The construction method without middle drift can simplify the construction organization process of double-arch tunnels, thereby improving construction efficiency. However, the mechanical evolution of the surrounding rock and support system, as well as the working mechanism of the support structure, remain unclear, which is unfavorable for structural design and construction safety control. Taking the Xinjia Tunnel as a case study, this paper establishes a three-dimensional nonlinear finite element model incorporating the complete construction sequence and the full set of support structures. The mechanical responses of the surrounding rock and support structure are calculated and analyzed under four working conditions with longitudinal spacings of the preceding and following tunnel faces set at L = 10.5 m, 36 m, 60 m, and 90 m. The stress transfer mechanism of the surrounding rock and the mechanical response evolution characteristics of each support component are systematically analyzed. The results show that the stress and deformation of the surrounding rock and support structure in a double-arch tunnel without middle drift exhibit significant asymmetry, and the stability of the goose-wing-shaped surrounding rock zone at the top of the middle partition wall is the key focus of construction safety risk control. A smaller longitudinal spacing between the preceding and following tunnel faces leads to a stronger superposition effect of construction disturbances and a significantly increased risk of support structure damage. The research findings can provide a theoretical basis and technical reference for the design and construction of double-arch tunnels without middle drift under similar geological conditions.

Pipe curtains are often used for pre-reinforcement to control surface settlement when excavating shallow-buried, large-span tunnels. There are two types of pipe curtain installation methods: longitudinal and transverse. The transverse pipe curtain construction can form a rigid transverse bearing system before tunnel excavation. This study investigates the deformation development of the pipe curtain, the soil pressure acting on the pipe curtain. The results indicate that the pattern of the deflection deformation of the pipe curtain is similar to the pattern of surface settlement. Considering the effect of soil arching between the pipes, a multilayer parabolic curve is used to characterize the shape of the soil arch. The soil arch can inhibit ground relaxation and reduce surface settlement. Based on the stress deflection theory, a multilayer parabolic arch model is established, and a method for calculating the loose soil pressure above the transverse pipe curtain is proposed. Combining this calculation method with the elastic foundation beam model of the Pasternak theory, a deformation calculation method for the pipe curtain at different excavation stages is developed. This method is compared and validated with field measurements and numerical simulation results. The proposed calculation method provides support for the design of transverse pipe curtains.

This study introduces a new method for calculating the surrounding rock pressure of twin-tunnels with small clear spacing under various working conditions. The proposed method unifies the calculation approaches for surrounding rock pressure under different working conditions into a single framework. A comparison is made between the new method and the standard recommended methods, and the installation procedures for field monitoring instruments of surrounding rock pressure are standardized. Laboratory experiments are conducted to correct the monitored values of surrounding rock pressure in the field. Additionally, the active-passive load separation method is employed to extract the active load component from the measured pressure values. The results show that the surrounding rock pressure calculated using the new method aligns more closely with actual engineering conditions. The variation trends and absolute magnitudes of surrounding rock pressure are smaller than those obtained using the standard methods. The corrected surrounding rock pressure values are closer to the true surrounding rock pressure. By comparing the active load derived from the measured values with the calculated surrounding rock pressure, the comparison results become more accurate. Furthermore, a three-way comparison among the calculated values from the new method, the standard recommended values, and the field-measured values confirms the correctness of the new method and its rationality and applicability in practical engineering projects.

