Earth Pressure Balance Tunnel Research Articles

Enhancing earth pressure balance tunnel boring machine performance with support vector regression and particle swarm optimization

This paper combined data-driven modeling and optimal control for performance enhancement of earth pressure balance tunnel boring machine (EPBM). Two coupled processes, EPBM advance rate (AR) and cutterhead rotation torque, are modeled using support vector regression (SVR). An optimal control framework was formulated to maximize the AR, solved with particle swarm algorithm. Using the Seattle N125 project as case study, it is found the SVR model can predict EPBM AR with R2 = 0.90, normalized root mean square error (nRMSE) = 0.30 and mean absolute percentage error (MAPE) = 31.2%, and R2 = 0.65, nRMSE = 0.59 and MAPE = 6.7% for torque prediction. Compared to human operator, EPBM with optimal control can increase AR by 0.6–23.3 mm∕min on average, accompanied by an average torque reduction of 83.1 kN - m. It is found higher cutterhead rotation and lower chamber pressure always contribute to faster tunneling, but the optimal total thrust force to apply depends on the chamber pressure.

A More Comprehensive Way to Analyze Foam Stability for EPB Tunnelling—Introduction of a Mathematical Characterization

In the tunnelling industry, a large share of the market is occupied by EPB (Earth Pressure Balance) machines. To operate this kind of machine, a radical change in the rheological behaviour of the excavated soil must be performed, and this is achieved by adding water, foam, and, eventually, polymers. The stability of the foam is assessed through a half-life test. The main limitation of this test is that only one value is used in the characterization of the foam degradation process, which is insufficient to describe the whole evolution of the phenomenon. The results of more than 270 tests were modelled through a five-parameter mathematical formulation that suited the experimental data. The results show that the influence of concentration on the stability of the foam is not always present and that the flow rate used during production bears an influence on the characteristics of the foam.

Predicting earth pressure balance (EPB) shield tunneling-induced ground settlement in compound strata using random forest

The prediction and control of the ground settlement induced by shield tunneling in compound strata is a crucial challenge for tunnel excavation. In this regard, a finite element model for the analyses of ground surface settlement induced by earth pressure balance (EPB) tunneling was established and validated by the field test. The influence of single influence factors, such as rock proportion in the face, support pressure, and tunneling angle, on the surface settlement above the tunnel face was systematically studied. To investigate the joint influence of different factors on ground deformation mechanism, the multi-factor analysis based on field data and the multiple regression (MR) method was conducted. However, the prediction accuracy of the MR method was limited. Therefore, a prediction approach of shield tunneling-induced settlement in compound strata based on a random forest (RF) was developed. Tunnel geometry, geological features, and operation parameters were investigated as input variables in the RF model to achieve this goal. The applicability effect of the RF-based method was proved by utilizing two data sets (i.e., the settlement ahead of tunnel face and final settlement). The results indicated that the prediction accuracy of shield tunneling-induced settlement in compound strata by the RF-based model is higher than the MR method. The relative importance analysis of variables indicates that advance rate, the cover depth of the tunnel ring, and pressure in the chamber were the most influential features for the settlement ahead of the tunnel face, while the thrust of the cutter head and the grout quantity were the most significant variables for the settlement after the segment assembling is completed. Overall, the conducted study could provide a helpful reference for the efficient excavation of EPB tunnels across compound strata.

Assessment of reliability and maintainability of earth pressure balance tunnel boring machine (EPBTBM) – An approach

Earth pressure balance tunnel boring machines (EPBTBM) are used in soft or mixed ground conditions often encountered in tunneling operations. The performance of EPBTBM could be enhanced to a great extent by appropriate time management, proper maintenance and reduced unproductive times such as idle hours and breakdown hours using reliability analysis. The reliability of a system primarily depends on time between failure (TBF) and maintainability is based on time to repair (TTR). Investigations on EPBTBM system performance were carried out for an irrigation tunnel through mixed strata conditions and the failure/repair data were collected. TBF and TTR data are found independent and identically distributed (IID). This implies that data are not following any particular trend or correlation. Therefore, these data are fitted in different types of probability distributions such as Gamma, Lognormal, Weibull with Easyfit and MATLAB tools. All the distributions are subjected to a Kolmogorov-Smirnov (k-s) test. Data analysis carried out on EPBTBM deployed in tunnels showed that TBF data and TTR data are found to best fit in General Gamma (4p) and General Gamma distribution respectively. Analysis indicated that ninety percent level of reliability of the system is at 2.64 h of operation whereas the mean time to repair is 5.78 h. The overall availability of the system was 67% which is comparatively less. Failure mode effects and criticality analysis (FMECA) was also carried out to find the critical part of the EPBTBM system. Disc cutter got the highest risk priority number indicating that it is the most vulnerable part of the EPBTBM. This paper reports that the availability of such systems can be increased to 80 percent by appropriate scheduling, preventive maintenance, and effective use of skilled manpower.

