Application of transient Rayleigh wave in detection of tunnel lining void area

Transient Rayleigh wave detection is a high-precision nondestructive detection method.At present,it has been widely used in shallow exploration,but rarely used in tunnel lining quality detection.Through the tunnel lining physical model experiment,the layout defects of the double-layer reinforcement lining area were detected and the Rayleigh wave velocity profile and dispersion curve were analyzed after data process-ing,which finally verified the feasibility and accuracy of Rayleigh wave method in detecting the tunnel lining void area.The results show that the method is not affected by the reinforcement inside the lining,the shallow detection is less disturbed and the accuracy is higher,and the data will fluctuate slightly with the deepening of the detection depth.At the same time,this method responds quite accurately to the thickness of the concrete,allowing for the assessment of the tunnel lining’s lack of compactness.This method has high efficiency,good reliability,and simple data processing,and is suitable for nondestructive detection of internal defects of tun-nel lining structure.

Time-dependent safety of lining structures of circular tunnels in weak rock strata

Following tunnel excavation and lining completion,fractured surrounding rock deforms gradually over time;this results in a time-dependent evolution of the pressure applied to the lining structure by the surrounding rock.Thus,the safety of the tunnel lining in weak strata is strongly correlated with time.In this study,we developed an analytical method for determining the time-dependent pressure in the surrounding rock and lining structure of a circular tunnel under a hydrostatic stress field.Under the proposed method,the stress–strain relationship of the fractured surrounding rock is assumed to conform to that of the Burgers viscoelastic component,and the lining structure is assumed to be an elastomer.Based on these assumptions,the viscoelastic deformation of the surrounding rock,the elastic deformation of the lining structure,and the coordinated deformation between the surrounding rock and lining structure were derived.The proposed analytical method,which employs a time-dependent safety coefficient,was subsequently used to estimate the durability of the lining structure of the Foling Tunnel in China.The derived attenuation curve of the safety coefficient with respect to time can assist engineers in predicting the remaining viable life of the lining structure.Unlike existing analytical methods,the method derived in this study considers the time dependency of the interaction between the surrounding rock and tunnel lining;hence,it is more suitable for the evaluation of lining lifetime.

Seismic performance of tunnel lining of side-by-side and vertically stacked twin-tunnels

The dynamic interaction between tunnel lining and its surrounding soil is a complicated issue as the magnitude of seismic wave from bedrock to the structure can be easily influenced by the geometrical layout and structural stiffness of the tunnel.A series of numerical analysis was conducted to study the dynamic response of the tunnel lining of side-by-side and vertically stacked double-tube tunnel since the inertia and kinematic interactions between the tunnel lining and the surrounding soil during an earthquake could induce excessive stresses to the lining itself due to the stiffness variation between the lining and the soil.Real earthquake ground acceleration was used as an input motion in the dynamic analysis.The interactive behavior of bending moment and axial forces,and the displacement of the tunnels were used to evaluate the effect of tunnel geometrical layout on the performance of the lining.It is found that the effect of earthquake on the axial thrust of the lining is insignificant,and there is a reduction of the bending moment in the lining due to the redistribution of the surrounding soil after the earthquake.

Application of GPR reverse time migration in tunnel lining cavity imaging

Correctly locating the tunnel lining cavity is extremely important tunnel quality inspection.High-accuracy imaging results are hard to obtain because conventional one-way wave migration is greatly aff ected by lateral velocity change and inclination limitation and because the diff racted wave cannot be accurately returned to the real spatial position of the lining cavity.This paper presents a tunnel lining cavity imaging method based on the groundpenetrating radar(GPR)reverse-time migration(RTM)algorithm.The principle of GPR RTM is described in detail using the electromagnetic wave equation.The finite-difference timedomain method is employed to calculate the backward extrapolation electromagnetic fi elds,and the zero-time imaging condition based on the exploding-reflector concept is used to obtain the RTM results.On this basis,the GPR RTM program is compiled and applied to the simulated and observed GPR data of a typical tunnel lining cavity GPR model and a physical lining cavity model.Comparison of RTM and Kirchhoff migration results reveals that the RTM can better converge the diff racted waves of steel bar and cavity to their true position and have higher resolution and better suppress the eff ect of multiple interference and clutter scattering waves.In addition,comparison of RTM results of diff erent degrees of noise shows that RTM has strong anti-interference ability and can be used for the accurate interpretation of radar profi le in a strong interference environment.

