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Foam is widely used in soil conditioning during EPB shield tunnelling to ensure the efficiency and safety of face support and muck flow. Currently, the concentration of the foaming agent solution is chosen based on the recommendations of foaming agent suppliers, the half-life time of foam or the properties of the foam-soil mixture. This paper studies the foaming agent concentration (Cf) choice taking surface tension into consideration. Test results show that the Cf should be higher than the critical micelle concentration (CMC) to offset the influence of the expanding surface area during foam generation in shield tunnelling. The comparison between the capillary rise method and the platinum plate method shows that this offset can almost be accomplished when Cf is higher than 1% and fully accomplished when Cf is higher than 3%. Foam generation tests show that despite the differences in foam generation methods, the foaming ability of foaming agent solution and the stability of the generated foam are close to optimum at Cf of 1% and reach optimum at Cf of 3%. Slump tests show that foam with Cf lower than 1% is not stable enough in the foam-sand mixture for shield tunnelling. Both slump tests and field validation show that when Cf is higher than 3%, the increase of Cf has limited influence on soil conditioning performance. [ABSTRACT FROM AUTHOR]

Controlling the ground settlement and building deformation triggered by shield tunnelling, particularly within water-rich strata, poses a significant engineering challenge. This study conducts a finite element (FE) analysis focusing on the ground settlement and deformation of adjacent structures (with a minimum distance of 2.6 m to the tunnel) due to earth pressure balance (EPB) shield tunnelling. The analysis incorporates the influence of groundwater through a 3D fluid–solid coupling model. This study assesses the effects of tunnelling on the behaviour of nearby buildings and introduces two mitigation strategies: the vertical partition method and the portal partition method. Their effectiveness is compared and evaluated. Our findings reveal that the deformation curves of the stratum and the building are influenced by the accumulation and dissipation of pore pressure. The vertical partition method reduced surface settlement by approximately 70%, while the portal partition method further minimized building deformation but required careful application to avoid issues like uplift. Both methods effectively mitigate the impacts of tunnel construction, with the portal partition method offering superior performance in terms of material use and cost efficiency. This research provides a scientific foundation and technical guidance for similar engineering endeavours, which is vital for ensuring the safety of metro tunnel construction and the stability of adjacent buildings. [ABSTRACT FROM AUTHOR]

Soil conditioning is essential for addressing the stratum applicability problem of earth pressure balance (EPB) shields. Under high water pressures, EPB shields spew water and soil when excavating coarse-grained strata. Typically, foam combined with polymers and slurry is used to solve spewing. However, in current techniques, slurry, foam, and the other agents are mixed with soil separately, their synergistic effect is seldom realized. In this study, an anionic surfactant was used to foam in bentonite slurry to form bubble–slurry to maximize the synergy between bubbles and slurry. The slump, volume stability, and permeability test of bubble–slurry-conditioned sand was conducted to examine the conditioning effect, and the stratum applicability of bubble–slurry was determined from the perspective of permeability. It was found that the conditioning effect of bubble–slurry in coarse gravel soil was excellent and could expand the applicability of EPB shields. The main stabilization mechanism of bubble–slurry is that bentonite particles provide a space barrier for bubbles. And three seepage modes of bubble–slurry-conditioned sand were innovatively defined, and the occurrence conditions of the three seepage modes were analyzed according to the permeability coefficient of sand, initial dynamic shear force of bubble–slurry, and hydraulic gradient. [ABSTRACT FROM AUTHOR]

Ground volume loss is an important parameter for settlement control and prediction in shield tunneling. This parameter is usually determined empirically, while its variation along the tunnel is not predictable. In this study, an analytical formula for ground volume loss is obtained based on physical derivation. The ground volume loss is divided into two components. Key operational parameters, i.e. face pressure, grouting pressure and grouting volume, are used to calculate the two components. The proposed formula is then validated against the field data collected from Changzhou Metro in thick silty clay layer. The validation shows that the new formula gives reasonable prediction of ground volume loss. Furthermore, the three model coefficients derived from physical calculation are very close to the regression coefficients based on statistical method. For EPB tunnels excavated in silty clay layer, the ground volume loss along a specific tunnel can be predicted by the new formula. The analysis also indicates that the surface settlement of EPB tunnel should be controlled by a balanced combination of varied operational parameters.