Asymmetric bifurcated tunnels serve as critical structures connecting mountain tunnels and transportation hubs. The intermediate rock wall in such tunnels features complex stress conditions and is prone to severe accidents such as instability and failure. Traditional methods based on the assumptions of symmetric twin tunnels and homogeneous rock masses cannot accurately calculate the minimum safe thickness, which has become a core technical bottleneck restricting the safe and efficient construction of such tunnels. This paper systematically investigates the stability evolution law of asymmetric intermediate rock walls and the determination method of safe thickness. The concept of stress gradient is introduced to establish a quantitative relationship model between rock wall thickness and stress distribution. Through four key improvements, namely span equivalence, buried depth equivalence, coefficient correction, and parameter equivalence, an analytical solution for the minimum safe thickness of asymmetric intermediate rock walls applicable to any number of heterogeneous overlying strata is derived. Based on a mountain tunnel project in Qingdao, multi-dimensional verification is carried out using FLAC3D numerical simulation and field acoustic wave velocity testing. The three-stage evolution law of stress gradient decaying as a power function with rock wall thickness is revealed. A high-precision stress correction coefficient formula with a coefficient of determination R²=0.980 is obtained by regression, and the minimum safe thickness of the intermediate rock wall in this project is determined to be 1.10 m. The calculation results of the analytical solution are applied to engineering construction management, and the rock wall forming effect is satisfactory in on-site construction. The critical thickness for plastic zone penetration determined by numerical simulation is 1.0–1.05 m, and the superposition thickness of the loose circle at the thinnest section obtained by field acoustic wave testing is 1.05 m, which is highly consistent with the analytical value. The research results can provide important theoretical support and engineering reference for the thickness optimization design and construction safety control of intermediate rock walls in asymmetric bifurcated tunnels.

Interconnected cave-fracture networks are widely developed in karst regions. The complex geometric characteristics and strong heterogeneity of these formations not only disturb groundwater flow fields but also alter the in-situ stress state of the surrounding rock, leading to the formation of irregular preferential seepage pathways. These factors pose significant challenges to the long-term stability and construction safety of underground tunnels. In this study, a hydro-mechanical (HM) coupled model is established using COMSOL Multiphysics to investigate the seepage behavior of tunnels under different geological conditions, considering the synergistic effects of cave-fracture systems. The model is validated against classical consolidation problems and fracture seepage cases, and the results show good agreement with analytical solutions and previous studies, indicating its reliability. The results show that increasing fracture density enhances network connectivity, resulting in higher seepage pressure on the tunnel lining accompanied by greater non-uniformity. In addition, larger cave radius, shorter cave distance, and higher cave pressure further increase the seepage pressure on the lining, with caves located above the tunnel exerting a more pronounced influence on the upper section. When a connected seepage pathway forms between the cave, fractures, and lining, the local seepage pressure is more likely to exceed the lining bearing capacity. These findings suggest that targeted anti-seepage measures, such as grouting reinforcement, are necessary in high-risk zones, and provide a basis for the adaptive design of tunnel linings in karst regions.

During the excavation of the underground cavern at the Baihetan hydropower station, significant time-dependent deformation of the surrounding rock was observed, posing a serious challenge to the long-term stability control of the caverns. In this study, numerical models of the layered excavation for typical monitoring sections in the main and auxiliary powerhouses on both banks of the Baihetan hydropower station were established using a viscoplastic damage model. The time-dependent deformation responses of the surrounding rock during the entire underground cavern excavation process were successfully simulated, and the deformation and failure mechanisms of the surrounding rock during layered excavation were analyzed in combination with field monitoring data. The results demonstrate that the maximum stress trajectories at the right-bank powerhouse under higher stress conditions exceeded those at the left-bank powerhouse by 6 MPa after the powerhouse excavation. A larger stress difference caused stress trajectories to move closer to the rock strength surface, therefore making creep failure more likely to occur in the right bank. Targeted reinforcement in high-disturbance zones of the right-bank powerhouse reduced the damage progression rate at borehole openings from 0.295 per month to 0.0015 per month, effectively suppressing abrupt deformations caused by cumulative damage. These findings provide a basis for optimizing the excavation design of deep underground caverns.

The accurate prediction of the surface settlement trough is essential for the safety assessment of tunnel construction in densely occupied urban areas. In this study, we propose an artificial intelligence model to predict surface settlement troughs induced by twin tunnelling. The proposed model includes a newly proposed formula for describing settlement trough and a new calculation method of loss. The model uses a Graphical Convolutional Neural network (GCN) to extract latent feature information from field monitoring data that shows the state of the surrounding ground before the second shield passage. The proposed model is verified by comparing its results to those of two other models. The analysis shows that the developed calculation method of loss and consideration of the state of surrounding ground significantly improve the prediction accuracy of surface settlement troughs. While adding more monitoring points can offer benefits, the performance gains become weaker as the number of monitoring points increases. Therefore, we recommend using 24 monitoring points for the proposed model as it strikes the optimal balance between performance and computational efficiency.

