Earth Pressure Balance Shield Research Articles

Quantitative evaluation of influence of the soil transportation mode on the excavation efficiency for a shield tunnel based on the Markov chain

The earth pressure balance (EPB) shield tunneling efficiency is greatly affected by the choice of soil transport mode. In this study, the influence of two soil transport modes, such as the continuous belt conveyor and rail train, on the efficiency of shield excavation was analyzed using the Markov chain model. A method was proposed to define the ideal and non-ideal excavation states and quantitatively evaluate the excavation efficiency of the two soil transportation modes of the EPB shield. Based on this model framework, a profitable Markov chain model was established to predict the expected profits of the two soil transportation modes. The Beijing Metro New Airport Line first-phase project was used as a case study to verify the model established. The results show that under the same conditions, the continuous belt conveyor soil transport mode can have a higher excavation efficiency and expected profit. This advantage gradually increases over time.

Experimental Investigation of the Effects of Aperture Ratio, Chamber Pressure and Soil Conditioning on the Clogging Behavior of EPB Shields

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.

A study on soil conditioning for cohesive soils with high content of clay minerals: Microscopic simulation and laboratory experiment

An earth pressure balance shield machine (EPB) is prone to blockage when tunneling in strata with high clay mineral content. Engineers often improve soil fluidity and reduce soil adhesion through soil conditioning to ensure the safe and efficient tunneling of shields. This paper selected montmorillonite, kaolinite, and illite as research objects and used molecular dynamics simulation and laboratory experiments. The interfacial mechanical behavior between anionic surfactant (sodium dodecyl benzene sulfonate, SDBS) solution and clay minerals was simulated at the microscopic scale and the improvement mechanism of SDBS solution on clay minerals was revealed. Then, the zeta potential of clay minerals and the stirring torque of the soil under different foam injection ratios (FIR) were tested. Accordingly, the conditioning effect of the SDBS solution at the macroscopic scale can be evaluated. The results show that the electrostatic adsorption and repulsion of ions occur on the surface of clay minerals, which makes it difficult for clay particles to aggregate into clusters. The SO3H- groups of the SDBS surfactant are the key factor to change the hydrophilicity of clay minerals. When different soil samples are improved to the same state, montmorillonite requires the largest FIR due to its microstructure.

Recycling the discharged soil from EPBS tunnels as a soil conditioner and its rheological behaviors

Earth pressure balance shield (EPBS) has become the mainstream construction method for metro tunnels in China, which inevitably generates a large amount of discharged soil, putting enormous pressure on the environment and the cost. In order to realize the resource utilization of shield dregs, this paper explored the feasibility of recycling the waste silty clay as a soil conditioner in EPBS tunnel construction. Mixtures of bentonite, wasted silty clay, carboxymethyl cellulose (CMC), and tap water in different ratios were prepared. Defining the substituted ratio as the mass ratio of silty clay to (bentonite + silty clay), the effect of the substituted ratio on the apparent viscosity, flow curve, rheological model and physical stability of bentonite-silty clay composite slurry were investigated. Furthermore, the relationship between slurry microstructures and macroscopic rheological properties was revealed by scanning electron microscopy (SEM). The results show that the constitutive relationship of bentonite slurry (0% substituted ratio) can be well described by Herschel-Bulkley model. The silty clay has a deteriorating effect on the rheological behavior and physical stability, and changes the flow pattern of the composite slurry. Microscopically, CMC makes the clay particles in slurries form agglomerates with larger particle sizes, and can absorb excess free water from the slurry, thus significantly increasing the yield stress and apparent viscosity. The recycling scheme proposed in this study can not only greatly reduce the procurement cost of raw materials, but also eliminate the transportation and disposal costs of discharged soil, which has significant environmental benefits and promising application prospects.

Investigation of disc cutter wear during shield tunnelling in weathered granite: A case study

Wear characteristics of disc cutter can vary significantly during shield tunnelling in different geological formations. This paper presents a case study on disc-cutter wear experienced during earth pressure balance shield tunnelling of the Guang-Fo intercity railway in Guangzhou, China. During excavation of the 2.8 km tunnel, 149 cutter inspections were conducted and 1163 cutters were consumed. Cutter wear morphology and cutter consumption was investigated. Cost-effectiveness of cutter replacement under different ground conditions was evaluated regarding time consumption. Field data revealed that cutter consumption varied with ground conditions, cutter mounting radii, and boring parameters. During mixed ground tunneling, it was noticed that the tips of cutter rings had been sharpened. An approximate estimation of when to inspect cutters during tunnelling can be determined using the field penetration index. Cutter replacement in hard rock ground was more cost-effective than in other ground conditions. The wear data provided here can serve as a reference for future operations in similar geological conditions.