Foam conditioning is a widely adopted technique in earth pressure balance (EPB) shield tunneling for the purpose of reducing sand permeability and preventing water spewing. The permeability of foam-conditioned sands differs from that of natural sands due to the presence of foam bubbles. This study investigated the effect of water pressure on the permeability of foam-conditioned sands using novel laboratory permeability tests. The water pressure, for the first time, is decoupled with the hydraulic gradient, owing to a newly developed permeameter with the controllable downstream hydraulic pressure in the laboratory. The results show that the permeability is significantly affected by the water pressure, and the effect is also predominantly dependent upon the foam injection ratio. The initial hydraulic conductivity increases with the increasing water pressure, while the initial stable period duration decreases. The water-plugging structure formed by foam bubbles and sand particles is prone to be damaged under high water pressure due to the shrinkage of foam bubbles. This means that the existing permeability tests with low water pressure underestimate the permeability of foam-conditioned sands. The underlying mechanism of water pressure in modifying the permeability of foam-conditioned sands is also examined from a particle-scale perspective. [ABSTRACT FROM AUTHOR]

Ground volume loss is an important parameter for settlement control and prediction in shield tunneling. This parameter is usually determined empirically, while its variation along the tunnel is not predictable. In this study, an analytical formula for ground volume loss is obtained based on physical derivation. The ground volume loss is divided into two components. Key operational parameters, i.e. face pressure, grouting pressure and grouting volume, are used to calculate the two components. The proposed formula is then validated against the field data collected from Changzhou Metro in thick silty clay layer. The validation shows that the new formula gives reasonable prediction of ground volume loss. Furthermore, the three model coefficients derived from physical calculation are very close to the regression coefficients based on statistical method. For EPB tunnels excavated in silty clay layer, the ground volume loss along a specific tunnel can be predicted by the new formula. The analysis also indicates that the surface settlement of EPB tunnel should be controlled by a balanced combination of varied operational parameters. [ABSTRACT FROM AUTHOR]

Both foam and bentonite slurry are commonly employed in soil conditioning to prevent water spewing during earth pressure balance (EPB) shield tunnelling. A novel calculation model to estimate the permeability coefficient of soils conditioned with foam and bentonite slurry is developed. In this model, bentonite particles are assumed to adhere to the foam film in a monolayer of bridging particles, and the permeation channels within the conditioned soil are updated using the Kozeny-Carman equation. A series of permeability tests on conditioned soil, varying conditioning parameters and water heads, validate the accuracy of the model. The measured results that the value of permeability coefficient can be well captured by the calculation results, which indicates that the model can accurately predict the permeability of soil conditioning by foam and bentonite slurry. While there is a slight reduction in accuracy for conditioning states with high foam and bentonite slurry injection ratios, the model remains conservative for tunnelling engineering safety due to calculated values consistently exceeding measured ones. Furthermore, the model performs well in cases of foam-conditioned soil from previous studies. Additionally, the model underscores the significant influence of bentonite slurry on the permeability coefficient by altering soil grain gradation. [ABSTRACT FROM AUTHOR]

At present, the determination of tunnel parameters mainly rely on engineering experience and human judgment, which leads to the subjective decision of parameters and an increased construction risk. Machine learning algorithms could provide an objective theoretical basis for tunnel parameter decision making. However, due to the limitations of a machine learning model's performance and parameter selection methods, the prediction model had poor prediction results and low reliability for parameter research. To solve the above problems, based on a large number of construction parameters of a composite section subway in Shenzhen, this paper combined dimensionality reduction data with service analysis to optimize the selection process of shield tunneling parameters, and determined the total propulsion force, cutter head torque, cutter head speed, and advance rate as key tunneling parameters. Based on an LGBM algorithm and Bayesian optimization, the prediction model of key tunneling parameters of an earth pressure balance shield was established. The results showed that the average error of the LGBM model on the test set was 8.18%, the average error of the cutter head torque was 13.93%, the average error of the cutter head speed was 3.16%, and the average error of advance rate was 13.35%. Compared with the RF model, the prediction effect and the generalization on the test set were better. Therefore, an LGBM algorithm could be used as an effective prediction method for tunneling parameters in tunnel construction and provide guidance for the setting of tunneling parameters. [ABSTRACT FROM AUTHOR]