Face failure of earth pressure balance (EPB) shield tunnels in granular soils is a great threat to the surroundings. This paper aims at investigating the influence of particle size distribution (PSD) on the failure mechanism from both the macroscopic and microscopic perspectives. The macroscopic investigation was carried out by performing several model tests which incorporated a miniature shield, and the microscopic investigation was conducted by utilizing the advantage of the discrete element method (DEM). The face failure of tunnels with C/D = 2.0 (C = tunnel cover depth; D = tunnel diameter) propagates to ground surface in three phases due to soil arching. PSD has influence on the timing of face failure, the size of failure zone and the soil arching. The proportion of strong contacts and the maximum soil arching increase with larger content of middle particles, while they decrease with larger content of fine particles. The soil arching maximizes when the local failure occurs and weakens in the transition phase, it extinguishes when propagating to Z/D = 0.4 (Z = depth from ground surface) as the global failure occurs regardless of PSD.

The aim of this paper is to provide a particle-scale insight into the face stability of shallow (C/D = 1.0; C = tunnel buried depth and D = tunnel diameter) earth pressure balanced (EPB) shield tunnels in cobble-rich soil, considering both the dynamic excavation process and particle size gradation. The work was performed by three dimensional discrete element method (DEM). Results show that particle size greatly influences the movement of cobble particles. The coarse particles serve as the skeleton of the ground and provide the stability. The erosion of fine particles has little influence on tunnel face stability, while the erosion of middle particles weakens tunnel face stability. The limit support pressure pf increases with increasing cutterhead rotating speed, and the normalized limit support pressure pf/gamma D (gamma = soil unit weight, D = tunnel diameter) obtained in this paper is larger than existing researches. The abrupt increase of middle particles in the muck composition can be recog-nized as the forewarning information against face failure.

This paper studied the subgrade spring stiffness and its influencing factors in the seismic deformation method of circular tunnel. Numerical calculations are performed for 3 influencing factors: stratum stiffness, tunnel diameter and burial depth. The results show that the stratum stiffness and tunnel diameter have great influence on the subgrade spring stiffness. The subgrade spring stiffness increases linearly with stratum stiffness increasement, and decreases with the tunnel diameter increasement. When the burial depth ratio (burial depth/tunnel diameter) exceeds to 5, the subgrade spring stiffness has little sensitivity to the burial depth. Then, a proposed formula of subgrade spring stiffness for the seismic deformation method of circular tunnel is proposed. Meanwhile, the internal force results of the seismic deformation method are larger than that of the dynamic time history method, but the internal force distributions of the two methods are consistent, that is, the structure exhibits elliptical deformation with the largest internal force at the conjugate 45 degrees position of the circular tunnel. Therefore, the seismic deformation method based on the proposed formula can effectively reflect the deformation and internal force characteristics of the tunnel and has good applicability in engineering practice.

This paper aims at revealing the failure mechanism of deep earth pressure balanced (EPB) shield tunnels in dry graded cobble-rich soil, considering the influence of kinematic differences of filler and skeleton cobble particle. The work was performed by three dimensional discrete element method (DEM). Results show that particle size greatly influences the movement of cobble particles. The coarse particles serve as the skeleton of the ground and provide the stability, while the fine and middle particles are filler materials. The erosion of fine particles has little influence on tunnel face stability, while the erosion of middle particles weakens tunnel face stability much. Existing researches overestimate tunnel face stability as the erosion of filler particles is ignored. The collapse developing from the tunnel face to the ground surface experiences three phases, i.e. local, ultimate and global. The proposed angles of 0.6(45 degrees+phi(p)/2) (phi(p) = the peak friction angle) to the horizon and 0.4(45 degrees-phi(p)/2) to the vertical are suggested to shape the local failure zone in the longitudinal and transverse direction, respectively. The collapse pressure and size of the failure zone increase with increasing cutter head panel opening ratio and rotating speed.

