Slurry Shield Tunneling Research Articles

Assessing clogging potential and optimizing driving parameter of slurry shield tunneling in clay stratum assisted with CFD-DEM modeling

This paper proposed a two-way coupled computational fluid dynamics (CFD) and discrete element method (DEM) approach to analyze the evolution of clogging in slurry shield tunneling quantitatively. The interactions between clay particles and slurry were considered by exchanging three interaction forces, including buoyancy force, pressure gradient force, and drag force. The CFD-DEM coupling approach was first benchmarked by comparing cutterhead torque and total thrust with field monitored data of a practical slurry shield tunnel project. The evolution process of the particle phase and the fluid phase over time was presented. The results indicated that fewer than 70% of the particles can be washed away in time by the circulating slurry. About 9% of the particles adhered to the submerged wall, resulting in increased cutterhead torque and thrust. Through parametric analysis, the influence of the shield driving parameters on the clay clogging behavior is further explored. The time history of cutterhead torque or thrust can be used as a criterion for judging whether clogging has occurred. Additionally, a new assessment method of clogging risk and an optimization strategy of driving parameters were proposed, which were intended to provide some guidance for similar projects.

Numerical analysis of slurry penetration and filter cake formation in front of tunnel face

The bentonite slurry penetration in front of the tunnel face significantly influences the face stability during slurry shield tunneling. In this study, the mechanism of slurry-soil interaction is clarified through the soil microstructural evolution. A transient model coupling the slurry transport and deposition theory is proposed to estimate the slurry penetration and filter cake formation. The time-dependent parameters are considered, including the flow parameters (flow path and pressure gradient) and the hydraulic resistance of soils (soil permeability, porosity, and shear resistance). The proposed model is compared with the infiltration column tests and shows a good agreement.Moreover, a parametric study is conducted to investigate the influence of the soil and slurry properties on the slurry-soil interaction. The results indicate the soil-bentonite relative size is the main factor influencing the formed filter cake type. The filter cake of type Ⅲ (external filter cake and penetration zone) could be formed when the bentonite particle size is between the range of 0.01d10–0.5d10. This type of filter cake has good tightness and could help to improve the face stability when tunneling by slurry shield machine.

A Modified Process Analysis Method and Neural Network Models for Carbon Emissions Assessment in Shield Tunnel Construction

This paper proposes a modified process analysis method that combines with the input–output method for carbon emissions assessment in slurry shield tunnel construction. The method was applied to analyze the carbon emissions generated during the construction procedures of a slurry shield tunnel. The results indicate that the carbon emissions from building materials account for the majority of the total emissions, while those from the shield machine and construction procedure are relatively small. In addition, BP and CNN-LSTM neural network models were established to validate the accuracy of the calculation results with model error of 0.1031. Finally, recommendations for reducing carbon emissions in the construction course of slurry shield tunnels are provided.

Slurry Discharge Pipeline Damage and Wear Due to Transporting Rock Particles during Slurry Shield Tunneling: A Case Study Based on In Situ Observed Results

Rock particles in excavated materials can damage and wear down slurry discharge pipelines when slurry shield tunneling occurs in a pebble layer and rock ground. This pipeline damage and wear, if not properly dealt with, can lead to a broken-down tunneling machine. Based on a slurry shield tunneling project in China, damage and wear were thoroughly examined. The observed pipeline wear and leaks, transported rock particles, mechanical properties, and flow rate of the carrier slurry were presented. The measured results showed that the wear rates of a straight pipe, a pipe along a curved tunnel, a 60° inclined pipe, and a 90° elbow pipe in pebble ground were approximately 0.71 mm/100 rings, 1 mm/100 rings, 2.14 mm/100 rings, and 4 mm/100 rings, respectively. When the machine drove into rock ground, the wear rates increased by one to two times, which could be attributed to the sharper particle shapes. Countermeasures to address these issues, such as adjusting the pipeline layout, welding reinforcement plates in advance, and preparing additional pipes, were highlighted. The wear rates of different types of pipes, the effectiveness of new pipeline fixing methods, and the probability of pipeline leaks in different strata were discussed in detail.

