Excavation Unloading Research Articles

Analytical solutions for stresses and displacements of tunnels under static and seismic loading

Analytical solutions can quickly and simply evaluate stresses and displacements in tunnels and are widely adopted in the preliminary design phase of tunnels. Tunnels must be able to withstand static loading and seismic loading. This paper proposes new analytical solutions for the stresses and displacements of tunnels under static and seismic loading. The tunnel lining is treated as a thick-walled cylinder. Moreover, the contact stiffness coefficient k0 and the stress release coefficient η0 are introduced to consider the arbitrary contact conditions at the ground–lining interface and the amount of excavation unloading. The new analytical solutions are verified by comparison with finite element analysis results. The results demonstrate the accuracy and applicability of the proposed analytical solutions. In addition, parametric analyses are conducted to explore the effects of the stress release coefficient η0 and stiffness coefficient k0 at the ground–lining interface, as well as the incident angle α of shear waves. The analysis results indicate that the proposed analytical solutions offer satisfactory performance and can be applied as an effective tool for preliminary tunnel design.

Experimental study of the coupling effect on segmental shield tunnel lining under surcharge loading and excavation unloading

Surcharge loading and excavation unloading are two common engineering disturbances affecting the adjacent tunnel in congested urban areas. The coupling effect on segmental shield tunnel lining under loading and unloading conditions is still unclear. In this paper, physical model tests conducted in 1-g plane strain conditions were designed and performed to study the coupling effect of loading and unloading on shield tunnel. The results show that the response of segmental shield tunnel linings will be significantly amplified under the coupling conditions compared with the individual disturbance. A coupling effect index was proposed to evaluate the coupling degree of different disturbances. The classification was also presented, divided into favourable, zero, and unfavourable coupling effects. The tunnel response has a more significant coupling effect in the case of loading first and unloading later. There is a coupling amplification effect of 128% and 144% for the tunnel horizontal convergence and the springline near the excavation.

Physical and numerical investigation on nonlinear mechanical properties of deep-buried rock tunnel excavation unloading under complicated ground stresses

The nonlinear stress and deformation response of tunnel excavation unloading under high and complicated geostresses are important features of deep rock tunnel engineering, and such conditions differ significantly from those of shallow tunnels. Additionally, several studies on deep tunnels have focused on mechanical behaviors of rock tunnels at certain depths, ignoring nonlinear responses of rock tunnels at various depths. In this study, physical model tests and three-dimensional (3D) numerical calculations concerning complicated ground stresses were conducted to investigate the nonlinear mechanical and strength properties of tunnels excavated in rock formations with varying buried depths. The generalized Zhang–Zhu criterion (GZZ) was found to appropriately describe the nonlinear laws of rock strength (yield) with stress levels on the π plane, and the correctness and reliability of the GZZ strength criterion were subsequently verified by comparing the deformation monitoring data of physical model testing with different strength criteria-based numerical results. These results were then used to examine the stress distributions, deformations, and failure characteristics of physical model tunnels under complicated stress conditions at various buried depths. The nonlinearity of stress and deformation of the model tunnels are directly related to stress conditions, especially to the horizontal tectonic stress. This indicates that the traditional linear design approaches and practical experiences related to shallow tunnels dominated by gravity stress are not applicable to deep-buried tunnels under high and complicated ground stress conditions.

Investigation of fatigue failure and energy characteristics of rock exposed to complicated stress disturbance paths: Cyclic stress amplitude effect

AbstractTo deeply reveal the differences in geomechanical responses of rock during mining exposed to complicated stress disturbance of excavation unloading and cyclic loading conditions, cyclic triaxial fatigue loads with four cyclic stress amplitude (CSA) (60, 100, 130, and 160 MPa) were performed on deeply buried rock. Results show that rock deformation, energy evolution, and failure modes are all influenced by the CSA. Additionally, it is found that all the strain energy increases with increasing CSA during rock failure. Especially, the dissipated energy increases greatly at the confining stress unloading stage (CSUS) than the fatigue loading stage (FLS), indicating the influence of unloading on rock damage accumulation. Moreover, a damage evolution model reflecting a first slow and then quick increase pattern was established using the dissipated energy. Finally, a transition failure mode from the dominant tensile‐shear to pure shear is found. It is suggested that much more energy needs to be consumed to induce shear cracks than tensile cracks.