This study aims at addressing the face failure of earth pressure balance (EPB) shield tunnels in dry dense sand by model tests and discrete element method (DEM) models. The model tests incorporated a miniature EPB shield which could fully reproduce the real tunnel construction of excavation and support. DEM models simulating the model tests were developed to capture the underlying face failure mechanism. Results show that both the limit support pressure obtained at chamber board and tunnel face increase with increasing C/ D ( C is tunnel cover depth, and D is tunnel diameter). The ratio of the former to the latter approximates 0.60 due to the soil retaining of cutter head panel, and it is independent of C/ D. The local face failure initializes around tunnel face and develops directly to the global failure outcropping the ground surface in one phase with C/ D ≤ 1.0, while the local failure develops to the global failure in three phases with C/ D = 2.0 due to the soil arching evolution. The soil arching gets weaker when it propagates upward, and the horizontal stress concentration in the longitudinal direction is stronger than the transverse direction due to the difference of arch foot.

To understand the fracture behaviors and failure process in rock strata with bedding planes, this study presents a comprehensive investigation into the failure modes and micro-crack classification of Mao phyllite under Notched Semi-Circular Bend (NSCB) testing. The research combines rock experiments with Acoustic Emission (AE) detection and numerical simulations to gain insights into the behavior of layered rock masses. The X-ray Diffraction (XRD) and the Scanning Electron Microscope (SEM) were adopted to quantify the mineral components and observe the fracture morphology, respectively. The results showed that as the bedding plane angles (θ) increasing, the peak load trended to decrease. And the AE characteristic represented several significant changes in the loading process. The force–displacement curve of NSCB specimens with bedding planes was divided into four stages, and the characters of each stage were summarized. The AE frequency analysis allowed for the differentiation of AE events into shear and tensile types, providing a detailed understanding of crack development. Additionally, a sensitivity analysis examined the effects of bedding plane parameters on failure behavior. Moreover, the study categorizes the failure modes into three distinct categories based on θ, explaining the unique characteristics exhibited by each category. This study provides a more comprehensive understanding to the fracture behaviors and failure process of layered rock, benefits to the construction of underground projects.

In this paper, a semi-analytical method is developed to solve ground movement due to twin tunnel construction addressing the limitation of zero-volume change in energy method. A novel deformation mechanism, capable of considering the volume change of soil, is developed. Validation of the proposed method is conducted through numerical simulations and field monitoring data. Numerical results indicate the method’s efficacy in accurately describing ground settlement attributed to twin tunneling. Moreover, the proposed method shows improved agreement with the numerical simulation as depth decreases and the prediction performance of the proposed method excels particularly in twin tunnel excavation within clay compared to sand. Furthermore, the method demonstrates superior prediction capabilities at the ground surface compared to the subsurface. Notably, the maximum settlement tends to occur adjacent to the first tunnel. However, it is important to note that increasing the clearance between the twin tunnels diminishes the prediction accuracy of the proposed method.

There are many prediction models for shield tunneling parameters, and there is a lack of comparative study on the applicability of various prediction models in the same stratum. This paper combines the three models of SVR, linear regression, and BP neural network in MATLAB software to train and learn the shield tunneling parameter data of the Shuai-Nei section of Hohhot Rail Transit Line 2 and predict the tunneling speed of the shield machine in this section. The results show that the SVR regression model has the lowest prediction accuracy and is unsuitable for predicting shield tunneling speed in this stratum. After the noise reduction of the input tunneling parameters, the BP neural network model and the linear regression model can better predict the tunneling speed of the shield machine in this stratum with the accuracy of the test set being 87%. Applying the BP neural network model and linear regression model to simulate the tunneling parameters of EPB shield in the water-rich round gravel stratum is good. Among them, the accuracy rate of the training set of the BP neural network regression model is 98%, which indicates that the nonlinear mapping ability and generalization application of the BP neural network are excellent.