Experimental Investigation of Face Stability of a Slurry Shield Tunnel Based on a Newly Developed Model Test System

Experimental Investigation of Face Stability of a Slurry Shield Tunnel Based on a Newly Developed Model Test System

Investigation of carbon emission in slurry shield tunnel construction based on modified process analysis method

Underground engineering, including shield tunnel construction, is a significant contributor to carbon dioxide emissions in infrastructure engineering projects. To better predict and control the carbon emissions associated with shield tunnel construction, this paper presents a novel calculation method: the modified process analysis method based on input-output and process analysis methods. To evaluate the effectiveness of the proposed method, a specific shield tunnel construction project was selected as a case study. The modified process analysis method was used to analyze the various factors that influence carbon emissions during the project’s construction phase. In addition, a neural network approach was applied to validate the accuracy of the calculation using the LSTM and BP neural network. The results demonstrate that the proposed method not only combines the strengths of traditional methods but also offers high accuracy and acceptable error rates. Based on these findings, several measures to reduce carbon emissions during shield tunnel construction are suggested, providing valuable insights for reducing CO2 emissions associated with infrastructure engineering projects. This study highlights the importance of adopting innovative approaches to reduce carbon emissions and promotes the implementation of sustainable practices in the construction industry. Through the use of advanced analytical methods, such as the proposed modified process analysis method, we can effectively mitigate the environmental impact of construction activities and make significant contributions to the global effort to combat climate change.

Method for calculating pressure losses in the pipelines of slurry shield tunneling based on coupled simulation of computational fluid dynamics and discrete element method

AbstractHydraulic mucking/transport or slurry pipelining are widely used in dredging, trenching, (deep sea) mining, tunnel boring, and many other applications. Slurry pipelining during slurry shield tunneling is a classic instance of complex hydraulic mucking systems, which is characterized by the complex rheological behavior of the slurry and the inclusion of large and irregular particles. These characteristics make it difficult to estimate the pressure losses along slurry pipelines, compared to other hydraulic mucking systems. This study established a powerful method for predicting the pressure losses in long‐distance slurry pipelining of slurry shield tunneling based on computational fluid dynamics–discrete element method (CFD‐DEM)‐coupled simulations. Rheological tests were carried out to provide reliable slurry flow parameters for CFD calculations. Irregular pebbles with different sizes were digitally scanned using 3D scanning technology, and the scanned models were applied to the DEM. This method was successfully applied to a slurry shield tunnel project in Beijing, China. The mechanical properties of the fluid phase and particle phase were revealed by modeling a typical slurry pipeline. A comparison of the pressure losses along slurry pipelines was made between the proposed method and field data, indicating the validity of the established method. The simulation results indicate that when the slurry flows through the elbow, a secondary flow will be formed due to centrifugal force. Pebbles of different sizes show different motion responses under the combined action of gravity and slurry coupling force. The turbulent flow of particles will increase the frictional pressure losses in the elbow area, compared to the straight section. This work provides a valuable reference for the layout of the slurry circulation system of similar projects.

A multi-field model for growth and dissipation of excess pore water pressure induced by slurry shield tunneling in semi-confined aquifer

This study focuses on the mechanism of slurry infiltration in front of the tunnel face during the construction of a slurry shield machine. A multi-field transient model is proposed to describe the growth and dissipation of the excess pore water pressure and the deposition of bentonite particles for slurry shield tunneling in the saturated semi-confined aquifer. Based on the conservation principle, the governing equations for the mass continuity of bentonite particles and volume continuity of slurry are derived with consideration of particle deposition and the elastic storage of the aquifer. The proposed model is validated by field measurements. The results indicate that the excess pore water pressure drops steeply close to the tunnel face and then gradually decreases to a stable value over several tunnel diameters due to the retardation of the infiltration zone. The retardation effect is positively correlated with the permeability coefficient of the soil and the filtration coefficient, while it is negatively correlated with the specific storage coefficient. The penetration distance of bentonite particles decreases with smaller soil pores and a higher slurry concentration. Besides, a quantitative equation is proposed to evaluate the influence of slurry concentration, filtration coefficient, and storability of the soil on slurry support efficiency.

