Xinjiang Wei

To explore the impact of ground load on existing shield tunnels and connected shaft structures, this study designed a testing device to identify the stress of shield tunnels and connected shaft structures under ground load. By controlling the location and magnitude of the load, we investigated the internal forces and deformations of the tunnels and shafts, as well as the displacement of the soil. Additionally, a three‐dimensional finite element model was established to reveal the internal forces and deformations of shield tunnels and connected shaft structures under uniform ground load, as well as the settlement of the soil. The finite element simulation results were highly consistent with the results of indoor model tests, demonstrating the consistency between these two methods. The results indicated that the internal forces and deformations of the shafts vary significantly with the magnitude of the surcharge load. When the surcharge is directly above the tunnel axis, the internal forces of the shafts decrease as the distance between the load and the shaft increases. Within the 2D eccentric range, the internal force changes caused by the load effect are significant, but outside the 3D eccentric range, the internal forces and bending moments of the shafts decrease significantly, and the magnitude of the changes is not obvious. Beyond the 2D range of the load area, the displacement of the soil decreases significantly, and the magnitude of the changes is not obvious. This reflects the significant impact of ground load on the shaft structure and highlights the tolerance of remote soil measurement points to load fluctuations. This study has strong reference significance for the excavation of vertical pipelines under shield tunnels.

The prevalence of T‐shaped structures comprising interconnected tunnels and shafts is steadily increasing. Through a two‐stage analysis approach, the influences of unloading and barrier action of the supporting system under the bedding and the sidewall of the foundation pit were analyzed, and a mechanical model considering the additional stress produced by the tunnel shaft under the action of the foundation pit excavation was developed. Utilizing the Mindlin solution, this study computed the additional stress on the adjacent tunnel shaft caused by foundation pit excavation, and the optimal equation for calculating the additional stress was identified using MIDAS GTS‐NX software. Through single‐factor variable analysis, the influencing factors, such as excavation length, width, excavation depth, minimum distance between the shaft and foundation pit, and stress loss rate, were varied, and the influence of unloading at the bottom and sidewall of the foundation pit on the additional stress of the shaft was calculated and examined. The following conclusions can be obtained: (1) when bedding is excavated on the side of the tunnel shaft, the influence of unloading on the bottom of the foundation pit is greater than that on the sidewall; (2) when the vertical shaft is subjected to a horizontal stress load on the side of the foundation pit along the depth direction, the joint of the tunnel and vertical shaft is subjected to the maximum stress; (3) when the turning point of vertical stress changes (from upward to downward) occurs below the surface, this indicates that the assembled pipe joint structure of the shaft experiences the risk of tension disconnection under the action of foundation pit unloading; and (4) sensitivity analysis of the horizontal and vertical additional stress factors reveals that the sensitivity of the distance between the shaft and the pit is the highest, followed by the excavation depth.

Proper design of special segments under cavity conditions is essential for successfully implementing the upward shield tunneling method. When constructing upward tunnels, the segment structure in the corresponding construction area of the horizontal tunnel is special. To study the internal force distribution on the structure under the worst possible conditions given by the upward shield tunneling method, MIDAS GTS NX software is used to numerically simulate the structure based on the condition of the cavity behind it to determine the most unfavorable radius of the cavity. Moreover, an analysis of the cavity location is conducted. The results demonstrate that when the cavity is located at the bottom of the arch with a radius of 1 m and near the top of the arch with a radius of 0.5 m, the special segment structure is under the most unfavorable combination of stresses. The most dangerous working condition occurs when there is a 0.5 m radius cavity on one side of the arch in the cross‐section of the special segment.

The development of underground space is fast because of the lack of space. To build shafts in underwater tunnels, the vertical tunneling method (VTM) was invented in 1974 in China, which can act as a freshwater intake or sewage outlet. During the operation of the VTM, the jacking force is one of the essential factors that draw attention. This paper conducts a numerical study of the jacking force and its influencing factors during the vertical tunneling process. First, based on the finite element software ABAQUS, a numerical model of the vertical tunneling process is established according to the VTM project in Beihai, China. Second, in accordance with the Latin hypercube sampling method and the multivariate significance analysis, the mechanical parameters are determined or back‐analyzed. Then, the calculated jacking force of the numerical model is compared with the measured jacking force. It turns out that the changing trend of the jacking force in the numerical model and the measured jacking force is relatively consistent. Finally, the influencing factors of the jacking force, such as elastic modulus and the angle of internal friction, are analyzed based on the numerical model. The results show that the elastic modulus and the angle of internal friction of soil are the main influencing factors of the jacking force. The secondary factors are Poisson’s ratio, static earth pressure coefficient, unit weight, and cohesion.

