•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.
When shield tunnelling is in a water-rich sand stratum with poor bearing capacity, instability is easily generated, and even ground collapses may occur. The variation of pore water pressure in a ...water-rich sand stratum during shield tunnelling was analyzed based on a large-scale cross-river shield tunnel in China, which was also investigated by a three-dimensional fluid-solid coupling finite element model. The results show that the influence range of the pore water pressure in front of the excavation face is approximately 2.0 times the excavation diameter and 1.5 times on both sides of the shield. The tunnelling steps would cause obvious variation in the pore water pressure and lead to great disturbance to the surrounding fine sand stratum. The quality of filter cake and the set of support pressure imposes an important impact on the nonlinear variation in the pore pressure, which could cause great disturbance to the stratum. To ensure the safety of the subsequent tunnelling in the fine sand layer, effective treatment should be taken.
A series of problems, including large deformation of supporting structures, shotcrete peeling, and yielding of the steel frame, occurs constantly during tunnel construction in soft clay layers. These ...problems considerably raise the risk of tunnel construction. Thus, the deformation characteristics of soft clay around the tunnel during construction must be investigated to guarantee safety. In this research, a field test was conducted on 50 sections of a tunnel, which was built in high-plasticity clay layers. Then, the deformation characteristics of surrounding rocks and influencing laws caused by burial depth, invasion thickness of soft clay, and atmospheric precipitation were discussed. Results indicated that surrounding rocks are most likely to undergo large deformation during tunnel construction when the burial depth of tunnel ranges from 1.5D to 2.5D (D is the tunnel excavation span). Tunnel deformation also increases rapidly when the invasion thickness exceeds 60% of the tunnel height. The ratio between clay thickness and burial depth of the tunnel is another crucial index that could cause a large tunnel deformation as it exceeds 0.25. In addition, a significant correlation was observed between tunnel deformation and rainfall during the construction period. The deformation of surrounding rocks increases rapidly with rainfall and will continually develop for 1-2 weeks when the average daily rainfall is greater than 4 mm.
Friction resistance usually constitutes one of the two main components for the calculation of required jacking force. This paper provides a new approach to predict the friction resistance of slurry ...pipe jacking. First, the existing prediction equations and their establishment methods and essential hypotheses used were carefully summarized and compared, providing good foundations for the establishment of the new model. It was found that the friction resistance can be uniformly calculated by multiplying an effective friction coefficient and the normal force acting on the external surface of the pipe. This effective friction coefficient is introduced to reflect the effect of contact state of pipe-soil-slurry, highly affected by the effect of lubrication and the interaction of pipe-soil-slurry. The critical quantity of pipe-soil contact angle (or width) involved may be calculated by Persson’s contact model. Then, the equation of normal force was rederived and determined, in which the vertical soil stress should be calculated by Terzaghi’s silo model with parameters proposed by the UK Pipe Jacking Association. Different from the existing prediction models, this new approach has taken into full consideration the effect of lubrication, soil properties (such as internal friction angle, cohesion, and void ratio), and design parameters (such as buried depth, overcut, and pipe diameter). In addition, four field cases and a numerical simulation case with various soils and design parameters were carefully selected to check out the capability of the new model. There was greater satisfaction with the measured data as compared to the existing models and the numerical simulation approach, indicating that the new approach not only has higher accuracy but is also more flexible and has a wider applicability. Finally, the influence of buried depth, overcut, and pipe diameter on the friction resistance and lubrication efficiency were analyzed, and the results can be helpful for the future design.
Block-in-matrix-rocks (bimrocks) are very complicated geological masses that cause many challenging problems during the design and construction of engineering projects, such as parameter ...determination and landsliding. Successful engineering design and construction depends on a suitable constitutive model and reliable design parameters for geological masses. In this paper, the vibration attenuation signal of welded bimrocks was obtained and studied using resonance test technology. Combined with a uniaxial compression test, a constitutive model was proposed to describe the mechanical behavior of welded bimrocks. On this basis, the relations between the dynamic elastic modulus and the physical parameters of bimrocks were established, which included macroscopic mechanical parameters and damage constitutive parameters. Consequently, a new technological process was proposed to provide quick identification of the mechanical properties of welded bimrocks. The results indicate that the dynamic elastic modulus is highly correlated with the rock block proportion (RBP) and uniaxial compression strength (UCS). It is an effective parameter to predict the strength of the bimrocks with high RBPs. Additionally, the proposed constitutive model, which is based on damage theory, can accurately simulate the strain softening behavior of the bimrocks. Combining the resonant frequency technology and the proposed constitutive model, the complete stress strain curve can be obtained in a rapid and accurate manner, which provides a further guarantee of the stability and safety of underground engineering.