Infiltration behaviour and microstructure of filter cake from sand-modified bentonite slurry

This study uses a setup involving a scanning electron microscopy (SEM) to examine the infiltration behaviour and mechanism of the formation of filter cake from sand-modified bentonite slurry. We began by determining whether a filter cake can be formed by using water–bentonite slurry in samples of the fine, medium, and coarse sand (Series 1). Water–bentonite–sand slurries with different mass ratios of bentonite and fine sand (B:S) were subsequently tested using the medium and coarse sand (Series 2). The results show that for Series 1, a thin filter cake (<0.1 mm after drying) with extremely low permeability (∼10−8 m/s) was formed only when the fine sand was used. The filter cake consisted of layers of dense bentonite platelets arranged perpendicular to the infiltration direction. Similar results were obtained for Series 2 between the medium and coarse sand. Within the range of 1:1.5 ≤ B:S ≤ 1:0.5, the low permeable filter cake was formed on the deposited layer of fine sand and bentonite platelet. Contrary to expectations, adding more sand (B:S = 1:2) did not complete the formation of filter cake, and instead led to continuous deposition of fine sand and bentonite platelet, which may cause inadequate effective face support during slurry shield tunneling.

Effects of seawater on the infiltration behavior of bentonite slurry into sand

As a support fluid, bentonite slurry has been extensively used in slurry shield tunneling to stabilize the tunnel face by forming a low-permeable filter cake. However, the infiltration behavior of bentonite slurry in complex conditions such as when suffering from seawater intrusion has not been fully understood. In order to investigate the infiltration behavior and filter cake formation under seawater intrusion, seawater-based bentonite slurries with increased salt content were prepared to represent different seawater intrusion severities. The rheological properties and physical stability of these slurries were evaluated. A series of infiltration column tests were conducted on two saturated sand beds with different particle sizes. Scanning electron microscope (SEM) was performed to unravel the microstructure of the filter cake. The infiltration column test results show that the low-permeable filter cake can be formed on the fine sand bed regardless of the salt content of the slurry. The salt shortened the time span of the mud spurt and made the filter cake more permeable to water. For the medium sand, the filter cake cannot be formed when the salt content exceeds 1%, because the deterioration of rheological properties of the slurry weakened its rheological blocking effect when infiltrating the sand. Based on the SEM images, it was found that the filter cake under seawater intrusion contained many well-connected inter-agglomerate channels that reduced its sealing quality. The obtained results can provide guidance and reference for the application of bentonite slurry when tunneling in a sea environment.

A new pressure gradient model of slurry shield pipeline system coupling with rheological properties and wall slip behaviour under sandy stratum

Pipeline transport is of uttermost importance during slurry shield tunneling, and the precise calculation of the pressure gradient of the pipeline system is the premise for the optimal design of industrial slurry pipeline system. This paper proposed a new muck slurry pressure gradient model of slurry shield pipeline system coupling with the wall slip behaviour based on the Herschel-Bulkley rheological model (H-B model) to estimate the pressure gradient of the muck slurry under sandy stratum. Effects of the sands volume fraction and average particle size on the rheological properties of the muck slurry in pipeline system were investigated to construct the H-B rheological parameters prediction models, which were coupled into the proposed pressure gradient model. Besides, the accuracy and dependability of the proposed model were validated through the engineering field tests and model tests, and the prediction results of the proposed pressure gradient model has an average error of 10.6% and 8.2% with engineering field test data and model test data respectively. The results indicated that the muck slurry is a typical thixotropic fluid. The sand volume fraction and particle size both significantly affect the rheological properties of the muck slurry, the yield stress and plastic viscosity of H-B rheological parameters increase exponentially with the sand volume fraction increasing and increase linearly with the average particle size increasing, the power law index of H-B rheological parameters increases linearly as average particle size and sands volume fraction increase. Ultimately, the research results can provide reasonable suggestions and references for the designing of pipeline system and efficient tunneling of slurry shield.

