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To reveal the dynamic evolution mechanism of hydraulic fracturing in hydraulic tunnels under different confining pressures and overcome the limitations of traditional tests and numerical methods in simulating complex crack propagation and multi-field coupling problems, a meshless numerical simulation framework suitable for hydraulic fracturing in hydraulic tunnels is constructed using the Smoothed Particle Hydrodynamics (SPH) method. Through the definition of the contact force transmission mechanism between matrix particles and water particles as well as the particle failure criterion, the dynamic tracking of the entire process of crack initiation, propagation, and penetration is realized. Multiple schemes with confining pressure ratios λ ( σ x / σ y ) of 0.2, 0.4, 0.6, and 0.8 are set up to systematically investigate the influence of confining pressure ratio on the hydraulic fracturing process. The research results show that the SPH method can effectively reproduce the evolution of complex crack networks in hydraulic fracturing and overcome the defects of traditional grid methods in dealing with discontinuity problems. The confining pressure ratio is a key parameter regulating the crack morphology. With the increase of λ , the crack network gradually transforms from a dendritic shape ( λ  = 0.2) to a m-shaped pattern ( λ  = 0.4) and a snowflake-like structure ( λ  = 0.6, 0.8). The increase in the proportion of horizontal stress significantly promotes the lateral propagation of secondary cracks. A common mechanism of “corner stress concentration dominating crack initiation” exists under different confining pressure ratios. Moreover, with the simultaneous increase of σ x and σ y , the degree of stress concentration decreases, and the crack propagation rate slows down. This study provides theoretical support and quantitative tools for the analysis of water-mechanics coupling disaster-causing mechanisms and engineering disaster prevention and control in hydraulic tunnels.

Understanding the time-dependent performance changes in tunnel supports through the monitoring data is crucial to predict the long-term stability of deep hydraulic tunnels. This paper provides an analytical solution for assessing the long-term stability of the diversion tunnels from the perspective of support permeability degradation. An evaluation factor Rgl, which denotes the ratio of the hydraulic conductivities of the grouted rock and the concrete lining, is proposed to analyse the degradation characteristics of tunnel support performance over time. The time-dependent changes in Rgl and its controlling factors are discussed using the monitoring data of the Jinping II diversion tunnel. The results show that there are ascending or descending trends of Rgl over time, linking to the evolutions of hydraulic conductivity in the grouted rock and the lining. The reliabilities of the analytical model for explaining the changes in tunnel support permeability are examined using numerical methods. The comparisons between the numerical solutions and monitoring results show good agreements in the hypothetical cases characterising the potential degradation of support of the diversion tunnel.HighlightsRatio of hydraulic conductivities between grouted rock and lining was proposed for diversion tunnels.Time-dependent hydraulic performance in support of the Jinping II diversion tunnel was analysed.Hydraulic conductivity degradations of diversion tunnel supports were simulated.

Under long-term water flow, the physical and chemical properties of hydraulic tunnel linings are more likely to deteriorate than those of road tunnels, thus affecting the normal operation of tunnels. Evaluating the durability of hydraulic tunnel linings can help to grasp the durability of tunnels in a timely and accurate manner and provide a basis for the routine maintenance of tunnels. This paper proposes new methods for evaluating the durability of hydraulic tunnel linings. Firstly, the types of tunnel defects are divided, the durability indices corresponding to the defects are selected scientifically, and a hydraulic tunnel durability evaluation index system is established. Then, the G2 method is modified by the entropy value method to make it a subjective and objective weighting method, which can make the weights fit the reality while the calculation is easy, and the TOPSIS method is modified by the gray correlation degree to optimize the judgment criteria between the evaluation scheme and the ideal solution. Finally, the practicality and accuracy of this method are verified by the calculation of the five sections of a tunnel with lining durability grades of A, B, C, B, and C, respectively, which matched the calculation results of the RAGA-PP method in the related literature.

