•A hybrid model considering the non-continuity of shield tunnel was established.•Effects of lateral unloading on stress and deformation state of tunnel joints was studied.•A deformation control ...standard for existing tunnel under lateral unloading was built.
Unloading during lateral excavation widely occurs in existing subway shield tunnels. Previous studies have focused on the overall stress and deformation of existing tunnels caused by nearby unloading. However, the stress and deformation state of tunnel segment joints have yet to be considered. This study considered the non-continuity of the shield tunnel lining and the interactions among tunnel segment, surrounding rocks and ballast bed. A hybrid model of a shield tunnel was established based on 3D nonlinear contact theory. The mechanical and deformation properties of the segments and joints of an existing shield tunnel under the influence of lateral excavation of the foundation pits were studied. Unloading during lateral excavation caused the cross section of the shield tunnel to generate vertical convergence and shift horizontally towards the foundation pit. An opening and dislocation in the joint, which caused the waterproof ability of the joint to decrease sharply, were observed. Meanwhile, stress at the segment joint increased sharply and caused local cracks in the segment lining. Axial and shearing force on the joint bolt also increased significantly. Based on existing subway regulations, the calculation results were combined to establish a deformation control standard for existing shield tunnels under lateral excavation. The rate of vertical convergence of the lining should be less than 3.68‰, and the rate of horizontal shift of the axis should be less than 0.53‰.
Squeezing failure of tunnels: A case study Cao, Chengyong; Shi, Chenghua; Lei, Mingfeng ...
Tunnelling and underground space technology,
07/2018, Letnik:
77
Journal Article
Recenzirano
The Laodongshan Tunnel, as a part of Guangtong-Kunming railway in China is one of the most important connections in the whole railway line. The large deformation and failure of primary support ...occurred in the early tunneling stage, and deformation characteristics and failure causes of primary support were obtained by means of field investigation and analysis. There have been many factors that result in the occurrence of squeezing failure of tunnel during construction, such as high in situ stress, soft-weak rock, groundwater seepage, weak support, delay closure of primary support, etc. The lessons to be learned is that for tunnels construction in weak rock mass under high in situ stress, the controlling principle of tunnel deformation ought to be based on combination of stress releasing and support resistance. The support type and method during tunnel excavation needed to be adjusted dynamically based on the pre-excavation geological forecast (e.g. tunnel seismic prediction (TSP), ground penetration radar (GPR), and horizontal pre-boreholes, etc.). Besides, many countermeasures were proposed during construction, including strengthened pre-support, improvement of support stiffness, grouting reinforcement, installing additional supports, closing the tunnel ring early, and timely installation of secondary lining. Finally, from feedbacks of monitoring results, squeezing failure of Laodongshan tunnel was effectively controlled. Thus these countermeasures can guarantee safety construction in weak rock mass under high in situ stress was achieved.
•The similitude criterion of model was derived from similarity theory.•The experimental program was planned in detail.•The mechanical characteristics of rock pressure, structural stress were ...studied.•The failure mechanism of shallow bias tunnel was investigated.
The similitude criterion for the model used in this study was derived from similarity theory and elasticity mechanics equations. An experimental program was designed and used to simulate the excavation of three unsymmetrical loading model tunnels with different bias angles. The change laws and distribution forms of structural stress and surrounding rock pressure, and the failure mechanism of lining and surrounding rock on shallow buried tunnels under unsymmetrical loading were then studied systematically. Results show that the surrounding rock pressure and structural stress both change constantly in the process of tunnel excavation, and the surrounding rock stress releases obviously near the excavation face, which presents a noteworthy biased feature and time–space effect. As bias angle increases, the difference of surrounding rock pressure between the shallow and deep side of tunnel increases, and compared with the Code method, the difference obtained from the model test and field test results are increasing at a faster pace, especially, the vertical surrounding rock pressure. Thus it can be seen that the solution of Code method underrates the bias feature of surrounding rock pressure, causing the tunnel to be underdesigned. The existence of unsymmetrical load transforms the state of structural stress, and the failure form differs in location. Moreover, owing to the effect of bias angle, the development of the morphologic traits of the lining structure should be closely monitored during tunnel construction, so as to take specific measurements. The failure process over the entire construction process can be described as follows: local movement→superficial crack in tensile area of deep side→shear slip develops in deep ground. The failure mode presents as an inverted cone, with the tunnel as the top and the slope as the bottom. And owing to the effect of bias angle, the rupture angle of shallow side is smaller than that calculated by the Code method, whereas the reverse is true for the deep side.