Based on the overlapping tunnel of Tel Aviv Red Line Light Rail Project in Israel, this study established a three-dimensional numerical model by using Rhino6 and FLAC3D software on the basis of analysis of on-site monitoring data, to investigate the ground settlement and stratum displacement laws of shield overlapping tunnel at different overlapping angles.Results show that when the two tunnels gradually transit from horizontal to overlapping state, with the increase of overlapping angle of the two tunnels, the accumulated settlement at the measuring points in the middle of the two tunnels increases, but the accumulated settlement is not only affected by overlapping angle, but also closely related to the stratum traversing situation.Under the stratum condition of the project, when the overlapping angle is 60°, the accumulated surface settlement reaches the maximum because both tunnels pass through the loose reservoir K1 stratum.When the overlapping angle of the rear-built tunnel is less than 45° after excavation, the width of the settlement trough increases.When the overlapping angle is larger than 45° and smaller than 60°, the width of the settlement trough remains unchanged.The width of settlement trough decreases when the overlapping angle is larger than 60°.When the overlapping angle reaches 90°, the settlement trough is symmetrical on both sides.After excavation of tunnels on both sides, when the overlapping angle of the tunnels reaches 75°, the strata on both sides change the direction of movement.This research offer reference for the design and construction of overlapping tunnels.

The water permeability of conditioned soils is one of the most essential properties for Earth Pressure Balance (EPB) tunnelling in coarse-grained soils. Permeability tests are conducted to study the influence of water heads on the permeability of foam-conditioned sands. The initial permeability coefficient of foam-conditioned sands increases with the water head, while the stable permeability coefficient and the initial stable period duration decrease. Meanwhile, a novel analytical model is proposed to estimate the initial permeability coefficient. In this model, the effect of the water head on the initial permeability coefficient is incorporated by calculating void ratios of the foam and effective diameters of foam bubbles under different water pressures. Experimental results are in close agreement with analytical solutions, indicating the excellent performance of the proposed calculation method. In addition, the physical mechanisms of how the water head affects the permeability of foam-conditioned sands are discussed from the contraction and evolution of foam bubbles.

Earth pressure balance (EPB) shield excavation in cohesive strata frequently encounters cutterhead clogging, consuming time and economic costs. Engineering practice shows that cutterhead aperture ratio, soil chamber pressure and soil conditioning have important influences on the clogging behavior of EPB shields. In order to study these factors, a series of laboratory tests were conducted using a setup that can closely simulate the shield tunnelling process. The indicators analyzed were excavation speed, torque, specific energy and soil distribution on the cutterhead. The results indicate that cutterhead aperture and cutter arrangement are important factors in clogging potential. Clogging creates a significant reduction in excavation speed. Torque is not a sensitive parameter reflecting clogging because of the effect of the consolidated soil on the cutters. The presence of soil chamber pressure makes it difficult to remove the clogging through the shearing action of the excavation surface. The injection of a sufficient amount of foam can reduce the risk of shield clogging, but it may lead to an increase of soil adhesion on the cutterhead. The amount of soil that adheres to the cutterhead cannot be used as a classification criterion for clogging potential. Based on experimental phenomena and established studies, a mechanical model called 'beam-bridge' was proposed to study the behavior of the soil through the cutterhead opening and damage form of clogging. The results will help to the excavation control of EPB shields. [ABSTRACT FROM AUTHOR]