Interface characteristics and stability mechanisms of bubbles in high-concentration bentonite slurry

Foam and slurry are important materials for preventing spewing of the earth pressure balance (EPB) shields, but their interaction at the interface level is not entirely understood. Herein, an anionic surfactant was used to foam bentonite slurry with a concentration of 10–30 wt%. The mechanism of the slurry on the foamability of the surfactant and the bubbles stability were analysed through surface viscoelastic modulus, ζ-potential, FT-IR, surface tension, contact angle, and microscopic observation. The results show that the bentonite slurry forms a stable three-dimensional grid structure, which increases the surface tension and free energy required for the formation of the gas–water interface. Surfactants are adsorbed on the bound water layer on the surface of bentonite particles. This increases both the electrostatic repulsion of system and the hydrophobicity of the particle and forms a multi-layered gas–water interface with high viscoelasticity, thus slowing down the drainage, coarsening, and coalescence.

Autonomous intelligent control of earth pressure balance shield machine based on deep reinforcement learning

In order to reduce the construction risk caused by human operation error and improve the geological adaptive ability of the shield machine, an autonomous intelligent control method is proposed for shield machine within the framework of interaction–judgment–decision based on Deep Deterministic Policy Gradient (DDPG) deep reinforcement learning in this study. Due to the strong nonlinear relationship between the shield machine's tunneling parameters, this research builds a deep reinforcement learning environment using mechanism model of sealed cabin pressure. DDPG agent model of the shield machine is established to replace the shield machine to interact and train with the geological environment. By minimizing the difference between the target pressure setting value and the sealed cabin pressure value, the dynamic balance between the sealed cabin pressure and the pressure on the excavation surface is realized, and the best strategy is obtained. Through real-time interaction with the geological environment, the method in this paper can dynamically adjust the tunneling parameters, accurately control the sealed cabin pressure, and has a strong geological adaptive ability. By realizing the intelligent decision-making of the tunneling parameters, it greatly improves the independent decision-making ability of the shield machine system, reduces the inaccuracy of human operation, and provides an effective guarantee for the efficient and safe operation of the shield machine. This study applies deep reinforcement learning technology to the control field of earth pressure balance shield machine, promotes AI technology, and provides a new idea for the development of AI construction technology in engineering field.

Intelligent real-time prediction of multi-region thrust of EPB shield machine based on SSA-LSTM

During the tunneling process of shield machine, the accurate determination of tunneling parameters is the guarantee of safe operation of shield machine. Through the real-time construction data analysis and mining of shield machine, the tunneling parameters at the next moment are obtained in real-time. Based on this, this paper proposes an intelligent real-time prediction method for multi-region thrust of EPB shield machine based on Sparrow Search algorithm-Long and Short-term Memory (SSA-LSTM). By correlation analysis of the construction big data, the data features with a great correlation of shield machine thrust are obtained. And it is used as the input of LSTM prediction model to explore the nonlinear relationship between inputs and output. SSA is used to optimize LSTM prediction model to establish a more accurate nonlinear relationship, and then the multi-region thrust of shield machine at the next moment is accurately obtained. The simulation results show that SSA-LSTM model can accurately predict the thrust of each region of shield machine at the next moment, and the prediction performance is better than other models. The method provides a reference basis for shield machine to implement accurate thrust regulation and provides a guarantee for effective control of Earth pressure balance (EPB) in sealed cabin to ensure construction safety, which has engineering application value.

