At present, the construction of large-span and super-large-span subway tunnels is increasing. Temporary support is often designed to reduce the span in the construction of those tunnels. However, temporary support needs to be removed before secondary lining is constructed due to the spatial overlap between them. The removal of temporary support is the weak situation, because the original stress balance of the structure will be broken. Improper construction can easily cause safety accidents such as tunnel collapse. In view of the lack of systematic research on the structural mechanical response and safety of the super-large-span tunnel constructed by the double-layer initial support arch-cover method at home and abroad, this paper takes the four-line parallel tunnel of Huahua section of Guangzhou Metro Line 11 as the basis project. Through the combination of theoretical analysis and numerical calculation, the mechanical response and safety of the structure during the dismantling of the tunnel are analyzed from both horizontal and vertical aspects. Finally, a scientific, reasonable, safe, efficient and rapid dismantling scheme was determined, and successfully passed the inspection of on-site construction. The results show that it is feasible to dismantle the super-large-span tunnel in urban soft stratum by using the scheme of 'first edge and then middle, and symmetrical demolition in cross direction, first three demolition and one demolition, and then one demolition and one demolition in longitudinal direction', which can provide reference and guidance for subsequent projects.

To address the challenge of structural deterioration caused by frequent cracking in the lining structures of deep-buried hydraulic tunnels in high-altitude areas, the enhancement of concrete's mechanical properties is investigated through the addition of fibers and determines the optimal fiber content for practical engineering application. Firstly, tests on the tensile, compressive, and flexural mechanical properties of basalt fiber-reinforced concrete (BFRC) with varying fiber contents were conducted, the variation patterns of concrete's tensile, compressive, and flexural mechanical properties under different volumetric fiber contents were obtained. Subsequently, a mesoscopic numerical model of fiber-reinforced concrete that truly reflects the microstructural factors such as aggregate shape, gradation, aspect ratio, fiber distribution, and initial defects was established. By comparing the mesoscopic numerical model with indoor axial tension test results, the mechanism of fiber reinforcement on the tensile strength of concrete was revealed. Finally, the optimal fiber content was analyzed. The results indicate that: Compared to the plain concrete, a fiber volume content of 0.2% is optimal, with the axial tensile strength, split tensile strength, and flexural strength of BFRC increased by 12.81%, 14.79%, and 21.26%, respectively. The error between the tensile strength of the fiber concrete predicted by the established mesoscopic numerical model and the indoor test results for plain (fiber) concrete is 4.24% (5.26%), and the model can accurately reflect the failure development process and macroscopic mechanical behavior of fiber-reinforced concrete specimens. The findings of this study can provide a reference for the design and application of basalt fiber-reinforced concrete structures.

The reloading mechanical properties of the surrounding rock in an underground energy storage cavern are crucial for determining the safety of underground energy storage projects. This study conducted triaxial loading and unloading tests on mudstone, as well as reloading tests on unloaded damaged mudstone. By employing testing and analysis techniques such as nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), the research investigated the impact of unloading effects on the reloading mechanical properties of mudstone and revealed the deterioration mechanisms of reloading damaged mudstone. The results indicate that the fractal dimension ultimately decreases as confining pressure increases, and the confining pressure's control over internal micro-cracks in the mudstone becomes more pronounced. With increasing unloading damage, small-size micro-pores inside the mudstone samples develop into medium-sized pores, resulting in a higher internal porosity. The greater the initial unloading damage, the larger the reduction in reloading strength of the mudstone. The degree of unloading damage progressively affects the failure mode of rock samples, transitioning from shear failure to shear-tensile failure, and eventually to tensile-shear failure with increasing unloading damage. A correlation between unloading damage degree, porosity, and reloading strength has been established, bridging the gap between microstructural damage and macro-strength deterioration in unloaded damaged mudstone. This finding provides a reference for delineating unloading damage zones and predicting reloading strength within unloading areas.