Field observations demonstrate that rapid unloading can induce intensive dynamic failure of a tunnel in a jointed rockmass. In this study, we demonstrate the characteristics of the dynamic failure and establish a series of validated discrete element models to investigate the mechanism of the dynamic failure. Our results indicate that the excessive release of strain energy and the complex nonlinear behavior of joints together resulted in the dynamic failure. Primarily, joints can enlarge the unloading disturbance zone (UDZ) and lead to the excessive release of strain energy, especially for a model with smaller joint spacing and higher unloading rate. Secondly, joints can significantly decrease the dependence of the dynamic effect on the unloading rate and expand the unloading time threshold that can induce the dynamic effect, more importantly, the nonlinear variation of joint stiffness not only induces vibration localization but also increases vibration amplitude and reduces vibration frequency.

The tunnel face collapses easily during tunnel construction in weak surrounding rock under high geo-stress. Identifying the instability mechanism of the face and proposing targeted control measures are of great importance to prevent the collapse of the face. Using the setting of the carbonaceous phyllite stratum of the Baima tunnel, a FLAC3D-PFC3D coupled model is used to analyze the influential factors on the stability of the face and optimize the original support scheme. The results show that the instability of the face is rooted in the collapse of the rock above the face, and the bench method with short cycle footage is more conducive to the stability of the face. Compared with single support mode of advanced small pipes, the combined support scheme of long bolts and small pipes can better control the displacement of the surrounding rock and prevent the collapse of the tunnel face. The numerical simulation and field surrounding rock monitoring results verify the rationality of the excavation method, the cycle footage, and the support system. This finding will be of great significance to guide design and construction in weak surrounding rock.

A large scale model test and numerical simulation using three dimensional discrete element method (3D DEM) were carried out in parallel to study the soil disturbance induced by earth pressure balanced (EPB) shield tunnelling in multilayered ground with soft sand lying on hard rock. The model test used a miniature EPB shield machine which could fully reproduce the real tunnel construction of excavation and support. The DEM model simulating the model test was developed to capture the microscopic mechanisms of ground deformation. The surface and subsurface settlement were closely recorded in model test and compared with those of homogeneous ground to clarify the influence of underlying rock on the movement of overlying sand. The volume ratio of rock to sand in excavated muck was closely recorded in DEM simulation to quantitatively analyse the over excavation of soft sand. Results show that both the surface and subsurface settlement trough in multilayered ground are narrower than those in homogeneous ground, while the subsurface settlement in multilayered ground is larger than that in homogeneous ground. The width parameter of settlement trough decreases nonlinearly along the gravitational direction. The over excavation of overlying soft sand is influenced by the ratio of hard rock to tunnel cross section and the rotating speed of shield cutterhead.

It is well known that the Strategic Petroleum Reserve in salt caverns is an important means to solve the potential petroleum shortage. However, salt mines in China are mainly lacustrine-layered structures with many mudstone interlayers. Meanwhile, the acid value of extracted crude oil has been increasing in recent years. The acid erosion of salt cavern interlayers by crude oil would affect the safety of bedded salt cavern oil storage. Therefore, combined with acoustic emission technology, the multistage creep mechanical behaviors of natural mudstone interlayer samples and the mudstone interlayer samples treated by oil erosion are studied in this paper. The internal structures of interlayer samples before and after oil erosion were systematically analyzed by SEM. The results show that petroleum acid (naphthenic acid) plays a significant deterioration role in the mechanical properties of mudstone interlayers during the petroleum reserve process. Compared with the uncorroded samples, the mudstone interlayers after oil erosion show obvious brittle failure characteristics. At low stress levels, the axial strain grows stepwise and the lateral strain is smaller than the axial strain. At high stress levels, however, the lateral creep is obviously higher than the axial creep. A large number of AE signals were generated at the initial loading stage for different stress levels. After the creep stabilized, the AE signals were significantly reduced. During the process of petroleum erosion, the internal pores in rocks continued to develop with the dissolution of many mineral particles. This suggests that crude oil storage in the bedded salt rocks could accelerate the deterioration of the surrounding rocks via erosion. This paper could provide basic research data and a reference for the construction of oil storage in the bedded salt rocks.