Tunnel Span Research Articles

Stability Analysis of Surrounding Rock and Initial Support of Tunnel Undercrossing Multi-Situational Goafs: A Reference of Construction Guidance

To ensure the construction and operational safety of tunnel undercrossing multi-situational goafs, the Huaying Mountain High-Speed Rail Tunnel, a critical section of the Xi’an-Chongqing High-Speed Railway, was taken as a case study. Based on a three-dimensional finite difference numerical simulation platform, twelve situations were established to analyze the effects of three factors: distance, scale, and angle. The stability analysis was conducted by examining the displacement and deformation characteristics of the surrounding rock, stress changes, and axial forces of the initial support for each situation. The results show that in tunnel undercrossing multi-situational goafs, the vertical deformation, horizontal convergence of the surrounding rock, and the maximum axial force of initial support are all affected. Within a certain range, changes in distance significantly impact subsidence and settlement deformation of the surrounding rock. However, as the distance increases, the horizontal and vertical displacements of the tunnel and the axial force of the initial support tend to decrease. Conversely, the scale and angle of the goaf have an opposite effect on the surrounding rock: as the scale and angle increase, the stability of the surrounding rock deteriorates. In this case study, when the distance exceeds 1.13 times the tunnel span, the influence of the goaf on the stability of the surrounding rock gradually decreases. When the angle exceeds 45°, vertical displacement decreases, and the increasing trend of horizontal displacement gradually diminishes. The conclusions of this paper can provide guidance for designing reinforcement schemes for tunnels crossing through multi-situational goafs. The findings provide valuable insights and guidance for similar engineering projects.

Model Test on the Collapse Evolution Law of Tunnel Excavation in Composite Strata with a Cavity

More complex geological conditions could be encountered with the construction of urban subway projects. At present, many subway tunnels have been built in composite strata with upper soft and lower hard layers, but the presence of a cavity in the strata increases the risk of collapse during construction. In this paper, a series of model experiments and discrete element methods were conducted to investigate the failure behavior of composite strata with a cavity caused by tunnel excavation disturbance. The influence of the distance between the cavity and vault (hd) and the distance between the soil–rock interface and vault (hr) on the collapse of the composite strata are analyzed. The research results indicate that tunnel collapse exhibits progressive failure because of the forming of a collapsed arch in the strata. If the hd is greater than the tunnel span (D), the arch can be stabilized without other disturbances. Additionally, the thickness of the tunnel rock layer affects the height of the collapsed arch significantly, as it is difficult to form a stable arch when the hr is less than 2/3 D. Finally, reasonable construction safety distances are proposed based on the possibility of forming a stable arch collapse in the tunnel and determining the range of the collapse.

Model tests and numerical modeling of the failure behavior of composite strata caused by tunneling under pipeline leakage conditions

Pipeline leakage is a frequent phenomenon in subway construction and has contributed to numerous underground collapses. In this study, the failure behavior of composite strata (i.e., an upper soft soil layer and a lower hard rock layer) caused by tunnel excavation under seepage conditions was investigated via indoor model tests and finite difference methods. It was found that the thickness of the hard overburden above a tunnel considerably influenced the stability of the tunnel. When the overburden thickness was less than half of the tunnel span (D), an increase in the seepage zone could induce the formation of sinkholes after tunnel excavation. When the overburden thickness was greater than 1D, a stable collapse arch was formed after tunnel excavation. Thus, strata between a tunnel overburden and a leakage zone should be reinforced to improve excavation stability.

Research on evolution characteristics of multi-information and stability of goaf under deep tunnel

In order to ensure the safety of construction and operation of the tunnel near the goaf. Based on dynamic numerical analysis, three-dimensional finite difference numerical simulation software was developed by FISH programming. Then, on basis of the construction of practical engineering, the safety distance and energy evolution mechanism in an underlying mining goaf area of deep tunnel are studied. The results show that according to the different safe distance in an underlying mining goaf area of deep tunnel, the tunnel structure is divided into danger zone, influence zone and safe zone. The relationship between the cumulative horizontal energy and stress ratio can be divided into three stages: acceleration, average stability and deceleration. The energy evolution of the three stages can also better reflect the damage and failure mechanism inside the surrounding rock mass. The measured field curve shows that when the safety distance is 12 m, the goaf has limited influence on the stability of the tunnel, and the construction in an underlying mining goaf area of deep tunnel is stable on the whole and meets the safety requirements. The best safe distance between the tunnel and goaf is about one time of the tunnel span.

