Shield Construction Research Articles

Ground Settlement of High-Permeability Sand Layer Induced by Shield Tunneling: A Case Study under the Guidance of DBN

During the shield construction of Harbin Subway Line No. 3 Project, the average ground settlement exceeds the warning value. In order to find the cause of settlement and improve it, this paper establishes a settlement prediction model to analyse the potential influencing factors based on the Deep Belief Network (DBN) and calculated the correlation degree between influencing factors and settlement through sensitivity analysis. It was found that the permeability coefficients of layer and cutter head torque are the main factors affecting settlement. Then, corresponding muck improvement measures were made according to the analysis of the layer conditions, which successfully reduced the permeability and shear strength of the muck, thereby controlling the surface settlement value within the warning range. The research results in this paper illustrate the applicability and robustness of DBN in tunnel engineering, and the related research ideas can be applied to other projects.

The life-cycle development and cause analysis of large diameter shield tunnel convergence in soft soil area

The tunnel convergence is an important index of the construction quality and operation safety. In the past, most of researches were focused on the shield tunnel lateral convergence and its effect on the safety at operation period. In this paper, by employing the 3D laser scanning test method and convergence ellipse fitting identification technology, the shield tunnel full space convergence can be tested and divided into three typical modes: transverse ellipse, oblique ellipse and vertical ellipse. Based on series of field tests in three large-diameter tunnels under construction and operation, the spatial distribution and life-cycle development laws of above three typical convergence modes were firstly seriously studied, the changes of the long axis, short axis and deflection angle of above convergence modes are given. Meanwhile, the generation mechanism and main cause is studied by analysis of machine-structure interaction, soil pressure and back-fill grouting action during shield construction. It is found, in the soft soil areas, the formation of different convergence modes mainly depends on the initial load condition at the stage of segments leaving the shield tail, and the following action of back-fill grouting will have little effect on the structure convergence, but most on the surrounding soil deformation. Subsequently, the convergence will continue growth with the short axis deformation greater than the long axis, and gradually develop towards symmetry with soil consolidation. Besides that, the obvious coupling relationship between the convergence and the longitudinal settlement can also be found after long-term operation from the tests. The above findings are very helpful to fully understanding the stress and deformation interaction during the large diameter shield tunnel construction and operation.

Study on Influence of Shield Side-piercing Construction on Pile Foundation of Nearby High-speed Railway Bridge

Taking the pile foundation of Suzhou Metro Line 3 passing through viaduct as the engineering background, the driving process of shield tunnel is simulated by finite element software. Through the analysis of surface settlement, bridge pier deformation and bridge pile foundation deformation in shield construction, the following conclusions are drawn: The foundation reinforcement “scheme of root pile + sleeve valve pipe grouting” has an obvious effect on reducing the ground settlement, the deformation of bridge piers and the deformation of pile foundation structure. The deformation and stress of the pile foundation of the bridge and the horizontal displacement of the pier above it are related to its distance from the tunnel, and the nearer it is, the greater its influence is. Whether the foundation is strengthened or not has little influence on the change law of axial force of bridge pile foundation, and the maximum axial force appears at the inflection point of horizontal displacement. Due to the excavation of shield tunnel, the bending moment of pile foundation changes in three sections along the depth direction, and the change of bending moment in the middle section is opposite to that on both sides.

Lattice Boltzmann simulation for grout filling process during simultaneous backfill grouting of shield in tunnel construction

Over the years, it has become obvious that simultaneous backfill grouting is of great significance to the design of the tunnel lining and process control of shield construction. It is essential to know the grout pressure distribution around the tunnel lining. The purpose of this study is to seek an alternative method to simulate the grout diffusion process and obtain pressure distribution of different time during simultaneous backfill grouting of tunnel construction. A lattice Boltzmann method (LBM) based free surface flow model is introduced and modified, and a Smagorinsky large-eddy simulation model serves to deal with the small-scale turbulent problem, which is a challenge for LBM to simulate the large-scale real-world engineering. The simulation results obtain the grout filling state with time and pressure distribution around the tunnel lining. Additionally, the grout pressures on site are measured at a ring in line 8 of Chengdu Rail tunnel, and the measurement results are also compared with ones taken from numerical simulation, it is showed that the numerical results are consistent with that of measurement on the whole. Through simulating different grouting strategies, it is concluded that for four-holes grouting schemes, the grouting pressures of upper holes are significant for the grout pressure distribution and should be paid most attention in construction.

