Adjacent Tunnel Research Articles

Deformation analysis and safety assessment of existing metro tunnels affected by excavation of a foundation pit

The excavation of a foundation pit considerably affects the adjacent structures and underground pipelines owing to the change in the stress state of the surrounding soil, resulting in deformation. The study of an actual engineering case was conducted to examine the influence of excavation on the deformation of adjacent subway tunnels. The finite element analysis software PLAXIS 3D was used to simulate the entire excavation process. The structural design of the foundation pit was optimized based on the simulation results to ensure the stability of the foundation pit and the safety of the existing subway tunnel structure. Finally, the safety evaluation of the excavation of the foundation pit that caused the deformation of the adjacent subway tunnel was performed. The influence of the excavation and unloading of the foundation pit on the subway tunnel is closely related to the distance between the subway and the foundation pit, the amount of earthwork excavated at one time, and the engineering geological conditions. The results of this paper can provide useful reference for the design optimization and safety assessment of similar projects.

Numerical analysis on rock blasting damage in Xiluodu underground powerhouse using an improved constitutive model

Damage effect on surrounding rock is unavoidable during blasting excavation of deep underground caverns. In order to accurately analyse the damage degree of surrounding rock caused by explosive load under high ground stress conditions, an improved damage constitutive model of rock mass is proposed based on dynamic finite element numerical simulation software, and the continuous change processes of the deterioration of rock mass material parameters and damage threshold under blasting dynamic disturbance are studied. Based on the monitoring data of blasting excavation vibration of underground powerhouse in Xiluodu Hydropower Station, the field measured data and numerical simulation results of several different constitutive models are compared, and the effects of different constitutive models and explosive loads on vibration propagation attenuation law of surrounding rock of this tunnel and adjacent tunnel are analysed. The results show that the calculated results of the improved damage constitutive model are closer to the field measured values. The damage depth of surrounding rock is mainly affected by the diameter of cartridge under the same charge quantity, and the change of the hole spacing will only affect the damage range of surrounding rock.

Influence of Tunnel Boring Machine (TBM) Advance on Adjacent Tunnel during Ultra-Rapid Underground Pass (URUP) Tunneling: A Case Study and Numerical Investigation

This study investigates the influence of subsequent tunnel boring machine (TBM)-driven processes on the responses of the first tunnel in twin-tunnel construction using the ultra-rapid underground pass (URUP) method. A comprehensive finite element analysis (FEA) is performed to simulate the URUP TBM tunneling, considering the non-uniform convergence caused by the TBM geometry, the tunnel face supporting pressure, and the tail-grouting pressure. The FEA model is validated by the monitoring results of the bending element of the first tunnel lining. The FEA results reveal that the grouting pressure of the second tunnel has significant influence on lining deformation of the first tunnel, while the face supporting pressure shows little effect. The relationship between the grouting pressure and the maximum bending moment of adjacent first tunnel can be fitted by linear function. A grouting pressure equals to the lateral earth pressure is able the reduce the variation of the bending element of the first tunnel during the TBM-driven process of the second tunnel. The bending element of the first tunnel shows a typical lognormal relationship with the face supporting pressure during the TBM advance of the second tunnel. A critical cover-to-depth ratio, under which the horizontal and vertical soil arching effect vanishes, can be deduced to be within the range of 0.55–0.60.

Estimation of Rock Load of Multi-Arch Tunnel with Cracks Using Stress Variable Method

In multi-arch tunnels, the increased rock load on the concrete lining of the main tunnel and side walls due to the excavation of adjacent tunnels is critical and must be considered in the design stage. Therefore, this study estimates the rock load of a multi-arch tunnel using two-dimensional numerical analysis, considering rock mass classifications, overburden, and construction steps. The rock load is estimated using two criteria: the factor of safety and stress variable. The rock load is underestimated when the factor of safety is applied to rock mass class III. However, the stress variable method reveals a reasonable rock load as overburden increases. Particularly, the rock load is estimated to be equal to the overburden in shallow tunnels and approximately 0.7 times the tunnel width in deep tunnels. Additionally, the crack-induced rock load is computed using back analysis at the excavation completion stage of adjacent tunnels, yielding the relation between the rock load height and the deformation modulus of the rock mass. Therefore, an accurate estimation of the rock load of multi-arch tunnels emphasizes the importance of a more economical and realistic design and must be addressed in the process of performance-based tunnel design.