A Study on the Formation Mechanism and Calculation Method of Surrounding Rock Pressure in Shallow-buried Loess Tunnel Considering the Influence of Vertical Joints

Vertical joints have significant effects on the stability of shallow-buried loess tunnel. Due to the presence of vertical joints distributed widely in loess, the formation mechanisms of surrounding rock pressure on shallow-buried loess tunnel and its computational method are far different from those in other types of soil tunnel. In this study, the distribution characteristics of surrounding rock pressure in shallow-buried loess tunnel were investigated, and discrete element software 3DEC was used to analyze the formation mechanism and development process of surrounding rock pressure in loess tunnel under the influence of joints. We correlated the stratum deformation characteristic with the frictional force caused by the mutual dislocation between joints, and revised the semi-empirical method (Chinese Code methods) in China. The results show that the measured data of radial surrounding rock pressure of shallow-buried loess tunnel are generally between 0–600 kPa, and 80% in the range of 0–200 kPa. The surrounding rock pressure increases with the burial depth, while the distribution of lateral pressure coefficient is relatively discrete. The existence of loess vertical joints (LVJs) aggravates the uneven distribution of the vertical pressure above the tunnel, and the reduction of the lateral earth pressure coefficient at rest under the influence of joints leads to an increase in pressure near the arch shoulder, these two constitute the essence of the joints action on the surrounding rock pressure. The surrounding rock pressure of the shallow-buried loess tunnel reaches the peak stress at about 0.5D from the tunnel face (D is the tunnel span), which is about 1.04–1.32 times of the initial stress. After the excavation of the upper step, the stress of the tunnel crown monitoring point decreased to about 60% of the initial stress. Compared with ignoring the influence of joints, the time of reaching the peak stress at the arch shoulder monitoring point is earlier, and the final stress release degree is smaller. The comparison of the analytical results (proposed method) with the numerical results and the field data exhibits good agreement, proving the proposed method's correctness.

Influence of Underground Excavation Expansion on Surrounding Rock Characteristics at Intersection of Ventilation Shaft and Tunnel: A Case Study

Existing railways can no longer meet transportation requirements, and it is an urgent need to expand old tunnels. However, the existence of ventilations shaft makes expansions face greater risks. This study analyzed the tangential stress change trend during the expansion process through field monitoring, and numerical simulation was used to analyze the changes in stress and displacement under different shaft depths and width–span ratios. The results show that as one approaches the tunnel face, the tangential stress in the arch foot and side wall of the J-2 and J-3 sections gradually increased, and the tangential stress in the arch foot and side wall of the J-1 section gradually decreased. The distance of the tunnel expansion’s influence on tangential stress is about 0.91 to 1.45 times the tunnel span. The largest value of vertical displacement had a linear relationship with shaft depth, and the largest value of horizontal displacement had a quadratic relationship with shaft depth. Changes in the width–span ratio only had a greater impact on the ventilation shaft section. These results can provide a reference for similar in situ expansion projects.

Amplification effect of near-field ground motion around deep tunnels based on finite fracturing seismic source model

Dynamic failure of rock masses around deep tunnels, such as fault-slip rockburst and seismic-induced collapse, can pose a significant threat to tunnel construction safety. One of the most significant factors that control the accuracy of its risk assessment is the estimation of the ground motion around a tunnel caused by seismicity events. In general, the characteristic parameters of ground motion are estimated in terms of empirical scaling laws. However, these scaling laws make it difficult to accurately estimate the near-field ground motion parameters because the roles of control factors, such as tunnel geometry, damage zone distribution, and seismic source parameters, are not considered. For this, the finite fracturing seismic source model (FFSSM) proposed in this study is used to simulate the near-field ground motion characteristics around deep tunnels. Then, the amplification effects of ground motion caused by the interaction between seismic waves and deep tunnels and corresponding control factors are studied. The control effects of four factors on the near-field ground motion amplification effect are analyzed, including the main seismic source wavelength, tunnel span, tunnel shape, and range of damage zones. An empirical formula for the maximum amplification factor ( α m ) of the near-field ground motion around deep tunnels is proposed, which consists of four control factors, i.e. the wavelength control factor ( F λ ), tunnel span factor ( F D ), tunnel shape factor ( F s ) and excavation damage factor ( F d ). This empirical formula provides an easy approach for accurately estimating the ground motion parameters in seismicity-prone regimes and the rock support design of deep tunnels under dynamic loads.

