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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]

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]

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]

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]

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]

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]

Soil conditioning of gravel-clay strata using foams for earth pressure balance (EPB) shield tunnelling is less well understood compared to conditioning for clay or sand strata. This lack of understanding can lead to muck clogging at the cutting wheel and increased wear on cutter tools. To address these issues, this study introduces the response surface methodology (RSM) to investigate the effects of various foaming agent components and their interactions on foam conditioning of gravel-clay strata that are widely distributed in Southeastern China. The controlled variables selected for this study include anionic surfactant sodium fatty alcohol polyoxyethylene ether sulfate, nonionic surfactant alkyl polyglycoside, foam stabilizer hollyhock gum, organic dispersant guar gum, and inorganic dispersant sodium tripolyphosphate. The responses evaluated are the foaming agent viscosity, foam volume, foam half-life time, adhesive force, and undrained shear strength. Fifty groups of laboratory tests, including apparent viscosity, Ross-Miles, liquid loss, improved cone pull-out, and vane shear tests, are conducted to characterize these responses. The surface tension measurements and microscopic observations are also performed to interpret foam performance. Ultimately, the optimal ratio of foaming agent components is determined using the desirability approach. The comparison of in-house and commercial foaming agents in both laboratory and in-situ tests, focusing on their conditioning effects on soil properties and tunnelling parameters, highlights the applicability of the proposed RSM models. This study offers new insight into the consequence and interaction of different foaming agent components and serves as a valuable reference for soil conditioning of gravel-clay strata during EPB shield tunnelling. [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]

• 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]

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Long-term settlement induced by EPB tunnelling studied with numerical simulations. Long-term settlement induced by tunnelling in soft cohesive subsoil is rarely considered as a subject for research despite substantial evidence that it increases considerably after construction. The paper aims to investigate the time effects of tunnelling with an earth pressure balance (EPB) shield by means of analysing subsoil deformation with the most up-to-date numerical tool. A three-dimensional numerical model with a detailed description of the geometrical and mechanical characteristics of the tunnelling process and the various materials involved was built. The computational model validated on a real-case was then used to simulate tunnelling in different groundwater conditions prior to construction. That was followed by a simulation of groundwater drawdown conditions after tunnelling with impermeable and permeable tunnel linings. The calculations show the development of the settlement over time in different groundwater conditions together with the significant influence of the permeability of the tunnel. [ABSTRACT FROM AUTHOR]

A major consideration in urban tunnel design is to estimate the ground movements and surface settlements associated with the tunnelling operations. Excessive ground movements may result in damage to adjacent buildings and utilities. Numerous empirical and analytical solutions have been proposed to relate the shield tunnel characteristics and surface/subsurface deformation. Numerical analyses, either 2D or 3D, have also been applied to such tunnelling problems. However, substantially fewer approaches have been developed for earth pressure balance (EPB) tunnelling. Based on instrumented data on ground deformation and shield operation from three separate EPB tunnelling projects in Singapore, this paper utilizes a multivariate adaptive regression splines (MARS) approach to establish relationships between the maximum surface settlement and the major influencing factors, including the operation parameters, the cover depth and the ground conditions. Since the method has the ability to map input to output patterns, MARS enables one to map all influencing parameters to surface settlements. The main advantages of MARS over other soft computing techniques such as ANN, RVM, SVM and GP are its capacity to produce a simple, easy-to-interpret model, its ability to estimate the contributions of the input variables, and its computational efficiency. [ABSTRACT FROM AUTHOR]

Tunnelling is often considered an exclusive field of experts with countless years of practice mostly because of a grey area existing between design carried out with sophisticated theories and construction ruled by practical judgement. Despite the auxiliary actions carried out during execution are acknowledged to affect significantly the interaction between soil and lining, limited effort is made to analyse their effects with an appropriate theoretical support, often preferring to perform broad estimate with arbitrary coefficients. The present paper aims to establish a common field between designers and executors, trying to take advantage on one side of the increased power of numerical computation, on the other side of the accurate control of equipment nowadays available. A three-dimensional numerical model is then built to simulate tunnelling with Earth Pressure Balance, characterising with great detail the properties of the various materials and the geometrical and mechanical characteristics of excavation. The calculation is superintended by a script that automatizes the assignment of input and the interpretation of output, allowing in this way to perform repetitive parametric analyses. The tool is firstly tested on a real case study, a section of the MRTA project of Bangkok, then applied to quantify the effects of the pressure applied on the tunnel face and of the gap left between lining and soil. The calculation shows the time evolution of the mechanisms induced by excavation and lining installation and their modification induced by a parametric change of the main technological factors. [ABSTRACT FROM AUTHOR]

