Soft ground tunnelling in urban areas is more frequently being performed using the shield method. Due to its great influence on both ground settlement and construction safety, face stability is one ...of the most critical problems in shield tunnelling. In this research, a series of 3-D finite element simulations were conducted using the Midas-GTSNX software in order to determine the required collapse pressure of a tunnel face during tunnelling in homogeneous or layered soils. In terms of homogeneous ground, the effects of different soil strength parameters, cover-to-diameter ratios and tunnel diameters were investigated. Based on the numerical results of 140 analyses, a new design equation has been derived to calculate the required face collapse pressure during tunnelling in a purely frictional soil or a c'-φ' soil above the groundwater table. The results of this equation are in close agreement with the results from available experimental tests and theoretical approaches, and hence the equation provides a very useful method for estimating face collapse pressure. Furthermore, the arching effect has been explicitly investigated, and the failure mechanisms ahead of the tunnel face have been presented for various cases. For layered ground, two stratification scenarios were considered. Each scenario comprised two strata; an upper and a lower stratum. The first scenario (Case 1) involved the upper stratum intersecting with the lower stratum at the tunnel crown, while in the second scenario (Case 2), both strata intersect at the tunnel axis. In each case, the shear strength parameters of the upper and lower strata were changed to study the influence of these variations on face collapse pressure. In Case 1, it was found that the face collapse pressure is far more sensitive to parameter variations in the lower stratum than those of the upper stratum. In Case 2, however, almost the same values of face collapse pressure were obtained for both sets of parameter variation. Furthermore, if the lower stratum is stronger than the upper stratum, the required face collapse pressure in Case 2 is greater than that in Case 1.
•Typical soft ground tunnel defects are described based on a tunnel inspection program.•Major factors contributing to tunnel defect generation are identified and discussed.•A framework is developed ...for tunnel defect analyses.•The mechanism of tunnel defect generation and functional degradation is explored.
This paper deals with the mechanism of soft ground tunnel defect generation and functional degradation. The subway mileage has increased dramatically worldwide, especially in China, in the past decades and will be continuously increasing in the next decades driven by the demands on underground space usage and the advancement of tunneling technique. Subway tunnels are vulnerable to a variety of defects which, individually or interactively, deteriorate the tunnel function for providing passengers with a safe and comfortable transportation means. Understanding the mechanism of tunnel defect generation and functional degradation and providing effective maintenance measures can slowdown the tunnel defect generation and prevent the tunnel defects from developing into catastrophic structural failure. This paper summarizes typical tunnel defects and major contributing factors to the defect generation based on the findings from an inspection program of 130km of soft ground tunnels in east China. A detailed example of the tunnel inspection and rehabilitation is presented. A framework is developed for analyzing the tunnel defects. The mechanism of tunnel functional degradation is explored associated with five critical links: environment, structure, components, joints, and materials, within a tunnel operation system. The five links individually deteriorate with time and interactively degrade the tunnel function. The research findings from this paper lay a foundation for developing practice guidance for tunnel maintenance.
•Eight groups of ground conditions for face failure are considered in model tests.•Three face failure modes and their generation conditions are proposed.•Evolutionary mechanisms of typical failure ...modes are revealed.•Extensive parametric analyses based on FELA combined strength reduction method.•Reasonable matched upper bench length and height are put forward.
Face stability protection is a critical issue during large cross-section tunneling. This study investigates the evolution mechanism of tunnel face instability with different ground conditions based on model tests and numerical simulations. Eight groups of model tests were conducted to study the failure behavior of the tunnel face and to characterize the ground displacement and stress distributions. Three failure modes including front extrusion, forward caving and backward caving were defined based on the model test results. Finite element limit analyses were performed using the strength reduction method to simulate the failure behavior. The simulation results were compared with those of the model tests, validating the effectiveness the numerical techniques used in the study. Parametric studies were conducted to investigate the effects of ground strength and unit weight, and tunnel unsupported length, height and burial depth on the tunnel face stability. The simulation results showed that the effect of the gravity load on face stability weakens with the increase in the burial depth due to the arching effects as observed in model tests. Since the ground strength, unsupported length and tunnel height significantly influenced the face stability, the bench method with different unsupported length was compared with the full-face method. The effects of upper bench length and height on the face stability under different ground conditions were investigated. This study explored the face failure mechanism of large cross-section tunnels excavated under different ground conditions. The results can provide a reliable reference to evaluate face stability and safety control during tunnel excavation.
