• Proceedings of the Korean Geotechical Society Conference
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• 2005.10a
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• pp.140-153
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• 2005

During the tunnel construction the major failure mode can be categorized as: tunnel failure just after the tunnel excavation without support, failure after application of shotcrete and finally failure after setting the concrete lining. The failure mode just after the tunnel excavation without support, can be further classified as : bench failure, crown failure, face failure, full face failure, failure due to weak strata and failure due to overburden. Moreover the failure after application of shotcrete is classified as heading face failure, settlement of shotcrete support, local failure of shotcrete lining and invert shotcrete. To find out the major causes of tunnel collapse, the investigation was done in case of the second phase of Seoul subway construction. The investigation results depicted that the major causes of tunnel collapse were due to the weak layer of rock/fault and sudden influx of ground water from the tunnel crown. While the investigation results of the mountain road tunnels construction have shown that the major causes of tunnel failure were inadequate analysis of tunnel face mapping results, intersection of faults and limestone cavities. In this paper some recent measurement in order to mitigate such tunnel collapse are presented

• Journal of Korean Tunnelling and Underground Space Association
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• v.18 no.5
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• pp.499-509
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• 2016

In recent years, the use of underground spaces becomes more frequent and the demands for urban tunnels are rapidly increasing. The urban tunnels constructed in the ground with a shallow and soft cover might be deformed in various forms on the face, which would lead, the tunnels to behavior 3-dimensionally, which may have a great impact on the longitudinal load transfer. The tunnel face might deform in various forms depending on the construction method, overburden and the heterogeneity of the ground. And accordingly, the type and size of the distribution of the load transferred to the ground adjacent to the tunnel face as well as the form of the loosened ground may appear in various ways depending on the deformation form of the tunnel face. Therefore, in this study was conducted model tests by idealizing the deformation behavior of the tunnel face, that were constant deformation, the maximum deformation on the top and the maximum deformation on the bottom. And the test results were analyzed focusing on the deformation of the face and the longitudinal load transfer at the ground above the tunnel. As results, it turned out that the size and the distribution type of the load, which was transferred to the tunnel as well as the earth pressure on the face were affected by the deformation type of the face. The largest load was transferred to the tunnel when the deformation was in a constant form. Less load was transferred when the maximum deformation on the bottom, and the least load was transferred when the maximum deformation on the top. In addition, it turned out that, if the cover became more shallow, a longitudinal load transfer in the tunnel would limited to the region close to the face; however, if the cover became higher than a certain value, the area of the load transfer would become wider.

• Proceedings of the Korean Geotechical Society Conference
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• 2003.03a
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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.

• Geomechanics and Engineering
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• v.15 no.2
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• pp.769-774
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• 2018

The growth of cities requires the construction of new tunnels close to the existing ones. Prediction and control of ground movement around the tunnel are important especially in urban area. The ground respond due to EPB (Earth Pressure Balance) pressure are investigated using the finite element method by ABAQUS in intersection of the triplet tunnels (Line 2, 3 and 4) of Mashhad Urban Railway in Iran. Special attention is paid to the effect of EPB pressure on the tunnel face displacement. The results of the analysis show that in EPB tunneling, surface settlement and face displacement is related to EPB pressure. Moreover, it is found that tunnel construction sequence is a great effect in face displacement value. For this study, this value in Line 4 where is excavated after line 3, is smaller than that line. In addition, the trend of the displacement curves are changed with the depth for all lines where is located in above and below, close to and above the centerline tunnel face for Line 2, 3 and 4, respectively. It is concluded that: (i) the surface settlement decreases with increasing EPB pressure on the tunnel face; (ii) at a constant EPB pressure, the tunnel face displacement values increase with depth. In addition, this is depended on the tunneling sequence; (iii) the trend of the displacement curves change with the depth.

• 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.

• 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 Korean Tunnelling and Underground Space Association
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• v.9 no.1
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• pp.83-89
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• 2007

As tunnel excavation generats stress release, a stability security of tunnel face is mainly important in case of tunnel excavation in the weak grounds. Using the steel bar or glass fiber pipe which had regular hardness, a face bolt method to reinforce previously is applied to an excavation object tunnel face aspect among measures methods regarding this. Therefore, used $FLAC^{3D}$ Ver. 2.1 on 5 Case of 0.5D (2EA), 1.0D, 1.5D, 2.0D with the length and 6 Case of 0, 20, 40, 60, 80, 100EA with the number of the bolt that a face bolt method was installed at these papers in the necessary weak grounds in order to review applicability of the tunnel face reinforcement method that used these face bolts, and executed three dimension continuous analysis.

• Journal of Korean Tunnelling and Underground Space Association
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• v.18 no.3
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• pp.273-282
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• 2016

This study estimates tunnel face pressure through existing 8 analytical equations and 3D numerical analysis, and compares and examines it. In general, the estimating tunnel face pressure of domestic shield TBM has been examined by a method according to analytical equation and empirical method, but numerical analysis is combined in a section passing complicated stratigraphic condition and special soil condition. Therefore, the researcher is to find a reliable method to examine of tunnel face pressure by confirming a correlation between tunnel face pressure estimated by equation and tunnel face pressure estimated by numerical analysis program. When tunnel face pressure is estimated, both analytical equation and numerical analysis were identically examined in soil conditions such as sandy soil and cohesive soil. In addition, existing analytical equation is used as equation, and 3D analysis copying construction process and shield tunnel as numerical analysis.

• Journal of Korean Tunnelling and Underground Space Association
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• v.8 no.2
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• pp.101-113
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• 2006

Arching effect occurs around the unsupported excavation surface near to tunnel face when a tunnel is excavated in a stable rock mass. If a weak fracture zone exists in front of tunnel face, a displacement occurs between tunnel face and weak fracture zone due to stress concentration. If three-dimensional absolute coordinates (longitudinal, transverse, vertical direction) is measured at tunnel face by geodetic method, the ground change in front of the tunnel face can be predicted by analysing three-dimensional absolute displacement. The purpose of this study is to verify the analysis method of three-dimensional absolute displacement by comparing the trend of displacement ratio at crown and sidewall of tunnel and the influence line/trend line of crown settlement compared with TSP results in the same section.

• Journal of Korean Tunnelling and Underground Space Association
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• v.8 no.1
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• pp.65-76
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• 2006

In this study, NATM can reduce the loosened ground near the tunnel face more than the other pre-existing tunnelling methods, because of rapid supporting by means of shotcrete and rock bolts. However, this method sometimes can not help for a unstable tunnel face with a unsupported caondition. In order to keep from that dangerous case, some excavation methods such as bench cut and drift advancing method are introduced, despite of high construction cost and period. So, this thesis is intended to introduce the new tunnel face shape, that is concave shaped face, and discusses its effects on the tunnel stabilization.