Influence and evaluation of saturated soil shield construction on existing highway

Abstract In order to study the shield construction under an existing highway, the initial displacement and the excess pore water pressure solution from Biot consolidation equation were used to derive the analytical solutions of the vertical deformation and pore water pressure of saturated soil caused by the frontal friction and side friction of the cutterhead. In addition, by introducing the layered method and combining it with other theoretical analytical equations, the expressions of total vertical deformation and total pore water pressure caused by tunnel excavation with different overlying materials were obtained. The simulation results on an engineering project showed that the angle α between the direction of the road and the propulsion axis of the shield had a significant influence on the surface settlement of the road. When the angle α was increased, the settlement curve had higher variation; but the variation would not exceed the maximum settlement value above the shield axis. When the road was perpendicular to the shield axis, a critical point of the road bulging and settlement was formed above the incision. Due to the grinding resistance of the cutterhead, the pore water pressure under the roadbed was distributed asymmetrically on both sides of the shield axis.

A remediation technology for arsenic contaminated soil slurry generated by slurry shield construction process

A remediation technology for arsenic contaminated soil slurry generated by slurry shield construction process

Experimental study on damage evolution characteristics of segment structure of shield tunnel with cracks based on acoustic emission information

Cracks are constantly observed in segments due to adverse geological conditions and loads during shield construction. However, crack propagation can have a negative impact on the stress deformation of the segment structures in the long run. Revealing the characteristics of energy release during crack propagation can effectively reflect the damage evolution process. Based on an investigation of segment cracks in a subway tunnel project, this paper conducts an in-depth analysis of AE data of segments through similar model tests, and explores the influence of cracks of different lengths, quantities and positions on the damage evolution of segments of shield tunnels. The results show that cracks reduce the overall stiffness of the structure when compared with that of a segment ring with no prepared cracks, and the elastic bearing range increases slightly, while the plastic bearing range decreases obviously. With the increase of the length and number of prepared cracks, the number of ladders of AE events decreases and the height of ladders increases, and the number of AE events of segments increases continuously at the critical point of instability. The damage of segments appears earlier, and the development process of the damage and is relatively gentle if cracks are located at the arch waist. By contrast, the damage of segments is more serious if cracks are located at the arch roof and arch bottom. The results provide theoretical basis for design optimization, disease analysis and evaluation of shield tunnel.

A static risk assessment model for underwater shield tunnel construction

The shield method is widely used in underwater tunnel construction. However, the shield construction process faces many uncertain risk factors that increase the level of safety risk during shield tunnel construction. Therefore, risk assessment has become a necessary task in the early stages of tunnel construction. In this study, a new risk assessment model for underwater shield tunnel construction is proposed that combines a normal cloud model with an entropy weight method. The model contains 20 assessment indexes and gives the range of each index corresponding to different risk levels, which comprehensively reflects the influencing factors of risk. By using a normal cloud model, the fuzziness and randomness of risk assessment data are taken into account effectively, and the reliability of assessment results is increased. Based on an entropy weight method, the rules and characteristics of the evaluation data in the model are considered, and the weight coefficient of the evaluation index is determined to avoid the subjectivity of the expert weight method. The risk assessment model is applied using the Xiangjiang shield tunnel as an example. The results show that the overall construction risk level of the Xiangjiang tunnel is level II, which is consistent with the site risk situation and shows that the model can objectively and accurately evaluate the construction risk level of an underwater shield tunnel.

Influence of shield tunnel construction on ground surface settlement under the condition of upper-soft and lower-hard composite strata

Taking the shield tunnel project of Guangzhou Metro Line 8 from Tongdewei Station to Shangbu Station as the research background, using the research method of finite element simulation and site monitoring, this paper analyses the influence rules of shield tunneling on ground subsidence under the condition of different hard rock height ratios. The research results show that in the process of crossing different hard rock height ratio composite stratum, as the hard rock height ratio decreases, the value of ground settlement decreases and settlement tank becomes shallow. The surface subsidence in different hard rock height ratio strata is obviously different, and the maximum difference is about 8.6 mm; The influence of the hard rock height ratio on the surface longitudinal settlement is mainly reflected in the position change of the beginning and the end of the settlement. With the increase of the hard rock height ratio, the shield construction reduces the amount of the surface longitudinal settlement and its influence range; Through the research, it is found that the hard rock height ratio in the 0-0.2 and 0.5-1 is the sensitive interval, and the settlement value in these two hard rock height ratio interval varies greatly. It is necessary to pay attention to the uneven settlement of the ground surface caused by shield construction in the sensitive hard rock height ratio interval. The research results of this paper can provide reference value for similar shield construction in upper soft and lower hard composite stratum.