Dynamic response of existing tunnel under cylindrical unloading wave

When an underground tunnel is excavated suddenly in the high-stress rock mass, the unloading wave will generate from the tunnel boundary and propagate into the surrounding rock mass, which can induce strong dynamic vibrations and even damage to the adjacent existing tunnel. This study aims to investigate the dynamic response of existing tunnel induced by cylindrical unloading wave resulting from neighboring tunnel excavation. A two-dimensional mathematical physical model was first constructed based on the superposition principle of coupled static and dynamic loads, in order to derive the formulas for calculating the dynamic stress, velocity and displacement around existing tunnel induced by cylindrical unloading wave. A two-dimensional numerical model established by particle flow code (PFC) was then implemented to verify the theoretical analysis, and further to investigate the failure characteristics of existing tunnel under coupled static in situ stress and dynamic unloading wave. A series of parametric studies were conducted in the theoretical and numerical models to investigate the influence of unloading rate, unloading path, in situ stress and excavation radius on the dynamic response of existing tunnel. The results show that the unloading wave induced by tunnel excavation in high-stress rock mass can induce strong dynamic response in the surrounding rock mass of adjacent existing tunnel, and the increase of stress level and unloading rate will aggravate the dynamic effect. The peak value of velocity induced by nonlinear unloading is larger than that induced by linear unloading. In addition, the excavation-induced unloading wave can induce dynamic failures in the surrounding rock mass of existing tunnel under high in situ stress. Therefore, the unloading wave induced by adjacent tunnel excavation should be taken into consideration when evaluating the stability of deep tunnels.

Longitudinal responses of shield tunnel subjected to surcharge considering dislocation

Surcharge due to improper soil dumping and irregular backfilling often causes excessive deformation along the adjacent shallow tunnel in soft-soil areas. Tunnel structures are usually modelled with beam elements where the bending of the concrete segments is considered while the shearing dislocation between segmental rings is usually ignored. In this work, a Timoshenko beam was used to consider such dislocation of a shield tunnel under surcharge and a Pasternak foundation was used to account for the tunnel–soil shear effect. A numerical method was then developed to assess the effect of surcharge loading on the longitudinal responses of the underlying tunnel based on a two-stage method. The proposed solutions were verified using a case history from Shanghai and a three-dimensional numerical simulation from the literature. Results of a further parametric study showed that the deformation induced by surcharge loadings would be underestimated without the shearing-induced dislocation of tunnel segments, which could overpredict the serviceability of the underlying tunnel in surcharge accidents.

Study on the influence of supertall building load on adjacent tunnel subsidence

Abstract. With the rapid growth of economy in coastal megacities, the construction of supertall buildings with deep pile foundations adjacent to tunnels in soft soil is inevitable. The additional subsidence of tunnel and long-term subsidence of soft soil was appeared caused by the supertall building load transferred through pile foundation. In this paper, a typical case in Shanghai where deep piles of supertall building group adjacent to a metro tunnel was selected, and the long-term monitoring data of tunnel deformation was collected and analysed. The layered subsidence of surrounding soft soil was analysed by using the monitoring data of extensometers in a land subsidence monitoring station near the study area. The results of data analysis showed that the construction of supertall building had a significant impact on the adjacent tunnel subsidence. Moreover, with the increase of the load transferred from building structure to pile foundation, the adjacent tunnel appeared sustained uneven subsidence. And the subsidence of tunnel in the study area mainly depended on the deformation of underlying soil.

