• Title, Summary, Keyword: tunnel face stability

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Evaluation of Tunnel Face Stability with the Consideration of Seepage Forces (침투력을 고려한 토사터널 막장의 안정성 평가방법에 대한 고찰)

  • 남석우;이인모
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.193-200
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    • 1999
  • Since Broms and Bennermark(1967) suggested the face stability criterion based on laboratory extrusion tests and field observations, the face stability of a tunnel driven in cohesive material has been studied by several authors. And recently, more general solution for the tunnel front is given by Leca and Panet(1988). They adopted a limit state design concept to evaluate the face stability of a shallow tunnel driven into cohesionless material and showed that the calculated upper bound solution represented the actual behavior reasonably well. In this study, two factors are simultaneously considered for assessing tunnel face stability: One is the effective stress acting on the tunnel front calculated by upper bound solution; and the other is the seepage force calculated by numerical analysis under the condition of steady state ground water flow. The model tests were performed to evaluate the seepage force acting on the tunnel front and these results were compared with results of numerical analysis. Consequently, the methodology to evaluate the stability of a tunnel face including limit analysis and seepage analysis is suggested under the condition of steady state ground water flow.

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Effect of Seepage Forces on the Tunnel Face Stability - Assessing through Model Tests - (침투력이 터널 막장의 안정성에 미치는 영향 연구 - 모형실험을 중심으로 -)

  • 이인모;안재훈;남석우
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.41-48
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    • 2001
  • In this study, two factors are simultaneously considered for assessing tunnel face stability: one is the effective stress acting on the tunnel face calculated by upper bound solution; and the other is the seepage force calculated by numerical analysis under the condition of steady-state groundwater flow. The seepage forces calculated by numerical analysis are compared with the results of a model test. From the results of derivations of the upper bound solution with the consideration of seepage forces acting on the tunnel face, it could be found that the minimum support pressure for the face stability is equal to the sum of effective support pressure and seepage pressure acting on the tunnel face. Also it could be found that the average seepage pressure acting on the tunnel face is proportional to the hydrostatic pressure at the same elevation and the magnitude is about 22% of the hydrostatic pressure for the drainage type tunnel and about 28% for the water-proof type tunnel. The model tests performed with a tunnel model had a similar trend with the seepage pressure calculated by numerical analysis. From the model tests it could be also found that the collapse at the tunnel face occurs suddenly and leads to unlimited displacement.

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Study on the Seepage Forces Acting on the Tunnel Face with the Consideration of Tunnel Advance Rate (터널 굴진율을 고려한 막장에서의 침투력에 관한 연구)

  • 남석우;이인모
    • Journal of the Korean Geotechnical Society
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    • v.18 no.5
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    • pp.221-228
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    • 2002
  • The stability of a tunnel face is one of the most important factors in tunnel excavation. Especially, if a tunnel is located under groundwater level, groundwater may flow into the tunnel face and seepage forces acting on the tunnel face due to groundwater flow may affect seriously the stability of the tunnel face. Therefore, the seepage pressure at the tunnel face should be considered fir the proper design and safe construction of a tunnel. In this paper, the effect of tunnel advance rate on the seepage forces acting on the tunnel face was studied. The finite element program to analyze the groundwater flow around a tunnel with the consideration of tunnel advance rate was developed. Using the program, the parametric study for the effect of the tunnel advance rate and hydraulic characteristics of the ground on the seepage forces acting on the tunnel face was made. From this study, it was concluded that the tunnel advance rate must betaken into consideration as an additional parameter to assess the seepage forces at the tunnel face and a rational design methodology fer the assessment of support pressures required for maintaining the stability of the tunnel face was suggested for undetwater tunnels.

