Narrow Results

Based on the stability and bifurcation theory of dynamical systems, the bifurcation behaviors and chaotic motions of the two-state variable friction law of a rock mass system are investigated by the bifurcation diagrams based on the continuation method and the Poincaré maps. The stick-slip of the rock mass is formulated as an initial values problem for an autonomous system of three coupled nonlinear ordinary differential equations (ODEs) of first order. The results of linear stability analysis indicate that there is an equilibrium position in the rock mass system. Furthermore, numerical results of nonlinear analysis indicate that the equilibrium position loses its stability from a sup-critical Hopf bifurcation point, and then the bifurcating periodic motion evolves into chaotic motion through a series of period-doubling bifurcations with the decreasing of the control parameter. The stick-slip and chaotic motions evolve into infinity in the end with some unstable periodic motions.

The coupling effect of seepage and temperature fields in fractured rock mass is a hot topic in the area of water conservancy, nuclear waste disposal and geothermal resources development. A coupling mathematical model of the seepage, flow temperature and rock mass temperature fields in the fracture network of rock mass is established based on the seepage and temperature interaction. A calculation program is developed and applied to calculate the seepage and temperature fields of the dam foundation of a water conservancy project. The interaction mechanism of the seepage, flow temperature and rock mass temperature fields is analyzed in this paper. Results show that the seepage field largely influences the temperature field, which can provide several suggestions for the deep underground disposal of nuclear waste, geothermal resources development and fractured rock mass in dam foundations. Considering the coupling effect of the seepage, flow temperature and rock mass temperature fields by the fracture network method is necessary.

To investigate crack initiation and propagation of rock mass under coupled thermo-mechanical (TM) condition, a two-dimensional coupled TM model based on the numerical manifold method (NMM) is proposed, considering the effect of thermal damage on the rock physical properties and stress on the heat conductivity. Then, the NMM, using empirical strength criteria as the crack propagation critical criterion and physical cover as the minimum failure element, was extended for cracking process simulation. Furthermore, a high-order cover function was used to improve the calculation accuracy of stress. Therefore, the proposed method consists of three parts and has a high accuracy for simulating the cracking process in the rock mass under the coupled TM condition. The ability of the proposed model for high accuracy stress, crack propagation, and thermally-induced cracking simulation was verified by three examples. Finally, the proposed method was applied to simulate the stability of a hypothetical nuclear waste repository. Based on the outcome of this study, the application of NMM can be extended to study rock failure induced by multi-field coupling effect in geo-materials.

Hydraulic fracture propagation directly affects the recovery rate of resources when hydraulic fracturing techniques are applied to exploiting unconventional oil and gas resources. Rock mass is the main engineering medium of hydraulic fractures and is generally considered to be of considerable spatial variability in physical and mechanical properties. Understanding the irregular propagation mechanism of hydraulic fracture in spatial heterogeneity rock mass is essential and beneficial to assess the recovery rate of oil or gas resources. This work develops a random phase-field method (RPFM) to simulate the irregular propagation of hydraulic fracture in spatially variable rock mass. The spatial variability of elastic modulus is characterized by the random field theory. Utilizing the advantages in modeling complex crack patterns and crack kinematics, the phase-field method (PFM) is used to predict the fracture propagation. Various anisotropic random fields of elastic modulus with different coefficients of variance and scales of fluctuation are generated via the Cholesky decomposition method. The random fields are subsequently implemented into COMSOL Multiphysics and combined with the PFM to investigate the hydraulic fracture propagation. This study investigates the influence of spatial variability of elastic modulus on the peak fluid pressure, fracture length, fracture area and fracture shape. It reveals that the spatial variability of elastic modulus has a significant influence on the propagation of hydraulic fractures, and the results provide a preliminary reference for hydraulic fracturing design with consideration of spatial variability of rock mass.

New plastic solutions for the bearing capacity factors of conical footings on rock masses obeying the Hoek–Brown yield criterion are presented in the paper by using the lower and upper bound finite element limit analysis. A conical footing has a cone apex angle and is penetrated into a rock mass. The considered dimensionless parameters include the cone apex angle, the yield parameter, and the geological strength index of a rock mass, where the effects of these dimensionless parameters on the bearing capacity factor are compressively investigated. The collapse mechanisms of conical footings on rock masses are examined and discussed in the paper.

Rapid development of tunnel boring machine (TBM) technology accelerates the extensive application of TBM in rocks and various geological conditions. TBM rock tunnelling is becoming a competitive approach, to the conventional drill and blast method. Over the years, many researches have been conducted to understand the rock fragmentation mechanism by TBM cutters, and to improve and predict the TBM performance in various rock masses. However, due to the complexity of natural grounds, TBM tunnelling still faces many challenges. This keynote paper examines rock fragmentation process, influence of joints and mixed face conditions on fragmentation and cutter force differentiation. Some of the studies show a prospect for improving TBM cutter and cutterhead design. The encountered problems in complex ground during TBM tunnelling are highlighted, and elaborated with examples. Finally, the needs for future research and development are proposed.

Two types of in-situ triaxial testing apparatus to measure stress-strain relationship of rock mass (i.e. large-scale apparatus and small-scale apparatus) are introduced. The large-scale apparatus comprises of inner and outer cells to apply confining pressure to the specimen, axial loading system which is generally installed in the ground surface, special instrumentation devices and data recording unit. The small-scale apparatus consists of a triaxial cell to measure deformations of the specimen under confining pressures and axial load and a separate axial loading system, which enables to apply axial load to the specimen in any depths. Axial and radial deformations of the specimen can be measured inside the triaxial cell and recorded with a data recording unit. For drilling the borehole, preparation of the specimen and performing the in-situ triaxial test, a special procedure is introduced. Some typical results of proof tests were presented. The results, which were similar to conventional laboratory triaxial tests, showed that the proposed test method was successful to measure average stress-strain relationships of a rock mass.