Study on the Creep Characteristics and Fractional Order Model of Granite Tunnel Excavation Unloading in a High Seepage Pressure Environment

The creep associated with unloading surrounding rock during the excavation of deep tunnels seriously affects the stability of the tunnel, and a high seepage pressure will aggravate the strength attenuation and structural deterioration of the surrounding rock. Based on the background of the excavation-induced unloading of the surrounding rock of a deeply buried granite tunnel with high seepage pressure, in this paper we carry out a triaxial unloading seepage creep test that considers the effects of both excavation disturbance and seepage pressure. We also analyze the mechanism of unloading and seepage pressure leading to sample failure and construct a fractional creep damage constitutive model that considers the unloading effect. The results include the following findings, firstly, seepage pressure will affect the creep deformation of rock for a long time, and the circumferential expansion of the granite creep process is more obvious than the axial expansion. Secondly, a high seepage pressure will reduce the rock bearing capacity. Under 0, 2 and 4 MPa seepage pressures, the long-term strength of the samples are 193.7 MPa, 177.5 MPa and 162.1 MPa, respectively. Thirdly, the rock damage factor increases with increasing seepage pressure, time and deviatoric stress. Finally, the rationality of a fractional-order model that considers the effect of unloading and seepage is verified by the test data. These research results may provide some reference for the stability analysis of surrounding rock during excavation in environments under high-stress and high-seepage-pressure.

Reaction and deformation mechanism of a slipping-stretching landslide: Example of the Liangtianao ancient landslide, Guangxi Province, China

Slipping-stretching landslides have long been recognized as a common type of landslide case, but such reactions have rarely been reported. There was a slipping-stretching landslide reaction at Liangtianao, Guangxi Province, China, and the geological background and deformation characteristics of the case were identified by detailed geological survey and long-term monitoring. A FEM model of the case was built using GEO5 to analyze the mechanism of reaction and deformation. The results are as follows. 1) The Liangtianao landslide is a bedding rock ancient landslide, which remained in a creeping state after the landslide occurred in geological history. 2) The new sliding surface in the Liangtianao landslide is basically consistent with the weak interlayers formed by ancient landslides, and the fault-type is a slipping-stretching type with the following deformation process: strain at leading edge by road excavation→ leading edge instability→ middle part shear and creep→ back edge tensile. 3) Micro-geomorphology, rock mass bedding, and weak interlayers are internal causes of the Liangtianao landslide reaction, while excavation unloading and rainfall are the external causes. The inclinometer indicates that the Liangtianao landslide is still highly sensitive to rainfall after the landslide reaction, which may trigger a secondary landslide reaction. 4) The numerical analysis results indicate that the maintenance of a passive state in the anti-sliding section is highly beneficial to maintaining basic stability after the landslide reaction; unloading only 1/20 of the sliding section makes the landslide stable in the long term.

Comparative experimental investigation on true triaxial double-sided fast unloading rockburst and biaxial compression under high stress

To explore the effect of rapid excavation unloading under high pressure on delayed rockburst, a true triaxial double-sided rapid unloading rockburst experiment and a biaxial compression failure experiment without unloading were carried out. The results show that the rapid unloading of excavation before the occurrence of rockburst will cause unloading damage, and the axial strain, acoustic emission (AE) count, and AE energy will suddenly increase at the unloading point. The AE source shows that the rapid unloading produces many tensile cracks, mainly distributed near the free surface. The axial peak stress of the rockburst and the stored strain energy before failure are higher than those of biaxial compression, so the ejection velocity of free surface fragments is also higher at failure.

Failure mechanism and stability analysis of an open-pit slope under excavation unloading conditions

Excavation unloading significantly contributes to rock slope failure in an open-pit mine. At present, there is no relevant theoretical study on the failure mechanism of the rock slope under excavation unloading. Therefore, in this study, based on the theory of fracture mechanics, the expression of the stress intensity factor at the crack tip on the rock mass at the vertical distance, h, from the slope top under excavation unloading is derived, the calculation method of the crack initiation angle is given, the expression of the ultimate safe height of the slope under unloading is obtained, and the ratio of the fracture toughness of the slope rock mass to the composite stress intensity factor at the crack tip on the rock mass is defined as the slope stability factor, which is verified by an engineering example. The results show that the crack initiation angle decreased when crack inclination was increased, and the crack initiation angle increased when the side-pressure coefficient, slope angle, and friction coefficient were increased. The ultimate safety height of the slope decreased first and then increased with the increase in the crack angle, and it was approximately linear with the crack length and inversely proportional with the slope angle. The stability coefficient calculated by this method is the same as that calculated by the limit equilibrium method and is small, which indicates the accuracy and rationality of this method. Results in this study can provide a theoretical basis for understanding and controlling the slope collapse disaster induced by excavation unloading.