An expert system for diagnosing fire-caused damages to reinforced-concrete tunnel lining

During the last four decades, reinforced-concrete structure failures have been happening widely for many reasons, such as increased service loads, war accidents, fire, and durability problems. The economic losses due to those failures are very high. An expert system is an interactive computer-based decision tool that uses both facts and heuristics to solve difficult problems based on knowledge acquired from experts. To realize these requirements, a logic programming visual basic language is used together with visual diagnosis. The expert system, Diagnosis of Fire-Caused Damages to Reinforced-Concrete Tunnel Lining (DFCDRCTL) was developed in this work for diagnosing the annual damages caused by fire. The program is used as an alternative of a human expert to make annual technical decisions in diagnosing fire damages at the second reinforced-concrete tunnel lining segment. It is concluded that the proposed DFCDRCTL expert system is easy to use, and is fast and helpful for engineers.

Semi-analytical solution for internal forces of tunnel lining with multiple longitudinal cracks

Longitudinal cracks on the tunnel lining significantly influence the performance of tunnels in operation.In this study,we propose a semi-analytical method that provides a simple and effective way to calculate the internal forces of tunnel linings with multiple cracks.The semi-analytical solution is obtained using structural analysis considering the flexural rigidity for the cracked longitudinal section of the tunnel lining.Then the proposed solution is verified numerically.Using the proposed method,the influences of the crack depth and the number of cracks on the bending moment and modified crack tip stress are investigated.With the increase in crack depth,the bending moment of lining scetion adjacent to the crack decreases,while the bending moment of lining scetion far away from the crack increases slightly.The more the number of cracks in a tunnel lining,the easier the new cracks initiated.

Dynamic interaction of twin vertically overlapping lined tunnels in an elastic half space subjected to incident plane waves

The scattering of plane harmonic P and SV waves by a pair of vertically overlapping lined tunnels buried in an elastic half space is solved using a semi-analytic indirect boundary integration equation method. Then the effect of the distance between the two tunnels, the stiffness and density of the lining material, and the incident frequency on the seismic response of the tunnels is investigated. Numerical results demonstrate that the dynamic interaction between the twin tunnels cannot be ignored and the lower tunnel has a significant shielding effect on the upper tunnel for high-frequency incident waves, resulting in great decrease of the dynamic hoop stress in the upper tunnel; for the low-frequency incident waves, in contrast, the lower tunnel can lead to amplification effect on the upper tunnel. It also reveals that the frequency-spectrum characteristics of dynamic stress of the lower tunnel are significantly different from those of the upper tunnel. In addition, for incident P waves in low-frequency region, the soft lining tunnels have significant amplification effect on the surface displacement amplitude, which is slightly larger than that of the corresponding single tunnel.

Pressure waves acting on wall of a tunnel and their impact on the tunnel's structural safety

The transient pressures induced by trains passing through a tunnel and their impact on the structural safety of the tunnel lining were numerically analyzed.The results show that the pressure change increases rapidly along the tunnel length,and the maximum value is observed at around 200 m from the entrance,while the maximum pressure amplitude is detected at 250 m from the entrance when two trains meeting in a double-track tunnel.The maximum peak pressure on the tunnel induced by a train passing through a 70 m^(2) single-track tunnel,100 m^(2) double-track tunnel and two trains meeting in the 100 m^(2) double-track tunnel at 350 km/h,are−4544 Pa,−3137 Pa and−5909 Pa,respectively.The aerodynamic pressure induced axial forces acting on the tunnel lining are only 8%,5%and 9%,respectively,of those generated by the earth pressure.It seems that the aerodynamic loads exert little underlying influence on the static strength safety of the tunnel lining providing that the existing cracks and defects are not considered.