Soft soils pose significant challenges to the environment and construction of infrastructure on them owing to their distinct characteristics such as low bearing strength, high water content, low ...permeability, and high void ratio. The stiffness modulus of soft ground soils (
G
s
) is one of the major considerations while designing geo-structures. The determination of the stiffness modulus of soft ground materials such as soils requires expensive machinery, more skilled labor, and consumption of time which is contrary to the current trends of sustainable development. Therefore, this paper presents the artificial intelligence (AI)-based sustainable solutions for the estimation of
G
s
using artificial neural network (ANN), gene expression programming (GEP), and multiple linear regression (MLR) techniques. In this regard, 199 samples of soft soil from different locations were retrieved and tested to determine basic soil attributes such as sand content (
S
), fine content (FC), liquid limit (LL), plastic limit (PL), water content (w), and bulk density (d) which were used as potential indicators for computing soft ground stiffness modulus. Many statistical tests, including
R
-square (
R
2
), root means square error (RMSE), and mean absolute error (MAE), were used to further substantiate the performance efficiency of computed prediction models. The findings show that the proposed models meet all accuracy-related acceptance requirements. However, ANN outperforms GEP and MLR. Further, to evaluate the specific impact of input factors, sensitivity and parametric tests were also executed.
•We categorized factors influencing tool wear in EPB-TBM tunneling.•We analysis effect of TBM operational parameters and geological factors on tool wear.•We analysis cutter life for EPB machine.•We ...found that soil conditioning, earth pressure, machine torque, thrust, penetration rate and rpm greatly reduce tool wear.•Improving soil conditioning parameters reduces tool wear even in coarse-grained soils.
Wear of cutting tools and other components of the pressurized face tunnel boring machine which come in contact with the muck is an important parameter in soft ground tunneling. This is due to the need for cutterhead inspection and tool maintenance under pressurized conditions or “hyperbaric interventions” which is a time consuming, dangerous, risky, and costly activity. This study investigates the tool wear of an EPB-TBM used in Tehran metro line 7 over the initial 6500m of tunnel. The ground along the tunnel alignment is mostly alluvial deposits, composed of gravely sand with clay/silt and sandy gravel with clay. In this project, the number of cutting tools replaced were 1169, including 654 rippers, 357 scrapers and 153 disc cutters. In this paper the influence of geological parameters and operational factors on tool wear is examined. The results show that by increasing TBM thrust, earth pressure, and torque, the cutter consumption generally increases. Also, soil conditioning plays an important role in controlling cutter wear, so that the tool wear can be reduced even in coarse-grained soils by improving the soil conditioning parameters. Quantitative analysis of these parameters for this project and a brief discussion of their implications is offered. Also, multiple linear regression analysis was used to seek the best correlation between the cutter wear and ground / opening variables, and correlation coefficient of up to 0.98 have been observed. This highlights the potential of developing models for tool wear prediction by multiple regression, if sufficient data is collected in soft ground tunneling projects.
•Sprayed waterproofing enables tension and shear transfer between lining layers.•load transfer can give efficiency gains in lining design, but not in all cases.•A guaranteed benefit of interface ...tensile bond is sharing of groundwater loading.•Composite action can induce tension in secondary from bending, reducing its capacity.•Concern about water saturation of membrane can be addressed by limiting water head.
Introduction of sprayed waterproofing has led to innovation of composite sprayed concrete lined (SCL) tunnels, where placing the waterproofing between primary and secondary layers gives potential for composite structural action, by transmission of tension, compression and shear stresses across the interface. Numerical analysis is required to design such structures taking into account soil-structure interaction and staged construction, but there is currently very limited guidance on how to conduct such analyses.