Experimental study and field validation on soil clogging of EPB shields in completely decomposed granite

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.

Effect of foam conditioning on performance of EPB shield tunnelling through laboratory excavation test

An appropriate application of a soil conditioning strategy during an earth pressure balance (EPB) shield operation is essential for reducing the downtime accompanied by improvements to the hydro-mechanical properties of the muck. Among the various additives used for conditioning, foam is the most widely adopted owing to its usability. In this study, to simulate EPB tunnelling and study the effects of ground and foam injection conditions in the laboratory, a lab-scale excavation test apparatus was devised. This apparatus was designed to vertically excavate the soil specimen with the simultaneous injection of foam and water, and discharge the conditioned muck. Subsequently, a series of excavation tests were conducted using artificial sandy soil to estimate the effects of the fine content, water content, and foam injection conditions, which can affect the performance of EPBs. Based on the measured torque, the weight loss of the cutter bits, and the slump of the muck from the excavation tests, the effects of foam injection parameters, namely, the foam injection ratio, foam expansion ratio, and concentration of foaming agent were discussed. The results show that the suggested laboratory excavation test can be used to qualitatively evaluate the effects of the basic soil properties and foam injection conditions on EPB shield tunnelling.

Face failure in cobble-rich soil: Numerical and experimental approaches on 1 g EPB reduced scale model

Recent years have witnessed several accidents associated with tunnel face failure in cobble-rich soil in the city of Chengdu, China. Due to its lack of cohesion, cobble-rich soil can be easily disturbed by shield tunneling. Based on the general conditions of the Chengdu Metro Line 1 project, the mechanisms of face failure of tunnels in cobble-rich soil driven with earth pressure balance (EPB) machines are studied. Specifically, we present results of tests carried out using a laboratory reduced-scale model of EPB tunneling operations in cobble-rich soil. The failure kinematics and limit face pressures are presented and analyzed. Then a three-dimensional (3D) discrete-element method (DEM) model, which is able to simulate the main EPB excavation processes is employed to gain further insight into the mechanisms of face failure in cobble-rich soil. Comparisons of these results with the observations based on previous studies are discussed. The results reveal a fundamentally different tunnel-face failure mechanism in cobble-rich soil in contrast with that in clayey or sandy soils. It shows that the ground movement during face failure is sudden in cobble-rich soil, which is different from the progressive mechanism in frictional–cohesive materials. The observed sinkhole at surface takes the shape of an oval, and the failure zone behind tunnel face extends almost as far as that ahead of the face, which is different from the observations in previous studies. The failure zone is found to be wider than that of sandy soils in both the transverse and longitudinal directions.

Laboratory Test of EPB Shield Tunneling in Mixed-Face Conditions

Field measurements and laboratory tests were carried out to study the mechanical behaviors of soil in response to earth pressure balance (EPB) shield tunneling in mixed-face conditions, i.e., layers of granite and sand. Specifically, an experimental investigation was carried out using a laboratory-scale model to replicate the EPB tunneling operations in such ground conditions. The effects of the ratio of rock/sand at the tunnel face (Tr/Ts) and the cover-to-diameter ratio (C/D) on the soil pressure in the chamber, ground displacement, and volume loss were examined. The results show that the soil's response to the mixed-face conditions differs significantly from that in the homogenous ground and highly depends on the ratio of rock/sand, especially for relatively shallow tunnels. As the ratio of rock/sand at the tunnel face increased, the pressure gradient in the chamber became more inconsistent. The translated cumulative curve does not always provide a good fit in the mixed-face conditions, particularly in cases when the rock/sand ratios are high. A smaller width of settlement trough has been observed for cases with a large rock/sand ratio compared to that in clay and cohesionless soil.

