İlk Kaplama Betonu Tamamlanmış Bir NATM Tünelinde Oluşan Ani Tabaka Hareketinin 3B Sayısal Modellemesi

Abstract

In this study, the causes of sudden deformations in the T-05 Tunnel located on the Ankara-Polatlı-İzmir YHT line were examined and the failure caused by the instabilities was analyzed with the 3D numerical model program (FLAC3D). It was determined that while the tunnel excavation was completed and the initial lining was not completed, deformations occurred on the tunnel lining as a result of the sudden loadings, threatening the stability of the tunnel. Thereupon, the tunnel was visited and measures were taken to prevent a complete collapse. First of all, the bottom half of the tunnel was filled with waste material. In order to understand the loading mechanism, studies were started and the drilling logs opened in and around the tunnel line were examined. Based on the rock mass classes determined on the tunnel route, the excavation support classes used during the construction of the T-05 tunnel were examined. Then, before starting the tunnel excavation, the excavation support systems determined during the project phase and the problems experienced while opening the tunnel (over breaks in the tunnel crown, cracks observed in the side walls) and the precautions taken were examined. The deformations observed during tunneling were instantly brought under control with the measures taken. 5 overbreaks were occurred at different locations during tunnel excavation at tunnel crown. The total volume of these leakage is 840 m³. Concrete injection was made in order to ensure the stability of the tunnel crown. In this way, the stability of the tunnel is ensured. However, as a result of the deformations and overbreaks, the support under the 8 m thick stronger layer approximately 12 m above the tunnel has decreased and probably layer separation has occurred. The deformations observed in the tunnel during the first excavation phase are the failures occured at the loose rock formation of immediate roof . With the overbreaks of the loose rock formation, the solid ground in the upper layer was emptied. The solid rock structure has been kept intact at this stage. Failures in solid rocks occur suddenly due to the brittle nature of the formation. Over time, the beam formed by the solid layer on the tunnel suddenly collapsed. The solid rocks, in which sudden failure was observed, were transferred dynamic loads to the tunnel by the leverage effect. Sudden deformations were observed in the tunnel on 16.11.2018, during the process when the tunnel was assembled approximately in the center and the final lining construction phase continued. A field visit was made by Prof. Dr. Bahtiyar ÜNVER and TCDD engineers and it was concluded that sudden instability occurred in the tunnel due to strata loading. In order to make the tunnel stable, first the bottom half of the tunnel was filled with waste material. The mechanism described above was analyzed on the generated 3D digital model. First of all, modeling was carried out using the excavation support class used in the phase before the formation of the deformations, and the deformations that occurred during the tunnel opening were also observed in the model. At this stage, the measures taken during the excavation (replacing the support systems) were added to the model and the model was solved again. It was observed that the deformations 24 % decreased after the model was solved. In the further part of the model, dynamic analysis package is used for 3D numerical modeling of sudden failures caused by layer movements. It has been observed that the dynamic modeling method used in the study is effective in the prediction of sudden deformations. It is thought that possible accidents can be prevented by dynamic modeling before excavation in the rock structure with the possibility of sudden deformation. Stress and deformations acting on the tunnel as a result of sudden loading were analyzed. As a result, it has been shown that the conditions observed in the tunnel can be largely analyzed using the numerical model.