Analytical study and field investigation on the effects of clogging in slurry shield tunneling

The pressurized slurry is the essential part of the slurry pressure balanced shield (SPBS) for stabilizing the working face and transporting the excavated soil. If clogging occurs in case of adverse ground conditions or equipment failure, a series of consequences will happen, such as sudden changes in slurry pressure, massive ground loss, and deformation or even collapse at tunnel workface. The present study reviewed and summarized the clogging mechanism of the slurry shield under different ground conditions. Then, based on the law of conservation of mass and Bernoulli’s Equation, an analytical approach was developed to investigate the effects of clogging on the slurry velocity and pressure along with the deformation of tunnel workface. Furthermore, a case study and field measurements were conducted to validate the analytical method. The significant findings drawn from the study can be summarized as follows: (1) The clogging of a slurry shield imposes severe effects on the slurry pressure balancing mechanism of shield tunneling. It can occur in both cohesive and non-cohesive ground with different formation mechanisms. (2) During the initial clogging, the slurry pressure above the spring line increases, and that below the spring line decreases. With the clogging deteriorating, the slurry velocity drops considerably at the top of the deposit area and escalates near the discharging port. The presence and increase of deposit lead to a sharp rise of slurry pressure in the slurry chamber, especially in the clogging region. (3) The reverse circulation of the slurry system (RCSS) is useful to clear the clogging. Still, the slurry deposits at the bottom will obstruct the slurry reverse flow, resulting in inward deformation of the tunnel face and a drastic increase in overcutting volume. (4) The ultimate volume deviation ratio of the slurry was proposed as a useful index to alarm the potential risk of collapse induced by the RCSS. (5) The timely adoption of RCSS is strongly suggested when any abnormal changes in the slurry pressure or the ultimate volume deviation ratio are observed for the first time.

Recycling waste sand from slurry shield tunneling: A sustainable filter aid for waste slurry dehydration

The slurry shield tunneling construction process produces waste slurry (WSL), which is often dehydrated using lime as a filter aid. However, a high pH limits the reusability of the mud cake produced, leading to environmental problems. Slurry shield tunneling also produces waste sand (WSA), which the present study evaluates as a substitute for lime via belt filter press testing. The ability of WSA to reduce the mud cake water content and porosity is found to be the same as that of lime, although double the amount is required. Specifically, the optimal amounts of lime and WSA addition are 2% and 4%, respectively, which produce similar total drainage rates and mud cake moisture contents and porosities. Both lime and WSA reduce mud cake macropores, increase small pores and improve compactness. The addition of >2% lime increases the bound water and moisture contents of the mud cake, while WSA reduces the bound water without affecting dehydration. Finally, the use of WSA instead of lime as a filter aid reduces the pH of the mud cake from strongly alkaline (about 12) to near-neutral (about 8). The results show that WSL can replace lime as a filter aid, providing a novel approach to WSL dehydration treatment and WSA reuse that has both environmental and economic benefits.

Influences of polyanionic celluloses and temperature on the rheological property of seawater slurries

