• Tunnel and Underground Space
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• v.30 no.1
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• pp.63-75
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• 2020

When tunnel is excavated via drilling and blasting, the excessive overbreak is the primary cause of personal or equipment safety hazards and increasing the cost of the tunnel operation owing to additional ground supports such as shotcrete. The practical management of overbreak is extremely difficult due to the complex causative mechanism of it. The study examines the relationship between rock mass characteristics (unsupported face condition, uniaxial compressive strength, face weathering and alteration, discontinuities- frequency, condition and angle between discontinuities and tunnel contour) and the depth of overbreak through using feed-forward artificial neuron networks. Then, Overbreak Resistance Factor (ORF) has been developed based on the weights of rock mass parameters to the overbreak phenomenon. Also, a new concept of tunnel overbreak management system using ORF has been suggested.

• Tunnel and Underground Space
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• v.18 no.4
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• pp.300-311
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• 2008

In this study, scaled model tests were performed to investigate the stability of three parallel tunnels. Seven types of test models which had respectively different pillar widths, tunnel sectional shapes, support conditions and ground conditions were experimented, where crack initiating pressures and deformation behaviors around tunnels were investigated. In order to evaluate the effect of pillar widths on stability, various models were experimented. As results, the models with shallower pillar widths proved to be unstable because of lower crack initiating pressures and more tunnel convergences than the models with thicker pillar widths. In order to find the effect of tunnel sectional shape on stability, the models with arched, semi-arched and rectangular tunnels were experimented. Among them rectangular tunnel model was the most unstable, where the arched tunnel model with small radius of roof curvature was more stable than semi-arched one. The model with rockbolt showed higher crack initiating pressure and less roof lowering than the unsupported model. The deformation behaviors of tunnels in the anisotropic ground model were quite different from those in the isotropic ground model. Futhermore, the results of FLAC analysis were qualitatively coincident with the experimental results.

• Proceedings of the Korean Society for Rock Mechanics Conference
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• 2002.10a
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• pp.31-42
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• 2002

최근 지하철 터널은 사용자 편의성, 도심지 접근성 및 원활한 교통처리 등을 고려하여 지반조건이 불리한 상황에서도 터널로 계획되는 경우가 많아지고 있다 따라서 시공중의 터널안정성확보, 굴착에 따른 인접구조물의 침하영향, 발파진동영향 등을 종합적으로 고려한 지보패턴 및 보조·보강공법의 결정이 매우 중요하나 정량적인 판단기준의 부재로 인하여 주로 경험적인 설계에 의존하는 경우가 많다. 본 연구에서는 도심지 지하철 터널의 복합적인 거동특성을 고려하기 위하여 여러 가지 예상위험요소의 정량화 방안을 제안하고, 다변량 통계분석기법을 활용하여 여러 가지 위험 요소들의 특성을 함축적으로 나타내는 소수의 총합적인 지표(안정성인자, 환경성인자)로 대표화 할 수 있음을 검증하였다. 안정성 인자 및 환경성 인자를 이용한 서울시 지하철 00공구 설계사례를 통해 정량화지표(Multiple Index)의 터널설계에의 적용성을 평가하고 이의 설계시 활용방안을 제안하고자 하였다.

• Tunnel and Underground Space
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• v.22 no.1
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• pp.43-53
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• 2012

Stability of cut-slope, the orientation and dimension of which are gradually changed, has been analyzed by employing the cross-section method capable of comprehensibly considering the lithological, structural and mechanical characteristics of slope rock. Lithological fragility is investigated by inspecting the drilled core logs and BIPS image has been taken to delineate the rock structure. Engineering properties of drilled-core including the joint shear strength have been also measured. Potential failure modes of cut-slope and failure-induced joints are identified by performing the stereographic projection analysis. Traces of potential failure-induced joints are drawn on the cross-section which depicts the excavated geometry of cut-slope. Considering the distribution of potential plane failure-induced joint traces blocks of plane failure mode are hypothetically formed. The stabilities and required reinforcements of plane failure blocks located at the different excavation depth have been calculated to confirm the applicability of the cross-section method for the optimum cut-slope design.

• Journal of Korean Society of Steel Construction
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• v.29 no.5
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• pp.353-359
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• 2017

The purpose of this study is to evaluate the strength and behavior of the composite member in case of concrete filled steel tube embedded in concrete for application concrete filled steel tube to steel rib support in tunnel. A total of six beam specimens were prepared for steel tube in-filled with plain concrete and aerated concrete, and static bending tests were performed. As a result, the member of concrete steel tube embedded with plain concrete showed higher strength than those with aerated concrete. However, it was found that the flexural strength of member with reinforcing bar around the steel tube is more influenced by the amount of the reinforcing bar than the type of the filled concrete.