Effect of conditioning schemes on mechanical properties of EPB shield soil

Shearing conditioned sands under different pressure conditions requires understanding their mechanical properties with various conditioning schemes to ensure feasible conditioning during earth pressure balance (EPB) shield tunnelling in sandy strata. This paper investigates the effect of bentonite slurry and foam, conditioned with sodium carboxymethyl cellulose (Na-CMC), on the workability of sand with varying water content, to obtain preliminary conditioning schemes. The conditioned sands with suitable workability are tested using a pressurized vane shear apparatus to obtain their mechanical properties, including compressibility, resilience, and shear strength before and after resiling. Results indicate that Na-CMC can reduce filtration loss of bentonite slurry and foam expansion ratio (FER) of foam, whereas enhance the stability of them. The foam injection ratio (FIR) of foam-conditioned sands and the maximum slurry addition of slurry-foam-conditioned sands decrease linearly with increasing the initial water content of sand, provided the conditioned sands have suitable workability. Increasing the FIR can remarkably increase the void ratio of conditioned sands, which maintains high compressibility and resilience, and low shear strength before and after resiling. The addition of bentonite improves the workability of sand. The conditioning schemes are feasible when the total stress is lower than a specific critical value. The critical total stress can be increased by increasing FIR and reducing the water content of sand or slurry-conditioned sands. This paper provides critical guidance for the design of conditioning schemes.

Analytical study on the seepage field of EPBS and preliminary discrimination of spewing

Considering the two-dimensional seepage effect of a soil chamber, the spewing problem of an earth pressure balance shield (EPBS) is analysed by an analytical method. In this paper, an explicit analytical solution of the two-dimensional shield seepage field considering the water level distribution at the cutter surface is derived by using the separation of variables method and the superposition principle, and the unknowns in the analytical solution are solved by constructing a non-homogeneous system of equations. The correctness of the solution is verified by using the experimental and numerical simulation results, and a parameter analysis was conducted to investigate the effect of shield geometry and water head at the center of the cutter surface. The results show that water head on the cutter surface, screw conveyor angle, screw conveyor length and screw conveyor in the water flow and water pressure with an approximate linear relationship, reducing the screw conveyor diameter is most conducive to prevent spewing. On the basis of previous studies, the analytical solution can calculate the critical permeability coefficient for the occurrence of a spewing, and use the undisturbed soil or predicted permeability coefficient to preliminarily discriminate whether a spewing will occur. By comparing with the shield spewing example and with the shield one-dimensional analytical solution, it is proven that spewing discrimination based on this analytical solution is effective and more accurate. Since the analytical solution considers the water level distribution on the cutter surface, it can be used to discriminate the spewing of shields of any size.

Evaluation and prediction of earth pressure balance shield performance in complex rock strata: A case study in Dalian, China

This research explores the potential for the evaluation and prediction of earth pressure balance shield performance based on a gray system model. The research focuses on a shield tunnel excavated for Metro Line 2 in Dalian, China. Due to the large error between the initial geological exploration data and real strata, the project construction is extremely difficult. In view of the current situation regarding the project, a quantitative method for evaluating the tunneling efficiency was proposed using cutterhead rotation ( R ), advance speed ( S ), total thrust ( F ) and torque ( T ). A total of 80 datasets with three input parameters and one output variable ( F or T ) were collected from this project, and a prediction framework based gray system model was established. Based on the prediction model, five prediction schemes were set up. Through error analysis, the optimal prediction scheme was obtained from the five schemes. The parametric investigation performed indicates that the relationships between F and the three input variables in the gray system model harmonize with the theoretical explanation. The case shows that the shield tunneling performance and efficiency are improved by the tunneling parameter prediction model based on the gray system model.

Upper bound analysis of longitudinally inclined EPB shield tunnel face stability in dense sand strata

Earth pressure balance (EPB) shield machine frequently drives along a route with a longitudinal inclination angle (δ) during tunnel construction. The tunnel face stability is a crucial issue for controlling the safety of tunneling. In this paper, the finite difference method (FDM) is used to investigate the face stability of longitudinally inclined shield tunnel in dense sand strata. Based on the numerical results of soil failure pattern in front of tunnel face, a novel two-dimensional (2D) failure mechanism is conceived following the upper bound theorem. It is assumed that the failure zone consists of an upper loosen zone, an arc-shaped shear band, and a triangular rigid body, which can calculate the active limit face support pressure pu. A scale factor for pu is introduced to quantify the three-dimensional (3D) behavior from the 2D calculation, with respect to different analysis approaches, e.g., numerical simulation, limit analysis, and limit equilibrium method. By incorporating the measure of scale factor, the calculation accuracy of the proposed 2D failure mechanism can be improved to provide reasonable prediction for longitudinally inclined tunnel.