Analytical solution of a circular lined tunnel with alterable mechanical property under hydrostatic stress and internal pressure

The influence of the interaction between surrounding rock and lining on the long-term behaviour of a tunnel in service is significant.In this paper,we proposed a mechanical model of the circular lined tunnel with the alterable mechanical property under hydrostatic stress and radially inner surface pressure of the lining.The alterable mechanical properties of the surrounding rock and the lining are embodied by the changing of their elasticity modulus with service time and radial direction of the tunnel,respectively.The proposed mechanical model is successfully validated by comparison with the existing theoretical models and the numerical simulation,respectively.The influences of the main parameters of the proposed mechanical model,such as the radial power-law indexes and the time-varying coefficients of the surrounding rock and the lining,as well as the radially inner surface pressure of the lining,on the interface displacement and pressure between surrounding rock and lining are investigated.The research results can provide some valuable references for timely diagnosis and correct evaluation of the long-term behaviours of a tunnel in service.

Simplified design of rock cavern concrete lining to resist shock loading

A simplified method was proposed for the design of concrete lining in underground rock cavern/tunnel against shock loading. The loading may result from the detonation of explosives on ground surface or ground penetration projectiles exploding adjacent to the cavern/tunnel. The resulting problem necessitates the solution of the dynamics of a beam loaded by a transient pressure uniformly distributed over the span. According to mechanical characteristics of the system with rock bolt and shotcrete, a dynamic support design method based on equivalent single degree of freedom （SDOF） was put forward. The SDOF method was applied to obtaining the maximum displacement at the mid-span of the beam, which is often the controlling factor in the blast-resistant design. In the formulation of the problem, the proposed method combines the phenomena of spalling and structural dynamics theory. An example is provided to demonstrate the applicability of this simplified method.

Analytical modeling of complex contact behavior between rock mass and lining structure

Based on the nondestructive test data of operating railway tunnels in China, this paper summarizes the basic characteristics of the complex contact behavior between the rock mass and lining structure. The contact modes are classified into dense contact, local non-contact, and loose contact. Subsequently, the corresponding mechanical model for each contact mode is developed according to its mechanical characteristics using the complex variable method. In the proposed mechanical model, a special algorithm is introduced to detect whether the local non-contact zone is re-contacted. Besides, a novel conformal mapping method is also proposed to accurately calculate the mechanical response of the concrete lining. Finally, the accuracy of the proposed method is verified by comparing it with the finite element method(FEM). Several parameter investigations are conducted to analyze the effects of different contact modes on the rock-lining interaction. The results show that:(i) the height of the local noncontact area does not have a significant effect on the contact stress distribution if no re-contact occurs;(ii) backfill grouting can reduce the local stress concentration caused by poor contact modes;and(iii) reducing the friction coefficient of the interface can lead to a more uniform distribution of internal forces in the concrete lining.

Rock Pressure on Tunnel with Shallow Depth in Geologically Inclined Bedding Strata

The method to calculate rock pressure to which the lining structure of tunnel with shallow depth is subjected in geologically inclined bedding strata is analyzed and put forward. Both the inclination angle of bedding strata as well as the internal friction angle of bedding plane and its cohesion all exert an influence upon the magnitude of the asymmetric rock pressure applied to tunnel. The feature that rock pressure applied to tunnel structure varies with the incUnation angle of bedding strata is discussed, At last, the safety factor, which is utilized to evaluate the working state of tunnel lining structure, is calculated for both symmetric and asymmetric lining structures. The calculation results elucidate that the asymmetric tunnel structure can be more superior to bear rock pressure in comparison with the symmetric one and should be adopted in engineering as far as possible.

Heteromaterial-gate line tunnel field-effect transistor based on Si/Ge heterojunction

A Si/Ge heterojunction line tunnel field-effect transistor （LTFET） with a symmetric heteromaterial gate is proposed. Compared to single-material-gate LTFETs, the heteromaterial gate LTFET shows an off-state leakage current that is three orders of magnitude lower, and steeper subthreshold characteristics, without degradation in the on-state current. We reveal that these improvements are due to the induced local potential barrier, which arises from the energy-band profile modulation effect. Based on this novel structure, the impacts of the physical parameters of the gap region between the pocket and the drain, including the work-function mismatch between the pocket gate and the gap gate, the type of dopant, and the doping concentration, on the device performance are investigated. Simulation and theoretical calculation results indicate that the gap gate material and n-type doping level in the gap region should be optimized simultaneously to make this region fully depleted for further suppression of the off-state leakage current.