A simplified method for estimating the longitudinal and circumferential behaviors of the shield-driven tunnel adjacent to a braced excavation

The construction of soil excavations will inevitably have an adverse effect on the adjacent tunnels. However, the structural responses of existing shield-driven tunnels adjacent to a braced excavation and their serviceability are still not fully understood. This paper presents a simplified method for evaluating the longitudinal and circumferential behaviors of the shield-driven tunnel adjacent to a braced excavation. To capture both shear and bending deformation of segmental linings, an appropriate model was adopted to study the longitudinal tunnel deformation features based on the Timoshenko beam theory. A continuous ring structure under a plane strain condition was used to determine the circumferential behavior of segmental lining. Case history was used to verify the developed simplified method. Systematic parametric studies were performed to develop a better understanding of the governing factors of the tunnel behaviors, including excavation-tunnel relative position and the retaining wall deflection. Furthermore, the serviceability limit state of shield tunnels was quantified using the defined three factors of safety, which enables a quick assessment of the excavation-induced tunnel damage potential. The contours of factor of safety can also be employed to determine the envelope of the excavation influence zones.

Hybrid Simulation of Seismic Responses of a Typical Station with a Reinforced Concrete Column

During the 1995 Kobe earthquake, damages were observed in the Daikai subway station and adjacent tunnels. It was the first large-scale underground structure that failed under the earthquake excitation. Numerical and experimental analyses have been conducted to study the failure process of the Daikai station. However, the issue of the scale ratio still exists in the shaking table tests of underground structures. In order to tackle this issue, a hybrid simulation technique is developed here to study the seismic performance of a typical subway station. Based on the previous research, it is found that the central column is the critical component of the structure. Therefore, a reinforced concrete central column is physically tested in the hybrid simulation process. On the other hand, the remaining parts of the structure and soil domain are numerically modeled at the same time. Four hybrid simulation cases are conducted with peak ground accelerations of 0.01 g, 0.1 g, 0.22 g, and 0.58 g. The test results of displacement and shear force are compared with the analytical results. Moreover, the good agreement between the test results and numerical results validate the accuracy of the proposed hybrid test method. After the hybrid simulation process, a quasi-static test is conducted to illustrate the mechanical properties of the central column after the earthquake excitation.

Prediction of the tunnel displacement induced by laterally adjacent excavations using multivariate adaptive regression splines

Excavations may cause excessive ground movements, resulting in potential damage to laterally adjacent tunnels. The aim of this investigation is to present a simple assessment technique using a multivariate adaptive regression splines (MARS) model, which can map the nonlinear interactions between the influencing factors and the maximum horizontal deformation of tunnels. A high-quality case history in Tianjin, China, is presented to illustrate the effect of excavation on the tunnel deformation and to validate the FEM. The hypothetical data produced by the FEM provide a basis for developing the proposed MARS model. Based on the proposed model, the independent and coupled effects of the input variables (i.e. the normalized buried depth of tunnels Ht/He, the normalized horizontal distance between tunnels and retaining structures Lt/He, and the maximum horizontal displacement of retaining structures, δRmax) on the tunnel response are analysed. The prediction precision and accuracy of the MARS model are validated via the artificial data and the collected case histories.

Dynamic response of rock-soil dam to blasting-induced vibration with different distances

In order to assess the dynamic performance of a rock-soil damn Zhejiang, Eastern China and suggest a proper blasting scheme for the adjacent tunnel excavation, a three-dimensional finite difference model was established, and dynamic simulation analyses were carried out. Results show that close blasting will have severe impact on the stability of the dam. The different part of the dam has different response to the blasting shock. The velocity response at the side close to the blasting is far larger than that of the other side. Local damage may occur in the dam body near the blasting point. The vibration amplitude of the dam due to the blasting decreases with the distance of blasting point to the dam. The peak vibration velocity reduces to less than 0.4 cm/s and the maximum permanent displacement of the dam decreases to no more than 0.1cm when the distance between the blasting point and the dam reaches to 180m. It indicates that the blasting at this distance has little influence on the stability of the dam. So, a safe blasting distance of 180m is suggested in the project.