• Tunnel and Underground Space
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• v.19 no.5
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• pp.388-396
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• 2009

Rock bolt is one of the most important supports for tunnelling to prevent excessive ground relaxation at the primary tunnel excavation stage. It forms a ground arch band by confining the ground around a tunnel. Rock bolt has various effects, such as support or hanging effect, internal pressure effect, arching effect, ground improvement effect etc. Most studies on rock bolt focused on the concept of support, but only a few researches on the ground reinforcing effect by pre-tensioning a rock bolts. In this study, large scale model tests are performed to investigate the ground reinforcing effect of rock bolts for regularly jointed rocks. Simple beam model was built to find out the reinforcing effect of jointed rocks, which was reinforced by pre-tensioned rock bolts. Settlement of model beam was analyzed through measuring its sagging for various installation intervals.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.2
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• pp.655-666
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• 2013

A number of highway projects are in progress in Korea to accommodate increasing transportation demands. As the highway route becomes more complex, some projects include tunneling through unconsolidated sedimentary rock. Since an unconsolidated sedimentary rock mainly consists of rock and ground mass, the behavior and characteristics in unconsolidated sedimentary rock tunnel are quite different from typical rock tunnel. However, construction case histories and rock classifications method on unconsolidated sedimentary rock tunnel had not been developed or studied domestically. Consequently the case studies and rock classification system for unconsolidated sedimentary rock are required to better understand its behavior for tunneling. In this study, rock mass classification method is proposed to identify unconsolidated sedimentary rock based on point load and slake durability tests. Based on this, the proposed method of unconsolidated sedimentary rock can be applied well through comparisons with the results of convergence measurement.

• Journal of Korean Tunnelling and Underground Space Association
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• v.26 no.4
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• pp.315-326
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• 2024

In this study, the Delphi method and AHP (analytic hierarchy process) were used to evaluate tunnel collapse risk from a comprehensive and multifaceted perspective. Influence factors were established through literature reviews, previous studies, and brainstorming sessions with expert groups, resulting in the construction of five main classification systems. A panel of 21 experts was formed, and three rounds of Delphi surveys were conducted to prevent errors and biases in the expert judgment process, thereby enhancing reliability. Ultimately, 14 influence factors were identified through CVR (content validity ratio) and COV (coefficient of variation) analyses of the experts' responses. Subsequently, the AHP method was applied to assess the relative importance of each influence factor and calculate the final composite weights. The timing of support and reinforcement had the highest priority, followed by groundwater inflow, joint conditions, support pattern levels, and auxiliary methods. These findings help identify the key factors affecting tunnel collapse risk and provide a foundation for developing strategies to improve tunnel safety.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.3
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• pp.433-452
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• 2019

This study deals with shotcrete reinforcing performance according to the amount of synthetic fiber (PP fiber) and proper evaluation method. The shotcrete compressive strength, flexural strength and flexural toughness were tested by setting the mixing amounts of steel fiber ($37.0kg/m^3$) and synthetic fiber (PP fiber) as parameters ($5.0kg/m^3$, $7.0kg/m^3$ and $9.0kg/m^3$). Particularly, circular panel flexural toughness test (Road and Traffic Authority, RTA) was performed to evaluate the shotcrete energy absorption capacity. As a result, the compressive strength and the bending strength of the steel fiber reinforced shotcrete were large, but the flexural toughness of the synthetic fibe (PP fiber) reinforced shotcrete was large. Therefore, synthetic fiber (PP fiber) reinforced shotcrete is considered to have a reinforcing effect comparable to that of steel fiber reinforced shotcrete. Analysis of the relationship between the flexural toughness and the energy absorption capacity of synthetic fiber (PP fiber) reinforced shotcrete revealed that the energy absorbing ability is exhibited at a flexural toughness lower than the allowable standard (3.0 MPa). (Class A: 2.55 MPa = 202J, Class B: 2.72 MPa = 282J, Class C: 3.07 MPa = 403J). As a result of this study, it can be concluded that the actual shotcrete support performance can be evaluated by evaluating the support performance of the shotcrete measured at less than the allowable standard (3.0 MPa) at the actual tunnel site.

• Tunnel and Underground Space
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• v.17 no.4
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• pp.266-284
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• 2007

Recently, many efforts have been made to construct the first unlined tunnel, without in-situ concrete lining, in Korea. However, the lack of reliability in the performance of shotcrete as permanent tunnel support prevented from its realization. Shotcrete has been regarded to have significant problems in field application and long term performance because of unsatisfactory strength level and durability compared to those of European countries. In this study, the high strength shotcrete satisfying compressive strength over 40 MPa and flexural strength over 4.5 MPa was developed from optimized mix design. The type of accelerators and the amount of silica fume were selected as the main factors in mixing process and the analyses were carried out up to the elapsed time of 2 years. In order to evaluate the short term durability of shotcrete, an array of laboratory test consisting of freeze-thaw, carbonation chloride penetration and permeability test was performed. For long-term durability tests, specimens have been put in an operated highway tunnel to expose them to the similar environment when they are actually used as an unlined tunnel support. From the strength and durability tests, it was found that only alkali-free based accelerator satisfied the target strength of this study and also, the developed shotcrete showed very high performance in its durability.