Ground loss is the volume of soil excavated in excess of designed excavation. It defines the boundary conditions for ground deformation field and thereby dominates the magnitude of tunneling-induced ground settlement. In practice, it is generally quantified by a parameter called ground loss parameter. To date, there is no unanimously agreed formulation for ground loss parameter despite its indispensable role in developing both empirical and analytical solutions for tunneling-induced ground settlement. Herein, a comprehensive field database is utilized to quantitatively assess existing formulations of ground loss, and to unravel its role in settlement prediction via inverse analysis. It reveals that remarkable errors can be generated by classical solutions for tunneling-induced ground settlement. This implies that the reliability of classical solutions for tunneling-induced settlement can be potentially improved with a more accurate formulation of ground loss. A data-driven formulation for ground loss is developed with aid of the random forest algorithm, and it can well capture the target value with an R-value equaling 0.84. The developed formulation is further implemented in the O'Reilly and New solution, yielding a hybrid model for settlement prediction. The hybrid model can accurately predict the actual settlement with an R-value of 0.84, outperforming the purely data-driven model and further confirming the accuracy of the proposed formulation of ground loss. [ABSTRACT FROM AUTHOR]

Soil conditioning is essential for addressing the stratum applicability problem of earth pressure balance (EPB) shields. Under high water pressures, EPB shields spew water and soil when excavating coarse-grained strata. Typically, foam combined with polymers and slurry is used to solve spewing. However, in current techniques, slurry, foam, and the other agents are mixed with soil separately, their synergistic effect is seldom realized. In this study, an anionic surfactant was used to foam in bentonite slurry to form bubble–slurry to maximize the synergy between bubbles and slurry. The slump, volume stability, and permeability test of bubble–slurry-conditioned sand was conducted to examine the conditioning effect, and the stratum applicability of bubble–slurry was determined from the perspective of permeability. It was found that the conditioning effect of bubble–slurry in coarse gravel soil was excellent and could expand the applicability of EPB shields. The main stabilization mechanism of bubble–slurry is that bentonite particles provide a space barrier for bubbles. And three seepage modes of bubble–slurry-conditioned sand were innovatively defined, and the occurrence conditions of the three seepage modes were analyzed according to the permeability coefficient of sand, initial dynamic shear force of bubble–slurry, and hydraulic gradient. [ABSTRACT FROM AUTHOR]

Abstract The spewing of a screw conveyor easily occurs from the earth pressure balance (called EPB) shield in a water‐rich sand stratum. This may lead to the collapse of the tunnel face and even serious subsidence of the ground surface. To understand the spewing mechanism of the shield screw conveyor and explore the critical hydraulic condition of soil spewing in a shield–soil chamber, a simplified theoretical model for the spewing of the screw conveyor was developed based on the equation of groundwater flow in the screw conveyor under turbulent state. Thus, coupling Darcy's law with Brinkman's equation, this model was implemented within the COMSOL Multiphysics framework. The underground water flow in the shield screw conveyor was simulated so as to obtain its velocity and flow rate. Numerical simulations show that the water pressure distribution is concentrated in the lower part of the soil chamber after the groundwater enters the soil chamber. When the groundwater enters the screw conveyor, its pressure gradually decreases along the direction of the screw conveyor. When the water flow reaches the stratum–shield interface, the flow velocity changes markedly: first increases and concentrates at the entrance of the lower soil chamber, plummets and stabilizes gradually, and increases again at the exit. The soil chamber and screw conveyor are significantly depressurized. It is also found that the soil permeability coefficient can be reduced to k

During EPB (earth pressure balance) shield tunnelling in cohesive soils, the excavated soil may stick to the cutters, causing clogging. Clogging could reduce the efficiency of tunnelling as the cutters may be covered with excavated soil, the opening ratio of the cutterhead may be reduced by the sticked soil, and increased torque may be required. This paper studies clogging by testing adhesion strength and tangential adhesion strength with modified pullout tests and direct shear tests, respectively. Strong weathered granite and 11 artificial soil samples are tested to check the feasibility of the modified pullout tests and direct shear tests in the clogging study. The repeatability and accuracy of the modified pullout test are improved by building a new soil container and using a new loading method. Soil conditioning by injecting chemical additives such as foam and dispersant into the excavated soil can be used to reduce the clogging potential during shield tunnelling. The influences of foam and dispersant on Atterberg limits, adhesion and tangential adhesion in strong weathered granite are also studied. Test results show that the Atterberg limits decreased in conditioned soil, and soil conditioning has a greater influence on tangential adhesion than on adhesion. A new clogging potential assessment method for conditioned soil is proposed based on the pullout and direct shear test results. Validations using strong weathered granite show that this method is more suitable for conditioned soils than the traditional methods. [ABSTRACT FROM AUTHOR]