Study on the Ultimate Supporting Force of Shield Excavation Face Based on Anisotropic Strength Theory

The determination of the ultimate supporting force of the shield excavation face is an important problem to be solved in shield construction. Considering that the tunnel burial depth ratio has a significant effect on the instability mode of the excavation face, the classic “wedge-prism” limit equilibrium model is improved. Based on the rotation effect of principal stress axis, the Casagrande anisotropic strength equation is introduced into the modified limit equilibrium model of “wedge-prism”, and then the limit equilibrium solution of the ultimate supporting force of shield excavation face in anisotropic soil is deduced. Finally, the influence of each calculation parameter on the ultimate supporting force is analyzed by examples. The research results show that the results of the modified “wedge-prism” calculation model proposed in this paper are slightly larger than those of the centrifugal test. If the influence of the instability mode of excavation face and the anisotropy of soil strength on ultimate supporting force of the shield excavation face is not taken into account, the calculation result will be unsafe. The limit supporting force of shield tunnel excavation surface has a simple linear relationship with the anisotropy ratio. When the anisotropy ratio is greater than 1, the ultimate supporting force of shield excavation face decreases first and then tends to be stable with an increase in the buried depth ratio. When the anisotropy ratio is less than 1, the law is reversed. The more obvious the anisotropy of soil strength, the greater the rate of change of ultimate supporting force. The limit supporting force of the shield excavation face decreases linearly with the exertion of loosening earth pressure, linearly decreases with the increase in soil cohesion, and decreases nonlinearly with the increase in the angle of internal friction in soil. The relevant conclusions will provide theoretical guidance for controlling the reasonable chamber pressure of shield tunneling, and ensure the safety of construction.

Research on Stress Characteristics of Segment Structure during the Construction of the Large-Diameter Shield Tunnel and Cross-Passage

With the intensive development of China’s high-speed railway network and intercity railway network, the construction of the large-diameter shield tunnels and cross-passages is gradually increasing. The construction of large diameter shield tunnels and the excavation of cross-passages puts forward higher requirements for the stability and safety of segment structure. Based on the Wangjing tunnel project, this paper studies the segment displacement and mechanical response of the shield tunnel with a diameter of 10.5 m in the process of shield construction and cross-passage construction. The results show that during the construction of large diameter shield tunnels, the vault and invert produce inward displacement, the invert uplift usually is more severe than the vault settlement, and the arch waist on both sides produces outward displacement. Near the segment K (capping block), the mechanical performance of the segment is close to that of the hinge or chain rod, which can only effectively transmit the axial force but cannot resist the bending moment and shear force. During construction of the cross-passage, the maximum deformation and stress of shield tunnel segment are symmetrically located at the interface of the main tunnel and cross-passage. The upper and lower edges of the segment at the interface tend to change from compression to tension. At the same time, the steel bars on the inside and outside of the segment vault and the arch waist change from compressive stress to tensile stress, which can easily lead to segment damage, so these positions can be reinforced by erecting section steel frames before construction.

Fuzzy Comprehensive Evaluation of Subway Shield Construction Risk Based on WSR and Combined Weighting

Aiming at the problem of risk evaluation of subway shield construction, a fuzzy comprehensive evaluation method based on WSR and combined weighting is proposed. Firstly, using the WSR methodology to analyze and select the risk indicators of the subway shield system from the three dimensions of “Wuli - Shili - Renli”, a risk assessment index system for subway shield construction was constructed. Then, fuzzy theory is introduced to establish a comprehensive evaluation model of subway shield construction based on combined weighting. Through the multiplication synthesis method, the weight values of the analytic hierarchy process and the entropy weight method are combined, and the principle of fuzzy relation synthesis is used to comprehensively evaluate the membership level. In the end, the shield tunnel in Qingdao Metro Line X is used as an example to calculate and demonstrate. The results show that the risk is at the “general” level, which is basically consistent with the actual situation. It shows the feasibility and applicability of the established evaluation method, which has guiding significance for reducing the risk of subway shield construction.

