboratory Phase II (CJPL-II), photogrammetry method and digital borehole camera were used to quantify key features of rock spalling including orientation, thickness of slabs and the depth of spalling. The failure mechanism was analysed through scanning electron microscope and numerical simulation based on FLAC3D. Observation results clearly showed the process of rock spalling failure: a typical spalling pattern around D-shaped tunnels after top-heading and bottom bench were discovered. The orientation and thickness of the slabs were obtained. The slabs were parallel to the excavated surfaces of the tunnel and were related to the shape of the tunnel surface and orientation of the principal stress. The slabs were alternately thick and thin, and they gradually increased in thickness from the sidewall inwards. The form and mechanism of spalling at different locations in the tunnels, as influenced by stress state and excavation, were analysed. The result of this study was helpful to those rethinking the engineering design, including the excavation and support of tunnels, or caverns, at high risk of spalling.

Application of D-CRDM Method in Columnar Jointed Basalts Failure Analysis

Columnar jointed basalt is a type of joint rock mass formed by the combined cutting effect of original joints and aphanitic microcracks. After excavation unloading, such rock mass manifested distinct mechanical properties including discontinuity, anisotropy, and proneness of cracking. On the basis of former research findings, this paper establishes a D-CRDM method applicable to the analysis of columnar jointed basalt, which not only integrates discrete element and equivalent finite-element methods, but also takes into account the coupling effect of original joints and aphanitic microcracks. From the comparative study of field monitoring data and strain softening constitutive model calculated results, it can be found that this method may well be used for the simulation of mechanical properties of columnar jointed basalts and the determination of rock failure mechanism and failure modes, thus providing references for the selection of supporting measures for this type of rock mass.