Deep Underground Science and Engineering最新文献

Due to the complexity of underground engineering geology, the tunnel boring machine (TBM) usually shows poor adaptability to the surrounding rock mass, leading to machine jamming and geological hazards. For the TBM project of Lanzhou Water Source Construction, this study proposed a neural network called PCA–GRU, which combines principal component analysis (PCA) with gated recurrent unit (GRU) to improve the accuracy of predicting rock mass classification in TBM tunneling. The input variables from the PCA dimension reduction of nine parameters in the sample data set were utilized for establishing the PCA–GRU model. Subsequently, in order to speed up the response time of surrounding rock mass classification predictions, the PCA–GRU model was optimized. Finally, the prediction results obtained by the PCA–GRU model were compared with those of four other models and further examined using random sampling analysis. As indicated by the results, the PCA–GRU model can predict the rock mass classification in TBM tunneling rapidly, requiring about 20 s to run. It performs better than the previous four models in predicting the rock mass classification, with accuracy A, macro precision MP, and macro recall MR being 0.9667, 0.963, and 0.9763, respectively. In Class II, III, and IV rock mass prediction, the PCA–GRU model demonstrates better precision P and recall R owing to the dimension reduction technique. The random sampling analysis indicates that the PCA–GRU model shows stronger generalization, making it more appropriate in situations where the distribution of various rock mass classes and lithologies change in percentage.

The utilization and development of urban underground space play a crucial role in optimizing the layout of civic architecture and enhancing the urban ecological environment, which contributes toward increasing the overall carrying capacity and promoting sustainable development in megacities. To delve into the research progress of urban underground space, knowledge maps were created using the information visualization software VOSviewer. The literature was systematically extracted from three prominent databases, namely, Web of Science, Scopus, and China National Knowledge Infrastructure. According to the bibliometric analysis of the co-citation and core words co-occurrence, the trends and challenges of research on urban underground space were identified. As highlighted by the results obtained, it still remains highly challenging to achieve interdisciplinary collaboration in urban underground space research; the research trends of urban underground space consist of collaborative planning and whole life cycle sustainable development, multisource geological data informatization and resource evaluation, infrastructure design optimization, and intelligent construction. The knowledge map, drawn using bibliometric methods, offers a quantitative analysis of literature retrieval across various levels. It is recognized as an essential tool for exploring and identifying challenges and trends in urban underground space.

Testing of large-sized specimens is becoming increasingly important in deep underground rock mechanics and engineering. In traditional mechanical loading, stresses on large-sized specimens are achieved by large host frames and hydraulic pumps, which could lead to great investment. Low-cost testing machines clearly always have great appeal. In this study, a new approach is proposed using thermal expansion stress to load rock specimens, which may be particularly suitable for tests of deep hot dry rock with high temperatures. This is a different technical route from traditional mechanical loading through hydraulic pressure. For the rock mechanics test system of hot dry rock that already has an investment in heating systems, this technology may reduce the cost of the loading subsystem by fully utilizing the temperature changes. This paper presents the basic principle and a typical design of this technical solution. Preliminary feasibility analysis is then conducted based on numerical simulations. Although some technical details still need to be resolved, the feasibility of this loading approach has been preliminarily confirmed.

Based on a comprehensive review of domestic and foreign literature, this article discusses the technical difficulties and development status of enhanced geothermal system (EGS) concerning the thermal energy extraction of deep hot dry rock (HDR) reservoirs and proposes suggestions for the research focus of numerical simulation of HDR reservoir stimulation. Additionally, it summarizes the existing methods and mainstream working fluids for HDR reservoir stimulation. The article emphasizes the significance of factors such as well location, production well depth, artificial fracture orientation, and complexity in optimizing the thermal production efficiency of the EGS. Furthermore, this article delves into a detailed discussion on the influence of fracture spacing, fracture permeability, fracture length, fluid injection rate, and injected fluid temperature on the performance of the EGS. In light of the thermo-hydro-mechanical coupling challenges associated with high-temperature reservoirs, it is suggested that future research efforts should focus on investigating the impact of thermo-induced stresses on the stability of the artificial fracture network within the EGS during long-term (>30 years) circulation of hot and cold fluids.

There is an urgent need to develop optimal solutions for deformation control of deep high-stress roadways, one of the critical problems in underground engineering. The previously proposed four-dimensional support (hereinafter 4D support), as a new support technology, can set the roadway surrounding rock under three-dimensional pressure in the new balanced structure, and prevent instability of surrounding rock in underground engineering. However, the influence of roadway depth and creep deformation on the surrounding rock supported by 4D support is still unknown. This study investigated the influence of roadway depth and creep deformation time on the instability of surrounding rock by analyzing the energy development. The elastic strain energy was analyzed using the program redeveloped in FLAC^3D. The numerical simulation results indicate that the combined support mode of 4D roof supports and conventional side supports is highly applicable to the stability control of surrounding rock with a roadway depth exceeding 520 m. With the increase of roadway depth, 4D support can effectively restrain the area and depth of plastic deformation in the surrounding rock. Further, 4D support limits the accumulation range and rate of elastic strain energy as the creep deformation time increases. 4D support can effectively reduce the plastic deformation of roadway surrounding rock and maintain the stability for a long deformation period of 6 months. As confirmed by in situ monitoring results, 4D support is more effective for the long-term stability control of surrounding rock than conventional support.