Aiming at the problem of insufficient bearing capacity of tunnel primary support system under unfavorable geology, such as stress concentration zone and broken structural zone, a composite support structure with stud shear connectors arranged at the interface between steel and concrete is proposed. According to the stress characteristics of tunnel support structure, a large eccentric compression test is carried out to explore the failure mode and bearing characteristics of composite support structure, and the bearing capacity of composite structure under different eccentricity conditions is analyzed by numerical simulation. The results show that when there is no stud shear specimen, the separation failure occurs between I-shaped steel and shotcrete. When the stud shear is arranged, the failure mode of steel reinforced concrete structure is concrete cracking and crushing, and the stud shear effectively limits the relative slip between the contact interface of steel and concrete. Compared with the natural bonding condition, the ultimate bearing capacity of the specimens with double-row stud shear connectors increased by 14.79%, and the lateral deflection decreased by 22.94%. The specimens showed better toughness, bearing capacity and bending stiffness. Under the same eccentricity, the arrangement of stud shear connectors can effectively improve the ultimate bearing capacity of the structure, and with the increase of eccentricity, the effect of stud shear connectors on the bearing capacity of the specimen under large eccentric compression is gradually enhanced. The research results can provide theoretical support for the initial support technology of tunnel.

Aiming at the geological characteristics of loess overlying red-bedded mudstone in the Gansu area, two kinds of industrial solid wastes, namely steel slag and mineral powder are adopted to carry out the research on the mechanical properties and the constitutive model of the modified loess-red bedded mudstone mixed fill by compacting and triaxial CU test. The results show that: (1) The maximum dry density of the mixed fill is the largest when the mass ratio of loess: red mudstone is 1 ∶ 3. (2) The shear strength of steel slag is maximum when the mass dosing of steel slag is 7.5%. The larger the mass dosing of mineral powder is, the larger the shear strength is. (3) It is verified that the strain hardening process of soil samples before and after steel slag improvement could be accurately fitted by the Duncan-Zhang model. (4) For the problem that the hyperbolic model promoted by Shen Zhujiang can not fit the strain hardening well, this paper proposes to use the staged fitting method with positive and negative solutions, and it is verified that this method can accurately fit the strain hardening process of the mineral powder amended soil.

The soil in the northwest loess irrigation area has been affected by the dual effects of dry-wet cycling and salt pollution, leading to significant deterioration of the loess properties in this region. Taking Q₃ loess contaminated by Na₂SO₄ as the research object, direct shear tests and scanning electron microscopy (SEM) are used to analyse the changes in mechanical properties and microstructure characteristics after different numbers of dry-wet cycles. The results show that: Under the dual effects of dry-wet cycling and salt pollution, the strength of the loess deteriorates significantly, and the microstructure changes significantly. Among them, the cohesion shows obvious deterioration, with a decline rate of up to 38.24% to 51.4%. With the increase of salt pollution and dry-wet cycling times, the proportion of individual particles in the loess body increases significantly, and the aggregates begin to disintegrate; Na₂SO₄ repeatedly crystallizes and dissolves during dry-wet cycling, thereby exerting a combined effect of salt swelling and salt erosion on the soil; among the two effects, the destructive effect of dry-wet cycling on the soil structure is greater than that of salt pollution. The research results provide an effective reference for engineering activities and pollution control in loess areas.

With the rapid development of infrastructure such as railways, highways, and water conservancy and hydropower projects in the western region of China, a number of extremely complex geological conditions and highly challenging ultra-deep buried tunnels have emerged. These developments place higher demands on the collection, analysis, and application of tunnel geological information. Geological information transparency technology plays a crucial role in ensuring safety, controlling costs, and optimizing construction progress, serving as key technical support for the smooth execution of ultra-deep buried tunnel projects. This paper systematically reviews the current development status of geological information detection technologies, covering remote sensing survey technology, geophysical exploration technology, and directional drilling technology in the pre-construction phase, as well as advanced geological forecasting technology during construction. The application scope and development directions of various technologies are analyzed. The paper also reviews the research status of multi-source geological information fusion technology and 3D geological modeling technology, detailing their progress and challenges in predicting geological conditions, model building, and applications. Finally, this paper looks ahead to the overall development of geological information transparency technology for tunnels: First, establishing a five-dimensional integrated detection system encompassing "space-air-ground-borehole-tunnel" to improve detection accuracy; second, developing intelligent geological monitoring technologies and equipment; and third, based on multi-source data fusion, constructing 3D geological models and applying virtual reality technology to achieve dynamic geological information visualization, thus enhancing the safety and efficiency of tunnel construction.

