• Tunnel and Underground Space
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• v.18 no.1
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• pp.58-68
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• 2008

The construction of an emergency spillway of Imha Dam is being in progress on the granite region including fault fractured zone. Considering that this tunnel is being excavated in three paralled rows, the pillar width between each tunnel and the face distance between each tunnel face were evaluated. The Influence of the fault fractured zone for the tunnel stability was investigated by numerical modelling in 3D. Various geophysical investigations and rock engineering field tests were carried out for these purposes. It was suitable that the second tunnel would be excavated in advance, maintaining the face distance between each tunnel face of minimum 25 m. The results of numerical modelling showed that the roof displacement and the convergence of the second tunnel were insignificant, and the maximum bending compressive stress, the maximum shear stress of shotcrete and the maximum axial force of rockbolt were also insignificant. Therefore, it was estimated that the stability of the spillway tunnel was ensured.

• Tunnel and Underground Space
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• v.11 no.1
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• pp.30-35
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• 2001

Focusing on the bench length, this paper presents the results of 3-dimensional elafto-plastic FE Analysis un tunnels of full face, mini-bench and short bench excavated in weathered rock. Influences of unsupported span, horizontal to vertical stress ratio, thickness of shotcrete on the behavior of rock and support were a1so studied. Results showed that displacements of mini-bench tunnels responded more sensitively to bench lengths than those of short bench. The effects of bench excavation on upper half displacement increased with longer unsupported span. Horizontal to vertical stress ratio showed a greater influence on displacement and preceding displacement ratio or sidewall rather than those of crown and invert.

• Journal of the Korean Geotechnical Society
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• v.19 no.2
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• pp.267-278
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• 2003

Artificial neural networks are efficient computing techniques that are widely used to solve complex problems in many fields. In this study, in order to predict tunnel-induced ground movements, Tunnel Behavior Prediction System (TBPS) was developed by using these artificial neural networks model, based on a Held instrumentation database (i.e. crown settlement, convergence, axial force of rock bolt, compressive and shear stress of shotcrete, stress of concrete lining etc.) obtained from 193 location data of 31 different tunnel sites where works are completed. The study and test of the network were performed by Back Propagation Algorithm which is known as a systematic technique for studying the multi-layer artificial neural network. The tunnel behaviors predicted by TBPS were compared with monitored data in the tunnel sites and numerical analysis results. This study showed that the values obtained from TBPS were within allowable limits. It is concluded that this system can effectively estimate the tunnel ground movements and can also be used f3r tunneling feasibility study, and basic and detailed design and construction of tunnel.

• Journal of Korean Tunnelling and Underground Space Association
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• v.19 no.4
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• pp.651-668
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• 2017

For initial stability of the tunnel, the primary support, Shotcrete and rockbolt shall be placed in the most appropriate time. This is because the role of such support plays a vital role in long-term and short-term tunnel stability. In this study, the rock bolt is an important supporting system that receives the external pressure generated by the stress relaxation during tunnel excavation as axial force and transmits it to the shotcrete on the tunnel excavation surface. Until now, most of the materials of rock bolts have been used in the field, but there have been many problems such as uncertain quality of Chinese materials entering the market, poor packing due to falling down of rock bolts when filled with mortar, and corrosion due to water. Therefore, in this study, we have developed a high strength steel pipe rock bolt using Autobeam material to solve and improve various problems of existing rock bolts. In order to evaluate the performance of the developed bolt, field tests were carried out and the existing mortar filler in order to improve the performance of the rock bolt, the design and construction criteria were studied and the results were included in this paper.

• Journal of Korean Tunnelling and Underground Space Association
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• v.15 no.5
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• pp.537-546
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• 2013

Traffic congestion in existing tunnels has increased due to increased traffic volume and enlarged vehicles. Enlarging existing tunnels has advantages over constructing new tunnels by reducing land purchasing costs as well as minimizing natural environment destruction. In fact, many overseas projects for enlarging existing tunnels have been reported. Thus, it appears that the demand on enlarging existing tunnels continues to rise in Korea in near future. Nonetheless, the studies related to the enlarged tunnels have been relatively rare since there have been few tunnel enlargement projects in Korea. In the present study, the tunnel behavior and the stability of rock pillar when enlarging existing parallel tunnels were investigated by performing FE analysis and using existing theory and empirical relationships. Four different enlarging cases, depending on the enlargement types and directions, were examined in the study. According to the results, for the tunnels with the same pillar width after enlarged, the uni-laterally enlarged tunnel indicated 5 to 20% higher crown settlement compared to the bi-laterally enlarged tunnel, and for the tunnel with the narrowest pillar, the highest shotcrete stress was observed. Also, the strength/stress ratio for rock pillar was more than 1.0 for all four enlargement cases, and the Matsuda's method was found to give higher strength/stress ratio by about 50% compared to the Peck's method.