Impact loads widely exist in practical engineering and often cause cumulative damage and cracks or even fracture failure of concrete structures with their repeated long-term action. This experimental ...research is conducted on the damage evolution characteristics of concrete under impact loads by regarding C50 nonreinforced concrete as the research object and using a self-developed drop-weight device with electromechanical impedance measurement technology. Results show the following. (1) Under low-energy impact, concrete damage has long continuous development process and remarkable cumulative effects. An apparently sudden break characteristic appears before failure. Under high-energy impact, concrete damage accumulates rapidly, and piezoceramic patch signals grow linearly. (2) The root mean square deviation (RMSD) of the concrete increases exponentially with impact times. Particularly, when the RMSD exceeds 0.075, the concrete damage process enters the rapid development stage and approaches the critical failure state. (3) Under the experimental conditions in this study, the relationship between the ultimate impact times (damage life) and impact heights of the concrete samples shows the development trend of the power function. The above results can provide reference for the research on service life prediction methods of concrete structures under impact loads.
The electromechanical impedance (EMI) technique based on the PZT transducer, in the past twenty years of research progress, has demonstrated its potential for cost-effective and high damage-sensitive ...structural health monitoring. This paper presents an application of EMI technique for damage evolution investigation of initial damaged tunnel invert concrete under high traffic cyclic loading. In the tests, six different levels of initial damage inside the specimens, as the influencing factor, were firstly created by applying a certain number of impacts with a free-falling iron ball. Then, these specimens were applied typical mechanical boundaries similar to that of tunnel invert concrete by an installation specially designed. Finally, the traffic cyclic loading carefully determined was exerted by the MTS815 hydraulic zero-control testing system, to investigate the evolution characteristic of piezoelectric signature of the PZT sensor. The experimental result shows that the conductance signature follows a consistent rightward shifting with the number of cycles increasing, and the larger the initial damage of the specimen, the more obvious the rightward shifting. It indicates that, in general, the traffic cyclic loading is not large enough to make the microcracks propagate to fatigue failure in tunnel invert concrete even if there is a large initial damage. On the contrary, with special mechanical boundaries, the local nominal stiffness of tunnel invert concrete will gradually increase. This phenomenon may be interpreted as an effect of generalized damage recovery. A new damage index, with values between 0 and 1, was then defined to quantify the damage level for quantitative analysis. It was found that the damage recovery behaviors for different initial damaged specimens can be well and uniformly described by an empirical expression, which may be helpful for the damage assessment of tunnel invert concrete under high traffic cyclic loading in the future.
During deep foundation pit construction, the structural clearance intrusion, which is caused by the complex formation conditions and the inefficient drilling equipment, is usually detected due to the ...vertical deviation of piles. To meet construction requirements, pile parts intruding into the structural clearance are supposed to be excised. However, the sectional flexural strength of the pile is bound to decrease with partial excision, which would reduce the bearing capacity of the enclosing structure during construction. In this paper, a theoretical derivation of the normal sectional flexural strength of the partially excised circular pile is proposed. The derivation adopts the assumption of the plane section and steel ring equivalence and can be solved by the bisection method. Furthermore, the calculation method is applied to the pile evaluation of a practical engineering; also, the method is verified by the numerical method. The application results show that the excision of rebar and pile’s sectional area will cause a rapid linear decline in the sectional flexural strength. After excising 18 cm radial thickness of the circular pile (ϕ800 mm) and 6 longitudinal rebars, the sectional flexural strength of the pile decreases to 58% from the origin, which cannot meet the support requirement. The analysis indicates that pile reinforcements must be carried out to maintain the construction safety.
•A detailed FDEM numerical method to simulate mechanical and fracturing responses of heterogeneous geomaterials with irregular inclusions is systematically developed.•A computational geometry method ...named CWSVM is proposed to control mesh quantity and quality.•A signed-distance-field-based discrete element method (SDF-DEM) is employed to approach the natural allocation and orientation of inclusions.•A combined constitutive model is proposed to consider the shearing hardening behaviour for the cohesive elements.•Effects of the interface strength on the mechanical and fracturing behaviours of inclusion-containing geomaterials are extensively discussed.
In this paper, a detailed FDEM approach to simulate the mechanical and fracturing responses of heterogeneous geomaterials with irregular inclusions is systematically developed. The inclusion surface morphology is first obtained through 3D scanning techniques. A computational geometry method, the curvature-weighted sphere Voronoi method (CWSVM), is adopted to control the mesh quantity and quality and ensure the efficiency and accuracy of the FDEM numerical model. A signed-distance-field-based discrete element method (SDF-DEM) is employed to approximate the natural distribution and orientation of inclusions. Heterogeneous geomaterials with large inclusion contents (such as 60% and 70%) are generated effectively and efficiently through this approach. Next, to model the fracturing process, a finite discrete element method (FDEM) model is developed by integrating cohesive elements into the mesh in a fast and efficient manner. In addition, a combined constitutive model is proposed to consider the shear-hardening behaviour of the cohesive elements. The proposed numerical approach is verified through comparison with experimental results, including the shape of inclusions and mechanical responses of geomaterials. The results demonstrate that both satisfactory precision and low calculation costs can be achieved using the proposed algorithm. The consequent simulation performance is verified through comparisons of observations and numerical results with experimental results for failure patterns and mechanical behaviours. In addition, the effects of the strength of the interfaces between the inclusions and matrix on the mechanical and fracturing characteristics of inclusion-containing geomaterials are analysed quantitatively. The mechanical strength decreases rather than increases with increasing content of inclusions when the interface strength is less than the matrix strength.
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