Deformation and failure of overburden soil subjected to normal fault dislocation and its impact on tunnel

In this study, a large-scaled model test was conducted to explore the deformation and failure mechanism of tunnel with fault rupture. The strain evolution, ground fissure and displacement, earth pressure and tunnel deformation are collected in the test. Then, tunnel mechanical behavior under the fault dislocation is revealed based on the test result. The results show that a prismoid-like 3D shear zone appeared within fault fracture zone. The tunnel experiences an elongated ‘S’-shaped deformation along longitudinal axis of tunnel and the displacement changes of sectional linings within the shear zone are more significant. In transversely, the linings exhibit an oval horse-shoe deformation pattern. Three types of failure mode defined as Type Ⅰ, Type Ⅱ and Type III, corresponding to slight damage, moderate damage and severe damage, are summarized transversely in this paper. Under normal fault dislocation, the tunnel within the shear zone experienced a combination failure of bending moment and shear force. But compression failure of linings far from the fault would occur with longitudinal cracks on tunnel subjected to normal fault. Taking fault width (Wf) and tunnel span (St) into consideration, a reasonable tunnel fortification length was proposed with maximum of 2.25Wf and 3.5St under a specific fault dip of 60°. In the model test, when the fault dislocation reaches 10 mm, a slight crack begins to appear on arch foot of lining and then dislocation of joint occurs. Thus, corresponding to a prototype tunnel, a safe vertical displacement to ensure no damage to tunnel should be 34.6 cm when a segmentally designed tunnel is subjected to normal fault dislocation.

Study on the Influence of the Blasting of Preceding Tunnel on the Pre-Penetration Surrounding Rock in Succeeding Tunnel with Small Clear Distance

To study the influence of the preceding tunnel blasting on the surrounding rock in the pre-penetration area of the succeeding tunnel, combined with the actual monitoring of the Kaifeng Mountain small clear distance tunnel, the relationship between the vibration velocity and the blasting center distance was derived according to the energy attenuation law, predicted the peak velocity of particle vibration on the pre-penetration surrounding rock, came up with the conception of surrounding rock seismic wave vibration velocity ratio, obtained its change situation, and then established a numerical model to further analyze the changes in vibration velocity, plastic strain, and stress. The results show the prediction formula of peak vibration velocity obtained from theoretical analysis of vibration velocity, which can well calculate the peak vibration velocity of some unmeasurable points in tunnel engineering and provide theoretical support for actual tunnel blasting engineering. Under the same clear distance conditions, the front blasting side of surrounding rock in the pre-penetration area of the succeeding tunnel is affected by blasting vibration more than the back blasting side, and the vibration velocity decreases gradually from the arch waist⟶arch shoulder⟶arch foot⟶vault (arch bottom), and the most unfavorable section is that the blasting surface is parallel to the surrounding rock section in the pre-penetration area of the preceding tunnel. The excavation of the lower step has greater destructive force than that of the upper step; with the clear distance that gradually increases, the impact of the preceding tunnel blasting on the surrounding rock in the pre-penetration area of the succeeding tunnel becomes smaller, and D (D is the tunnel span) is the minimum safe clear distance.