Study of the Growth Process of Original Crack in the Surrounding Rock of Tunnel under the Adjacent Explosive Load

The surrounding rock damage of a tunnel under adjacent explosive load is often manifested as the growth of the original crack. In order to thoroughly understand the crack growth mechanism, in this study, the growth process of the original crack was investigated in detail by the dynamic caustics experiment. The experimental study shows that the growth of original crack is the result of the combined action of explosion stress wave and explosive gas. The quasi-static stress generated by the explosive gas was superimposed on the weakened stress field, resulting in the formation of the peak of the main stress difference in the surrounding rock, moving towards the adjacent tunnel in the form of an arc wave. With the arc wave moving towards the original crack, the growth rate of the original crack increases rapidly. During the period of 200 μs to 250 μs, the crack growth rate oscillates at the peak, and its size is approximately equal to the moving speed of the arc wave. Based on the experimental results and microscopic damage mechanics, the high stress concentration at the original crack tip under the stress wave first causes damage localization and the weakest chain is formed by the penetration of the damage localization zones by microcracks by special distribution law. Subsequently, the original crack starts to initiate and grow along the direction of the weakest chain.

3D Discrete Element Method Modelling of Tunnel Construction Impact on an Adjacent Tunnel

Closely-spaced side-by-side mechanized twin tunnels are commonly used in urban metro transit systems. The aim of this paper was to investigate the influence of tunnel construction on an adjacent existing tunnel using the 3D discrete element method (DEM) and propose a new method to protect the existing tunnel. The first tunnel at the left side was initially included in the DEM model. The construction of the second adjacent tunnel at the right side was then modelled in phases. Displacement, stress variation, porosity variation, and coordination of surroundings of the first tunnel were investigated. The results showed that the excavation of the second tunnel had a great negative impact on the existing tunnel surroundings when the distance of twin tunnels was less than the diameter of the tunnel. The maximum horizontal displacement occurred at the angle of 45° and was equal to 103 mm. The influenced area of the existing tunnel is the surroundings between the tunnel crown and the spring line on the second tunnel side near the tunnel lining. The composite reinforcement method consists of pasted fiber reinforced plastic (FRP) sheet method and grouting in a specific area; thus, the structure safety and the normal operation of the existing tunnel nearby can be guaranteed. The protective method can give reference for similar project.

Study on the behaviour of pre-existing single piles to adjacent shield tunnelling by considering the changes in the tunnel face pressures and the locations of the pile tips

In the current work, a series of three-dimensional finite element analyses have been conducted to investigate the behaviour of pre-existing single piles in response to adjacent tunnelling by considering the tunnel face pressures and the relative locations of the pile tips with respect to the tunnel. Via numerical modelling, the effect of the face pressures on the pile behaviour has been analysed. In addition, the analyses have concentrated on the ground settlements, the pile head settlements and the shear stress transfer mechanism at the pile-soil interface. The settlements of the pile directly above the tunnel crown (with a vertical distance between the pile tip and the tunnel crown of 0.25D, where D is the tunnel diameter) with a face pressure of 50% of the in situ horizontal soil stress at the tunnel springline decreased by approximately 38% compared to the corresponding pile settlements with the minimum face pressure, namely, 25% of the in situ horizontal soil stress at the tunnel springline. Furthermore, the smaller the face pressure is, the larger the tunnelling-induced ground movements, the axial pile forces and the interface shear stresses. The ground settlements and the pile settlements were heavily affected by the face pressures and the positions of the pile tip with respect to the tunnel. When the piles were inside the tunnel influence zone, tensile forces were induced on piles, while compressive pile forces were expected to develop for piles that are outside the influence zone and on the boundary. In addition, the computed results have been compared with relevant previous studies that were reported in the literature. The behaviour of the piles that is triggered by adjacent tunnelling has been extensively examined and analysed by considering the several key features in substantial detail.