Design and investigation of dopingless double-gate line tunneling transistor: Analog performance, linearity, and harmonic distortion analysis

The tunnel field-effect transistor (TFET) is proposed by using the advantages of dopingless and line-tunneling technology. The line tunneling is created due to the fact that the gate electric field is aligned with the tunneling direction, which dramatically enhances tunneling area and tunneling current. Moreover, the effects of the structure parameters such as the length between top gate and source electrode, the length between top gate and drain electrode, the distance between bottom gate and drain electrode, and the metal position on the on-state current, electric field and energy band are investigated and optimized. In addition, analog/radio-frequency performance and linearity characteristics are studied. All results demonstrate that the proposed device not only enhances the on/of current ratio and reduces the subthreshold swing, but also offers eight times improvement in cut-off frequency and gain band product as compared with the conventional point tunneling dopingless TFET, at the same time;it shows better linearity and small distortions. This proposed device greatly enhances the potential of applications in dopingless TFET.

Elastoplastic Analysis of Circular Tunnel in Saturated Ground Under Different Load Conditions

When a tunnel is excavated below the groundwater table,groundwater flows in through the excavated wall of the tunnel and seepage forces act on it.These forces significantly affect the ground reaction curve,which is defined as the relationship between the internal pressure and radial displacement of the tunnel wall.This study investigates analytical solutions for seepage forces acting on the lining of a circular tunnel under steady-state groundwater flow.Considering the tunnel’s construction or service period and boundary conditions,the direction of maximum principal stress changes,and the input stress of the Mohr-Coulomb criterion varies.The stress distribution and yield range of the surrounding soils and linings are studied.The first,second,and third critical inner pressures are defined and evaluated.The influence of the seepage field on the plastic radius,first critical pressure,and stress distribution of the tunnel is analyzed.It is shown that during the construction period,the seepage force promotes the expansion of the yield area,whereas during the service period,the opposite is the case.The first critical pressure increases nearly linearly with the distant water pressure.The radial stress distribution decreases clearly in comparison with that when the seepage force is not considered,and the reduction is more prominent when internal pressure increases.The tangential stress distribution increases clearly compared with that when the seepage force is not considered.

Stone Columns and Tensioned Anchors to Completely Eliminate Tunnels Trough Settlements

The complex tunnelling constructive environment in urban area in similar green field situations is faced through analytical evaluations in order to control the design calculation process and subsequently manage the interventions techniques with the aim of totally reducing the typical settlements trough above the tunnel either during the construction stage or during the serviceability stage. Recently, the author has proposed an operative and mathematical method by an opportune choice of tensioned anchors to control the tunnel lining settlements. In order to completely eliminate the remainder typical soft soil trough which is normal to the line of the tunnel, it is here proposed to use and properly calculate the interventions of stone columns by the SAVE （silent, advanced, vibration-erasing） Compozer method, in combination with the anchorages.

Adhesion performance of alkali-activated material for 3-dimensional printing of tunnel linings at different temperatures

Robotic-based technologies such as automated spraying or extrusion-based 3-dimensional(3D)concrete printing can be used to build tunnel linings,aiming at reducing labor and mitigating the associated safety issues,especially in the high-geothermal environment.Extrusion-based 3D concrete printing(3DCP)has additional advantages over automated sprayings,such as improved surface quality and no rebound.However,the effect of different temperatures on the adhesion performance of 3D-printed materials for tunnel linings has not been investigated.This study developed several alkali-activated slag mixtures with different activator modulus ratios to avoid the excessive use of Portland cement and enhance sustainability of 3D printable materials.The thermal responses of the mixtures at different temperatures of 20 and 40℃ were studied.The adhesion strength of the alkali-activated material was evaluated for both early and later ages.Furthermore,the structural evolution of the material exposed to different temperatures was measured.This was followed by microstructure characterization.Results indicate that elevated temperatures accelerate material reactions,resulting in improved early-age adhesion performance.Moreover,higher temperatures contribute to the development of a denser microstructure and enhanced mechanical strength in the hardened stage,particularly in mixtures with higher silicate content.