This paper reviews use of numerical analysis to simulate composite SCL tunnels, focussing on soft ground tunnelling. It introduces types of sprayed membrane, their benefits in design and current industry practice for simulating the sprayed membrane interface. Numerical strategies for simulating composite action and their verification against laboratory test data are then described. Recommendations are made of design principles to optimise design of SCL tunnels with spray-applied waterproofing. Further opportunities for research on this topic are discussed.
•Eco-friendly & biologically inactive basalt fiber is chosen to improve cemented clay.•Basalt fiber enhances ductility, UCS and peak deviatoric stress of cemented clay.•Optimal amount of basalt fiber ...is identified for UCS and triaxial shear behaviour.•Bridging of basalt fiber & cementing of hydration product makes a great contribution.•Interface interaction between basalt fiber and cemented matrix is main mechanism.
The polypropylene fibers, which are currently attracting enormous attention in various geotechnical applications, carry a risk of aging under an integrated effect of heat, oxygen, light and other environmental factors, causing potentially infrastructure failure. An eco-friendly and biologically inactive material – basalt fiber, which has excellent natural resistance to aging and can eliminate aging-associated disasters, deserves more attention in geotechnical field. However, quite few studies are available on the beneficial reuse of basalt fibers to improve the engineering performance of soils. Therefore, this study aims to incorporate the sustainable basalt fiber and clarify how its inclusion impacts the mechanical properties and microstructure of cemented kaolinite. The experimental programs are comprised of three types of tests, i.e. two to examine the compressive strength and triaxial shear behavior and one to evaluate the microstructure properties. The results indicate that the basalt fiber reinforcement plays an essential role in enhancing the compressive strength and peak deviatoric stress of cemented and uncemented kaolinite. The inclusion of basalt fibers improves the ductility and weakens the brittleness of cemented kaolinite. The compressive strength increases with basalt fiber content and curing time, and reaches the peak at the fiber content of 0.2%, followed by a reduction due to the formation of weak zone at higher fiber content. The peak deviatoric stress is elevated until reaching the maximum at the basalt fiber content of 0.4%, after which further addition of basalt fiber tends to reduce its reinforcing effect. The peak deviatoric stress increases as the basalt fiber length is shortened and the confining pressure is raised. The strength gain of cement-basalt fiber inclusion is much more than the sum of strength increase induced by them individually. The combination of basalt fiber and cement has the virtues of both cement-stabilized and basalt fiber-reinforced kaolinite. The SEM analysis reveals that the mechanical interaction in the form of interface bonding and friction between kaolinite particle, cement hydration product and basalt fiber is the dominant mechanism controlling the reinforcement-cementation benefits. The bridging effect (reinforcement) of basalt fibers and binding effect (cementation) of hydration products make a major contribution to the formation of stable and interconnected microstructure, which results in an evident improvement in the mechanical behaviour of cemented kaolinite. The combination of basalt fiber and cement stabilization would be an innovative and effective method for geotechnical engineering works such as soft ground improvement.
•The semi-analytical solutions of three-dimensional ground movements are proposed.•The nonuniform convergence and the ovalization are considered.•The accuracy and efficiency of the proposed method ...are verified.•The influences of typical factors on the ground movements are illustrated.
This paper presents the semi-analytical solutions to predict the three-dimensional ground movements of shallowly buried tunnels in clayey and sandy soils based on the virtual image technique. Both the ground nonuniform convergence and the tunnel ovalization are taken into account in the proposed method. The accuracy of the proposed method is verified by comparing with the results from field monitoring, numerical simulation and existing solutions. The results indicate that the transverse surface settlements mainly occur within H/tan(45° + φ/2) + R from the tunnel axis on both sides of the tunnel, and the variations of the longitudinal surface settlements are mainly produced within ± 2H from the tunnel face. The internal friction angle, Poisson’s ratio, cover depth and tunnel diameter have significant effects on the surface and subsurface movements according to the parametric analysis. The proposed method can serve as a practical tool in the preliminary design of tunnels and provide a theoretical basis for ensuring the construction safety of actual engineering.
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