• Journal of Korean Tunnelling and Underground Space Association
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• v.22 no.5
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• pp.543-561
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• 2020

This paper is the result of the study on the effect of the support structure of the tunnel steel rib. In tunnel excavation, the top and bottom half excavation methods result in subsidence of steel rib reinforcement due to insufficient support of steel rib reinforcement when the ground is poor after excavation. The foundation of the steel rib installed in the upper half excavates the bottom part of the base, causing the subsidence to occur due to various effects such as internal load and lateral pressure. As a result, the tunnel is difficult to maintain and its safety is problematic. To solve these problems, steel rib support structures have been developed. For the purpose of verification, the behavior of the supporting structure is verified by model experiments reduced to shotcrete and steel rib material similarity, the numerical analysis of ΔP and ΔP generated by bottom excavation by Terzaghi theoretical equation. As a result, it was found that the support structure of 20.100~198.423 kN is required for the 10~40 m section of the depth for each soil of weathered soil~soft rock. In addition, as a result of the reduced model experiment, a fixed level of 50% steel rib deposit of steel rib support structure was installed. The study shows that the installation of steel rib support structures will compensate for uncertainties and various problems during construction. It is also thought that the installation of steel rib support structure will have many effects such as stability, economy, and air reduction.

• Journal of Korean Tunnelling and Underground Space Association
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• v.8 no.1
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• pp.31-40
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• 2006

The entrance and the exit structures of tunnels are often constructed on unfavorably soft soils (colluvial soils) as a result of environment-friendly design highlighted in recent years. For construction of such a tunnel, it is essential to secure sufficient bearing capacity of the lining supports as well as that of the surrounding soils. In this regard, H-shape steel-ribs with high stiffness are commonly used for lining supports. However, it was the past convention to ignore the effect of the steel-ribs in numerical evaluation of the structural safety. This study is intended to show how the shotcrete stresses are relieved by the steelribs, on the basis of numerical data obtained from 3-dimensional finite element analysis. The effect of steel ribs to shotcrete stresses is examined at different levels of application rates, i.e., 0%, 50%, 75% and 100% of the total stiffness. The data obtained from numerical analysis was compared with in-situ measurement. The effect of st eel ribs to shotcrete stresses was verified and appropriate total stiffness was proposed in the range of 50%~75%.

• Tunnel and Underground Space
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• v.18 no.3
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• pp.226-238
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• 2008

The steel ribs which are used to enhance the supporting capability of the shotcrete are estimated to be very effective, but their characteristics depending on the types of steel support are not well understood enough to be considered in the design stage. This paper describes the behavior of the shotcrete reinforced by various types of steel supports. Through flexural toughness test, major strength parameters such as flexural tensile strength, equivalent flexural tensile strength and residual tensile strength were obtained and used in the numerical analyses. Test results show that steel rebar was not as dependable as H-beam or lattice girder but close examination of the test results revealed that the specimen was failed in shear because of the shorter span than desired. Therefore tests on the properly dimensioned specimens are necessary for valid evaluation of the steel rebar reinforced shotcrete. In the first set of numerical stability analyses, shotcrete and steel supports were modelled separately. Then compared with the second set of analyses in which shotcrete and steel supports were regarded as a composite material. The two results coincided reasonably and this equivalent model turned out to be useful.

• Geotechnical Engineering
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• v.13 no.2
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• pp.155-168
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• 1997

Lattice girder is a new steel support developed in Europe for the replacement of an existing H-shaped steel set, which is installed after tunnel excavation. Lattice girder has the following several advantages : 1. Lattice girder minimizes the amount of shotcrete shadow which happens to occur behind a steel support. 2. A triangular shape of lattice girder makes shotcrete placed efficiently. 3. Lattice girder provides a good bond strength for shotcrete, which makes the composite structure of lattice girder and shotcrete behave monolithic, and therefore, the rock load can be supported effectively by the lattice girder system, This paper presents the results from a model wall test, a strength test for shotcrete shot on the model wall and a strength test for the bond between lattice girder and shotcrete. These tests proved that lattice-girder system is superior to H-shaped steel-set system concerning the shotcrete rebound rate, the developed shotcrete strength and the adhesion characteristics.