Study on Deformation Law of Deep Foundation Pit with the Top‐Down Method and Its Influence on Adjacent Subway Tunnel

Based on a long‐term comprehensive instrumentation program, the performance of an excavation pit constructed by the top‐down method in downtown Kunming was extensively examined. The measured excavation responses included the deflections of diaphragm wall, vertical column movement, slab axial force, building settlement caused by ground deformation, and the influence of pit excavation on the adjacent subway tunnel. This paper analyses the monitoring data of the existing construction stage. Based on the analyses on the data of field and numerical simulation, the following major findings were obtained: (1) the relationship between the measured maximum wall deflection, δhm, and excavation depth, H, in this study is δhm = 0.06%H ~ 0.27%H, which is quite different compared with the relationship of soft‐soil pit δhm = 0.02%H ~ 1.2%H, but closer to the normalized curve of rock‐socketed pile δhm = 0.01%H ~ 0.45%H and rock‐socketed diaphragm wall δhm = 0.031%H ~ 0.129%H. (2) The relationship between the maximum settlement of column (δp) and excavation depth (H) is δp = −0.09%∼0.04%H. The maximum distortion between the diaphragm wall and the column is less than 1/500 of the limit range proposed by Bjerrum. (3) The impact range caused by excavation is about 3.8 times the maximum excavation depth. The ground settlement around the foundation pit is groove type, and the maximum settlement point is located at 2.7 times the maximum excavation depth. (4) The excavation of the foundation pit leads to the maximum vertical settlement of 2 mm and maximum horizontal displacement of 5.2 mm in the subway tunnel; the maximum change of axial force and bending moment are 8.8 kN (the vertical direction) and 6.4 kN·m/m (the horizontal direction), respectively.

Study on Ventilation System Linkage Control Strategy in a Double‐Hole Tunnel Fire

In order to prevent smoke from flowing backward and penetrating into adjacent tunnel through cross channels during evacuation in case of fire accidents in highway tunnels, it is extremely necessary to implement the linkage control scheme for double‐hole tunnel ventilation system. Therefore, six linkage control schemes for ventilation system are proposed in this article. Firstly, these six schemes were numerically simulated through Fire Dynamics Simulator (FDS) to obtain the flow velocity, flow direction, and smoke distribution under different schemes when the peak power of fire accidents is 30 MW. And two better schemes were selected for further investigation based on the evacuation conditions in the upper stream of fire source. Secondly, a 1 : 10 scale model experiment was conducted to test these two schemes, and the results from numerical simulation and model experiment were compared and verified. Finally, the location of fire accident, the time to develop stable opposite airflow, and the number of open cross channels were analyzed to confirm the applicability of the recommended scheme. The results show that the optimal scheme is to switch on the fans in the upstream of the fire accident and the fans on both sides of the cross channel in the non‐fire tunnel to form positive pressure to control the smoke flow. This scheme ensures a stable and uniform smoke flow in the cross channel and personnel’s safety during evacuation.

Three-dimension numerical analysis of large-span bridges’ pile foundation adjacent to an existing tunnel

In the process of urban transportation construction, it is inevitable that the new bridges will be adjacent to the existing tunnels. According to the spatial relationship between the new bridge and the existing tunnel, a three-dimension finite differential model of the pile-tunnel interaction is established by a numerical simulation method. The influence of the upper load of the new bridge on the adjacent tunnel is simulated, with the results of the numerical simulation method and the Boussinesq method are compared. It is found that the results of numerical simulation are verified by the Boussinesq method, with the deviation less than 10%. Moreover, the horizontal deformation of the tunnel is central-symmetrically distributed, and the symmetrical point of the horizontal deformation and the position of maximum vertical deformation are both near the midpoint of the two cushion caps. Besides, the deformation of the tunnel is mainly vertical. Last but not least, the influence of the construction and operation of the pile foundation of the bridge on the rails in the existing tunnel meets the control criteria. Therefore, the methods of analysis and parameters in this paper can provide a reference for similar projects.