Structural integrity assessment of shield tunnel crossing of a Railway Bridge using orthogonal experimental design

This paper focuses on the specific project of shield construction across the existing Wuzhun Railway Bridge between Xuanrendun Station and Dadiwobao Station on Urumqi Metro Line 1. A three-factor and four-level model of the Wuzhun Railway Bridge foundation reinforcement scheme is designed based on the principle of the orthogonal experiment. The calculation of each reinforcement scheme is conducted by using FLAC3D numerical simulation software. The optimal reinforcement scheme of the Wuzhun Railway Bridge foundation through orthogonal design-direct analysis and variance analysis is obtained. The reinforcement of the Wuzhun Railway Bridge is carried out according to the optimal scheme, and the rationality of the reinforcement scheme is verified by comparing and analyzing the numerical simulation data with the monitored data received from the construction site. The research results serve as a guide in the construction of future tunnel shield crossings for the existing railway, and the developed research methodology provides a reference for similar tunnel crossing projects.

A Risk Assessment Method for Metro Shield Tunnel Construction Based on Interval Number

In recent years, metro construction is flourishing in China. In the construction process of the metro shield tunnel, there are various complex potential risk factors fraught with many uncertainties. Therefore, it is crucial to conduct a risk assessment on urban metro shield construction. To improve the accuracy of the risk assessment results of metro construction projects, a risk assessment model based on interval numbers is proposed from the perspective of uncertainty. The experts’ evaluation information is represented by the interval number, and then the operation rule and membership function of the interval number is used to calculate the risk level of each potential risk factor. Afterward, the weight of each evaluation index is calculated based on the interval number and the C-OWA operator. From the bottom of the evaluation system, the layer-by-layer operation is carried out to determine the risk level of the assessment index of each layer, and the risk grade is divided according to the risk matrix. Taking a section of the shield tunnel of Shenzhen Metro Line 12 as an example, the proposed method is used to evaluate the construction risk, and the potential risk factors that require the implementation of risk prevention measures in the project are obtained. The calculation results show that the assessment model is reasonable and reliable, and has high stability.

Investigation of the mechanical behavior of soft clay under combined shield construction and ocean waves

This study aims to investigate the mechanical behavior of soft clay under both shield construction and long-term dynamic wave loading in the practical construction of breakwaters to protect coastal power plants. Continuous rotation of the principal stress axis due to cyclic wave loading is reproduced by altering the control loads (that is, axial load, torque, inner cell pressure, and outer cell pressure) in a hollow cylindrical shear apparatus (HCA). In the HCA, the confining pressure is progressively reduced to replicate the stress release induced by ground loss during shield construction. The HCA test results show that both long-term wave loading and shield construction significantly affected the dynamic stiffness of soft clay. Under the combined effects of wave loading and shield construction, the post-cyclic shear strength and stiffness of the soft clay are reduced more significantly compared to the isolated effect of wave loading, especially at lower confining pressure. The degradation of the dynamic secant modulus, Ed, and the initial tangential modulus, Ei, is indicated to be synchronous at the end of the cyclic loading test, which implies the unique disturbance of the combined action of shield construction and dynamic wave loading on the microstructure of soft clay considered in this study. Based on the experimental observations, a simple and effective constitutive model is also developed to reproduce the degradation of stiffness and shear strength of soft clay induced by shield construction and dynamic wave loading.

Numerical simulation of freezing effect and tool change of shield machine with a frozen cutterhead

A shield machine with freezing function is proposed in order to realize tool change operation at atmospheric pressure. Furthermore, the transformation project of freezing cutterhead and tool change maintenance method are put forward. Taking the shield construction of Huanxi Power Tunnel as an example, a numerical analysis of the freezing cutter head of the project was carried out. The results show that when the brine temperature is −25 °C, after 30 d of freezing, the thickness of the frozen wall can reach 0.67 m and the average temperature drops to −9.9 °C. When the brine temperature is −30 °C, after 50 d of freezing, the thickness of the frozen wall can reach 1.01 m and the average temperature drops to −12.4 °C. If the thickness of the frozen wall is 0.5 m and the average temperature is −10 °C, as the design index of the frozen wall, the brine temperature should be lower than −28 °C to meet the excavation requirements in 30 d. Analyzing the frozen wall stress under 0.5 m thickness and −10 °C average temperature condition, the tensile safety factor and compressive safety factor are both greater than 2 at the most dangerous position, which can meet the tool change requirements for shield construction.