At present, the determination of tunnel parameters mainly rely on engineering experience and human judgment, which leads to the subjective decision of parameters and an increased construction risk. Machine learning algorithms could provide an objective theoretical basis for tunnel parameter decision making. However, due to the limitations of a machine learning model’s performance and parameter selection methods, the prediction model had poor prediction results and low reliability for parameter research. To solve the above problems, based on a large number of construction parameters of a composite section subway in Shenzhen, this paper combined dimensionality reduction data with service analysis to optimize the selection process of shield tunneling parameters, and determined the total propulsion force, cutter head torque, cutter head speed, and advance rate as key tunneling parameters. Based on an LGBM algorithm and Bayesian optimization, the prediction model of key tunneling parameters of an earth pressure balance shield was established. The results showed that the average error of the LGBM model on the test set was 8.18%, the average error of the cutter head torque was 13.93%, the average error of the cutter head speed was 3.16%, and the average error of advance rate was 13.35%. Compared with the RF model, the prediction effect and the generalization on the test set were better. Therefore, an LGBM algorithm could be used as an effective prediction method for tunneling parameters in tunnel construction and provide guidance for the setting of tunneling parameters.

Spewing is a key issue during earth pressure balance (EPB) shield tunneling in a water-rich sandy stratum. This paper proposed the bubble-slurry as a new type of soil conditioning agent. The foamability and stability of the bubble-slurry were studied, and the volume stability, fluidity, and permeability of bubble-slurry-conditioned sands with hydraulic gradient of 2 to 10 were analyzed. The stability mechanism was explored by the zeta potential and microstructure of the bubble-slurry system. The formulation of bubble-slurry with a half-life of 121–383 h was determined through laboratory tests. The results of soil conditioning experiments showed that the bubble-slurry-conditioned soil exhibited appropriate fluidity, excellent volume stability and low permeability in a certain period. The use of bubble-slurry can save approximately 3.0–8.2% of the slurry consumption compared with the method of slurry conditioning. The zeta potential and the distribution of slurry particles of the bubble-slurry system were key mechanisms affecting the stability of the bubble-slurry. The high absolute value of zeta potential of the system increases the dispersion of bubbles, and the slurry particles uniformly distributed on the bubble liquid film and in the Plateau boundary are conducive to enhancing the anti-disturbance ability and inhibiting the decay of bubbles. [ABSTRACT FROM AUTHOR]

To control tunneling risk, the prediction of the surface settlement rate induced by shield tunneling using earth pressure balance plays a crucial role. To achieve this, ten independent variables were identified that can affect the amount of settlement. The nonlinear relationship between maximum ground surface settlements and ten influential independent variables was considered in artificial neural network (ANN) models. A total of 150 genuine datasets derived from the Southern Development Section of the Tehran Metro Line 6 project were used to train, validate, and test ANN techniques. Hence, the ground surface settlements of the mentioned project were predicted by the most accurate back propagation ANN technique. Ultimately, the importance level of different influential parameters on ground settlement at tunneling is relatively determined based on the results of the optimal neural network. The results used in this paper to evaluate the relative importance of each variable involved in the rate of ground surface settlement demonstrate that the parameters of grout injection and permeability equivalent to the proportions of approximately 16.91% and 5.07% have the highest and lowest impact, successively. [ABSTRACT FROM AUTHOR]