Stability evaluation for the excavation face of shield tunnel across the Yangtze River by multi-factor analysis

  • Xue, Yiguo;Li, Xin;Qiu, Daohong;Ma, Xinmin;Kong, Fanmeng;Qu, Chuanqi;Zhao, Ying
    • Geomechanics and Engineering
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    • v.19 no.3
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    • pp.283-293
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    • 2019
  • Evaluating the stability of the excavation face of the cross-river shield tunnel with good accuracy is considered as a nonlinear and multivariable complex issue. Understanding the stability evaluation method of the shield tunnel excavation face is vital to operate and control the shield machine during shield tunneling. Considering the instability mechanism of the excavation face of the cross-river shield and the characteristics of this engineering, seven evaluation indexes of the stability of the excavation face were selected, i.e., the over-span ratio, buried depth of the tunnel, groundwater condition, soil permeability, internal friction angle, soil cohesion and advancing speed. The weight of each evaluation index was obtained by using the analytic hierarchy process and the entropy weight method. The evaluation model of the cross-river shield construction excavation face stability is established based on the idea point method. The feasibility of the evaluation model was verified by the engineering application in a cross-river shield tunnel project in China. Results obtained via the evaluation model are in good agreement with the actual construction situation. The proposed evaluation method is demonstrated as a promising and innovative method for the stability evaluation and safety construction of the cross-river shield tunnel engineerings.

A study on the evaluation method and reinforcement effect of face bolt for the stability of a tunnel face by a three dimensional numerical analysis (터널막장안정 평가기법 및 막장볼트의 보강효과에 관한 수치해석적 연구)

  • Kim, Sung-ryul;Yoon, Ji-Sun
    • Journal of Korean Tunnelling and Underground Space Association
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    • v.11 no.1
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    • pp.11-22
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    • 2009
  • Tunnel excavation with several sections and appropriate auxiliary measures such as face bolt and pre-grouting are widely used in case of weak and less rigid ground for the stability of a tunnel face during excavation. This papers first described the evaluation methods proposed in technical literature to maintain the tunnel face stable, and then studied by FEM analysis whether face reinforcement is need in what degree of ground deformation and strength features for the stability of a tunnel face when excavating by full excavation with sub-bench. Lastly, a three dimensional FEM analysis was performed to study how the tunnel face itself and the ground around the tunnel behave depending on different bolt layouts, length of bolts, number of bolts. There were relative differences in comparison of results on the stability of a tunnel face by a theoretical evaluation methods and FEM analysis, but the same in reinforced effect of face. It was found that the stability of a tunnel face can be obtained with face bolt installed longer than 1.0D (tunnel width), bolt density of about 1 bolt per every $1.5\;m^2$ (layout of grid type), and reinforcement area of $120^{\circ}$ arch area of upper section.

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Evaluation of tunnel face stability based on upper bound theorem (상한치 이론에 근거한 터널 막장의 안정성 연구)

  • Lee, In-Mo;Lee, Jae-Sung;Nam, Seok-Woo
    • Journal of Korean Tunnelling and Underground Space Association
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    • v.5 no.1
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    • pp.3-11
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    • 2003
  • Face stability of a tunnel is a main concern during tunnel excavation. However, there has been only a few studies on this problem while a lot of researches on the support systems have been carried out. In addition, when tunneling is performed below the groundwater level, the groundwater flows into the tunnel so that the seepage forces generated on the tunnel face might give rise to a serious potential for the face instability. In this study, the face stability was evaluated by simultaneously considering two factors: one is the effective stress calculated by upper bound theorem; the other is the seepage forces acting on the tunnel face obtained by numerical analysis under the condition of steady-state groundwater flow. Tunneling in difficult geological conditions often requires auxiliary techniques to guarantee safe tunnel excavations and/or to prevent damage to structures and services around the tunnel. The steel pipe-reinforced multistep grouting has been recently applied to tunnel sites in Korea. Face stability of a tunnel with the steel pipe-reinforced multistep grouting was also analyzed in this study.