Experimental study of foam infiltration from foam-sand mixture into saturated sand for EPB shield tunnelling: A mechanism study

This paper investigates the foam infiltration behavior from foam-sand mixture, resulting from foam conditioning of excavated sand inside of the excavation chamber of an Earth Pressure Balance (EPB) shield, into saturated sand at the tunnel face. The laboratory investigation was conducted using one dimensional element tests that simulate foam infiltration from the mixture into the saturated sand in front of the tunnel face. Pore pressures at different locations are measured near the sand surface, representing the tunnel face, while the sand column could provide a comparable hydraulic gradient to field conditions. Influences from foam injection ratio (FIR), remaining water content and sand type are examined. In the case of fine sand, it is found that there is little foam infiltration from the foam-sand mixture into the sand surface. Sedimentation of the foam-sand mixture on top of the saturated sand column is the dominating process that results in a dense mixture containing less liquid content with a smaller permeability. While for coarse-grained sand, foam infiltration will result in a foam infiltration zone with a lower permeability than the sedimented foam-sand mixture. Plastering effect at the tunnel face will increase with an increase in FIR and a decrease in remaining water content. Further analysis indicates that the sand matrix plays an important role in determining the permeability of a foam-sand mixture, as a higher density of sand results in a more tortuous seepage path for water. Mechanisms are discussed and implications for field applications are described.

Effects of the physical characteristics of foams on conditioned soil’s flow behavior: A case study

Earth pressure balance shield (EPBS) is the excavation technique responding to the technical construction complexity of the future line 16–1 of the Grand Paris Express project. Its principle is based on the capacity of the excavated ground to transmit the forces needed to counterbalance the earth and water pressures of the working face. Additives are injected into the soil; on the cutting wheel, in the excavation chamber and in the screw conveyor. Their role is to assure the soil conditioning by modifying the soil’s workability and its hydromechanical and rheological parameters. Therefore, the soil conditioning produces a plastic medium able to transmit the pressure to maintain the front face. In addition, as a second objective, the additives aim to overcome uncertainties in the soil. Depending on the case, the additives can reduce the clogging, internal friction, abrasiveness, and permeability of the soil. The additives can include water, surfactant, polymer and bentonite. The combination of water and surfactant with or without polymer produces the foam. In order to increase the excavation perfomance, laboratory prelimenary tests are performed to deduce the optimal foam parameters. Since no test measuring the foam characteristics is standardized yet, this article sums up the tests mostly used in foam characterisation; half-life measurements, rheological tests, microstructural observations. As for the conditioned soil, slump test is frequently used by the tunnelling industry to recommend the optimal soil conditioning parameters in terms of workability. This test has been adapted in this study and the results are verified by rheological tests, then compared to the real foam injection data used for the project and their effect on mechanical parameters. For a specific type of excavated soil, the quality (type) of foam recommended by the laboratory work falls into the same choice of additive based on in-situ tunnelling excavation data. These findings are explained by the foams characteristics. Therefore, a correlation between foam and the conditioned soil (sandy soil with silt and low percentage of clay) with foam is deduced at different levels: micro, meso and macro (real) scales.

The New Bubble-Slurry for Sand Conditioning during EPB Shield Tunnelling: A Laboratory Scale Study

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.

Research on optimizing performance of new slurries for EPBS soil conditioning based on response surface method

Earth pressure balance shield (EPBS) construction process produces a large amount of untreatable shield dregs. Transporting these dregs directly to the landfill not only increases the costs, but also causes severe environmental problems. In order to improve the utilization rate of fine-grained shield dregs, a method of using waste silty clay to partially replace bentonite as a conditioner is proposed. Bentonite, wasted silty clay, additives, and tap water are selected to prepare the bentonite- silty clay modified slurry (BSC) and its various properties are tested. The whole tests are completed by the phased test method. Firstly, four additives are added into silty clay slurry for modification, and sodium carbonate is determined as the optimal additive and its dosage scheme. On this basis, the quantitative function relationship between the content of materials and the performance of BSC is established according to response surface method (RSM), and the performance optimization of BSC is completed. Finally, the microstructure of slurry is studied by scanning electron microscope (SEM) to explain the modification mechanism of BSC by sodium carbonate. The results show that the slurry-making capacity of silty clay can be modified effectively when the mixing ratio of sodium carbonate is 1%–3%. The regression model established by RSM is in good agreement with the experimental results, which can effectively predict the BSC properties under different material contents. When the mass ratio of water: bentonite: silty clay: sodium carbonate is 1000:58.91:297.97:3.14, the performance of BSC is close to that of bentonite slurry with soil–water ratio of 1:10. This study provides a feasible way for the recycling discharged soil from EPBS tunnel construction.