The cutterhead of an earth pressure balance (EPB) shield usually relies on a large torque when tunneling through a water-rich and high-permeability sand layer. Sandy soils may lead to severe wear of the cutter tools and make the screw conveyor vulnerable to spewing. In this study, we investigate the characteristics of the soil conditioned by means of foam, bentonite slurry, and carboxymethyl cellulose solution. The mechanical and friction properties of the conditioned soil were tested by the triaxial shear, Laboratoires Central des Ponts et Chaussees (LCPC), and sliding tests, under considering the influence of soil consolidation time. The test results showed that the conditioned sand presents dilatancy, and its cohesion enhances but its internal friction angle decreases. The changing trend of these properties is more prominent with the increase of consolidation time but they stabilize after 12 h. The starting torque of the shield machine in 3-h shutdown reaches 60% of the rated torque, while the starting torque in 6-h shutdown exceeds 80% of the rated torque. To avoid a large starting torque, we suggest that the cutterhead is idled for a short time within 12 h after the shield machine is shutdown. Finally, the theoretical calculation method of torque on cutterhead was developed, and the prediction by such method is in good agreement with the in situ measured data in the cutterhead. [ABSTRACT FROM AUTHOR]

Based on the construction practice of the Beijing Metro Line 10 shield tunneling project, this paper describes research on soil improvement technology for Beijing stratum characteristics (sandy gravel stratum) and covers similar engineering conditions. It also describes the development of a new type of mud improver. Based on the laboratory tests with bentonite as the base mud and different additives, the effects of guar gum, CMC, xanthan gum, and clay medium particles on mud performance are analyzed. Then, two kinds of mud were used to conduct indoor simulated muck improvement tests (mixing test, slump test, sliding plate test, and adhesion resistance test), and the improvement effects of new mud and ordinary mud applied in pebble/round gravel and sandy soil layers were compared. The results show that xanthan gum is the best material to improve the performance of slurry, using the contrast test of bentonite-based slurry and different additives. The optimum slurry preparation scheme is 4% bentonite, 0.2% xanthan gum, 0.04% soda ash, and 1% clay particles. Using indoor simulated muck improvement tests (mixing test, slump test, slide plate test, and adhesion resistance test), the improvement effects of applying new mud and ordinary mud in pebble/round gravel and sandy soil layers are compared, and the advantages of the new mud in the application of the above two formations are verified. Among them, the new slurry has great advantages for improving the two parameters of the soil adhesion resistance coefficient and slump during shield tunneling. When the improved soil mass reaches the flowing plastic state, the proportion of new mud added to different soil mass is different. The proportion of new mud added to improved pebble/pebble soil is 28%, and that of sand and clay is 32%. It can be seen that new mud is more suitable for improving pebble/pebble soil. [ABSTRACT FROM AUTHOR]

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When excavating tunnels in the clay stratum by using earth pressure balance (EPB) shields, the cutterhead is prone to clogging and may lose its ability to excavate. To investigate the clogging mechanism, a cutterhead device equipped with a driving system was developed to emulate the cutterhead behavior of the EPB shield in conditioned soils. The influences of water content, consolidation time, and soil conditioning factors on clay adhesion were evaluated and compared based on general laboratory and cutterhead model tests. The mixing test results showed that the water content influenced considerably clay adhesion. A water content interval near the liquid limit caused a decrease in the adhesion coefficient, and the change in water content was more effective than that in clay conditioning. Direct shear tests revealed that soil conditioning could reduce clay strength, and the effect was evident when the consolidation time was less than 6 h. A peeling point was observed in the adhesion curve of the clay and steel blocks. The peeling point for unconditioned clay was greater than the corresponding shear failure strength, which indicated that the clay had failed before it was detached from the steel block surface. However, contrasting results were obtained for conditioned clay. The cutterhead model test also showed that the water content had the greatest influence on clay adhesion according to the torque and thrust. [ABSTRACT FROM AUTHOR]

Soil conditioning of an earth pressure balance shield in a saturated sand layer aims to impart adequate flow plasticity and impermeability. Thus, the effect of soil conditioning on the saturated sand layer was analyzed for Harbin Metro Line 2 in the tunnel section project from Harbin North Station to Dagengjia Station, and the optimal soil conditioning ratio was obtained through experiments on medium sand using foam, bentonite, and carboxymethyl cellulose. In addition, the microscopic pores and shape characteristics of the soil before and after conditioning were compared using computerized tomography (CT) scanning and scanning electron microscopy (SEM) to evaluate the microscopic effect of soil conditioning. The current research results revealed that the soil conditioning reduced the particle size dispersion and improved the particle gradation, and the conditioned soil particles were more uniform than the original soil sample. Moreover, the CT scanning images processed using the threshold method illustrated that the pores of the conditioned soil were independent of each other, no continuous channels were formed, and the porosity of each layer was under 4%. In addition, the SEM images confirmed that the clay mineral composition improved the angularity and sharpness of the sand particles, and the flocculent aggregate structure, formed in combination with the sand particles, reduced the soil permeability. Furthermore, the practical application of the optimal ratio of conditioning materials, obtained experimentally and verified through the aforementioned observation, yielded proper results in the field construction, and the spewing of the screw conveyor during the shield tunneling process was avoided. [ABSTRACT FROM AUTHOR]

