e conversion section of the cross passage and shaft is a priority concern in the stress transformation of a tunnel structure during subway underground excavation. In the construction of Subway Line 5 in Xi'an, China, the main line in the loess layer was constructed through the cross passage from the subway shaft of the Yue Deng Pavilion-San Dian Village Station tunnel section. Numerical simulation and field measurement were adopted to study the construction stability of the cross passage and shaft under two possible construction methods: the "shaft followed by cross passage construction" method and the "cross passage parallel shaft construction" method. e results showed that the surface deformation and plastic zone of the surrounding rock are similar under the two construction methods. However, of the two, the "cross passage parallel shaft construction" method was more advantageous in controlling the structural deformation of the original shaft and the stress distribution of the horsehead structure.e field monitoring data showed that the surface settlements and the deformation of the original shaft structures meet the requirement of control standards under the "cross passage parallel shaft construction" method.

Pipe jacking technology has the advantages of fast construction speed, high economic benefit, and small impact on the urban environment, and mechanical vibration and mechanical soil interaction will lead to the settlement of upper part of pipe jacking and surrounding soil. So, the solution of soil settlement problem is of great significance for underground engineering construction. The soil loss, additional stress, and friction during the construction of rectangular pipe jacking are the main factors that cause the surface settlement. Aiming at the problem of surface settlement caused by pipe jacking in no. 6 subway in Kunming, the formation deformation caused by additional stress and friction force on excavation surface during pipe jacking process was analyzed by Mindlin’s displacement solution, and the formation deformation caused by soil loss was analyzed by random medium theory. Considering the independence of the three influencing factors, the superposition of the three types of stratum deformation was the superposition model of surface settlement, and the model was compared with the empirical method and the measured settlement data to verify its practicability and reliability. The results showed that the surface vertical settlement curve was roughly in the shape of “S,” which can be divided into uplift occurrence area, sensitive settlement area, stable settlement area, and rebound settlement area. Friction force and additional stress caused the uplift of stratum in the front of construction area, and soil loss caused the subsidence of stratum in the rear of construction area. Rectangular pipe jacking in weak stratum caused the soil to move towards the excavation surface, easily causing micro-overexcavation.

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The employment of bentonite modified loess (BML) is a common method of constructing the anti-seepage lining of landfills in the loess region of China, and its long-term secure performance is threatened by wetting–drying (W–D) cycles. Taking the remolded loess (RL) and BML with 15% in mass of bentonite as research objects, the W–D cycles test, scanning electron microscope test and direct shear test were carried out to analyze the effects of W–D cycles on the microstructure and shear strength of samples. The regression equations between strength and micro-pore structure parameters were established by the multivariate linear stepwise regression method. The damage mechanism of BML after W–D cycles was studied by establishing damage degree models based on pore area ratio and cohesion. Results indicate that the water absorption and expansion of bentonite effectively block the intergranular pores, resulting in more medium and small pores and more pronounced surface contact of particles. After W–D cycles, the particle arrangement of samples before and after bentonite modification tends to be loose. Both the pore area ratio and fractal dimension increase and tend to stabilize after five cycles. The BML exhibits lower pore area ratio and greater fractal dimension while its cohesion and internal friction angle show more significant decrease after W–D cycles than those of RL. The damage variables based on pore area ratio and cohesion well describe the W–D induced damage of loess before and after modification from macro- and micro-scale perspectives. The damage degree of samples increases with W–D cycles, but the increment decreases.

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