In order to explore the optimization of air environment and ventilation design parameters in China's extra-long tunnels, this study first analyzed the statistical data of pollutant emission from motor vehicles in China, combined with literature research, and found that the concentration of NO_x in China's tunnels was relatively high, and gradually became the most concerned pollutants in tunnel ventilation. In this study, a field study was carried out in the Yanglin extra-long tunnel in Yunnan. The results show that the peak NO₂ concentration exceeds the ventilation design limit by 2.1 times during the test period, and the emission factors of CO, NO₂ and PM of gasoline vehicles are 0.79 g/(km·veh), 0.04 g/(km·veh) and 10.0 mg/(km·veh), respectively. diesel vehicles are 2.18 g/(km·veh), 1.27 g/(km·veh) and 149 mg/(km·veh), respectively. Compared with the pollutant emission values of domestic and foreign tunnel ventilation design standards, it is found that the current standard values in China are too large. According to the measured emission factors, the required air volume is calculated, and the result is more than 50% lower than the required air volume in Guidelines for Design of Ventilation of Highway Tunnel, which is similar to the required air volume in Standard for the Design of Road Tunnels, and the control item of the required air volume is NO₂ concentration. The results of this study can provide reference for the calculation of pollutant emission and air demand in tunnel ventilation design in China.

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.

In order to investigate the plastic yielding mode of surrounding rock caused by the excavation of shallow twin tunnels, the displacement and additional surface force joint controlling the Schwarz alternation method and complex function method are adopted to solve the elastic stress function of surrounding rock of shallow twin circular tunnels through iterative cycles as an example. The expression of plastic stress components of surrounding rock is obtained based on Mohr-Coulomb failure criterion. The elastoplastic solution of the radius of the plastic zone is determined preferentially by using the stress continuity condition at the interface of the elastic-plastic zones around the shallow twin circular tunnels. The analytical solution for the distribution range of the plastic zones around the shallow twin circular tunnels is established, according to the interconversion relationship between polar coordinates and right-angle coordinates. The rationality and applicability of the analytical solution are verified by numerical simulation results and field measurement results of engineering application. The influence of the center spacing of the twin tunnels on the plastic zones around shallow twin circular tunnels are also analyzed. The results show that the analytical solution in this study can be used to solve the problem of predicting the distribution range of the plastic zones around shallow twin tunnels in actual engineering, and meets the requirement of 20% engineering accuracy, fits well with the numerical simulation results, and has a high calculation accuracy. The distribution range of the plastic zones around shallow twin circular tunnels are positively correlated with the center spacing s of the twin tunnels. Based on the distribution pattern of the plastic zone around shallow twin circular tunnels under the influence of this factor, when the plastic zone reaches the critical state of penetration, the reasonableness of the calculation results of the distribution range of the plastic zones is preliminarily judged. It provides theoretical guidance for similar tunnel engineering design calculation and deformation control of surrounding rock.

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.

Understanding the resistance coefficient along the tunnel wall is helpful to optimize the ventilation design of tunnel construction and improve the rationality of ventilation scheme. The influence of the height, shape and spacing of rough elements on the resistance coefficient along the tunnel wall is studied. Through the model test and the numerical model, the difference between the empirical formula and the numerical simulation results was explored. The results show that: With the higher height of the rough element on the tunnel wall, the influence on the resistance coefficient along the tunnel wall is less. The shape of rough elements on the tunnel wall has a great influence on the resistance coefficient along the tunnel, and the semi-spherical rough elements have the smallest resistance coefficient along the way. Comparing the calculated results of empirical formula with the numerical simulation results, the difference between them is relatively stable, which is related to the height of rough elements. The correction coefficient α is proposed for the empirical formula, and the corresponding relationship between α and the average roughness height Δ of the wall is α = 1.29+0.024 8Δ.

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.