For the safety and efficiency of earth pressure balance (EPB) shield tunnelling, the dynamic characteristics of the muck in the excavation chamber and the screw conveyor should be considered. However, the velocity and pressure distribution of the muck in the EPB shield, reflecting muck flowability and face stability, are rarely observed at the jobsite. Therefore, a computational fluid dynamics (CFD) model with a modified inlet boundary was established to investigate the muck characteristics in the chamber and the screw conveyor during tunnelling in a full chamber mode. A compressible Bingham model was adapted to describe the muck compressibility and rheology. The time-dependent tunnelling parameters obtained from the simulation excellently matched the field-measured data. The simulation results show the nonuniform velocity distribution of the muck at the excavation face and the low-velocity zones where clogging potentially occurs. The muck pressure reaches a peak at the excavation face and then decreases along the cutterhead openings, chamber and screw conveyor for inflow. The simulation also captures the pressure imbalance at the cutterhead and bulkhead, which may destabilize the excavation face. The proposed discharge efficiency is 56.7% in the simulated condition, indicating that tunnelling with these muck properties has no risk of muck spewing. [ABSTRACT FROM AUTHOR]

• Reveals spatial motion trajectories and distribution patterns of granules during shield tunneling. • Introduces novel metrics for evaluating the cutterhead's cutting and guiding performance. • Clarifies the impact of shield tunneling parameters on the movement characteristics of granules. An investigation into the movement characteristics of granules during Earth Pressure Balance (EPB) shield tunneling contributes significantly to understanding the interaction between the shield machine and the soil. This understanding also enables researchers to gain deeper insights into the cutting and guiding effects of the shield cutterhead on the soil, thus providing a scientific basis for optimizing shield tunneling parameters and cutterhead designs. To achieve this, indoor model shield tunneling experiments were conducted using independently developed shield tunneling equipment and spatial positioning devices developed by our research group. The research investigated the spatial movement trajectories, radial diffusion, and axial movement characteristics of granules at the face subjected to the action of a spoke-type cutterhead. Furthermore, the study proposes the use of average diffusion degree and movement rates of granules as metrics for evaluating the cutting and steering performance of the cutterhead. The influence of cutterhead rotational speed and advance rate on the movement characteristics of granules was further explored based on these two indicators. The study's findings reveal the following: (1) Granules at the face predominantly exhibit a spiral movement trajectory when the EPB shield tunneling machine operates in a dynamic equilibrium state. (2) The cutting and steering performance of the cutterhead's various areas is enhanced progressively from the center to the outer edge, as evidenced by a gradual decrease in the average diffusion degree of granules and an increase in the average axial movement rate (AMR). (3) The AMR and average diffusion degree of granules are influenced by both the rotational speed of the cutterhead and the advance rate of the shield machine. Optimal matching of the rotational speed and advance rate maximizes the average AMR of granules, minimizes the average diffusion degree, reduces the torque on the cutterhead to its lowest point, and consequently, optimizes the efficiency of shield tunneling. This research presents a novel perspective and methodology for understanding the movement characteristics of granules during shield tunneling operations. The outcomes of this study provide valuable insights for the structural design of shield machine cutterheads and the optimization of tunneling parameters. [ABSTRACT FROM AUTHOR]

