• Journal of the Korean Geotechnical Society
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• v.17 no.5
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• pp.165-172
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• 2001

본 연구에서는 하천인접구간에서와 같이 지하수가 풍부한 지반에서 시공되는 터널의 막장 안정성을 평가하기 위하여 터널 막장에 작용하는 힘의 두 가지 요소를 고려하였다. 하나는 극한해석 중 upper bound solution으로부터 산출된 유효응력이며, 또 하나는 지하수의 정상류 흐름조건을 고려한 수치해석으로부터 산출된 침투력이다. 지하수가 풍부한 토사지반에서의 터널 시공시 터널 막장에 작용하는 힘을 구하기 위하여 침투력을 고려한 극한해석의 해를 구한 결과 터널 막장의 안정성을 유지하기 위한 최소 지보력은 터널 막장에 작용하는 유효응력과 침투력의 합으로 나타낼 수 있었다. 또한 터널 막장에 작용하는 평균침투압은 지하수위에 비례하여 작용하는 것으로 나타났으며, 이를 실내모형 실험 결과를 통하여 검증하였다. 지하수의 정상류 조건 하에서의 토사터널에 대한 실내모형 실험 결과, 터널 막장에 작용하는 침투력은 수치해석 결과 비슷한 양상을 보여주어 제안된 이론의 타당성을 입증하였다.

• 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.

• Journal of the Korean Geotechnical Society
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• v.28 no.4
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• pp.79-89
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• 2012

This paper presents experimental and numerical studies on the closure ratio of tunnel face to reduce pressure and displacement occurring at tunnel face during tunnel excavation. In experimental study, model tests are carried out according to the closure ratio of tunnel face and tunnel depth. Model test results are analyzed and interpreted by numerical calculation in order to verify results obtained from experimental and numerical studies. It is clearly found that tunnel face stability increases with the increase of the closure ratio of tunnel face. The results also show that tunnel face is stable when the closure ratio of tunnel face is larger than 80%. This research will be very useful to develop the economical tunnel face closing system.

• 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.

• Tunnel and Underground Space
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• v.18 no.6
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• pp.427-435
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• 2008

In this paper, review was made for the excavation method and optimum bench length for unstable tunnel face in case of rock classification type V in order to make the best use of in-situ bearing capacity. 3D FEM analyses were performed to investigate the influences on the tunnel face and adjacent area with regard to the pattern and number of bolts when face bolts were used as a supplementary measure. As a result of this study, full section excavation method with sub-bench is effective in reducing the displacement greatly due to early section closure. Displacement-resistant effects in accordance with the bolting patterns are grid type, zig-zag type and then circular type in order of their effect. And horizontal extrusion displacement of tunnel face reduces as the number of bolts increase. A grid type face bolt covering $1.5m^2$ of tunnel face could secure the face stability in case of full section excavation method with sub-bench.

• 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.

• Tunnel and Underground Space
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• v.28 no.2
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• pp.186-192
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• 2018

The German Tunnelling Committee (DAUB) recently published new recommendations for face support calculations; Zdenek Zizka and Markus Thewes of Ruhr University Bochum actively discuss and explain these new recommendations where these recommendations are intended to assist in choosing between various calculation methods which are also dependent on ground conditions. The guidelines also discuss important scientific approaches with face stability calculations, mainly those due to earth pressure and groundwater pressure on the tunnel face. This paper aims to explain these recommendations through Zdenek Zizak and Markus Thewes's discussion on these Tunnel Face Support Calculation Methods.

• Tunnel and Underground Space
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• v.18 no.5
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• pp.366-374
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• 2008

Tunnelling under water table induces many geotechnical problems because of groundwater. In subsea tunneling, reduction of face stability can induce flooding in the vicinity of a fracture zone characterized by high permeability and high water pressure. In this study, the effects of high water pressure on the stability of a tunnel face in a limited zone with high permeability(hazardous zone) are analyzed. On the basis of the 'advance core' concept, the seepage force acting on a hypothetical cylinder ahead of a tunnel face is modeled. This study focuses on the hydraulic behavior of the ground ahead of the tunnel face by three-dimensional steady-state seepage analyses. The impact of the hazardous zone on the seepage force and stability of the tunnel face are simulated and analyzed. In light of the analysis results, it is estimated that the distance from the tunnel face to the exterior boundary limit, which the seepage force significantly affects the stability of the tunnel face, of a hypothetical cylinder is approximately 5 times the tunnel radii. Despite the restrictive assumptions of this study, the results are highly indicative regarding the risks of hazardous zones.

• Journal of Korean Tunnelling and Underground Space Association
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• v.2 no.2
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• pp.32-40
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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.

• Journal of Korean Tunnelling and Underground Space Association
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• v.5 no.4
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• pp.323-336
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• 2003

Recently, the development of tunnel reinforcement method has been required relating to the shallow tunnelling in soft ground. In this study, the improvement method on tunnel stability is proposed by evaluating the efficiency of face reinforcement which enables to control extrusion of advance core, however, it is often neglected in urban tunnelling under the poor ground conditions. Systematic pre-confinement ahead of the face improves the tunnel stability, subsequently, displacement of the crown and surface settlement can be restrained by proper method. 3-dimensional numerical analysis including horizontal reinforcement modelling on a face is applied to estimate the behaviour of a tunnel in relation to the ground and reinforcement conditions. Consequently, extrusion at the face decreases significantly after using the horizontal reinforcement and the effect of reinforcement is much increased in case of applying the supplemental reinforcement ahead of the face together. Especially, confinement effect around the tunnel and the core is proved by means of the core reinforcement in poor ground conditions.