Excavation unloading response of cylindrical rock-like specimen with axial joints: laboratory experiment and numerical simulation

Abstract Joints have a significant influence on the deformation and failure mechanism of the surrounding rock. To reveal the influence of axial joint on mechanical response of the roadway surrounding rock after excavation unloading, the deformation and failure characteristics of cylindrical rock-like specimen (CRLS) with an axial joint was studied through laboratory tests and numerical simulation. Also, the influence of joint size and position on strain variation, secondary stress evolution and plastic zone distribution of the CRLS were analyzed. Results show that the axial joint can promote the deformation of the surrounding rock on the inner side of the joint, while hindering the deformation of the surrounding rock of the roadway on the outer side of the joint. In addition, the size and position of the axial joint have a significant influence on the distribution of secondary stress and plastic zone of the surrounding rock. The stress relief zone is mainly located between the joint and the excavation profile, whereas the plastic zone is mainly distributed at both ends of the joint and between the joint and the excavation profile. Finally, the tangential stress concentration can be alleviated by choosing proper distance between the joints and the roadway, and the failure mode of the surrounding rock between the joint and the excavation profile transmitted from tensile failure to shear failure with the axial joint moves away from the roadway. The research results could provide technical reference for roadway support and disaster prevention in a deep jointed rock mass.

An improved 3D DDA method considering the unloading effect of tunnel excavation and its application

Tunnel excavation unloading is the primary factor in disasters such as collapse and large deformations of surrounding rock. Therefore, it is a key issue to study the deformation mechanism of the surrounding rock in the process of tunnel excavation unloading. However, the original three-dimensional discontinuous deformation analysis method (3D DDA) can only simulate the displacement of the surrounding rock after the completion of tunnel excavation, which is not suitable for studying the mechanism of the surrounding rock’s deformation caused by tunnel excavation. Therefore, this paper proposes a virtual block removal algorithm that moves the blocks out of the calculation area while ensuring the consistency of the array and develops an improved 3D DDA method (3D DDA_exc) by embedding the algorithm into the original 3D DDA program, which is suitable for the simulation of tunnel excavation. In addition, based on the free fall models of static and sliding blocks, we compare the results of the 3D DDA_exc simulation with theoretical calculation results, which are basically the same. Then, both the 3D DDA_exc method and the discrete element method (DEM) are used to simulate the excavation of the tunnel in a blocky rock mass, and the rock displacements around the tunnel hole are basically the same, which further illustrates the accuracy of the 3D DDA_exc method in simulating tunnel excavation. Finally, taking the Huangjiazhuang Tunnel of the Rizhao-Lankao high-speed railway as a case study, we simulated block collapse during the excavation process of the tunnel using the centre diaphragm method (CD method). Compared with the result of the 3D DDA method without the process of tunnel excavation, block collapse can be avoided if the dangerous blocks are supported and reinforced during the excavation of part ① and part ② of the tunnel, which was of guiding significance to the safe construction of the tunnel.

Effect of disturbed frequency on rock failure and energy characteristics exposed to triaxial fatigue and multistage unloading confining pressure (TF-MSUCP) conditions

Rock is always encountered with disturbed stress, the respective excavation unloading and fatigue loading induced rock failure have been widely studied, yet the alternative fatigue loading and unloading on rock failure were not well understood. Herein, a series of triaxial fatigue and multistage unloading confining pressure (TF-MSUCP) tests were performed on granodiorite specimens with different disturbed frequency (i. e., 0.05, 0.1, 0.2, and 1.0 Hz). The stress strain responses, deformation, strain energy evolution, damage propagation, and mesoscopic failure pattern were investigated systematically. Testing results show that rock volumetric deformation, fatigue lifetime, dissipated energy, damage accumulation, and the failure modes were all influenced by the disturbed frequency (DF). The radial strain contributed greatly to the volumetric strain in the unloading confining pressure stage (UCPS) compared to the cyclic loading stage (CLS). The dissipated energy and failure pattern suggest that low disturbed frequency is prone to induce severe failure, in terms of greater radial deformation and complicated crack network. The relationship between the dissipated energy rate and confining pressure was established, and it is found that the dissipated energy rate is larger at UCPS than at CLS. A dissipated energy based damage evolution model was established, the model reveals the influence of disturbed frequency on damage accumulation. Post-test CT images revealed that the failure mode changes from mixed tensile-shear failure to pure shear failure with the increase of disturbed frequency.