Investigation of the support constraint effect and failure instability law of tunnels constructed using the New Austrian tunneling method

The application of a reasonable numerical calculation method is the key to accurately analyzing tunnel rock-support interactions. In this paper, we address the support constraint effect of tunnels and analyze the influence of related factors based on the confinement convergence method. Rupturable support models are developed using FLAC3D to intuitively show the numerical calculation results of tunnels. The results imply that the virtual supporting force generated by the support constraint effect should be considered in two-dimensional rock tunnel model calculations, and that the supporting force of the support should be increased by 2–3% of the maximum supporting force. Boundary effects should be considered in the three-dimensional tunnel model calculations, in which the influence range of the model boundary effect is nearly 1.5 times the tunnel span. A comparison of the field monitoring data and numerical calculations of the Baoshan tunnel project shows that the numerical results that consider the support constraint effect are in better agreement with the actual project situation. The rupturable support models can also reflect the stress and failure evolution law of supports, and provide support for the accurate evaluation of tunnel engineering stability.

Force-Chain-Based Analysis of Factors Influencing Pressure Arch Around Tunnel Under Earthquake

The force chain method is a powerful tool to analyze the pressure arch of a tunnel, which has important influence on the seismic stability of tunnel structures. The force distribution around the tunnel before and after an earthquake was observed with numerical simulation and shaking table test, and then some parameters related to the pressure arch around the tunnel were analyzed. The discrete element method software PFC was used to numerically simulate the variation of shape and height of pressure arch with tunnel depth before and after an earthquake. The effect of tunnel depth, span, section shape, lining stiffness, seismic spectrum, and seismic amplitude on the shape and height of pressure arch was numerically simulated. The numerical results show that the tunnel depth, span, section shape, and seismic amplitude have great effect on the pressure arch shape, while the lining stiffness and seismic spectrum has little effect on the pressure arch shape. In addition, the shaking table tests were conducted to observe the pressure arch shape before and after an earthquake, and the experimental results were applied to verify the numerical simulation results.

Analytical Solutions for the Mechanical Responses of Shallow Double-Arched Tunnel Subjected to Symmetric Loads

Double-arched tunnel is a special kind of tunnel structure which is widely adopted for shallow rock tunnels. The internal forces within a double-arched tunnel are difficult to be determined due to its special geometry and complex interactions with surrounding rock masses. This paper presents a set of analytical solutions for determining the internal forces within the lining structure of shallow double-arched tunnel subjected to symmetric external loads. The double-arched tunnel is decomposed into three main parts, i.e., the arch rings, the side walls, and the middle wall. The force method and the elastic foundation beam model are employed for the arch rings and the side walls, respectively, while the middle wall is treated as a cantilever beam fixed at the bottom. Analytical solutions for the internal forces of the three parts are derived separately, which are continuous at the conjunctions of different parts. The derived analytical solutions are verified by comparing with the FEM simulation results. Finally, parametric studies are performed to investigate the influences of burial depth, elastic resistance coefficient, opening angle, and tunnel span on the internal forces based on which some recommendations are provided for the construction and design of the double-arched tunnels. The derived analytical solutions provide fast estimations for the internal forces and deformation of the double-arched lining structures, which will be a useful tool for design optimization.

Quantitative evaluation of the constraint effect and stability of tunnel lining support

The surrounding rock stress release rate in a two-dimensional (2D) rock tunnel model and boundary problems in a three-dimensional (3D) rock tunnel model are the main factors that affect the results of modeling rock tunnels. We investigated the constraint effect of supporting structures and boundary effects of the 3D model based on the confinement convergence method. A rupturable lining unit was developed using FLAC3D to study the influence of the constraint effect on the surrounding rock stress release and influence range of the boundary effects. The results show that the 2D model stress release enhances the virtual support force generated by the support constraint effect by 3–5% of the initial rock stress. A comparison of field monitoring and numerical calculations of the Millin tunnel project shows that the 3D model boundary effect is nearly 1.5 times the tunnel span. The rupturable lining unit directly reflects the stress characteristics of concrete lining in practical engineering and provides support for the quantitative evaluation of tunnel engineering stability.