• Journal of Korean Tunnelling and Underground Space Association
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• v.16 no.5
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• pp.459-470
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• 2014

Now-a-days, the trend in constructing tunnels is to build more deeper, more longer tunnels of greater cross-sections. That's why, the demand of "Early-high-strength shotcrete" is very high because of their advantage of attaining higher strength immediately after excavation, which controls the ground subsidence. So, this study reveals the supporting phenomena of early-high-strength shotcrete, using three-dimensional numerical analysis. The crux of this study can be applied practically in construction sites also. Support Performance of two different qualities of shotcrete was checked out, by keeping the general shotcrete's thickness constant and comparing it with early-high-strength shotcrete's thickness decreasing it gradually in five steps, and analysing/comparing the support performance in all cases. Effect of using early-high-strength shotcrete was analysed to save the cost of steel sets, which are widely used for supporting the ground before the hardening of general shotcrete. The results of numerical analysis on the performance of early-high-strength shotcrete show that, it behaves more effectively under worse ground conditions and it can support the ground more conveniently than steel sets, before the shotcrete is hardened.

• Tunnel and Underground Space
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• v.9 no.3
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• pp.204-213
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• 1999

NATM(New Austrian Tunnelling Method) uses a support system of shotcrete, rockbolt and steel support, which are installed after tunnel excavation. Recently, a lattice girder among these support system is used in tunnelling. A lattice girder is a new steel support developed in Europe for the replacement of an existing H-shaped steel set, which is reported to have some problems in installation. This is a triangular shape welded with steel rods and is a light-weight support system which enables fast and easy installation of porepolling. The major advantage of a lattice girder is the good bonding with shotcrete. In this study, to evaluate the applicability of a lattice girder in tunnelling in Korea, field tests were performed at a high speed railway tunnel with a large section. Also, features of lattice girder in field tests were compared with those of a H-shaped steel set respectively. Field tests proved that a lattice girder fully supported the initial earth pressure developed right after excavation and limited ground deformation effectively.

• Journal of Korean Tunnelling and Underground Space Association
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• v.10 no.3
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• pp.269-282
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• 2008

In general, the effects of steel ribs are not considered in the numerical analysis of tunnel design. However, attempts have been increased recently to consider these effects in the analysis of shallow tunnels in soft ground, based on the fact that the steel ribs embedded in the shotcrete take a role to support some portion of the redistributed load due to excavation. In such analyses, the steel ribs can be considered in four different methods: (1) a conventional method where the steel ribs are not considered, (2) a method using the equivalent composite cross section in which the bending moment of shotcrete is not considered, (3) a method using the equivalent composite cross section in which both the compressive stress and the bending moment for the shotcrete and steel rib are considered, and (4) a method using beam elements for the shotcrete and the steel rib, respectively. These methods are adopted in the numerical analysis using FLAC 2D to investigate stresses of both the shotcrete and the steel rib. The overall results show that the analyses are more practical and economical when the effects of steel rib are considered fer the methods (2), (3), and (4). Since the results of those analyses considering steel rib capacity may be different according to the ground condition, it will be necessary to consider the appropriate method among them in accordance with design conditions.

• Journal of the Korean Geotechnical Society
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• v.28 no.10
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• pp.17-25
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• 2012

Shotcrete, rockbolt, and steel rib are installed to support ground after tunnel was excavated. These are important supports for NATM applied tunnels. Recently, lattice girder is increasingly used because it is easily installed. In this study, we developed a new lattice girder by increasing contacting area between spider and rod. To verify the effect of the new lattice girder, the 3-point and 4-point flexural strength tests were carried out for LG-$50{\times}20{\times}30$, LG-$70{\times}20{\times}30$, LG-$95{\times}22{\times}32$. As a result, in case of contacting area, strength of new SGS lattice girder is 17.95% higher than that of original lattice girder. In case of weakness point, strength of new SGS lattice girder is 19.37% higher than that of original lattice girder.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.37 no.2
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• pp.357-367
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• 2017

Bending and axial forces simultaneously occur at the cross-section of a shotcrete lining reinforced with steel supports due to the tunnel geometry. The shotcrete has changing flexural stiffness depending on the axial forces and, as a result, severely nonlinear behavior. The mechanical properties of a shotcrete-steel composite also depend on the type of steel support. This study presents a fiber section element model considering the effect of axial force to evaluate the nonlinear behavior of a shotcrete-steel composite. Additionally, the model was used to analyze the effects of different types of steel supports on the load capacity. Furthermore, a modified hyperbolic model for ground reaction, including strain-softening, is proposed to account for the ground-lining interaction. The model was validated by comparing the numerical results with results from previous load test performed on arched shotcrete specimens. The changes in mechanical responses of the lining were also investigated. Results show a lining with doubly reinforcement rebar has similar load capacity as a lining with H-shaped supports. The use of more materials for the steel support enhances the residual resistance. For all types of steel reinforcement, the contribution of steel supports during peak load decreases as the ground becomes stiffer.

• 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.