The earth pressure balance (EPB) shield cutterhead structure, which features an opening ratio and opening distribution as a core, seriously affects tunneling stability and tunneling efficiency. This paper presents a new model for the soil using visco-plastic fluid theory, and then introduces the model into the computational-fluid-dynamics model to comprehensively analyze the cutterhead structure, which consists of the soil, the cutterhead, the working chamber, and the screw conveyor. Based on this model, the stability situation and tunneling performance of multiple schemes of the cutting head structure are analyzed by changing the opening ratio and the opening distribution on the cutterhead. In this study, a new method for design and analysis of the EPB-shield cutterhead structure is proposed that fits changes in geologic conditions. The results will be helpful for engineers and manufacturers of more efficient machines and for carrying out tunneling projects with more stable EPB-shield cutterheads, and it will reduce the influences of changing geologic conditions during all stages of tunnel construction. [ABSTRACT FROM AUTHOR]

• The process of clogging can be divided into three stages. • Clogging can be detected by monitoring torque and excavation speed during tunnelling. • Soil conditioning can reduce clogging, but excess injected foam could be harmful. • The opening at the centre of the cutterhead is of vital importance. Clogging may occur inside the shield machine during earth pressure balance (EPB) tunnelling in cohesive soils. Soil conditioning can be used to reduce clogging potential and ensure the efficiency of the shield tunnelling. The current methods used to assess the clogging potential mainly focus on pure clayey soils or artificial mixed soils, the clogging in natural weathered soils is less studied. This paper studied the clogging when tunnelling in completely decomposed granite by laboratory tests, followed by field validation. In the laboratory tests, the torque and excavation speed were recorded and the soil distribution on the cutting device were analyzed. It appeared that when clogging happens, the excavation speed drops and the fluctuation in speed increases, while the torque rises as well as the fluctuations in the torque. Clogging first covers the cutters, then fill in the intervals between cutters, and finally blocked the openings on the cutting device. The opening at the central part of cutting device is of vital importance to prevent clogging. The process of clogging can be divided into three stages, the initial stage, the stabilized stage and the clogging stage. In-situ data from Guangzhou Metro Line 21 showed that clogging on cutterhead was effectively avoided when using the recommended soil conditioning scheme. Furthermore, the clogging evaluation method and process of clogging proposed are consistent with field validation and with current major clogging assessment method. [ABSTRACT FROM AUTHOR]

When Earth Pressure Balance (EPB) shields pass through clayey ground, the working tool is frequently clogged, causing a low tunnelling rate and accelerating cutter disk abrasion. Clay clogging is considered to be due to the high tangential adhesion strength between the clay and metal. This paper investigates the changes in the tangential adhesion strength with the consistency index under different total normal stresses and with different dispersant contents using a mixture of bentonite and kaolin (mass ratio of 1:1). The results showed that the tangential adhesion strength first decreased significantly and then remained almost constant with an increase in water content. Injection of the dispersant could decrease the tangential adhesion strength of clay. The tangential adhesion strength decreased gradually with a lower dispersant content (<4%) and then remained nearly unchanged. The tangential adhesion strength increased slowly with a consistency index lower than 0.5–0.6 and then increased significantly. Considering the effect of dispersant on the Atterberg limits of clay, the tangential adhesion strength between the soil and metal with different dispersant contents was virtually identical at the same consistency and under a normal stress. The tangential adhesion strength showed an exponential relationship with the normal stress and consistency index when the consistency index of clay was lower than 1. The reduction of average tangential adhesion strength showed a linear increase as that of liquid limit increased. The slope of the line increased when the normal stress increased. These results indicate that changes in the liquid can represent the effect of dispersant on the adhesion of clay. From the test results, it can be considered that a supporting medium with a consistency index lower than 0.5 is suitable for EPB shields. Since soil with a consistency index lower than 0.4 is easily squeezed out of a screw conveyor, a supporting medium with a consistency index of 0.4–0.5 is ideal for EPB shields, which is in agreement with the conclusion proposed by Hollmann and Thewes. [ABSTRACT FROM AUTHOR]

Settlement control is an essential part of the tunnel construction process. This paper proposes two novel computational models based on the Random Forest (RF) algorithm for supporting automatically steering Earth Pressure Balanced (EPB) shield. The first model is utilized for predicting tunneling-induced settlement and the other estimates shield operational parameters. A PSO-RF hybrid algorithm which intergrates the Particle Swarm Optimization (PSO) and the RF algorithms is proposed to optimize shield operational parameters when the settlement exceeds the tolerated value. The proposed models are adopted in the case study of Changsha Metro Line 4 project. The results indicate that the predicted settlements show great agreement with the measured settlements. The face pressure and grout filling are the most significant shield operational parameters to control the settlement as a result of Global Sensitivity Analysis (GSA). The anomalous settlement (≥10 mm) can be controlled under tolerated value after the face pressure and grout filling values are optimized by the PSO-RF hybrid algorithm. Simultaneously, the consistency of the face pressure and grout filling values calculated by the PSO-RF and the grid search method demonstrates the feasibility and applicability of proposed hybrid algorithm. • Random forest models are established for predicting tunneling-induced settlement and shield operational parameters. • A hybrid algorithm is proposed for optimizing shield operational parameters. • Combining settlement and operational parameters prediction models achieves the automatic control of shield machine. [ABSTRACT FROM AUTHOR]