• Journal of the Korean Geosynthetics Society
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• v.16 no.4
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• pp.151-161
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• 2017

A TSL (Thin Spray-on Liner) has a higher initial strength and faster construction time than conventional cementitious shotcrete. Because of its high adhesion and tensile strength, the TSL reinforced concrete show a characteristic like composite materials. In this study, to consider an application to the conventional design method, ASD (allowable stress design), numerical study was used. In the numerical analysis, material and contact properties were adopt from previous studies. Then a thickness of concrete in the tunnel was evaluated with the TSL reinforced case by the ASD concept. In other words, bending compressive stress, bending tensile stress and shearing force of the concrete were considered to determine a thickness of concrete lining by the given boundary conditions. From the numerical analysis, there was no tendency to show by the ASD because the ASD is based on the elastic theory while the TSL typically contributes to reinforcement after yielding.

• Tunnel and Underground Space
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• v.29 no.5
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• pp.346-355
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• 2019

When evaluating pillar stability in very closely spaced tunnels, a local safety factor (strength/stress ratio) at the minimum width has been widely used. Tension bolts have been frequently applied as reinforcement for the cases where safety factors are less than 1.0 from FEM stress analysis. However, the local safety factor shows a constant value irrespective of the change in pillar width/tunnel diameter (PW/D) and the safety factor of the pillar is underestimated because the variation of deviation stress is relatively small even when the pre-stressing is applied to the tension bolt. In addition, the average safety factor proposed by Hoek and Brown(1980) was reviewed, but the pillar safety factor was relatively overestimated when the width of the pillar was increased. As an alternative, the SRM safety factor using shear strength reduction method shows the effect of changing the safety factor in the case of no reinforcement and tension bolt reinforcement as the pillar width/tunnel diameter(PW/D) changes. The failure shape is also similar to the previous limit theory result. In this study, the safety factor was evaluated without considering rock bolt and shotcrete to distinguish reinforcing effect of tension bolt.

• Tunnel and Underground Space
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• v.22 no.2
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• pp.130-143
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• 2012

Cablebolts used to be employed as auxiliary supports where long or high capacity bolts are needed, but become competitive by the improvements in supportability and easiness in handling. Based on the test results obtained from various researches, the performance of the cablebolts was analyzed numerically while varying lengths and fixing conditions. The supporting effecte is assessed by monitoring displacements and stress taken place in shotcrete. When cablebolts are grouted without being tensioned, supporting effect was not as good as that of rockbolts. But, their supportability was good enough to substitute rockbolts if tensioned properly. Post grouting right after tensioning of the cablebolts shows reduction in supportability, but long term stability could be achieved without losing supportability if grouted when the bolt is far enough from the face. Further study is necessary including laboratory and in-situ tests under various conditions to use cablebolts as main support in tunnels.

• Journal of Korean Tunnelling and Underground Space Association
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• v.14 no.1
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• pp.55-77
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• 2012

A Modern Rock TBM is a tunnel excavation method combining the conventional tunnelling method with the mechanized tunnelling method. It is a hybrid system that excavates a tunnel with TBM and supports the ground by ring beam, wire mesh, rock bolt, shotcrete, i.e., conventional tunnelling method. In the Modern Rock TBM, a ring beam is similar to a steel rib in NATM in the way that uses H-beam. But using a ring beam is more effective than a steel rib because it is installed in a closed-circle. Therefore, improving the performance of the ring beam is a key factor for achieving tunnel stability. In this respect, this study introduces a pressurized ring beam that might be functioning more effectively by confining convergence during tunnel excavation. In order to verify the effect of the pressurized ring beam, a three-dimensional numerical analysis was conducted. The numerical analysis confirms an increase in the minimum principal stress and reduction in the plastic strain that triggers excessive displacement. The analysis result also indicates a decrease in the relative displacement occurring after installing the ring beam, and expansion in spacing between the ring beams.

• Journal of Korean Tunnelling and Underground Space Association
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• v.8 no.4
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• pp.289-305
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• 2006

This paper presents the effect of excavating methods on tunnel behavior. As part of this study, it is preliminarily focused on the comparison of two different excavation methods, center diaphram (CD) method and ringcut (RC) method. Especially, the purpose of this research is to study the behavioral mechanism of two tunnels which share the same construction environment but different excavating method. Two numerical analysis models with the same tunnel section and material properties are compared in this study, and they are analyzed by 3D finite element analysis. In each model, face stability, crown displacement, ground settlement, and shotcrete-lining stress are computed, then the general behavior of CD method and RC method is studied. The results indicate that the CD method tends to be effective in controlling tunnel displacement while the RC method is more effective in controlling ground settlement. Design implications of the findings from this study are discussed.