Evaluation of the Adaptability of an EPB TBM to Tunnelling through Highly Variable Composite Strata

The adaptability of an earth pressure balance (EPB) tunnel boring machine (TBM) to complex geological strata needs to be evaluated when a tunnel project is set up. It is helpful for better design and more suitable technical parameters of the machine. Because the factors that influence adaptability are numerous and interrelated, their effects also differ; hence, a quantitative assessment of adaptability is generally difficult. In the present study, a method for quantitatively evaluating the excavation adaptability of an EPB TBM was developed using an analytic hierarchy process (AHP) and fuzzy comprehensive evaluation. The method first involves an initial selection of factors that may influence tunnelling. Based on the expert questionnaire and their influence weights determined by AHP, the 10 evaluation indexes from 25 primary indexes were selected to establish the evaluation index system (EIS) including a target, criteria, and index level. Fuzzy membership functions are then derived one after another based on the effect of each evaluation index on the EBP TBM tunnelling process. A fuzzy mathematical method was finally presented to determine the adaptability of an EPB TBM excavation performance. The proposed method was applied to two special sections of diameter 6.98 m excavated by two EPB TBMs in the Shenzhen Metro Line 11 project. The geological exploration shows that these two tunnel sections pass through typical composite strata in Shenzhen. The determined adaptability grades of the EPB TBMs used for composite strata were confirmed by the actual tunnel excavation. It indicates that the developed method is reasonable and useful to evaluate tunnelling adaptability of EPB TBM.

Residual water content of excavated soil in EPB tunnelling

The water content of the excavated soil in EPB (Earth Pressure Balance) tunnel construction controls the amount of conditioning of the excavated material in the excavation chamber. The water content influences crucial performance parameters (conditioning, wear, support pressure, etc.). With regard to foam conditioning, this means that the less water enters the excavation chamber, the drier the soil-foam mixture with constant foam properties. Especially in cohesionless soils below the groundwater level, the amount of free pore water is quite high. The cutting process, the effective pressure state and the use of conditioning agents result in flow processes of pore water and foam, which can lead to the displacement of water from the pore space at the tunnel face. Therefore, only residual moisture is present when the soil enters the excavation chamber. However, how much water enters the excavation chamber is still not determinable and not considered designing soil conditioning for the TBM drive.In this study, the initial water content was linked to an analysis of the residual water content entering the excavation chamber during EPB advance. Foam Penetration tests with different cohesionless soils were performed on the laboratory scale to analyze the residual water content. From the laboratory tests, water contents for the design of soil conditioning concepts can be determined for specific tunnelling situations.

The effect of rock weathering and transition zones on the performance of an EPB-TBM in complex geology near Istanbul, Turkey

The new Istanbul Airport is being constructed over an old coal field area, 35 km away from the city center. The construction of a rail link between the city and the airport entails the excavation of tunnels in the areas of complex geology, creating several problems for tunnel construction. This paper summarizes the effect of different grades of weathering of an interbedded sandstone-mudstone-siltstone sequence and transition zones on the performance of an earth pressure balance tunnel boring machine (EPB-TBM) during this work. The variation of EPB pressure, TBM thrust, torque, penetration, and specific energy values were analyzed in different lithologies and weathering zones and with respect to the attitude of transition zones between different materials. Normalized thrust and torque values considered in respect of penetration rate and EPB pressure showed that the attitude of transition zone on TBM performance is strictly dependent on the position of each geologic formation in the respect to the other, inclined or horizontal. To be specific, in an inclined transition zone, the thrust and torque values were almost double compared to the values obtained in a horizontal transition zone in the same lithology. This was probably due to increased over-break in inclined transition zones, which created loose material that accumulated in front of the cutter head. However, further case studies would be required definitely to confirm this. Mean normalized thrust and torque values were almost half in moderately/highly weathered sandstone compared to fresh/slightly weathered sandstone. The highest mean penetration of 28 mm/rev and mean lowest specific energy of 2.4 kWh/m3 were obtained in transition zone between claystone with silty sand and highly weathered interbedded sandstone-mudstone-siltstone. Due to impact effects, increased disc consumption was experienced where passage was made from low strength due to high strength formations, with up to one disc per 115 m3 of excavated material in the transition zones included in the study. The relatively high production rates were achieved, which attributed to a combination of logging daily the face conditions accompanied by appropriate adjustments of soil conditioning agents (FIR-foam injection ratio and FER foam expansion ratio values) and anti-clogging agents to control the sticking behavior of the spoil. The impact of different ground conditions on EPB-TBM performance in different projects is also discussed in relation to the results obtained in this study. It is believed that this research will help to manage better mechanized tunneling activities in similar conditions.

Assessment of differential subsidence harmful effects on large bridge structures during the underground space development

The purpose of the study was to confirm the capability of numerical simulation of soils shift and bridge structures elements deformations during underground construction. The main goal of this simulation was to assess the harmful effects of mining operations on the existing bridge load-bearing elements. Modeling of bridge structures undermining by earth pressure balance tunneling boring machines (EPB TBMs) was carried out both at the design stage for predicting deformations and at the stage of underground work completion.

