• Geomechanics and Engineering
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• v.29 no.3
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• pp.301-310
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• 2022

Accurate prediction of mixed ground conditions ahead of a tunnel face is of vital importance for safe excavation using tunnel boring machines (TBMs). Previous studies have primarily focused on electrical resistivity surveys from the ground surface for geotechnical investigation. In this study, an FE (finite element) numerical model was developed to simulate electrical resistivity surveys for the prediction of risky mixed ground conditions in front of a tunnel face. The proposed FE model is validated by comparing with the apparent electrical resistivity values obtained from the analytical solution corresponding to a vertical fault on the ground surface (i.e., a simplified model). A series of parametric studies was performed with the FE model to analyze the effect of geological and sensor geometric conditions on the electrical resistivity survey. The parametric study revealed that the interface slope between two different ground formations affects the electrical resistivity measurements during TBM excavation. In addition, a large difference in electrical resistivity between two different ground formations represented the dramatic effect of the mixed ground conditions on the electrical resistivity values. The parametric studies of the electrode array showed that the proper selection of the electrode spacing and the location of the electrode array on the tunnel face of TBM is very important. Thus, it is concluded that the developed FE numerical model can successfully predict the presence of a mixed ground zone, which enables optimal management of potential risks.

• Tunnel and Underground Space
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• v.27 no.4
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• pp.185-194
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• 2017

This technical paper is a study on the unique displacements of Shotcrete Lining at the mined tunnel during excavation period through deep consideration with real time data from monitoring instrumentations correlation with the numerical analysis to identify the rock stresses and the rock spring points at the working face of the Conventional tunnelling by the Drill and Blast, based on the geological face mapping results of the project NS2, Transmission cable tunnel project in Singapore. The created geometry of numerical model was prepared to the real mined tunnel construction site including, vertical shaft, construction adit, tunnel junction area, and 2 enlargement caverns. The convergence measurements by the monitoring instrumentation were performed during the tunnel excavation and shaft sinking construction stages to guarantee the safety of complicated underground structures.

• 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.

• Geotechnical Engineering
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• v.10 no.2
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• pp.41-56
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• 1994

Under the groundwater level, the tunnel face is affected by the seepage force and the groundwater flow may cause a trouble to the tunnel support systems. The appropriate methods of analysis and design in the tunnel face and the lining, considering groundwater flow according to tunnel drainage condition are presented in this thesis. First, the effect of seepage on the stability of tunnel face was studied. Seepage force was estimated by the 3-D finite element analysis and the stability of tunnel face was checked by analytical method. Furthermore, using the finite difference method the stress and displacement on the face were computed for either case, where the seepage force is or is not considered, and the effect of seepage on the tunnel face stability was evaluated. Second, the effect of seepage force on the tunnel lining when construction is finished and steady state seepage flow occurs was studied and a design methodology considering seepage effect was made. Consequently, in case where the groundwater level remains almost unchanged and the steady state groundwater flow occurs, the proper countermeasures for face staility are required according to the condition of groundwater flow. Moreover, the tunnel lining should be designed and constructed considering the seepage force occuring by the groundwater flow toward the tunnel linings.

• Tunnel and Underground Space
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• v.15 no.1 s.54
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• pp.39-46
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• 2005

The RMR ratings, one by horizontal pre-boring in a tunnel and another by tunnel face mapping, are compared at the fault zone in a tunnel. Generally. the horizontal pre-borings were so effective as to forecast reasonably the supporting patterns after tunnel excavation. But the maximum difference in RMR ratings estimated by two methods was about 50 at a certain section of a tunnel. The differences were analyzed on each parameter of the RMR system: the rating differences were 24 in the condition of discontinuities, 15 in the RQD and 13 in the uniaxial compressive strength of rock. To minimize the gap between RMR by pre-borings and by face mappings, it is necessary to select the horizontal pre-boring location where tunnel stability could be critical and to evaluate in detail the sub-parameters of the condition of discontinuities.

• Proceedings of the Korean Geotechical Society Conference
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• 1993.03a
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• pp.9-14
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• 1993

Practical solutions that are available today for assessing stability of tunnel working face are largely based on the concept of critical stability ratio. The accuracy of a prediction of the soil behavior in the working face, thus, depends on the ability of the solution to completely and accurately describe the stress fields or kinematics generated by the excavation and the accuracy of the undrained shear strength of the soil introduced in the computation. This paper reviews the selected solutions describing stability of the tunnel heading in squeezing ground, and suggests a reference solution which is established based on comparison of the solutions and field data on stability of tunnel headings in clays. Although dealing with the shear strength determination is an important companion part of the geotechnical prediction for stability of the tunnel heading in clays, this part is beyond the scope of this paper at this time.

• Tunnel and Underground Space
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• v.5 no.2
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• pp.95-103
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• 1995

In this paper the stress redistribution around tunnel face was analyzed by using a three-dimensional finite element model. The effects of in-situ stress levels, excavation sequences, stiffness difference between the hard ground and the weak zone on the stress redistributions were considered. Displacement and stress changes at tunnel crown, side wall, and invert were investigated throughout the sequential excavation. To show ground response, percentage of the displacement and stress variations are used as a function of normalized distance that is between the face and monitoring section. Preceding displacements and stress variations were presented to be adopted in the two-dimensional tunnel analysis.

• The Journal of Engineering Geology
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• v.17 no.2 s.52
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• pp.197-204
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• 2007

Tunnel deformation happens by excavation. After installation of support, tunnel is gradually stabilized over time. Effect of excavation on tunnel behavior decreases as increase of distance from face. If the time that the displacement converges by tunnel stabilization is estimated, processes after stabilization can be advanced and economic loss can be reduced. In this study, the distance of displacement convergent point from face in the tunnel constructed on sedimentary rock is estimated using control chart method. As the results of analysis using a control of chart, displacements in a sedimentary rock tunnel are converged within 100 m from each tunnel face.

• Tunnel and Underground Space
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• v.13 no.6
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• pp.476-485
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• 2003

The purpose of this study is to present an analysis method for the prediction of the change of ground conditions. To this end, three-dimensional convergence displacements is analyzed in several ways to estimate the trend of displacement change. 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 fracture zone exists a specific trend of displacement change is known to be occurred from the results of the existing researches. The existence of a discontinuity, whose change in front of the tunnel face, can be predicted from the ratio of L/C (longitudinal displacement at crown divided by settlement at crown) etc. Therefore, the change of ground condition and the existence of a fracture zone ahead of tunnel face can be predicted by monitoring three-dimensional absolute displacements during excavation, and applying the methodology presented in this study.

• Journal of Korean Tunnelling and Underground Space Association
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• v.8 no.3
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• pp.197-204
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• 2006

A large sectional tunnelling method using Shield TBM is expected to be popular as domestic demand of long tunnel gets growing. Although a shield tunnelling method has been recognized as prominent method in consideration of stability and applicability in shallow and poor ground, the cases of accident and constructional trouble have been often happened due to unexpected poor ground condition, or selection and use of improper shield machine. Especially, troubling cases at tunnel face are frequently occurred, so supporting pressure control of tunnel face would be the main issue for securing safer and more efficient tunnel excavation using Shield TBM. In this point, we carried out the numerical feed-back analysis to compare the ground deformation pattern with theoretical result at tunnel face.