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Chemical corrosion and hydraulic fracturing are important ways to improve the permeability of the enhanced geothermal systems. In this work, we developed a discrete element code for simulating the hydraulic fracturing of three different fracture network models after chemical corrosion. Mechanisms of the crack extension and coalescence in the field-scale models with fracture network were systematically revealed, in which the effects of corrosion time, inclination of the fracture (γ), pH value and hydraulic pressure were considered. The results show that the simulated cracking behaviors of granites agree well with those obtained through Brazilian tests and triaxial compression tests, verifying the validity of the developed discrete element code. During hydraulic fracturing, the impact of γ on the fracture network connectivity increases with the increment of hydraulic pressure (Pin). With the increase in hydraulic pressure, crack propagation accelerates whereas the connectivity of fracture network models decreases. The reasons are that the excessive hydraulic pressure causes the crack to spread in an unfavorable direction and increases the effect of γ on the connectivity of models of fracture networks. As the pH value increases from 2 to 12, the crack growth rate first decreases and subsequently raises. The numbers of cracks per time step of sparse mixed fracture networks with artificial fractures (SM), dense mixed fracture networks with artificial fractures (DM) and dense mixed fracture networks without artificial fractures (NDM) are in the ranges of 0.06601–0.1259, 0.05176–0.2072 and 0.1727–0.6846, respectively. The connectivity of SM is better than that of NDM under the same hydraulic fracturing conditions, indicating that artificial fractures can greatly improve the fracture network connectivity and the utilization efficiency of EGS.

The harmonious development of urban rail transit, underground space engineering, and social economy is the key to regional sustainable development. Based on synergetic theory, this paper constructs the coupling coordination evaluation system of “rail transport-social economy” composite system at the scale of a city cluster. With this system, the coupling and coordinated development pattern and characteristics of “rail transit-social economy” in the Yangtze River Delta city cluster from 2002 to 2020 were analyzed. The paper makes a horizontal comparison with the Beijing-Tianjin-Hebei city cluster and the Pearl River Delta city cluster, as well as analyzes the differences in development and existing problems, and puts forward policy suggestions for rail and urban underground space development. The results show that: (1) The rail transit of 11 cities in the Yangtze River Delta shows a “step by step” development pattern. That is “national central city, provincial city, second-tier city, third-tier city, etc.”, accompanied by periodic changes of coupling and coordination degree. In addition, there is also the phenomenon of unbalanced development within the region; (2) From 2002 to 2020, the development of rail transit in the three city clusters shows a situation of “the overall supply is lacking and lags behind the social economy for a long time”. Among them, the Pearl River Delta city cluster has the most serious lags. Multi-channel financing, speeding up the construction of the rail transit scale according to local conditions, and improving operating efficiency are considered to be the keys to solve this problem; (3) In general, the coupling between rail transit and the social economy in the Yangtze River Delta city cluster is better than that in the Beijing-Tianjin-Hebei city cluster and the Pearl River Delta city cluster, but the coordination is at a slight disadvantage.

This paper outlines the construction process mechanics (CPM) principle for analysing the stability of rock tunnels and presents finite element method (FEM) numerical simulation and prediction on the deformation and failure of the rock masses surrounding tunnels under various rock mass properties and excavating and support conditions. Based on numerical modelling, a series of predicting curves for rock mass response and deformation are obtained, which provides the basis of guiding the design and construction of rock tunnels in Taiwan.

Based on comprehensive monitored data obtained from typical tunnel projects, and by using engineering experiences of experts together with numerical modeling results, the precedent type analysis (PTA) technique developed was for back-analyzing in situ stress and elastic modulus of rock masses surrounding tunnels. Then, the PTA is used as a supporting technique in the method of intelligent back analysis (IBA). The IBA method can integrate the mechanism analysis with experimental identification. Such integration is based on ‘certainty factors’, and is implemented with the computer program BMP90 with a global inference engine. The program BMP90 can solve a series of back analysis problems—such as identifying potential problems to be encountered during back analysis, assessing the strategy to be adopted, selecting criteria for back analysis, and ensuring convergence of back analysis results. Based on the engineering applications to approximately 100 tunnel projects in China, it is found that the IBA method supported by the PTA technique can be successfully applied to tunnel projects, i.e. back-analyzing the lateral in situ stress factor (λ), and the elastic modulus ( E ) of rock masses. The method of IBA supported by PTA and its application to a tunnel project are presented in this paper.

The complicated geological conditions and geological hazards are challenging problems during tunnel construction, which will cause great losses of life and property. Therefore, reliable prediction of geological defective features, such as faults, karst caves and groundwater, has important practical significances and theoretical values. In this paper, we presented the criteria for detecting typical geological anomalies using the tunnel seismic prediction (TSP) method. The ground penetrating radar (GPR) signal response to water-bearing structures was used for theoretical derivations. And the 3D tomography of the transient electromagnetic method (TEM) was used to develop an equivalent conductance method. Based on the improvement of a single prediction technique, we developed a technical system for reliable prediction of geological defective features by analyzing the advantages and disadvantages of all prediction methods. The procedure of the application of this system was introduced in detail. For prediction, the selection of prediction methods is an important and challenging work. The analytic hierarchy process (AHP) was developed for prediction optimization. We applied the newly developed prediction system to several important projects in China, including Hurongxi highway, Jinping II hydropower station, and Kiaochow Bay subsea tunnel. The case studies show that the geological defective features can be successfully detected with good precision and efficiency, and the prediction system is proved to be an effective means to minimize the risks of geological hazards during tunnel construction.