Discussion: Challenges of earth pressure balance tunnelling in weathered granite with boulders

Discussion: Challenges of earth pressure balance tunnelling in weathered granite with boulders

3D numerical simulation of consolidation induced in soft ground by EPB technology and lining defects

This paper analyses with a sophisticated numerical model the long-term phenomenology induced by Earth Pressure Balance tunnelling in compressible, low permeability soils. The complex soil-structure interaction determined by the EPB technology is analysed with a three-dimensional model that closely reproduces the sequence of face pressurisation, excavation, lining installation and tail grout injection and simulates the hydro-mechanical soil coupled response with a non-linear, irreversible, anisotropic hypoplastic model. After validation on a documented case study taken from literature, the model has been applied to a real example, MRTA Project in Bangkok. The study reveals that short and long-term settlements induced by tunnelling cannot be decoupled, being both governed by the fluid-soil-lining interaction and resulting from the time dependent stress transfer among the different elements. In this scenario over pressurising face and tail void reduces the immediate soil relaxation at the expenses of increasing the long-term settlements. Local or global defects have then been hypothesized on the lining waterproofness considering different initial groundwater conditions, soil permeability models and sets of EPB operative parameters. While concentrated lining defects play a negligible role, extensive imperfection noticeably increases settlements, volume loss and bending moments on the lining, with the same function of the relative soil-lining permeability.

Experimental study on the stability of foam-conditioned sand under pressure in the EPBM chamber

Proper soil conditioning is critical for effective earth pressure balanced tunnel boring machine (EPBM) operation. Foam as a soil conditioner has been extensively used in EPBM tunneling to modify the excavated soil properties. A critical characteristic of foam-conditioned soil is stability, i.e., the ability to maintain the engineering properties throughout the residency time (30–90 min) in the mixing chamber. A comprehensive suite of experiments on foam-conditioned soil was conducted at both microscale and macroscale to investigate the fundamentals of foam-soil interaction and engineering properties of foam-conditioned soil. A foam-soil capture device and an optical microscope were used to capture bubble-grain images at a microscale under pressure. A pressurized testing chamber (PTC) was used to examine the stability of the mechanical properties of foam-conditioned soil. The compressibility, vane shear strength, pore pressure, and effective stress of foam-conditioned soil with elapsed time were obtained from the PTC test. The bubble-grain image analysis results reveal that foam is more stable when mixed with soil than by itself, indicating that soil particles help stabilize foam bubbles. Soil particles create a barrier to bubble coalescence and coarsening. The PTC test results show that the engineering properties of foam-conditioned soil remained constant over the 60-minute test duration. The results suggest that the accumulated “air bubble” at the top of the mixing chamber during EPBM tunneling is not due to the de-stabilization of foam bubbles when mixed with soil.

Laboratory tests on conditioning the sandy cobble soil for EPB shield tunnelling and its field application

During the construction of the interconnection tunnel from Tianqiao Station to Yongdingmen Station of Metro Line 8 in Beijing, the Earth Pressure Balance (EPB) shield machine had to excavate in sandy cobble soil. Because the sandy cobble soil has varying moisture content along the tunnel route and shows the characteristics of loose particle structure, large pores and no cohesion between the cobble particles, it is difficult to stabilize the support pressure acting on the cutting face and transport the muck. To effectively and efficiently use the EPB tunnelling in such situations, a series of laboratory tests on conditioning the sandy cobble soil to a reasonable quality were conducted, and the influence of water content and additive amount on the conditioning effectiveness was studied. Based on the test results, the optimal soil conditioning schemes corresponding to the water content of different soil groups were proposed. Finally, the laboratory test results were used to guide the soil conditioning in the field and the measured field data showed that the conditioned material fulfills the required standard for an EPB application.