Rockburst and microseismic activity in a lagging tunnel as the spacing between twin TBM excavated tunnels changes: A case from the Neelum-Jhelum hydropower project

A reasonable spacing between tunnels is very important in deeply buried tunnels, as it may mitigate the risk of rockburst occurrence. This spacing is particularly important for tunnels excavated using a tunnel boring machine (TBM), as the methods available for controlling rockburst occurrence are inflexible and limited using these machines (compared to tunnels excavated using the drilling and blasting (D&B) method). In this paper, a rare example was studied in situ in which the spacing between twin tunnels was changed to mitigate the rockburst risk during TBM excavation. First, the characteristics of the rockburst and microseismic activity were investigated in three sections of the lagging tunnel before, during, and after the spacing between the tunnels was changed (we concentrated on the lagging TBM696 tunnel, which is part of the Neelum-Jhelum hydropower project). We observed a significant occurrence of rockburst and microseismic activity in the lagging tunnel at the original spacing of the twin tunnels in both the sandstone and siltstone lithologies, and thus a consideration of enlarging the spacing between the twin tunnels is reasonable. A more detailed investigation was then conducted to explore the effects of tunnel spacing on rockburst occurrence and microseismicity. The rockburst frequency and microseismic radiated energy due to excavation unloading generally decrease as the spacing between the twin tunnels increases (substantially at first and then more slowly as separation increases). In particular, the microseismic radiated energy exhibits a power-decreasing type in both the sandstone and siltstone. Increasing the spacing beyond a certain point, therefore, exerts a very limited effect on the microseismic radiated energy and occurrence of rockbursts (increments of 16–21 m and 15–20 m in sandstone and siltstone, respectively). Consequently, in practice, a very reasonable approach would be to enlarge the spacing of twin tunnels by 22.48 m. Thus, microseismic monitoring is a useful technique for assessing the rockburst risk and deriving a reasonable value for tunnel spacing. This study can be used as a guideline to derive reasonable tunnel spacings when using not only TBMs but also D&B.

Influence Law of Foundation Pit Excavation on Stress of Surrounding Tunnel Bolt

Due to the existence of a tunnel next to the foundation pit, the soil surrounding the foundation pit deforms into the foundation pit due to the excavation unloading during the process of foundation pit excavation, which causes the deformation of the tunnel. The supporting structure of the tunnel can ensure the deformation stability of the tunnel, so it is necessary to study the stress of the tunnel bolt supporting structure caused by the excavation of the foundation pit. In this paper, the numerical simulation method was adopted to study the influence of the distance between the foundation pit and tunnel and the width and depth of foundation pit excavation on the stress of the tunnel bolt, and the following results were obtained: (1) When the distance between the tunnel and the foundation pit changes, the axial force of the bolt changes accordingly. The axial force of the left bolt first increases and then decreases, and the largest axial force is located in the middle of the bolt. (2) With the increase in the excavation width of the foundation pit, the stress of the left bolt does not change much. For the left bolt, with the increase in excavation depth, the stress of the left bolt does not increase monotonically but shows a trend of increasing first and then decreasing. When the excavation depth exceeds a certain value, the influence of excavation depth on lateral soil displacement gradually decreases. With the increase in excavation depth, the axial force of the top bolt decreases first and then increases.

Numerical Analysis of Instability Mechanism of a High Slope under Excavation Unloading and Rainfall

High slope simulation analysis is an essential means of slope engineering design, construction, and operation management. It is necessary to master slope dynamics, ensure slope safety, analyze slope instability mechanisms, and carry out slope stability early warning and prediction. This paper, aiming at the landslide phenomenon of the high slope on the left bank of a reservoir project, considering the influence of stratum lithology, fault, excavation unloading, rainfall, and water storage, establishes a refined finite element model that reflects the internal structure of the slope. The fluid-solid coupling numerical simulation analysis of the high slope is carried out. Based on this, the failure mechanism of the slope under excavation unloading and heavy rainfall is explained. The application of an engineering example shows that under the combined action of excavation unloading and rainfall infiltration, the in-plane saturation of the structure formed at fault at the trailing edge of the excavation slope surface increases, the pore water pressure increases, and the shear strain concentration area appears at the internal structural surface of the slope. The shear strain concentration area extends along the structural surface to the front and rear edges of the slope, resulting in landslide damage.