Integrated geophysical technology is a necessary and effective means for geothermal exploration. However, integration of geophysical technology for large-scale surveys with those for geothermal reservoir localization is still in development. This study used the controlled source audio-frequency magnetotelluric method technology for large-scale exploration to obtain underground electrical structure information and micromotion detection technology to obtain underground wave velocity structure information. The combination of two detection technologies was used for local identification of geothermal reservoirs. Further, auxiliary correction and inversion constraint were implemented through the audio magnetotelluric sounding technology for maximum authenticity restoration of the near- and transition-field data. Through these technology improvements, a geothermal geological model was established for the Binhai County of Jiangsu Province in China and potential geothermal well locations were identified. On this basis, a geothermal well was drilled nearly 3000 m deep, with a daily water volume of over 2000 m³/day and a geothermal water temperature of 51°C at the well head. It is found that predictions using the above integrated geophysical exploration technology are in good agreement with the well geological formation data. This integrated geophysical technology can be effectively applied for geothermal exploration with high precision and reliability.

Excavation gaps around the front shield can be generated during shield construction, resulting in significant ground settlement. Traditional synchronous grouting slurries are unsuitable for filling these gaps during tunneling under existing subway lines. To address this issue, experiments are conducted on mix characteristics and hardening properties of slurries with variations in fineness and contents of fly ash. The experimental and computed tomography scan results yield the following findings: (1) fly ash with high fineness can effectively reduce the early strength of slurries and enhance their injectability. This improves the filling effect on micropores in the slurry and ultimately enhances the final hardening strength. (2) Fineness of fly ash controls the process of slurry hydration. The higher the fineness of fly ash, the more visible the exothermic hydration of slurry and the earlier the highest temperature peak appears. (3) Fly ash with high fineness can effectively increase the density and consolidation rate of slurries, resulting in greater improvement in slurry strength, particularly when the ratio of fly ash to cement (m_f/m_c) is 0.75. (4) Fly ash with high fineness can effectively decrease the likelihood of appearance of pores in the slurry, optimize the pore structure, and enhance the strength of slurries after consolidation.

The mechanical properties of rocks weaken under dry–wet cycles. This weakening may significantly modify the safety reserve of underground caverns or reservoir bank slopes. However, meso-damage has not been carefully studied based on micromechanical observations and analyses. Therefore, in this study, meso-damage of a yellow sandstone is investigated and a meso-damage-based constitutive model for dry–wet cycles is proposed. First, computed tomography scanning and uniaxial compression tests were conducted on yellow sandstones under different dry–wet cycles. Second, the evolution of rock mesostructures and the damage mechanism subjected to dry–wet cycles were simulated using the discrete element method with Particle Flow Code in 2 Dimensions (PFC2D) software. Third, a constitutive model was proposed based on the meso-statistical theory and damage mechanics. Finally, this constitutive model was verified with the experimental results to check its prediction capability. It is found that the radius and number of pore throats in the sandstone increase gradually with the number of dry–wet cycles, and the pore structure connectivity is also improved. The contact force of sandstone interparticle cementation decreases approximately linearly and the continuity of the particle contact network is continuously broken. The meso-deformation and strength parameters show similar declining patterns to the modulus of elasticity and peak strength of the rock sample, respectively. This meso-damage-based constitutive model can describe well the rock deformation in the initial pressure density stage and the damage stage under the coupling effect of dry–wet cycles and loads.

Due to their high reliability and cost-efficiency, submarine pipelines are widely used in offshore oil and gas resource engineering. Due to the interaction of waves, currents, seabed, and pipeline structures, the soil around submarine pipelines is prone to local scour, severely affecting their operational safety. With the Yellow River Delta as the research area and based on the renormalized group (RNG) k-ε turbulence model and Stokes fifth-order wave theory, this study solves the Navier–Stokes (N–S) equation using the finite difference method. The volume of fluid (VOF) method is used to describe the fluid-free surface, and a three-dimensional numerical model of currents and waves–submarine pipeline–silty sandy seabed is established. The rationality of the numerical model is verified using a self-built waveflow flume. On this basis, in this study, the local scour development and characteristics of submarine pipelines in the Yellow River Delta silty sandy seabed in the prototype environment are explored and the influence of the presence of pipelines on hydrodynamic features such as surrounding flow field, shear stress, and turbulence intensity is analyzed. The results indicate that (1) local scour around submarine pipelines can be divided into three stages: rapid scour, slow scour, and stable scour. The maximum scour depth occurs directly below the pipeline, and the shape of the scour pits is asymmetric. (2) As the water depth decreases and the pipeline suspension height increases, the scour becomes more intense. (3) When currents go through a pipeline, a clear stagnation point is formed in front of the pipeline, and the flow velocity is positively correlated with the depth of scour. This study can provide a valuable reference for the protection of submarine pipelines in this area.