The upward shield method in horizontal tunnels has the advantages of small ground space occupation, low construction cost, and short construction period, which expresses substantial economic benefits. The special segments of horizontal shield tunnels corresponding to the upward shield method construction area have special design requirements, and different internal force calculations and analyses should be carried out. To determine the mechanical behavior of the special segment structure of horizontal shield tunnels during the construction of upward shield tunnels, the theoretical calculation formula of the internal force of the special segment structure is deduced based on the average uniform rigidity ring method. Moreover, the bending moment and axial force of special segments are numerically simulated and analyzed using Midas GTS NX software. Finally, the theoretical formula and finite element calculation results are compared and analyzed. The results show that the internal force values of the open segment ring and the adjacent segment ring of the special segment are quite different during upward shield tunnel construction. Furthermore, compared with the arch crown and arch bottom of the segment ring, the stress at the arch waist of the segment ring is the most unfavorable. The stress difference between the open segment ring and the adjacent segment ring may cause unbalanced deformation, leading to damage to the segment ring radial joint. In the design and construction, attention should be given to the shear strength of the segment ring radial joint and the bending stiffness of the ring joint at the arch waist.

The jacking force is one of the important parameters affecting the construction process in the vertical tunnelling method. To study the dynamic changing process of jacking force with the jacked distance and the influencing factors of the maximum jacking force, both the indoor model test and numerical simulation were conducted. In the model test, we investigated the influence of the height of the soil and water content. The results indicated that the higher the overburden height was, the greater the jacking force. Moreover, the water content enhanced the compressibility of the soil and had little effect on the maximum jacking force. Additionally, the coupled Eulerian–Lagrangian (CEL) approach was used to simulate the vertical jacking construction. In the numerical simulation, we investigated two construction factors (jacking speed and the standpipe outer diameter) and four soil parameters (cohesion, internal friction angle, elastic modulus, and Poisson’s ratio). Before that, the CEL simulation results were compared with test data to prove the rationality of the CEL approach, and the two were in good agreement. The results showed that among the six influence parameters, according to the influence degree on the maximum jacking force from large to low, the outer diameter of the standpipe, internal friction angle, cohesion, elastic modulus, jacking speed, and Poisson’s ratio were ranked. In addition, the jacking speed of numerical analysis was suggested to be 0.2 m/s. The research results in this paper can provide a reference for the construction of the vertical tunnelling method.

In order to research the theory for the variety of transverse forces of the adjacent shield tunnels caused by foundation pits excavation, the effect mechanism of foundation pit excavation on the adjacent shield tunnel was analyzed. The sidewall unloading model of the foundation pit, considering the deformation of the retaining structures, was introduced to calculate the additional stress of soil caused by foundation pit excavation. On this basis, the additional confining pressure variation model of the adjacent shield tunnel was established, considering the influence of the longitudinal deformation. Take the deep foundation pit project by the side of the shield tunnel of Hangzhou Metro Line 2 as a case study, the variation in confining pressure distribution of the adjacent shield tunnel caused by foundation pit excavation was analyzed, and a simplified finite element model was established to calculate the internal force of the segment ring structure. Moreover, the influence factors were analyzed, such as the deformation of the foundation pit retaining structure, the clearance between the foundation pit and the adjacent tunnel, and the buried depth of the tunnel. The present study suggests that the foundation pit excavation reduces the confining pressure of the adjacent shield tunnel, increases the absolute value of bending moment and shear force, and decreases the axial force at the top and bottom of the tunnel’s segment ring. With the increase in the deformation of the foundation pit’s retaining structure, the absolute value of the additional confining pressure on the adjacent tunnel increases, and the response of the bending moment to the foundation pit excavation unloading is more obvious than the variation in the confining pressure. When the buried depth of the adjacent shield tunnel is deeper than the excavation depth of the foundation pit, the influence of the excavation on the tunnel will be obviously weakened. With the decrease in the distance between the pit and tunnel, the influence of the excavation on the tunnel will be enhanced.