•Rapid image acquisition equipment for small-section hydraulic tunnels with different cross-section forms and diameters.•Improved YOLOv5 object detection model incorporating DenseNet and ECA modules.•A “point-ring-section” detection method based on panoramic images.•Accurate identification and positioning of the boundary of large-area defects. Small-section hydraulic tunnels are characterized by small spaces and various section forms, under complex environments, which makes it difficult to carry out an inspection by the mobile acquisition equipment. To resolve these problems, an arbitrarily adjustable camera module deployment method and the corresponding automatic image acquisition equipment with multi-area array cameras are proposed and developed. Such method enables the acquisition of full-length surface images of the hydraulic tunnels with different cross-section forms and diameters by a one-way travel, and the overlap rate and accuracy of the acquired image sets meet the requirements of three-dimensional reconstruction and panoramic image generation. In addition, to improve the speed and accuracy of traditional algorithms for tunnel surface defects detection, this paper proposes an improved YOLOv5s-DECA model. The algorithm introduces DenseNet to optimize the backbone feature extraction network and incorporates an efficient channel attention ECA module to make a better extraction of features of defects. The experimental results show that mAP, and F1-score of YOLOv5-DECA are 73.4% and 74.6%, respectively, which are better than the common model in terms of accuracy and robustness. The proposed YOLOv5-DECA has great detection performance for targets with variable shapes and can solve the problem of classification imbalance in surface defects. Then, by combining YOLOv5-DECA with the direction search algorithm, a “point-ring-section” method is established to allow rapid identification of common surface defects by detecting them layer by layer with the bottom image of the stitched panorama as the seed. The presented method in this paper effectively solves the problem that a single image fails to show the overall distribution of the defects and their accurate positioning in a whole large tunnel section and the effective features of defects in an excessively large panoramic image size are difficult to be captured by the neural network. Field applications demonstrated that the presented method is adequate for high-precision and intelligent surface defect detection and positioning for different small-section hydraulic tunnels such as circular, arch-wall, and box-shaped hydraulic tunnels.

In the present study, a physical model test was used to investigate the cracking characteristics of the surrounding rocks of hydraulic tunnels under high geothermal conditions. Based on similarity theory, a similar material was developed to simulate the intact and hard rocks in thermo-hydro-mechanical (THM) coupling fields; this material was used to cast a large-scale model tunnel. A new loading system, including a temperature loading system and a water pressure loading system, was designed to improve upon conventional laboratory hydraulic tunnel testing and provide a means of better understanding the fracture behavior of solid media. An acoustic emission (AE) monitoring system, thermocouples and osmometers were used to reveal the real-time evolution of the temperature field, seepage pressure field and crack propagation. The test results showed that compared to a hydraulic tunnel without high geotemperature, the critical internal water pressure of the hydraulic tunnel under high geothermal conditions decreased significantly. After the initiation of main cracks was induced by hydraulic fracturing, due to THM coupling, a certain number of secondary cracks initiated and developed between the main cracks. In this process, the trend of the secondary crack tip deviated from the radial direction, resulting in a more developed and complicated evolution of cracks (including main cracks and secondary cracks). This was different from the cracking behavior of surrounding rocks without a high geothermal gradient, in which only a few main cracks due to hydraulic fracturing were observed. An obvious multifield THM coupling effect was observed under high-geotemperature conditions, and cracks propagated and damage increased in the surrounding rocks in a discontinuous and step-like manner. The number and source locations of the cracks recorded by the AE monitoring system were in good agreement with the evolution of the temperature field and seepage pressure field. In addition, FLAC3D numerical simulations based on the thermo-hydro-mechanics and damage (THMD) model revealed that high geotemperatures and in situ stresses significantly influenced on the cracking mode of the surrounding rocks of hydraulic tunnels under high geotemperatures. The results obtained in this study provide a better understanding of the cracking characteristics of the surrounding rocks of hydraulic tunnels under high geothermal conditions and are useful for the design of hydraulic tunnels.

Compared with highway tunnels, hydraulic tunnel linings are in the water environment for a long time, and their lining materials and structures are more vulnerable to damage. Therefore, a comprehensive scientific durability evaluation of hydraulic tunnel linings is of great significance for the safe operation and daily maintenance of hydraulic tunnels. This paper proposes a new method for evaluating the durability of hydraulic tunnel linings. The paper first constructs a hydraulic tunnel concrete lining durability evaluation system, taking into account the feedback between the indices, and using the analytic network process (ANP) to calculate the weights of each index, as well as multiple-expert scoring to reduce its one-sidedness and subjectivity. Considering the randomness and fuzzy nature of the evaluation, the cloud model was used to modify the matter–element theory to evaluate the durability of the hydraulic tunnel lining. Finally, an example application was carried out, and the durability classes of five segments of the Nawei Tunnel were calculated as III, II, II, II, and III. The results were compared with the evaluation results of the method in the related literature, which proved that the method has good accuracy in evaluating the durability of hydraulic tunnel linings.

For the cross-fault hydraulic tunnel, rationally simulating the surrounding rock and rock-fault interface, revealing its destabilization risk areas, and evaluating its safety degree are of great significance for its engineering design and construction. Based on the Zienkiewicz-Panda yield criterion and the Drucker-Prager plastic potential function, an elastic-plastic damage constitutive model of the surrounding rock was established, and its rationality was proved. The failure modes of the rock-fault interface were divided into normal disengagement damage, abrasion damage, and snip damage. The peak shear strengths of the structural surface under different shear failure modes were deduced, which were combined with the Desai thin-layer contact element model to form the numerical simulation method of the rock-fault interface. Finally, an evaluation index called the element dangerous coefficient (EDC) was proposed, which is capable of evaluating the element’s safety degree in the whole deformation and damage process. The proposed methods were applied to a cross-fault hydraulic tunnel, and some conclusions were obtained as follows. (1) The value of the EDC gradually decreases from the excavation surface to the interior rock mass, which indicates that the danger degree of the rock mass is gradually decreasing. (2) The rock-fault interface is the weak area, and the contact surface near the excavation surface is disengaged after the excavation, and the EDC value reaches 2.0. (3) The EDC index is capable of quantitatively evaluating the element’s hazard level in the elastic and plastic zones at the same time and giving the element’s hazard coefficient.