As an increasing number of submarine tunnels are being constructed by the slurry shield tunnelling method, seawater intrusion into slurries and variations in strata temperature are inevitably encountered during construction. They will lead to obvious deterioration in rheological properties of slurries, which can be adverse for the filter cake formation and even cause tunnel face instability. Considering the design of the slurry parameters and the actual slurry support effect, it is essential to select a suitable slurry ingredient and investigate the rheological properties of seawater slurries under different temperature environments. Using polyanionic celluloses (PAC) as the additive to seawater slurries was proposed in this paper. The influence of PAC contents on the rheology and physical stability of seawater slurries was investigated through laboratory experiments, with PAC contents varying from 0 to 0.4%. The rheology enhancement mechanism of PAC was also analyzed. Furthermore, the flow pattern and constitution relationship of seawater slurries with 0.4% PAC were studied at different temperatures from 2℃ to 30℃. The results show the presence of PAC can significantly enhance the viscosity and physical stability of seawater slurries. The seawater slurries incorporating 0.4% PAC and 12% Na-bentonite can maintain an excellent colloidal rate with 97.5% and a high viscosity, which was recommended to be a prospective mixing proportion used in slurry projects. Additionally, the flow characteristic of seawater slurries with different PAC contents can be described by the power law model very well. Moreover, with the increase in temperature, the rheological index showed an increasing trend, but the consistency coefficient gradually decreased. The constitutive equation of seawater slurries considering the effect of temperature was proposed. This research will provide significant guidance for the selection of slurry parameters and the establishment of seawater slurry infiltration theories during the construction of submarine tunnels.

A novel evaluation model for carbon dioxide emission in the slurry shield tunnelling

Excessive carbon dioxide emission leads to climate change and severely threaten social sustainability. A novel model for evaluating carbon dioxide emission in slurry shield tunneling is presented. Firstly, the carbon dioxide emission from a slurry shield tunnel during construction was calculated using the emission coefficient method. Then, for predicting carbon dioxide emission in slurry shield tunneling, a support vector regression integrated sparrow search algorithm model was proposed. Random forest is used to select the input variables of the model. The predicted model accuracy is validated by comparing it to multiple machine learning models. The results show that the model can accurately predict carbon dioxide emission in the shield tunneling with acceptable errors. The influencing factors for carbon dioxide emission in the slurry shield tunneling are disscussed. The recommendations for low-carbon tunnel construction are given. This study provides reference for guiding the low-carbon tunnel construction.

Laws and Numerical Analysis of Surface Deformation Caused by Excavation of Large Diameter Slurry Shield in Upper-Soft and Lower-Hard Composite Stratum

Due to the large cross-section design of large-diameter shield tunnels, most of the rocks and soils it crosses are composite strata with upper soft and lower hard. In order to reduce the construction cost of shield working shafts, large-diameter shield launching is usually buried at a shallow depth. Based on the typical large-diameter slurry shield tunnel, the following research results were obtained according to field monitoring and PLAXIS 3D finite element simulation. (1) The electronic level is used to monitor the surface settlement, and the field monitoring data were obtained; the surface settlement duration curve at the axis of the shield tunnel during the construction period can be divided into four stages: pre-deformation, shield passing, shield tail exit and shield moving away, of which the surface settlement accounts for the largest proportion during the shield passing. (2) In order to ensure the accuracy of the numerical simulation results, the linear shrinkage of the shield needs to be considered in the modeling. (3) The maximum surface settlement value at the center of the tunnel increases with the increase of the support pressure; when the support pressure exceeds 300 kPa, the surface uplift and the settlement caused by the formation loss will offset, and the surface settlement will decrease instead. The maximum surface settlement value is inversely proportional to the grouting pressure, but with the increase of the grouting pressure, the maximum uplift of the surface continues to increase. (4) With the numerical simulation of excavation step construction, the surface uplift increases with the increase of grouting pressure and shield radius, and decreases with the increase of shield buried depth.

Effect of Viscosity on Slurry Infiltration in Granular Media

The infiltration of slurry in granular media plays an important role in face stability during slurry shield tunneling. This study investigates the effect of slurry viscosity in Fujian standard sand through a series of column tests combined with microstructural investigation, which remains unclear. The viscosity of slurry covers a wide range, from 1.8 to 2,248 mPa·s. Slurry particles in the low-viscosity test (1.8 mPa·s) fail to form an impermeable cake due to the loose stacking of slurry particles retained in sand voids, corresponding to a slight water pressure difference and relatively large permeability. For the high-viscosity test (2,248 mPa·s), an impermeable cake forms at a shallow depth; however, this internal cake is vulnerable to be disturbed and damaged due to its shallow thickness. Ultimately, one recommended viscosity, 56 mPa·s, is identified as a transition state with two-stage intact clogging, exhibiting an effective infiltration depth and intact internal cake that forms simultaneously. Experimental observations show a negative water pressure difference in the high-viscosity test, attributable to the siphonic principle. Furthermore, a measurement of slurry samples and solutes using the material observation apparatus could provide the microstructure evidence to interpret the effect of slurry viscosity in infiltration clogging. This study offers a reliable reference for interpreting the effect of viscosity on slurry clogging in infiltration performance.