Investigation of divergence tunnel excavation according to horizontal offsets between tunnels

In most cases in urban areas, construction of divergence tunnel should take into account proximity to existing tunnel in operation. This inevitably leads to deformation of adjacent structures and surrounding ground. Preceding researches mainly dealt with reinforcing of the diverging section for the stability including the pillar. This has limitations in investigating the interactive effects between existing structures and surrounding ground due to the excavation of the divergence tunnel. In this study, the complex interactive behavior of pile, the operating tunnel, and the surrounding ground according to horizontal offsets between the two adjacent tunnels was quantitatively analyzed based on conditions diverged from operating tunnel in urban areas. The effects on ground structures confirmed by analyzing the ground surface settlements, pile settlements, and the axial forces of the pile. The axial forces of lining in operating tunnel investigated to estimate their impact on existing tunnel. In addition, in order to identify the deformation of the surrounding ground, the close range photogrammetry applied to the laboratory model test for confirming the underground displacements. Two-dimensional finite element numerical analysis was also performed and compared with the results. It identified that the impact of excavating a divergence tunnel decreased as the horizontal offset increased. In particular, when the horizontal offset was larger than 1.0D (D is the diameter of operating tunnel), the impact on existing structures further reduced and the deformation of surrounding ground was concentrated at the top of the divergence tunnel.

Finite Element Analysis of the Influence of Deep Foundation Pit Excavation Construction on Adjacent Subway Tunnel Structure

Foundation pit excavation construction is a widespread underground engineering activity in urban construction. More and more foundation pit projects are adjacent to existing subway tunnels. Through the engineering example, the thesis analyzes the influence of deep foundation pit construction supported by double-row piles on the structural safety of adjacent subway tunnels. Firstly, the risks of the project are analyzed, and then the structural safety control standard is determined according to similar engineering and related specifications. On this basis, the finite element numerical simulation software is used to calculate and analyze the deformation and force of the subway tunnel structure during the foundation pit construction, and the safety impact of the subway tunnel structure during the foundation pit construction is evaluated.

Numerical Analysis on the Behavior of Existing Tunnels Subjected to the Undercrossed Shield Tunneling at a Small Proximity

Shield excavation in a proximity to existing tunnels inevitably causes deformation and stress change of the adjacent tunnels. Based on the project of new metro line in Beijing where the shield tunnel was constructed under existing tunnel, a modified equivalent stiffness model for the existing tunnel is proposed. Based on this, a three‐dimensional finite element is established, and subsequently the proposed modified equivalent stiffness model and the previous model are implemented into the finite element model (FEM), respectively, and then the effectiveness of it is verified through comparison with the field measurements. The results indicate that the modified equivalent stiffness model has excellent agreement with the monitoring value. The impacts of key parameters including undercrossing angle, vertical spacing, and soil parameters on the crown settlement of the existing tunnel are investigated. The maximum settlement of the existing tunnel crown decreases as the undercrossing angle increases. The decease of vertical spacing between the newly constructed tunnel and the existing tunnel leads to an increase of the peak value of crown settlement. As for the soil parameters, friction angle φ is the dominant parameter affecting crown settlement of the existing tunnel whereas the cohesion c is least influential. During the process of route selection, it is suggested to keep the vertical spacing between new tunnel and the existing tunnel greater than the external diameter of the new metro tunnel. For the situation of undercrossing an existing tunnel within an extremely small clearance, it is recommended to strengthen the soil as necessary.