Subjected to the coupling action of multiple hazards in hydraulic engineering, hydraulic tunnels may be corroded and damaged to varying degrees during their service lives, which will decrease the seismic performance of these structures. However, the research and seismic design of significant hydraulic engineering projects focus on investigating the structural response based on the design material parameters, which may overestimate the seismic capacity of structures during their service lives. In this paper, research is performed to identify the effect of hydro-chemo-mechanical corrosion on the seismic performance of hydraulic tunnels with different burial depths. A plastic damage model of time-varying concrete degradation induced by the hydro-chemo-mechanical effect is first determined and implemented, and the endurance time acceleration records are generated in MATLAB. Then, a study of the endurance time relationship of hydro-chemo-mechanical corrosion-affected hydraulic tunnels, considering the fluid–structure-surrounding rock interaction systems throughout the service period, is undertaken to directly associate the structural response with the predefined evaluation index. Moreover, this research constructs 3D time-varying fragility surfaces considering the hydro-chemo-mechanical effect and seismic intensity. The results show that the relative displacement of hydro-chemo-mechanical corrosion-affected hydraulic tunnels is larger than that of nonaffected hydraulic tunnels. Hydro-chemo-mechanical effect-induced material deterioration will lead to an increase in the cumulative damage (crack) area and damage degree of hydraulic tunnels. Additionally, the seismic fragility analysis shows that the longer the service time of hydro-chemo-mechanical corrosion-affected hydraulic tunnels, the more likely they are to collapse. Hence, attention should be given to improving the aseismic capacity of hydro-chemo-mechanical corrosion-affected hydraulic tunnels in future seismic design and performance assessments.

The construction time of concrete structures in hydraulic tunnels is long, the construction environment is complex, and there are many influencing factors. The requirements for construction quality are high not only to meet the strength requirements but also to meet the design requirements of erosion resistance, crack resistance, and seepage resistance according to its specific operating environment. Therefore, evaluating the construction quality of concrete structures in hydraulic tunnels is of great significance. Considering the randomness and fuzziness of factors affecting the construction quality of concrete structures in hydraulic tunnels, this paper proposes a comprehensive evaluation model based on combined weighting (CWM) and uncertainty measurement theory (UM). The improved analytic hierarchy process (IAHP) and the CRITIC method are used to determine the subjective and objective weights of evaluation indicators. Combined weighting is based on the principle of minimum entropy, and the UM method is used to evaluate the construction quality level. Finally, taking a hydraulic tunnel as an example, its construction quality grade is calculated to be III, according to the evaluation model proposed in this paper, which matches the engineering reality, and a comparative study is made with the mixture element topology theory at the same time. It is verified that the evaluation model can scientifically and reasonably evaluate the construction quality level of concrete structures in hydraulic tunnels.

Rapidly developed deep learning methods, widely used in various fields of civil engineering, have provided an efficient option to reduce the computational costs and improve the predictive capabilities. However, it should be acknowledged that the application of deep learning methods to develop prediction models that efficiently assess the nonlinear dynamic responses of cross-fault hydraulic tunnels (CFHTs) is lacking. Thus, the objective of this study is to construct a rational artificial neural network (ANN) prediction model to generate the mass data and fragility curves of CFHTs. Firstly, an analysis of 1080 complete nonlinear dynamic time histories via incremental dynamic analysis (IDA) is conducted to obtain the mass data of the drift ratio of the CFHT. Then, the hyper-parameters of the ANN model are discussed to determine the optimal parameters based on four examined approaches to improve the prediction capacity and accuracy. Meanwhile, the traditional probabilistic seismic demand models of the predicted values obtained by the ANN model and the numerical results are compared with the statistical parameters. Eventually, the maximum likelihood estimation couping IDA method is applied to assess the seismic safety of CFHTs under different damage states. The results show that two hidden layers, ten neurons, and the ReLU activation function for the ANN model with Bayesian optimization can improve the reliability and decrease the uncertainty in evaluating the structural performance. Moreover, the amplitude of the seismology features can be used as the neurons to build the input layers of the ANN model. It is found through vulnerability analysis that the traditional seismic fragility analysis method may overestimate the earthquake resistance capacity of CFHTs compared with maximum likelihood estimation. In practical engineering, ANN methods can be regarded as an alternative approach for the seismic design and performance improvement of CFHTs.