Properties of Slurry Shield Tunnel Sand and Its Application in Large Flow Concrete.

The amount of Slurry shield tunnel slag (SSTS) from the Beijing East Sixth Ring Road renovation project is about 3 million m3, and it is mainly fine and silt sand. In order to realize its resource utilization, the properties of SSTS and the performance of concrete with strength grades from C30-C60, which used the mixed sand compound with SSTS and Coarse Manufactured Sand (CMS) as a fine aggregate, were investigated. The results showed that SSTS’ fineness modulus is 1.2, its clay content is 17.0% but its composition is mainly Inert Silt (IS), and SSTS replaced with 40% of the mass of CMS can obtain a mixed sand with a fineness modulus of 2.7 and a clay content of 7.0%. The morphological and filling effects of SSTS and IS will improve the workability and durability properties of concrete with no adverse effects on the compressive strength. On the other hand, clay lumps in SSTS adversely affect the workability, early cracking properties, and freeze resistance of concrete, which can be alleviated by dewatering and crushing the clay lumps in SSTS.

Laboratory model test of slurry shield tunnelling in saturated sandy soil

Experimental tests were carried out using a laboratory-scale model of a slurry shield machine to investigate the mechanical behaviours of soil in response to excavation in saturated sandy ground. The experimental model includes a miniature slurry shield machine that can approximate real tunnel construction, replicating the in situ methods of cutterhead excavation, temporary support with pressurised slurry at the face, advancement of the shield machine and a slurry circulation system. The effects of slurry characteristics, soil permeability and advance rate of the shield machine on the ground displacement, slurry infiltration and excess pore water pressure were analysed and these data were compared with predictions in previous studies. The results confirm that the model tests can effectively simulate the principal operations of a real slurry shield machine. Compared to the test results presented, previously proposed models tend to overestimate the increase rate of pore water pressure in front of the tunnel face, especially in cases of soil with lower permeability, higher concentrations of bentonite mixed slurry and lower advance rates. The slurry-infiltrated zone in front of the shield machine is non-uniform across the tunnel face and extends radially to a larger area than the tunnel diameter. The shape of the slurry-infiltrated zone can be categorised as one of two typical types: (a) cone; (b) bulb.

Three-dimensional tunnel face stability considering slurry pressure transfer mechanisms

In slurry-shield tunneling, the bentonite slurry tends to infiltrate into the soil skeleton ahead of the tunnel face, causing a forward seepage flow associated with excess pore pressures in the ground. Both the slurry infiltration and the forward seepage flow significantly impact the tunnel face stability. In the present paper, a kinematic approach of limit analysis with the classical three-dimensional discretized rotational failure mechanism is employed to assess the face stability of a slurry-shield-driven tunnel. Three situations of slurry infiltrations that influence the face pressure transfer mechanisms were considered: a full filter cake, a penetration zone, and pure groundwater flow without a filter cake or a penetration zone. The pore pressure distributions surrounding the tunnel face were firstly obtained using numerical calculations. Afterward, the incorporation of numerically obtained pore pressure distributions into the work rate calculations allows considering a three-dimensional forward seepage flow ahead of the tunnel face, which provides a rigorous upper-bound estimation to the tunnel face safety factor determined by the strength reduction technique. The proposed method is validated by comparing the calculated results with those provided by previously published literature. The effects of excess slurry pressures, hydraulic conditions, pressure drop coefficients, and soil shear strength were investigated. Several design charts are given for practical application.