Study on the Influence of Excavation Unloading on Pile Depth in Sandy Soil

With the development of urban construction, uplift pile has been widely used in underground structure bearing buoyancy. The pile is buried deep underground, and the foundation pit is excavated on the pile foundation, the stress redistribution of the soil after the foundation pit excavation will reduce the lateral stress of the pile at the bottom of the pit, resulting in the reduction of friction resistance, but this effect is limited to a certain depth at the bottom of the foundation pit. Referring to the unloading influence depth corresponding to the vertical unloading coefficient in the spring back calculation of foundation pit and introducing the unloading influence depth corresponding to the horizontal unloading coefficient, it is deduced that the relationship between the horizontal unloading coefficient and the vertical unloading coefficient is based on the friction angle. Based on the bearing capacity control and numerical simulation, it is determined that the corresponding depth when the horizontal unloading coefficient is 0.95 is the unloading influence depth. The depth to width ratio of excavation has a great influence on unloading influence depth, and the internal friction angle of soil has little influence on unloading influence depth, while the influences of pile diameter and pile-soil friction coefficient can be ignored. The formula of unloading influence depth is derived in this paper. Comparing the unloading influence depth of sand and soft soil, the unloading influence depth of sand is greater than that of soft soil.

Experimental investigation of failure process and characteristics in circular tunnels under different stress states and internal unloading conditions

In this study, we investigated the failure process and evolution mechanisms in circular tunnels under a complex stress path of deep in-situ stress + excavation unloading + stress adjustment using a simulation test based on loading first and then drilling (LFTD) mode. This test was performed on cubic sandstone specimens using a true-triaxial testing system combined with a self-developed drilling unloading test device. For comparison, another set of sandstone specimens, similar to the original specimens but with Φ 25 mm circular through-holes, were processed to perform another set of simulation tests based on drilling first and then loading (DFTL) mode. Under constant lateral stress , compared with the DFTL test, the initial failure vertical stress was lower, and the surrounding rock was more prone to failure in the LFTD test, indicating that drilling unloading causes damage to the surrounding rock, thereby inducing a strength-weakening effect. Under constant lateral and vertical stresses, sidewall failure in the LFTD test was more severe. The sidewall's initial failure vertical stress difference between the LFTD and DFTL tests increases with increasing lateral stress or initial hydrostatic pressure . The effect of drilling unloading becomes more apparent under a higher initial stress state. Comparing the same types of simulation tests under different stress states, the initial failure vertical stress increases with the lateral stress or initial hydrostatic pressure, inducing a strength-strengthening effect in the surrounding rock. Comparing the strength-strengthening effect caused by increasing the lateral pressure or initial hydrostatic pressure and the strength-weakening effect caused by drilling unloading shows that the strength-strengthening effect is more evident.

Rock fracturing observation based on microseismic monitoring and borehole imaging: In situ investigation in a large underground cavern under high geostress

During the excavation of deep rock engineering projects under high geostress, the brittle failure of hard rock often occurs. To understand the excavation-induced fracturing process of the surrounding rock of a large underground powerhouse, in situ observation schemes have been proposed, including microseismic (MS) monitoring, imaging with a digital panoramic borehole camera, multipoint extensometer measurements and on-site surrounding rock failure investigation. First, a MS system is used to capture the microfractures inside the rock mass, revealing the potential high-risk failure regions. The spatial distribution of MS events shows that the excavation-induced MS events are progressively clustered in the upstream spandrel and downstream foot of the powerhouse, approximately parallel to the direction of the maximum principal stress. Second, the digital panoramic borehole camera and multipoint extensometer are arranged in the high-risk region to further reflect the rock mass fracture evolution and deformation response characteristics in the monitored section. The borehole camera observations of the top arch and sidewall indicate that the excavation unloading not only causes surrounding rock fracturing near the periphery of the cavern but also induces deep fracturing at a certain depth from the periphery of the cavern. From these comprehensive observation results, the mechanism of rock mass mechanical behaviour is clarified; that is, the macroscopic deformation or failure characteristics of the surrounding rock are directly related to the evolution of microfractures. The research results of this paper are valuable for the stability analysis and excavation optimization of similar underground caverns.

