• Journal of Korean Tunnelling and Underground Space Association
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• v.11 no.2
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• pp.141-150
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• 2009

When constructing a subsea tunnel in discontinuous rock mass, fluid flow in joints has a great influence on the behavior of the tunnel so that hydro-mechanical coupled analysis should be performed for the stability estimation. In practice, relaxed rock load is generally used for the design of tunnel concrete lining. In a continuum analysis, a method based on the distribution of local safety factor around a tunnel was proposed for the estimation of a potential relaxed zone. However, in the case of discontinuous rock mass in which joints are developed, the whole stability of tunnels depends on the behavior of the joints. In this study, therefore, a method is proposed for the estimation of a potential relaxed zone occurred by the excavation of a tunnel in discontinuous rock mass. The suggested method is validated by sensitivity analysis and the comparison with the results of continuum analysis.

• Journal of Korean Tunnelling and Underground Space Association
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• v.14 no.3
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• pp.231-246
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• 2012

Recently, the construction of tunnel frequently involves neighboring weak ground conditions. In this case, the stabilized ground could be relaxed by the excavation of tunnel. This will create issues in terms of stability of tunnel. Major factors determining the stability of tunnel can be the direction (angle) of weak zone, the distance between tunnel and boundary of weak zone and so on. In this study, by quantifying the displacement and crack propagation during the excavation of tunnel constructed neighboring weak zone, the influence of the direction of weak zone and the distance between tunnel and boundary of weak zone on the mechanical behavior of tunnel is investigated. A series of experimental scaled model tests by changing the direction of weak zone and the distance between tunnel and boundary of weak zone, are performed and analyzed under the condition of homogeneous material. The results show that as the angle between ground surface and boundary of weak zone moves from horizontal to perpendicular plane, displacement near tunnel increases. An increased distance between tunnel and boundary of weak zone induces displacements near tunnel to decrease and stabilizes beyond a certain level of distance. These findings verify and extend the earlier studies quantitatively. Finally, an appropriate distance between tunnel and boundary of weak zone according to the angle of weak zone is justified. This fundamental insight provides the basis for a more rational design of tunnel neighboring weak ground conditions.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.6
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• pp.825-835
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• 2019

When the tunnel is designed, the ground is classified into several grades and the corresponding support system is applied according to the support pattern table. However, a simple pattern application based on rock grade does not take into account the longitudinal stress transitions occurring at rock grade boundaries. In this study, three-dimensional numerical analysis was performed to estimate the stress change in the longitudinal rock grade change of NATM tunnel, and the influence zone of load transfer was investigated using the influence line and trend line. As a result, the downward change of rock grade in the direction of tunnel excavation occurs in the range of 0.35~0.7D from low-strength rock to high-strength rock around the grade change boundary. It is necessary to apply a downward pattern of about 1.0D to the safety direction in consideration of the influence range of 0.35D to 0.7D.

• Journal of Korean Tunnelling and Underground Space Association
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• v.10 no.3
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• pp.207-219
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• 2008

In urban areas, it is very often to excavate ground adjacent to existing structures for the construction of new buildings. Deformation and vibration induced by such construction activities may cause damages to the existing structures and petitions from citizens. To secure safety of the existing structures, particularly of tunnels, establishment of general guidelines on vibration have been crucial concerns, although some institutions have their own guidelines which are not generally accepted. This study aims establishing guidelines for tunnel safety due to blast-induced vibration. Numerical methods are adopted for this study. Blast load equation proposed by International Society of Explosive Engineers (2000) is used to decide detonation pressure. Analysis models were obtained from the construction cases of Seoul Metros. By performing dynamic numerical analysis, vibration velocity of an existing tunnel is evaluated. The numerical results are verified by comparing with the field measurement data obtained in excavation sites adjacent to an existing tunnel. Based on the results vibration safety zone is proposed. Influence circle for vibration velocity is drawn and the area not exceeding the allowable vibration velocity is established.

• Journal of Korean Tunnelling and Underground Space Association
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• v.4 no.1
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• pp.71-78
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• 2002

In this study, the influence of the presence of discontinuity planes on the load transfer mechanism and the pattern of loosening zone was studied based on the laboratory test. The trap-door and the reaction plates are installed as the bottom plane of the model box. The vertical discontinuity plane is installed in the dry sand. Various overburden heights and locations of discontinuity planes are applied as major factors in this study. The results show that at higher overburden heights over about 1.5 times the excavation width, the ratio of the transferred stress to the insitu stress converges to a certain value even if the overburden height increases further. The results also show that the discontinuity plane gives relatively larger influence on the load transfer mechanism, that produces the unsymmetrical load concentration, when the discontinuity plane locates within the tunnel width. When the discontinuity plane locates outside the tunnel width, the unsymmetrical load concentration is reduced considerably.