Pixel-level crack segmentation of tunnel lining segments based on an encoder–decoder network

Regular detection and repair for lining cracks are necessary to guarantee the safety and stability of tunnels.The development of computer vision has greatly promoted structural health monitoring.This study proposes a novel encoder–decoder structure,CrackRecNet,for semantic segmentation of lining segment cracks by integrating improved VGG-19 into the U-Net architecture.An image acquisition equipment is designed based on a camera,3-dimensional printing(3DP)bracket and two laser rangefinders.A tunnel concrete structure crack(TCSC)image data set,containing images collected from a double-shield tunnel boring machines(TBM)tunnel in China,was established.Through data preprocessing operations,such as brightness adjustment,pixel resolution adjustment,flipping,splitting and annotation,2880 image samples with pixel resolution of 448×448 were prepared.The model was implemented by Pytorch in PyCharm processed with 4 NVIDIA TITAN V GPUs.In the experiments,the proposed CrackRecNet showed better prediction performance than U-Net,TernausNet,and ResU-Net.This paper also discusses GPU parallel acceleration effect and the crack maximum width quantification.

Interface failure of segmental tunnel lining strengthened with steel plates based on fracture mechanics

Segmental tunnel lining strengthened with steel plates is widely used worldwide to provide a permanent strengthening method.Most existing studies assume an ideal steel-concrete interface,ignoring discontinuous deformation characteristics,making it difficult to accurately analyze the strengthened structure’s failure mechanism.In this study,interfacial fracture mechanics of composite material was applied to the segmental tunnel lining strengthened with steel plates,and a numerical three-dimensional solid nonlinear model of the lining structure was established,combining the extended finite element method with a cohesive-zone model to account for the discontinuous deformation characteristics of the interface.The results accurately describe the crack propagation process,and are verified by full-scale testing.Next,dynamic simulations based on the calibrated model were conducted to analyze the sliding failure and cracking of the steel-concrete interface.Lastly,detailed location of the interface bonding failure are further verified by model test.The results show that,the cracking failure and bond failure of the interface are the decisive factors determining the instability and failure of the strengthened structure.The proposed numerical analysis is a major step forward in revealing the interface failure mechanism of strengthened composite material structures.

Characteristics of the crack tip field in high-speed railway tunnel linings under train-induced aerodynamic shockwaves

High-speed railway tunnels in various countries have continuously reported accidents of vault falling concrete blocks.Once the concrete block falling occurs,serious consequences follow,and traffic safety may be endangered.The aerodynamic shockwave evolves from the initial compression wave may be an important inducement causing the tunnel lining cracks to grow and form falling concrete blocks.A joint calculation framework is established based on ANSYS Fluent,ABAQUS,and FRANC3D for calculating the crack tip field under the aerodynamic shockwave.The intensification effect of aerodynamic shockwaves in the crack is revealed,and the evolution characteristics of the crack tip field and the influence factors of stress intensity factor(SIF)are analyzed.Results show that(1)the aerodynamic shockwave intensifies after entering the crack,resulting in more significant pressure in the crack than the input pressure.The maximum pressure of the inclined and longitudinal cracks is higher than the corresponding values of the circumferential crack,respectively.(2)The maximum SIF of the circumferential,inclined,and longitudinal crack appears at 0.5,0.68,and 0.78 times the crack front length.The maximum SIF of the circumferential crack is higher than that of the inclined and longitudinal crack.The possibility of crack growth of the circumferential crack is the highest under aerodynamic shockwaves.(3)The influence of train speed on the SIF of the circumferential crack is more than 40%.When the train speed,crack depth,and crack length change,the change of pressure in the crack is the direct cause of the change of SIF.