As metro lines continue to expand rapidly in urban areas, the excavation of twin tunnels in shallow depths using shield tunnelling methods has become widespread. By analysing field data obtained from an actual shield tunnelling project, it has been observed that the post-ground settlement occurring over the preceding tunnel during the excavation of the following tunnel in silty sand is approximately 42% of the green field settlement, which cannot be disregarded. Accurate approximation of the post-ground settlement is useful for preventing any damage due to excessive deformation and to determine the total ground settlement profile during twin tunnel construction stage. And yet, only a few number of studies have focused on investigating and predicting the post-ground settlement that occurs during twin tunnel construction in soft soils. Therefore, this study develops a transparent model using the multi-gene genetic programming (MGGP) method, enabling the prediction of post-ground settlement during twin tunnelling. Comparative analysis demonstrates that the proposed model is user-friendly and capable of generalising to unseen data. The reliability of the MGGP-based model has been validated through sensitivity and parametric analyses. Additionally, when estimating post-settlement during twin tunnelling, it is essential to consider the spacing between twin tunnels, soil cohesion, and crucial operational parameters of the shield, such as torque and face pressure. [ABSTRACT FROM AUTHOR]

To control tunneling risk, the prediction of the surface settlement rate induced by shield tunneling using earth pressure balance plays a crucial role. To achieve this, ten independent variables were identified that can affect the amount of settlement. The nonlinear relationship between maximum ground surface settlements and ten influential independent variables was considered in artificial neural network (ANN) models. A total of 150 genuine datasets derived from the Southern Development Section of the Tehran Metro Line 6 project were used to train, validate, and test ANN techniques. Hence, the ground surface settlements of the mentioned project were predicted by the most accurate back propagation ANN technique. Ultimately, the importance level of different influential parameters on ground settlement at tunneling is relatively determined based on the results of the optimal neural network. The results used in this paper to evaluate the relative importance of each variable involved in the rate of ground surface settlement demonstrate that the parameters of grout injection and permeability equivalent to the proportions of approximately 16.91% and 5.07% have the highest and lowest impact, successively.

• Revealed the failure process of cobble particles with different diameters. • Evaluated the roles of cobble particle with different diameters in keeping tunnel face stable. • Clarified the influence of panel opening ratio and rotating speed of cutter head on face stability. This paper aims at revealing the failure mechanism of deep earth pressure balanced (EPB) shield tunnels in dry graded cobble-rich soil, considering the influence of kinematic differences of filler and skeleton cobble particle. The work was performed by three dimensional discrete element method (DEM). Results show that particle size greatly influences the movement of cobble particles. The coarse particles serve as the skeleton of the ground and provide the stability, while the fine and middle particles are filler materials. The erosion of fine particles has little influence on tunnel face stability, while the erosion of middle particles weakens tunnel face stability much. Existing researches overestimate tunnel face stability as the erosion of filler particles is ignored. The collapse developing from the tunnel face to the ground surface experiences three phases, i.e. local, ultimate and global. The proposed angles of 0.6(45°+ φ p /2) (φ p = the peak friction angle) to the horizon and 0.4(45°- φ p /2) to the vertical are suggested to shape the local failure zone in the longitudinal and transverse direction, respectively. The collapse pressure and size of the failure zone increase with increasing cutter head panel opening ratio and rotating speed. [ABSTRACT FROM AUTHOR]

The muck flow behavior in the screw conveyor determines the pressure in the excavation chamber. The pressure in the chamber should be reduced to the atmospheric pressure at the outlet of the screw conveyor. Otherwise, the soil and water mixture would spew out form the screw conveyor, resulting in the pressure loss and face failure. In this study, three-dimensional numerical analyses of the muck transport performance of screw conveyor were performed using Computational Fluid Dynamics (CFD). The muck was assumed to behave as a Herschel-Bulkley (H-B) plastic fluid. A new model coupling rheological and wall slip models was developed and validated by a laboratory test. The error of the pressure transfer and the flow rate between simulation and test is around 3.8%. Through parametric analysis, the influence of the rheological parameters of muck and the working parameters of the screw conveyor on the muck flow behavior is further explored, and a spewing failure surface is given for the control of rheological parameters of the conditioned soil. The results in this study could provide some suggestions for soil conditioning and the optimization of screw conveyor performance. [ABSTRACT FROM AUTHOR]