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Reduced-Scale Model Tests on the Behavior of Tunnel Face Reinforced with longitudinal reinforcements (수평보강재로 보강된 터널 막장의 거동에 관한 축소 모형실험)

  • 유충식;신현강
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.79-86
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    • 2000
  • This paper presents the results of a parametric study on the behavior of tunnel face reinforced with horizontal pipes. A series of reduced-scale model tests was carried out to in an attempt to verify previously performed three-dimensional numerical modeling and to investigate effects of reinforcement layout on the tunnel face deformation behavior The results of model tests indicate that the tunnel face deformation can significantly reduced by pre-reinforcing the tunnel face with longitudinal members and thus enhancing the tunnel stability. In addition, the model tests results compare fairly well with those from the previously performed three-dimensional finite element analysis. Therefore, a properly calibrated three dimensional model may effectively be used in the study of tunnel face reinforcing technique.

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An improved collapse analysis mechanism for the face stability of shield tunnel in layered soils

  • Chen, Guang-hui;Zou, Jin-feng;Qian, Ze-hang
    • Geomechanics and Engineering
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    • v.17 no.1
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    • pp.97-107
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    • 2019
  • Based on the results of Han et al. (2016), in the failure zone ahead of the tunnel face it can be obviously identified that a shear failure band occurs in the lower part and a pressure arch happens at the upper part, which was often neglected in analyzing the face stability of shield tunnel. In order to better describe the collapse failure feature of the tunnel face, a new improved failure mechanism is proposed to evaluate the face stability of shield tunnel excavated in layered soils in the framework of limit analysis by using spatial discretization technique and linear interpolation method in this study. The developed failure mechanism is composed of two parts: i) the rotational failure mechanism denoting the shear failure band and ii) a uniformly distributed force denoting the pressure arch effect. Followed by the comparison between the results of critical face pressures provided by the developed model and those by the existing works, which indicates that the new developed failure mechanism provides comparatively reasonable results.

STABILITY OF TUNNEL WORKING FACE IN SQUEEZING GROUND

  • 손근종;손준익
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.9-14
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    • 1993
  • Practical solutions that are available today for assessing stability of tunnel working face are largely based on the concept of critical stability ratio. The accuracy of a prediction of the soil behavior in the working face, thus, depends on the ability of the solution to completely and accurately describe the stress fields or kinematics generated by the excavation and the accuracy of the undrained shear strength of the soil introduced in the computation. This paper reviews the selected solutions describing stability of the tunnel heading in squeezing ground, and suggests a reference solution which is established based on comparison of the solutions and field data on stability of tunnel headings in clays. Although dealing with the shear strength determination is an important companion part of the geotechnical prediction for stability of the tunnel heading in clays, this part is beyond the scope of this paper at this time.

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Evaluation of Face Stability of Tunnel with Steel Pipe-Reinforced Multi-step Grouting (강관다단 그라우팅으로 보강된 터널의 막장 안정성 평가)

  • 이인모;이재성;남석우
    • Proceedings of the Korean Geotechical Society Conference
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    • pp.273-280
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    • 2003
  • Tunneling in difficult geological conditions is often inevitable especially in urban areas. Ground improvement and reinforcement techniques are often required to guarantee safe tunnel excavations and/or to prevent damage to adjacent structures. The steel pipe-reinforced multi-step grouting method has been recently applied to tunnel sites in Korea as an auxiliary technique. In this study, the face stability with steel pipe-reinforced multi-step grouting was evaluated by simultaneously considering two factors: one is the effective stress acting on the tunnel face calculated by limit theorem and limit equilibrium method; the other is the seepage force obtained by means of numerical analysis. The study revealed that the influence of the steel pipe-reinforced multi-step grouting on the support pressure in dry condition is not significant while there is relatively a large amount of reduction in seepage forces by adopting the technique in saturated condition. The effect of the anisotropy of permeability on the seepage force acting on the tunnel face was also estimated by conducting the coupled analysis. It was found that a higher horizontal permeability compared with the vertical one causes reduction in the seepage force acting on the tunnel face.

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