Analysis of Ground Settlement in Metro Tunnel Construction with Shield Tunneling Method

With the continuous development of underground space, subway tunnel construction methods are constantly updated and developed. And shield tunnel method has become one the most common methods of subway tunnel construction. However, shield construction is often accompanied with the occurrence of surface subsidence. There may be a collapse of the surface when the surface settlement reaches a certain amount and the up buildings on the surface may be seriously influenced. The construction model was established based on FLAC3D software by using the engineering geological characteristic parameters of the study area and the observation data of ground subsidence during construction, which was compared and verified with the field vmonitoring value, and the subsidence was simulated and predicted. Using FLAC3D to establish a settlement prediction model for tunnel excavation and record the space-time changes of surface subsidence, it can better reflect the land subsidence situation and law in the horizontal and vertical directions of the axis of the subway tunnel.

Analysis on construction of the subway shield underpassing the underground pedestrian passage at close range

Numerical simulation analysis was conducted on the shield tunneling construction of the right line between Tangdong station and Huangcun station of Guangzhou rail transit line 21, the right line subway shield underpassing the underground pedestrian passage at close range. The numerical simulation vertical settlement of the ground and floor outside the foundation pit was compared with the field monitoring. The results show that the numerical analysis of the trend of land subsidence is consistent with the field monitoring, the thickness of the tunnel roof overburden has a great influence on the ground settlement. The ground settlement is mainly subsidence with thick overburden, while the thin overburden, the ground settlement is mainly uplift; the Equivalent Circular Zone formed by simultaneous grouting and secondary grouting has a great influence on the ground settlement. During the site construction, the tunneling parameters should be adjusted according to the site monitoring, and parameters such as soil and water pressure on the tunneling surface, overburden thickness, synchronous grouting pressure and secondary grouting pressure should be taken into comprehensive consideration to control the tunneling of shield underpassing the existing structures at close range.

Dynamic Risk Analysis in Metro Construction Using Statistical Process Control

Metro construction is normally carried out in complex engineering geological environment, so it can generate various risk events. In the process of metro construction, a scientific risk dynamic analysis is indispensable to reduce and control risks. In order to analyze the risk in metro construction more scientifically and reasonably, in this study, a new risk dynamic analysis method for metro construction is proposed using statistical process control. The method can analyse the risk level according to the process’s capacity index and identify the characteristics of risk variation according to the statistical control chart. The risk level and the characteristic of risks may vary with dynamical updating of monitoring data, so the conclusion of risk evaluation for a time interval can be drawn and corresponding safety measures can be ascertained. The method ushers statistical process control, so the random factors in risk evolution can be considered fully. Then, the method is applied to the risk analysis of shield construction under the Beijing-Tianjin intercity railway in Beijing Metro Line 8, a typical risk problem in the traffic construction. The variation of the risk level and the characteristic of risks can be evaluated reasonably because the dynamical randomness is considered. Moreover, whether risk control measures should be taken and what the effective measures are can be ascertained explicitly.

Numerical Analysis of Stress and Temperature Fields in a Composite Stratum Based on a New Method of Shield Construction for Safety and Environmental Protection.

Safety and environmental protection are key issues in shield construction. Due to wear, the cutter of a shield machine must be changed after a period of excavation. In order to realize the tool change operation of a shield machine at atmospheric pressure, a method of cutter head freezing of the shield machine is described in this paper. The finite element simulation method is used to analyze the construction of a shield machine with a frozen cutter head in a composite stratum. For a composite stratum with uneven hard and soft layers in a ratio of 1:1, the stress and temperature fields are analyzed, and the stress change around the hob is recorded. Through numerical simulation, the change of the temperature field around the shield machine is determined in real time. As time goes on, the temperature around the shield machine decreases, and the frozen range expands. When the temperature field in a specific point reaches a critical value, the temperature at that point will remain constant, and the stress field around the cutter head will also tend to become stable. The isothermal region of the soil presents an annular distribution, and the final temperature tends to be stable and gradually increase as the distance from the frozen cutter head increases. The final temperature of the monitoring area reaches a stable value corresponding to −26.5 °C, the axial depth of the frozen wall is more than 2.5 m, the minimum frozen radius is 3.2 m, the stress distribution around the cutter head is unbalanced, the maximum stress is measured in the hard rock layer, and the stress around the cutter head at the hob level indicates that tool change is necessary. Compared with the traditional method, the construction method of a frozen cutter head is more effective and more environmentally friendly. Further research will allow a broad application of this method in shield excavation in a composite stratum.