Deformation behavior analysis of tunnels opened in various rock mass grades conditions in China

The [BQ] method is a rock mass classification method to evaluate the quality of the rock mass and determine the construction parameters. This method is more empirical and cannot provide predictions for the deformation of tunnels after excavation. To predict the surrounding rock deformation of deep-buried tunnels by using the [BQ] method in China, first, data of 52 tunnels were collected and analyzed to determine the relationship between the grades of the surrounding rock, excavation method, burial depth, tunnel span, and surrounding rock deformation. Second, the equivalence of different surrounding rock grades to the range of geological strength index (GSI) scores were determined using methods, such as fitting GSI to another classification system RMR and RMR to BQ, and considering the correction factors of BQ values. This approach provides the basis for theoretical calculations based on the Hoek–Brown strength criterion. On the basis of the Hoek–Brown strength criterion, a theoretical approach to the deformation of surrounding rock under three failure models, namely, elastic–brittle–plastic, strain-softening, and elastic-perfectly-plastic, is presented when considering the installation time of primary support and the volumetric force of bolts. Finally, the theoretical approach is analyzed and compared with the measured data to verify its feasibility. Moreover, the effects of burial depth, grades of surrounding rock, support parameters, support time, and deformation allowance of the surrounding rock are analyzed. Analysis results can provide some guidance for the prediction of surrounding rock deformation of deep-buried tunnels in China.

Engineering geological characterisation of rock masses for design of excavation method and support system of Tanju Tunnel, West Nusa Tenggara, Indonesia

This paper presents experiences with design and construction process of the Tanju Tunnel, West Nusa Tenggara. The objective of this research was to characterize the volcanic and intrusive rock masses for designs of the tunnel excavation method and support system. Engineering geological investigations were carried out, upon which the tunnel stand-up time was estimated and the tunnel excavation method and support system were determined based on the rock mass classifications of RMR (Bieniawski, 1989). The investigation results showed that the tunnel construction area consisted of colluvium, andesite, and tuff breccia. The rock masses were dominated by the andesite, which had UCS of intact rock ranging from 200 to 300 MPa classified as strong intact rock. Based on the RMR value, the andesite was classified as a good quality rock mass. The tunnel had unsupported roof stand-up time approximately 20.000 hours for a 3 m tunnel span. The recommended tunnel excavation method was full face, 1.0-1.5 m advance, and complete support 20 m from face, while the support systems were locally bolt in crown 3 m long and 2.5 m spacing, with occasional wire mesh, 50 mm thick shotcrete in crown where required.

Support pressure assessment for deep buried railway tunnels using BQ-index

Estimation of support pressure is extremely important to the support system design and the construction safety of tunnels. At present, there are many methods for the estimation of support pressure based on different rock mass classification systems, such as Q system, GSI system and RMR system. However, various rock mass classification systems are based on different tunnel geologic conditions in various regions. Therefore, each rock mass classification system has a certain regionality. In China, the BQ-Inex (BQ system) has been widely used in the field of rock engineering ever since its development. Unfortunately, there is still no estimation method of support pressure with BQ-index as parameters. Based on the field test data from 54 tunnels in China, a new empirical method considering BQ-Inex, tunnel span and rock weight is proposed to estimate the support pressure using multiple nonlinear regression analysis methods. And then the significance and necessity of support pressure estimation method for the safety of tunnel construction in China is explained through the comparison and analysis with the existing internationally widely used support pressure estimation methods of RMR system, Q system and GSI system. Finally, the empirical method of estimating the support pressure based on BQ-index was applied to designing the support system in the China’s high-speed railway tunnel—Zhengwan high-speed railway and the rationality of this method has been verified through the data of field test.

Calculation of surrounding rock pressure of large section underground excavation metro station based on the construction process

The traditional calculation method of the surrounding strata pressure of the tunnel does not consider the tunnel construction process. The excavation of the large-section tunnel mostly adopts the section excavation method, which is to excavate and support at the same time. The stress state of the pressure of surrounding strata changes gradually during the excavation process. The traditional calculation results of tunnel surrounding strata pressure are generally large. In this paper, the large-section tunnel is simplified into three parts, and the degree of influence (η1, η2, η3) of the excavation height h and width b of the advance pilot hole and the distance d between the advance and rear pilot tunnels on the surrounding rock pressure calculation are analyzed. The regression equation for calculating the influencing factors (tunnel depth H, tunnel span B, internal friction angle ϕ, and rock weight γ) of surrounding rock pressure (p) is established. Through the numerical analysis, the influence coefficient η is obtained, the weight coefficient is introduced, and the computing formula of the pressure of surrounding strata of the large-section tunnel excavation is derived. The formula is used to the computing of the pressure of surrounding strata with a large-section subway station and the correct conclusion is obtained.