Before refracturing, due to the long-term injection and production of old wells, the distribution of regional formation pressure shows non-uniform variations. It is urgent to coupling consider this non-uniform stress evolution in the subsequent refracturing. For this purpose, taking the Chang-6 reservoir in the W block of Changqing Oilfield as an example, an in-situ stress evolution model under long-term injection and production of vertical wells is established using the Fast Lagrangian Analysis of Continua. The simulated stress field is then imported into a hydrofracture numerical model based on the discrete lattice method for modeling fracture propagation of refracturing, achieving an integrated simulation of in-situ stress evolution and hydraulic fracturing evolution. The results show that: (1) After the production of well WJ, the pore pressure around this well decreases by about 4 MPa, and the two horizontal principal stresses experience a similar synchronous reduction, but the decreasing magnitude is only about 2.5 MPa. This indicates that production will result in a decrease in the total stress but an increase in the effective stress. (2) Hydraulic fractures tend to propagate towards the depleted area preferentially. The engineering measures, such as slowly injecting fluid or shutting in before refracturing to increase the formation pressure in the depleted area, are recommended on site, thereby avoiding or reducing refracturing fractures extending into these areas. (3) As the injection time increases, the primary growth of refracturing fractures transitions from area expansion to width expansion, indicating that a short-duration, high-volume refracturing should be adopted. On the one hand, maximizing reservoir transformation can be achieved in a short time, on the other hand, increasing the injection rate can promote the even expansion of multiple fractures.

At present, the actual pollutant emissions under slag removal stage have not been fully considered in the ventilation of tunnel construction, greatly increasing the required air volume for tunnel construction and causing huge energy waste. On-site testing of CO concentration and wind speed was conducted based on a certain tunnel, the changes in CO concentration over time under different processes were studied, and the effects of engineering vehicle emissions and fan air supply on the distribution of CO concentration and wind speed in the tunnel were analyzed. The results show that: The CO concentration on the palm face remained basically unchanged within 25 minutes after blasting, about 200 mg/m³; During the slag removal stage, the CO concentration on the palm surface decreases linearly and reaches the standard limit (30 mg/m³) after 90 minutes of ventilation; The actual required air volume for tunnel construction during the slag removal stage is much lower than the standard requirements. As the supply air volume increases, the CO concentration in the return air section decreases exponentially. Based on the research results, an empirical calculation formula for CO concentration in highway tunnel construction ventilation is proposed, which takes into account the coupling effects of CO emissions and supply air volume.

After being disturbed, the regenerated rock mass is prone to looseness and fragmentation, but under the support effect, the regenerated rock mass still has a certain bearing capacity, especially with significant improvement in post peak bearing capacity, which is the foundation for controlling the stability of the regenerated rock mass roadway. To study the bearing characteristics of recycled rock mass and the influence of support on the bearing capacity of recycled rock mass, a four-factor mixed level orthogonal test was designed for uniaxial loading of recycled rock mass. The bearing mechanism of recycled rock mass is analyzed. The significance of anchor spacing, rod diameter, anchor length, and metal mesh on the post peak weakening trend and bearing capacity of recycled rock mass is tested based on orthogonal analysis of variance. The results show that: Under the conditions of no support and partial anchor support, the regenerated rock mass undergoes compression shear failure under uniaxial loading, while some anchor support regenerated rock mass undergoes splitting failure, and the metal mesh constrained regenerated rock mass specimen undergoes plastic sliding failure. The residual strength of the regenerated rock mass after fracture exhibits fluctuating characteristics. The stability of the post peak bearing stage depends on the friction effect of multiple fracture surfaces, and the support reinforcement friction effect resists the sliding and dislocation of the broken block under external loads. The metal mesh has a significant impact on the post peak bearing capacity of the recycled rock mass, while the change in anchor rod parameters only affects the weakening trend of the strength of the recycled rock mass.