In the case of sudden surcharge loading, shallow shield tunnels in areas with soft soil experience substantial deformation responses. It is very important to understand the different loading modes and control measures above the shallow shield tunnels in soft soil for improving the safety of tunnel structure and reducing the influence of deformation. In this study, a three-dimensional numerical model of shallow shield tunnels in soft soil is established with FLAC. Ground and tunnel deformations are analyzed under different loading modes, and the effects of different deformation control measures are also studied. The numerical simulations in this paper show that the surface and tunnel deformation responses vary when induced by different loading modes above shallow shield tunnels in soft soil. After surface hardening with a 20-cm-thick layer of C20 concrete, the surface settlement is effectively controlled, and the uneven longitudinal settlement of the tunnel vault is improved. However, controlling the height of the surcharge is the most direct deformation control method. When the height of the surcharge is reduced from 6 m to 4 m and 2 m, the maximum ground settlement is reduced by 37.8% and 69.4%, respectively, and the maximum longitudinal settlement of the tunnel vault is reduced by 35.3% and 65.2%, respectively. During the operation of shallow shield tunnel in soft soil area, sudden surcharge loading should not be allowed. In the inevitable case, the surcharge loading on one side of the tunnel should be prevented and the surcharge loading height should be strictly limited.

As it is affected by many uncertain factors, the construction risk of deep foundation subway station pits involves fuzzy and random uncertainties. Considering the fuzzy and random uncertainties involved in risk evaluation, an improved fuzzy comprehensive evaluation method combining a triangular cloud model and the probability density function (PDF) is proposed in this study. First, with reference to the actual situation of deep foundation pit construction, the sources of construction risk are identified, and a construction risk evaluation index system is established. Second, the Delphi method is used to analyse the importance of each index of the evaluation object in order to obtain the evaluation data. The fuzzy best worst method (FBWM) is used to calculate the weight of the evaluation indices. Then, the triangular cloud model is used to represent the risk grade membership function. In addition, the fuzzy comprehensive evaluation method is used to comprehensively evaluate the construction risk of deep foundation pits. The fuzzy comprehensive evaluation vector is obtained for the indices possibility (P) and loss (C), and the weighted average value of the vector’s risk grade is calculated. Finally, probability analysis is carried out using PDF to determine the risk grade of P and C, and thus, to determine the risk grade of deep foundation pit construction. This method optimises the risk evaluation process of deep foundation pit construction and realises the visualisation of the comprehensive evaluation results, making the risk evaluation process transparent and convenient for use by risk analysts. This method is applied to predict the construction risk grade of a deep foundation pit project in Nanning, China, and the prediction results are consistent with the actual situation.

The construction of new tunnels poses a threat to the operational safety of closely existing tunnels, and the construction mode of parallel undercrossing over short distances has the most significant impact. In this study, a new double‐line shield tunnel parallel undercrossing of existing tunnels in Hefei, China, is taken as an example. A three‐dimensional (3D) numerical model using FLAC^3D finite difference software was established. The dynamic construction of the new double‐line shield tunnel undercrossing the existing subway tunnel over a short distance and in parallel was simulated. The pattern of existing tunnel settlement and change in lining stress caused by the shield tunnelling process were analyzed. The reliability of simulation was verified through field‐monitoring data. Finally, based on the numerical model, the effects of change in stratum sensitivity on the settlement of existing tunnel, lining internal force, and surface settlement are discussed. The results show that during shield tunnelling, the maximum ground settlement is 3.9 mm, the maximum settlement at the arch waist of existing tunnel near the new tunnel is 7.75 mm, and the maximum vault settlement is 5.38 mm. The maximum stress of lining of existing tunnel before the excavation is 7.798 × 10⁵ Pa. After the construction of double‐line shield tunnel, the maximum stress of lining is 1.124 × 10⁶ Pa, an increase of 44% than that before the construction. The surface settlement and tunnel settlement are sensitive to the weakening of soil layer strength, and lining stress is not affected by the weakening of soil layer strength. The field‐monitoring results are consistent with the numerical simulation results, and the model calculation is reliable. This study plays an important role in ensuring construction safety and optimizing the construction risk control of a tunnel.

Polymers as a new chemical grouting material have been widely used in fractured rock mass; however, the understanding of polymer diffusion characteristics still needs to be further improved. In order to study the diffusion mechanism of foamed polymer slurry in rock fissures, the radial diffusion model of polymer single crack grouting is derived in consideration of the factors such as grouting volume, crack width and expansion rate. The influence of different factors on slurry diffusion radius, diffusion pressure and flow rate is analyzed, The diffusion model is verified by finite element numerical simulation. The findings show that (1) The results of slurry diffusion radius, pressure and velocity distribution at different times under different working conditions in the present model are in good agreement with the analytical solution; (2) The diffusion pressure is directly proportional to the grouting volume and expansion multiple, and inversely proportional to the crack width. In addition, diffusion pressure decreases with the increase of diffusion distance, and the pressure at the corresponding distance increases slowly with time, and finally tends to be stable; (3) For the same section, the radial velocity decreases slowly with the increase of time; for different sections, the flow velocity increases sharply with the increase of the distance between the section and the central axis of the grouting hole.