• Journal of Korean Tunnelling and Underground Space Association
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• v.16 no.1
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• pp.91-104
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• 2014

An Excavation Damaged Zone (EDZ), in which rock properties are permanently changed due to blasting impact or stress redistribution, can influence the behavior and stability of structures. In this study, the mechanical stability of an underground opening was simulated by using FLAC, which is a two-dimensional modeling code, with a consideration of EDZ. A sensitivity analysis was also carried out with fractional factorial design. From the modeling, it was found that the behavior and the stability of an underground tunnel are strongly dependent on the existence of the EDZ. The sensitivity analysis showed that the key parameters affecting the factor of safety around the tunnel are in-situ stress ratio, depth, cohesion, reduction ratio, internal friction angle, and height and width of the tunnel. It is necessary to consider the EDZ, which can significantly affect mechanical stability in tunnel design.

• Journal of Korean Tunnelling and Underground Space Association
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• v.14 no.5
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• pp.529-545
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• 2012

Three-dimensional (3D) finite element analyses have been performed to study the behaviour of a single pile to open face tunnelling in stiff clay. Several key factors such as tunnelling-induced ground and pile settlement, and shear transfer mechanism have been studied in detail. Tunnelling resulted in the development of pile settlement larger than the Greenfield soil surface settlement. In addition, due to changes in the shear transfer between the pile and the soil next to the pile with tunnel advancement, axial force distributions along the pile change drastically. The apparent allowable pile capacity was reduced up to about 30% due to the development of tunnelling-induced pile head settlement. The skin friction on the pile was increased with tunnel advancement associated with the changes of soil stresses and ground deformation and hence axial pile force distribution was reduced. Maximum tunnelling-induced tensile force on the pile was about 21% of the designed pile capacity. The zone of influence on the pile behaviour in the longitudinal direction may be identified as ${\pm}1$-2D (D: tunnel diameter) from the pile centre (behind and ahead of the pile axis in the longitudinal direction) based on the analysis conditions assumed in the current study. Negative excess pore pressure was mobilised near the pile tip, while positive excess pore pressure was computed at the upper part of the pile. It has been found that the serviceability of a pile experiencing adjacent tunnelling is more affected by pile settlement than axial pile force changes.

• Journal of Korean Tunnelling and Underground Space Association
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• v.9 no.3
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• pp.219-228
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• 2007

Scaled model tests were performed to explore the influence of eccentric load and lateral earth pressure on tunnel behavior and their results were verified through numerical analyses. As a method for reducing the eccentric load acting on tunnel, an eccentric supporting system (ESS) was proposed and its applicability was investigated. Experimental results showed that displacement decreased overall and the load inducing initial cracks increased as the eccentric supporting system was applied. The maximum eccentric vertical load which impacted the stability of tunnel was also increased. The test results on the influence of lateral earth pressure on tunnel behavior showed that the general aspect of displacement and crack growth changed significantly depending on the coefficient of lateral earth pressure. In addition, the weak zone In view of stability varied as well.

• Journal of Korean Tunnelling and Underground Space Association
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• v.6 no.1
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• pp.71-83
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• 2004

The purpose of this study is to present an analysis method for the prediction of the changes m ground conditions. To this end, three dimensional convergence displacements are analyzed in several ways to estimate the trend of displacement changes. Three-dimensional arching effect is occurred around the unsupported excavation surface including tunnel face when a tunnel is excavated in a stable rock mass. If the ground condition ahead of tunnel face changes or a weak zone exists, a diagnostic trend of displacement change is observed by the 3 dimensional measurement and numerical analysis. Therefore, the change of ground condition and the existence of a weak zone ahead of tunnel face can be predicted by monitoring 3-dimensional absolute displacements during excavation, and applying the methodology (the ratio of L/C, $C/C_o$, etc.) presented in this study.

• Journal of Korean Tunnelling and Underground Space Association
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• v.22 no.1
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• pp.23-46
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• 2020

In the current work, a series of three-dimensional finite element analyses have been carried out to understand the behaviour of pre-existing single piles to adjacent tunnelling by considering the tunnel face pressures and the relative location of pile tips with respect to the tunnel. The numerical modelling has analysed the effect of the face pressures on the pile behaviour. The analyses concentrate on the ground settlements, the pile head settlements, the axial pile forces and the shear stress transfer mechanism at the pile-soil interface. The head settlements of the pile (the vertical distance between the pile and the tunnel: 0.25D, where D is the tunnel diameter) directly above the tunnel crown with the face pressure 50% of the in-situ horizontal soil stress at the tunnel springline decreased by about 38% compared to corresponding settlements with a face pressure 25% of the in-situ horizontal soil stress at the tunnel springline. Furthermore, it was found that the smaller the face pressure, the larger the tunnelling-induced ground movements and the axial pile forces were and the higher the degree of the shear strength mobilisation at the pile-soil interface. When the piles were outside the tunnel influence zone, compressive pile forces were developed due to tunnelling. It has been found that the ground settlements and the pile settlements are heavily affected by the face pressures and the position of the pile tip relative to the tunnel. In addition, the computed results have been compared with relevant studies previously reported in literature. The behaviour of the piles has been extensively examined and analysed by considering the key features in great detail.