Assessment of the arching effect and the role of rock bolting for underground excavations in rock masses—a new numerical approach

In very jointed rock masses, the ground arch developed around the underground opening is an arch-shaped structure to maintain the excavation stability, and the rock bolting has been attributed as one of the dominant approaches to enhance the arching effect. Through 2D finite element method (FEM), numerical studies were performed to investigate the arching effect and the importance of rock bolting. Based on the distribution of the mean stress σm, the boundary of the ground arch is determined without several limitations existed in other methods and the artificial arching effect of rock bolting could be assessed directly. Impacts of the cover depth Ht, the tunnel span D, the in situ stress ratio k, and the rock mass strength (frictional strength ϕ) were first revealed. The enhancement provided by the rock bolting were then directly evaluated through the artificial ground arch. The formation of the artificial arch is carefully studied considering the bolt spacing and the bolt length. Results show that σm could be used as a reliable indicator to assess the arching effect. Rock bolting reinforces the surrounding rock mass and contributes to the formation of the sub-arch therein. With enough reinforcement, the sub-arch overlaps each other and a global artificial arch could form to enhance the arching effect.

Numerical Parametric Study of Countermeasures to Alleviate the Tunnel Excavation Effects on an Existing Tunnel in a Shallow-Buried Environment near a Slope

This paper studies the influence law of existing tunnels on the construction of intersecting new tunnels in a shallow slope burial context through 3D numerical analysis. The emphasis is on exploring the effect of new tunnels constructed in 54 conditions, including three ratios of overburden to tunnel height (C/H), three ratios of slope distance to tunnel span (D/W), two backfilling conditions of the existing tunnel (“hty” and “htn” conditions), and three magnitudes of surface loads (10 kPa, 20 kPa, and 30 kPa), on the deformation of lateral slopes and the overlying road. As the results show, the rigidly separated area between the existing and newly built tunnels in parallel to the excavation direction was precisely the sensitive area affected by the existing tunnel backfilling condition. The road settlement simulations perpendicular to the excavation direction revealed that various C/H and D/W ratio combinations controlled the shape and size differences of the settlement trough curve. This was because the C/H ratio primarily controlled the effective span and height transition of the newly built tunnel, whereas the D/W ratio mainly controlled the intersection position of the tunnels. Next, model A-A (“hty” condition) was identified as the only feasible construction model among all models in accordance with the engineering safety control criteria. Lastly, comparison of monitoring data with simulations found a slight difference in the distribution pattern between the two. Nevertheless, the final maximum settlement fully satisfied the construction control requirements overall. Aside from proving the correctness of simulation results, the present study also sets an excellent referential example for similar projects.

A Numerical Study on Parameters Affecting Seismic Behavior of Cut and Cover Tunnel

Underground structures and facilities constitute the underlying and infrastructural part of modern urbanization and are used for a wide range of applications, including urban underground railroad and urban road tunnels, silos etc. Underground structure’s construction method depends on such factors as the ground conditions, the ground water conditions, the tunnel span and the buried depth. The cut and cover method is an engineering technique for underground construction with low buried depth in urban areas. Furthermore, some urban underground facilities have been constructed in areas with risk of seismic activity which be resistant to both static and seismic loads. Therefore, seismic response of such shallow tunnels which are usually constructed by cut and cover method is of great importance and is investigated in this paper. Numerical analyses were performed on a shallow tunnel subjected to S-wave by using FLAC software, and the effects of different parameters are investigated. The maximum forces and bending moments in the tunnel lining during the earthquake motion for two earthquakes i.e., Tabas and Gilroy were compared and it was found that at all levels of maximum acceleration of input motion, seismic forces on tunnel structures during the Tabas earthquake were more critical which is related to the Arias intensity of the input motion. Increasing the buried depth of shallow tunnel and the enlargement of the tunnel span lead to more seismic forces on tunnel structures. Furthermore, with the increasing rigidity of the tunnel to the surrounding soil, additional dynamic forces in the tunnel lining will be more, however the stresses in the tunnel lining are reduced.