Double-wheel trench cutter is a professional construction equipment of underground diaphragm wall, and its tooth arrangement system is its core component. The parameter design of tooth arrangement system is closely related to rock breaking performance. In this study, a three-dimensional simulation method for milling rock and soil with milling wheel is developed using the continuum-discontinuum element method (CDEM). On this basis, considering the correlation between free surface and milling, the mechanism of the influence of cutting sequence on rock breaking performance of tooth arrangement system is studied, and a numerical generalization model of rock entry sequence is established under the condition of associated milling, in which the number of adjacent free surfaces is preset as the main influencing factor. The results show that: (1) The curves of milling force and milling depth obtained by milling wheel milling rock and soil mass verify the correctness of the numerical simulation method. (2) Compared with the intact rock mass, the peak load of rock samples with single adjacent preset free surface and double adjacent preset free surface decreased by 32.5% and 68.2% respectively, and the energy consumption of rock breaking decreased by 19.8% and 56.6%, respectively. (3) In the tooth arrangement system, the larger the ratio of the number of pick against double adjacent preset free surfaces to the total number of picks, the lower the overall energy consumption of the tooth arrangement system, and the energy consumption of the heterogeneous tooth arrangement system is lower than that of the sequential one. The research results provide a basis for improving the rock breaking process and tooth arrangement system optimization of double-wheel trench cutter.

The construction of urban subway tunnel inevitably produces disturbance to surrounding rock and causes ground surface settlement. Dynamic prediction of ground surface settlement caused by tunnel excavation is an important method to ensure the safety of above-ground buildings and tunnel construction. In view of the difficulty of accurate dynamic prediction of ground surface settlement during tunnel construction, based on the definition of longitudinal excavation coefficient γ, a dynamic prediction model of lateral ground surface settlement is established. The model can accurately describe the variation of the settlement of the same monitoring location with the advancement of the tunnel face, and then realize the dynamic prediction of the ground surface settlement at the construction site. The results show that under certain constraints, this model can be degenerated into Peck model and stochastic medium theory prediction model. The accuracy and applicability of the dynamic prediction model are verified by on-site construction. The tunnel can be divided into three affected segments longitudinally (i.e., intense influence, moderate influence, and mild influence) based on the obtained γ, which well reflected the influence degree of the excavated tunnel face on the same monitoring section at different positions. Through the analysis of the influence of the buildings and isolation piles on the ground surface settlement curve, it can be found that the building and its adjacent ground surface present the characteristics of cooperative deformation and joint bearing. Moreover, installing geological drill isolation piles on the side of the tunnel can reduce the ground surface settlement of that side up to 71.9%. The research results have a certain guiding and reference significance for the on-site construction of the Central Yunnan Water Diversion Project and similar projects.

Understanding of convection heat transfer characteristics of fractures is of great significance to improve the heat extraction efficiency of carbonate reservoir. Previous studies on convection heat transfer of fluid flowing through rock fractures are on either granite or sandstone. Limited experimental research has been performed on carbonate fractures after acidizing etching. In this work, an improved test method is developed to analyze the convection heat transfer characteristics of carbonate fractures after acidizing etching under real-time high temperature and high confining pressure. In this method, the traditional test method of convection heat transfer coefficient is improved by monitoring the temperature of inner fracture surface and flowing water. Two thermocouples are especially arranged inside the sample to monitor the temperature of inner fracture surface along the flow direction, and two other thermocouples for the inlet and outlet water temperatures. The results show that the temperature differences between the fracture surface and the flowing water are significantly dependent on confining pressure, fracture roughness and flow rate, and the maximum temperature difference could be reached 2.2 ${}^{\circ }$C, which leads to a significant difference in the convection heat transfer coefficient between the traditional and improved test methods. A larger number of pores, caves, and micro-fractures caused by acid etching are observed on the fracture surface by scanning electron microscopy. The special fracture morphology of carbonate is totally different from those of granite and sandstone in previous studies, and can increase the convection channel and increase the contact area with flowing fluid and results in the inapplicability of the hypothesis to carbonate fractures after acidizing etching. The present work could improve the knowledge of convection heat transfer characteristics of carbonate fractures.