Impact of Brittle Creep Failure on Time-Delayed Characteristics of Rockburst.

In this research, the combination of theoretical approach and numerical simulation was employed to comprehensively understand the initiation mechanism of time-delayed rockburst and analyze the time-delayed failure laws for surrounding rock after excavation unloading without prompt support. The investigations are principally at the angle of time and space, which refers to the creep property and damaged scope for surrounding rock. For the theoretical method, the analytical elastic and elastoplastic models for deep tunnel cross section and the creep model for brittle rock material from a microscopic view were combined. It was found that the time-delayed failure for surrounding rock resulted from the damage accumulation with crack development during the creep process. The surrounding rock with the elastic state was more stable than that in the plastic zone and the creep duration increased with growing distance from the center of tunnel section. Based on the theoretical creep model, the numerical simulation ulteriorly analyzed the brittle creep duration on the key positions. The surrounding rock tended to fail more in the strong excavation damage zone (SEDZ) than that in the weakly damaged zone (WEDZ), and brittle creep failure mainly occurred on the excavation border (EB) in a short space of time. In addition, the increase in the radius for tunnel cross section and the higher in situ stress distribution around the opening led to the acceleration of the creep process for surrounding rock, and the irregular cross-section shape of the tunnel caused the local damaged range extension and decreased the duration for time-delayed failure.

Deep-Seated Deformation Mechanisms of Excavated Slope and Control Methods: A Case Study

High slope stability control is a crucial technical issue in water conservancy and hydropower project construction. This issue is relevant throughout the entire engineering-related survey, design, construction, and operation process. This paper reports a recent case in which significant deep-seated deformation occurred during slope excavation. Several unfavorable structural surfaces were revealed throughout the slope’s undercutting process, particularly fault fz39, which ran diagonally through the entire slope from top to bottom. On the slope surface, damage such as cracking, bulging, and local slippage appeared. The measured value of multipoint displacement meters increased in a stepwise pattern with each blast, and the maximum measured value exceeded 100 mm. The studied slope’s whole excavation and reinforcement process is described in detail. We analyzed the deformation mechanism of the excavation slope by combining the geological conditions, real-time safety monitoring findings, and site deformation characteristics. The slope reinforcement scheme was optimized based on the preceding analyses. We used a three-dimensional numerical calculation program to simulate the slope excavation and unloading process to reproduce the current mechanical state of the slope, evaluate the reinforcement measures, and provide a useful reference for decision making. The analysis shows that the deformation behavior of the studied slope is closely related to fault fz39 and fracture L920, and presents a typical wedge failure mode. The deep-seated deformation of the excavation slope occurs at the junction of the wedge and bedrock, which is mainly affected by unfavorable geological structure surfaces, excavation unloading, and blasting vibration. The reinforcement scheme of unloading at the top of the slope and large-tonnage cables are established. The numerical analysis results showed that the design of the slope reinforcement measures was appropriate and ensured slope stability. The work described in this paper deepens our understanding of the occurrence of large deep-seated deformation on the structural-controlled slope under the action of excavation and improves the design philosophy.

Model Test on Approaching the Construction of Multi-Line Overlapping Shield Tunnels for Up- and Down-Crossing

Multi-line overlapping shield tunnels have a complicated layout in the underground space, and the interaction mechanism between tunnels is very complicated. Based on the construction formation of multi-line overlapped shield tunnel with short-distance up- and down-crossing, indoor model tests were conducted to analyze ground settlement (caused by excavation unloading and stratum loss) and the longitudinal deformation law of existing tunnels based on the drainage method, according to the characteristics and control requirements of the short-distance construction of shield tunnels. The results show that as long as the construction strategy of first-down and then-up crossing is adopted, no sudden change in ground settlement would occur. Furthermore, neither the changes in transient settlements nor the final ground settlement would be too large, which is different from the strategy of first-up and then-down crossing; the first-up and then-down crossing strategy generally increases the curvature of the deformation curve of the existing tunnel, and the deformation of the existing tunnel will cause obvious repeated vibrations. This provides theoretical guidance for the design and construction of multi-line overlapped tunnels.