•Shield tunnel lining damage induced by soil-water inrush is studied.•Numerical simulation matches with field results reasonably.•Strength reduction method have been applied for the consideration of ...soil loss.•Lessons learned from this incident have been presented.
This paper reports a case study on shield tunnel lining damage induced by soil-water inrush occurred in Tianjin Metro Line 1, China through both field monitoring and numerical simulation. This incident was triggered by the non-watertight boring work of thru holes adjacent to the cross passage between the twin tunnels. Under high hydraulic gradient, the seepage-prone weak zone was formed and extended, then the outburst of soil-water slurry was occurred. Measures including plugging engineering cotton, injecting quick-setting cement and welding partition plate of steel segments, had been taken but in vain. The outburst of soil-water slurry induced soil movement around the cross passage, thus leading to the damage of tunnel lining and ground surface settlement. After sealing the water ingress holes, stabilization methods including surface grouting and inside tunnel back grouting were applied. The mechanisms of segment lining damage and the effectiveness of stabilization are investigated through both numerical simulation and field monitoring data analysis. Lessons learned from this incident have been discussed, thus providing reference for potential shield tunnelling under similar engineering conditions.
•An improved analytical method for evaluating the responses of shield tunnel due to adjacent excavation is proposed.•The proposed method is verified by three case histories.•The verified method is ...applied to a practical project, construction optimization is also performed.•A series of parametric analyses are conducted.
Adjacent excavations may adversely impact existing shield tunnels. Therefore, evaluating the induced responses of existing tunnels is critical. Previous analytical methods have generally treated shield tunnels as Euler–Bernoulli or Timoshenko beams resting on Winkler or Pasternak foundations to simulate tunnel–soil interactions. However, these methods do not consider excavation–induced disturbance to the surrounding soil and lack sufficient theoretical bases for determining subgrade parameters. This study proposes an improved analytical method that utilizes a Timoshenko beam to simulate a shield tunnel, and that considers the bending and shearing effects on a tunnel. The Vlazov foundation model is introduced to simulate tunnel–soil interactions and to further consider induced disturbance to the surrounding soil. The consistency between the measurements obtained from three case histories and the predictions verifies the proposed method. Then, the verified method is applied to a practical project. Results indicate that the predictions are generally consistent with the in situ measurements. Construction optimizations are also performed to meet the safety requirements and to expedite the construction progress. Finally, parametric analyses are conducted, and their results reveal that the factors, including excavation–tunnel relative distance, ground Young’s modulus and tunnel buried depth, exert considerable influences on the underlying shield tunnel.
The inherent spatial variability of soil properties has a non-negligible effect on the geo-structures system. The quantitative evaluation on the uncertainty of the geo-structure system and its ...influence on the longitudinal responses of a shield tunnel induced by the adjacent excavation are seldom investigated in the past. This work aims to perform the probabilistic evaluations on the excavation-induced tunnel performance using the proposed probabilistic framework. An improved Timoshenko beam-based soil-tunnel interaction model and its finite element solutions incorporating variability of ground stiffness are developed as the deterministic model in the probabilistic framework. A lognormal random field discretized by Karhunen-Loève expansion method is adopted to characterize the spatial variability of subgrade reaction coefficient. The sparse polynomial chaos expansion (SPCE) method is combined with the global sensitivity analysis (GSA) for establishment of an analytical metamodel with a reduced dimension of input variables. Subsequently, with the aid of Monte Carlo simulations, the probabilistic analysis for the tunnel performance is implemented using the SPCE metamodel to reduce the computational effort. The results show that the spatial variability of ground stiffness plays an important role in the longitudinal performance of the shield tunnel suffering from an adjacent excavation. As the spatial soil variability strengthens, the failure probability of the shield tunnel increases obviously. The maximal displacement and minimum curvature radius of the shield tunnel seem to be the robust indicators for assessing the tunnel performance due to the conservative evaluations from that compared with joint deformations.
