The deformation suppressing effect of mountain tunnel reinforcement works for deformed mountain tunnels in service are evaluated by numerical analysis. It is found that the back-filling has a limited effect by itself. It is found that the rock-bolting has the effect even by itself, and that the effect increases as the number of rock bolts increases, and there is a length that the effect does not increase any more, which is related to the size of the fracture region. It is found that the effect of the liner reinforcement is small by itself, but the effect becomes apparent when liner reinforcement is used in combination with the back-filling. It is considered that the liner reinforcement should be used in combination with the back-filling and the rock bolting, only when the deformation is large and large effect is required. Based on these results, the selection criteria of reinforcement work and the prediction method of reinforcement effect are proposed so that it can be applied in practice. Moreover, the validity of the method is confirmed by the verification analysis of the actual deformation tunnel.
Tunnel displacement monitoring, a common and mandatory practice amid tunnel excavation, has shown to provide valuable information for evaluating or predicting ground conditions near tunnel face as well as information for assisting decision-making to select and apply proper tunnel support system during excavation. This feature might serve well even for high overburden tunneling, where little geotechnical information could be available in advance of excavation. One of the advantages of extracting information from monitoring data is to predict final tunnel convergence from initial few displacement readings: finding relationships between the initial and the final displacement. In order to fully exploit the monitoring data, it is crucial to observe tunnel displacements in proper interval, locus, and especially, timing. Along with analyzing correlation between monitoring data and actual tunnel support behavior, this paper emphasizes the importance of most initial displacement monitoring, as well as presents renewed support application criteria specific to excavation of solid sedimentary rock masses with foliation or bedding structures. Tunneling experiences in the Akaishi mountain range, central Japan, have shown the validity of this analytical approach.
We considered the index for evaluating soundness against lining concrete spalling toward streamlining and mechanization of the hammering test. From the laboratory test, the following facts were found: the frequencies of the low-order vibration modes of the defect, which may be on the verge of destruction, become considerably low, and it is difficult to hear the vibration sound of the low-order modes from hammering sound. Besides, the following facts were found from the on-site measurements in Shinkansen tunnel: there was a considerable variation in the frequency of the lowest-order mode of the defects, and many of the defects are safe enough for spalling. The inspection criteria of the hammering test are vague, and the evaluation is on the safe side. Therefore, the frequency of the lowest order mode of the defect can be used as an index for evaluation, and it can contribute sufficiently to the efficiency of the inspection.
The earth retaining is often designed using beam spring model assuming the retaining wall is a beam element, the strut is an elastic spring element, and the ground is a spring. A coefficient of horizontal subgrade reaction on the excavation side is set to a constant value, which is obtained from the in-situ test. At the excavation site where the soft alluvial cohesive soil was thickly deposited, it was confirmed by back analysis based on the measured values that the horizontal subgrade ground reaction becomes smaller as the displacement of the earth retaining wall increases. This method does not clarify any specific mechanism in stress-deformation in the braced excavation.
In this paper, the dependence of horizontal subgrade reaction on the displacement of the earth retaining wall was analyzed from the field measurement data and the in-situ test data. It was found that the wall displacement can be appropriately predicted by reducing the initial horizontal subgrade reaction to 30% near the bottom of the excavation. The results emphasize the dependence of the horizontal subgrade reaction on the wall displacement. Also, the behavior of the ground was simulated by the elastoplastic FEM analysis. It was seen that a tendency of an expansion near the bottom of the excavation and a compression tendency at a depth further from the bottom of the excavation could be reproduced with the elastoplastic FEM analysis. The dependence of the horizontal subgrade reaction on the displacement of the retaining wall due to the excavation is minimal.
This paper studies the interaction effects between a newly excavated shield tunnel near the Baba entrance of the Metropolitan Expressway and the existing twin deep tunnels of the Yokohama Kita Line. It was a challenging issue to understand the influence of the proximity tunnel excavation of the Baba entrance on the existing main tunnels of the Yokohama Kita line.
In this study, a three-dimensional numerical model of Baba entrance underground ramp is prepared. In the model, the excavation of new tunnel route is divided into four zones according to the (a) degree of tunnel alignment in vertical and horizontal directions, (b and c) volume and magnitude of grouting pressure, a prescribed displacement was introduced to the soil around the tunnel in each zone. The predicted lining deformation of existing main tunnels induced by new excavation was compared with the field measurement results. The proximity tunneling effects are discussed by using the comparison between the numerical simulation results and the field observations with the 3D variations of the separation in the twin tunnels.
Safe and cost-effective excavation of mountain tunnels requires accurate prediction of geotechnical condition ahead of the tunnel face and preventive auxiliary methods or appropriate support members should be adopted correspondingly. Authors have so far developed a method to predict the change in ground stiffness ahead of the tunnel face by means of inclination monitoring which can be applied without any hindrance to the construction. Applicability and validity of the method have been verified through numerical simulation and field measurement tests. This study performs parametric numerical simulations in which ground stiffness at the face under excavation and ahead of the face, and tunnel overburden are considered as variables in order to clarify the geotechnical condition required for the application of the proposed prediction method. The study also discusses a quantitative estimation method for the tunnel crown settlement by measured inclination. This leads to further effective use of ground prediction results in which allowable deformation volume and appropriate auxiliary methods required can be determined beforehand. Effectiveness of the estimation method is investigated through the post-evaluation of field measurement data.