Laboratory Studies on Soil Conditioning of Sand in the Mechanized Tunneling

Abstract Earth pressure balance (EPB) tunnel boring machines have become increasingly common in the last decade for the construction of tunnels in soft soils. The performance of these machines is so different that they have a major effect on the outside conditions of drilling operations. They use the excavated soil in a pressurized head chamber to apply support pressure to the tunnel face during excavation. A machine may be designed to work in ideal ground conditions. However, natural soils rarely have these properties, and conditioning of the soil is usually necessary to alter its properties to suit the machine. Tunnel excavation is not impossible when the unconditioned soils around the tunnel are coarse and sticky. Effective soil conditioning significantly improves machine performance and control of the soil flow through the screw conveyor. This research presents experimental investigations of soil conditioning for soil/foam and foam/polymer/soil. Methods are also provided to establish a suitable conditioning procedure for determining the optimum foam injection ratio (FIR) for these soils and for studying the effective parameters in soil conditioning such as shear strength and plasticity paste of conditioned soils. Direct shear strength tests were conducted on a small scale, and the plasticity paste of soil was studied by slump tests. The slump tests show that the plasticity of sand/foam and sand/foam/polymer is dependent on the initial moisture content, type of soil, etc. Thus, these parameters are important for achieving optimum FIR and suitable conditioning. In addition, direct shear strength tests show that the injection of foam to sand Type C caused a reduction of 51 % in the internal friction angle. Thus, foams are suitable for modifying soil. Increasing the FIR by more than a special value (optimum FIR) did not change the shear strength significantly.

Face Stability Analysis for the Earth Pressure Balance Method in Nonhomogeneous Inclined Soil Layers: Case Study

Earth pressure balance tunnel boring machines (EPB TBMs) were successfully used for tunnel excavation in loose grounds. Determining the minimal support pressure in the tunnel face is one of the most critical steps in tunneling by the earth pressure balance method. This study aimed to investigate the tunnel face stability of the Dez–Ghomrood water transfer tunnel analytically and numerically. During the water transfer tunnel project from Dez to Ghomrood, a very weak rock mass similar to an alluvial zone was encountered. Considering the depth of the tunnel and also the layering of the ground, calculating the proper support pressure of the tunnel face was significantly complex. The analytical method was based on the slope of layers relative to the tunnel face and the limit equilibrium of forces. Numerical analyses were conducted based on three-dimensional finite-element simulations using the PLAXIS 3D tunnel. The results demonstrate that the formation of soil arching yields to the curvilinear distribution of the weight of upper levels, therefore, pressure reduction at the tunnel face. The geomechanics parameters of the ground, shield geometry, and ground convergence in the excavated section are the determining factors in calculating the support pressure of the tunnel face. The slope of ground layers should also be considered in the tunnel face failure mechanism and in support pressure. The operational support pressure is compared to the analysis results, and the good accuracy of the method is confirmed.

As-Encountered Prediction of Tunnel Boring Machine Performance Parameters using Recurrent Neural Networks

The earth pressure balance tunnel boring machine (TBM) is advanced excavation machinery used to efficiently drill through subsurface ground layers while placing precast concrete tunnel segments. They have become prevalent in tunneling projects because of their adaptability, speed, and safety. Optimal usage of these machines requires information and data about the soil of the worksite that the TBM is drilling through. This paper proposes the utilization of artificial intelligence and machine learning, particularly recurrent neural networks, to predict the operational parameters of the TBM. The proposed model utilizes only performance data from excavation segments before the location of the machine as well as its current operating parameters to predict the as-encountered parameters. The proposed method is evaluated on a dataset collected during a tunneling project in North America. The results demonstrate that the model is effective in predicting operation parameters. To address the potential issue of gathering sufficient data to retrain the model, the possibility of transferring the trained model from one tunnel to another is tested. The results suggest that the model is capable of performing accurately with minimal or even no re-training.

A Rapid Experimental Procedure to Assess Environmental Compatibility of Conditioning Mixtures Used in TBM-EPB Technology

Earth Pressure Balance (EPB) Tunnel Boring Machines (TBM) are currently the most widely used machines to perform tunnel excavation, particularly in urban areas. This technology involves the injection of chemicals as conditioning mixtures, which commonly raises concerns limiting the reuse of soils after excavation. This study deals with the prospect of a simplified, rapid and replicable methodology for the evaluation of the biodegradability of these conditioning mixtures. For this purpose, the biodegradation of three commercial conditioning mixtures was investigated in closed bottle tests by investigating the effect of different mixtures dosages and two different inocula (soil humus and Bacillus Clausii). While using soil humus as inoculum, a comparative study of biodegradation of the three investigated mixtures was successfully carried out; in the case of Bacillus Clausii, it was not possible to make a comparison between the different formulations in a short time. The adoption of soil humus satisfied only the criteria of rapid test, while the Bacillus Clausii, as specific inoculum, can meet the criteria of replicable results. For this reason, in the second part of this experimental study, a rapid and replicable procedure was proposed and validated. A kinetic study of organic carbon removal was also carried out.