Notched beam specimens were loaded by the three-point bending test device, and the effects of different volume contents and combinations of steel fibers on the tensile properties of hybrid steel ...fiber-reinforced self-compacting concrete (HSFRSCC) were studied. The failure law and strain field distribution of the specimens were studied by digital image correlation (DIC) technology. Moreover, the curves between the load and crack mouth opening displacement (CMOD) of 18 groups of hybrid steel fiber-reinforced concrete specimens were obtained, and the stress-strain curves of 18 groups of specimens were derived from the load-CMOD curves. The results show that both single and hybrid steel fibers can improve the crack deformation resistance and tensile properties of concrete, but hybrid steel fibers have a more significant improvement effect. Only when the content of steel fiber is more than 0.6% can it have a more obvious postpeak descending section, and hybrid steel fiber has higher postpeak deformation capacity and flexural toughness. The fundamental reason why concrete with hybrid steel fibers has better tensile properties is that micro and macro steel fibers cooperate with each other to resist cracks, improving the toughness of concrete after cracking. Finally, the mechanism of different size and volume content of steel fiber was analyzed from the micro level, which can be used as a reference for the engineering design of HSFRSCC in the future.
•The function mechanism of DLPS is explored.•Deformation transferring path, working state, and failure modes are analyzed.•The mechanical model was established and deformation expressions were ...obtained.•Engineering design principles were proposed.
The Dongping No.1 Tunnel of the Guangdong–Foshan Loop Line Intercity Railway in China passes an existing large-scale underground pipe gallery in water-rich sandy strata through the undercutting method. During construction, controlling the underground pipe gallery deformation is difficult because the water-rich sandy strata can easily become unstable. To deal with this problem, this paper proposes a double layer pre-support system (DLPS) consisting of pipe shed and horizontal jet grouting piles, whose function mechanism is explored from the viewpoint of passive protection and active reinforcement. The deformation transferring path of the DLPS is described as follows: tunnel excavation → lower pre-support deformation → deformation of the soil between the upper and the lower pre-supports → upper pre-support deformation → underground pipe gallery deformation. Considering the influence of geological conditions, the proposed DLPS has two failure modes, namely, excessive settlement of the tunnel vault and excessive deformation of the overlying underground pipe gallery. The mechanical model and analytical method of the DLPS were established according to the theory of beam resting on elastic foundation. Comparing the computed and measured settlement values revealed that the established model is reliable because it could reflect the interaction characteristics of the upper pre-support, the lower pre-support, and the middle soil. Finally, multiple factor analysis was conducted to ascertain the influence of typical calculating parameters, and the deformation characteristics of the DLPS under different influence factors were obtained. The design suggestions for key parameters could provide reference for similar practical engineering in the future.
Although the sealant performance of segment joints has an important influence on the durability of shield tunnels, it is rarely discussed theoretically. In this study, sealant performance tests of ...segment joints were designed and conducted on the basis of the stress and deformation characteristics of segment joints of a metro tunnel. The sealant performance of segment joints was obtained at different joint openings and dislocation deformations. A stress–seepage coupling model for segment joints was established, and the corresponding coupling equation for contact interface was deduced. The relevant parameters of the coupling equation were determined using the test results. The coupling model was used to analyse the influence of joint opening and dislocation on the sealant performance of joints. The opening and dislocation deformation of segment joints could considerably reduce the sealant critical pressure of the joints. Consequently, the sealant capability of the joints significantly declined, and the seepage flow velocity rapidly increased.
The weak interlayer, as a problematic geological body during tunnel construction, greatly influences the propagation of the blasting stress wave, the blasting excavation qualities, and the explosion ...efficiency. A series of numerical models were established to study the changes in the propagation process of blasting stress waves and the failure morphology of the surrounding rock mass, aiming to reveal the weak interlayer’s influence mechanism. The result indicates that the weak interlayer’s existence reduces the propagation velocity and stress peak of the stress wave at barred zones but strengthens the peak stress at reflection zones, which leads to an asymmetrical distribution of rock damage. Furthermore, the type and distribution of the weak interlayer were classified and generalized into four types. The tunnel blasting outlines under different types of weak interlayers are derived through numerical modeling for designing references. A strategy to resist tunnel overbreak and underbreak was proposed combined with previous work. The actual blasting solution is compared to the designed blasting solution with optimised blasting parameters.