Experimental Study on the Effects of New Foam on the Improvement of Sandy Soil for Earth Pressure Balance Shield

In this paper, starting from the foaming amount, half-life and other indicators, the surfactant with excellent performance is optimized, and the optimal compound ratio and appropriate foam stabilizer, viscosity enhancer, and additives are studied. A new type of foam agent formula for the EPB shield is developed. The performance of the new foam agent is compared with that of domestic foam and imported foam from the aspects of foaming rate, stability, micromorphology and temperature resistance, which verifies that it meets the parameter requirements and advantages of a foam agent for shield tunneling. Then, by means of laboratory tests, such as the mixing test, friction coefficient test, adhesion resistance test, slump test and direct shear test, the improvement effect of the new foam agent is compared with existing foam agent products at home and abroad, and the improvement effect of the new foam agent on soil is evaluated. The formulation of the new foam agent is as follows: 1.6% sodium dodecyl sulfate (SDS) + 8% dodecyl dimethyl betaine (BS-12) + 7% dodecanol + 0.06% guar gum + 0.6% ammonium chloride. The foaming rate, stability, microstructure and temperature resistance of new foam meet the requirements of shield construction. New foam, imported foam and domestic foam have advantages in sand and soil improvement experiments, but from an economic point of view, the new type of bubble is better than the other two. A new type of shield foam agent is prepared to meet the needs of construction. There is a big advantage in terms of price.

Numerical investigation on the spewing mechanism of earth pressure balance shield in a high‐pressure water‐rich sand stratum

AbstractThe 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 < 2.6 × 10−4 cm/s through appropriate soil improvement, which can effectively prevent the occurrence of spewing disasters.

Clogging Prevention of Slurry–Earth Pressure Balance Dual-Mode Shield in Composed Strata with Medium–Coarse Sand and Argillaceous Siltstone

The slurry–earth pressure balance dual-mode shield has an earth pressure balance (EPB) and slurry shield functions. Based on a shield tunnel project of Guangzhou Metro Line 12 in China, this study investigates the clogging prevention of a slurry–earth pressure balance dual-mode shield in a composed stratum with medium–coarse sand and argillaceous siltstone. The results show that the slurry mode was not applicable to the composed stratum with medium–coarse sand and argillaceous siltstone. The excavated soil accumulated easily in the slurry chamber, causing shield clogging. The total thrust force of the shield increased significantly, the tunneling speed gradually decreased to 0, and the torque of the cutterhead increased slightly after the slurry shield was clogged. The fluctuation in the total thrust force, the cutterhead torque, and the tunneling speed also increased significantly. The EPB mode is recommended for composed strata with medium–coarse sand and argillaceous siltstone. The dispersible foam agent and water needed to be used for soil conditioning. The injection amount of foam and water was determined according to the status of the mud discharged by the screw conveyor. Water absorption can be used to characterize the water absorption capacity of particles larger than 0.15 mm. The ideal soil state was that the consistency index of the particles smaller than 0.15 mm was less than 0.5 to prevent the EPB shield from clogging. The water absorption of soil with a particle larger than 0.15 mm should be removed when calculating the consistency index.

A coupled deep learning approach for shield moving performance prediction of underground tunnel construction

Recently, deep learning methods have attracted great attentions for geotechnical engineering, especially for the safety construction of underground tunnels. In this regard, the control and regulation of operational parameters are vital for safety and intelligent tunnel construction. In this study, hybrid data-driven models on the basic of optimization approaches [particle swarm optimization (PSO), multilayer hierarchical heterogeneous PSO] and deep learning methods (long short-term memory and gated recurrent unit neural network) were developed. The optimization approaches were utilized to obtain the optimal parameters of deep learning methods. Apart from that, geological data sets were extended to match the scale of operational data sets using Kriging method, which considered the effects of local geology comprehensively. The developed hybrid models (DHMs) were utilized to estimate the speed of earth pressure balance (EPB) shield machine for a tunnel project in China. Results indicate that errors in estimated results by DHMs are acceptable in geotechnical engineering. Besides, the DHMs exist potentials to cope with time-series data sets from underground tunnel project. After that, Pearson correlation coefficient was used to evaluate the relative importance of influential factors on target variable. Results show that penetration has significant correlation relationship with excavation speed. The geological and operational parameters are equally important for the prediction performance of DHMs. The developed DHMs can provide a reference for regulating shield moving performance and attaining safety construction of underground tunnels.