Comparison of tunnel boring machine (TBM) and classic excavation method in squeezing rocks in T26 tunnel

Determining the correct excavation and support method when tunneling under squeezing rock conditions is one of the main problems of tunneling. The concept of squeezing is a concept that begins when the threshold shear value is exceeded, develops over time, and is considered risky when the strain exceeds 1%. In this study, the T26 Tunnel (Ankara-Istanbul High Speed Train Project Project km: 216+260 to 222+360), where both classical and Tunnel Boring Machine (TBM) excavation methods were applied, was examined in order to determine the most appropriate excavation and support method in the squeezing rock. Within the scope of the studies, the geology of the T26 Tunnel route and its immediate surroundings was re-evaluated and the existence of swelling accompanying the squeezing feature of the rocks was questioned. Laboratory tests have shown that the swelling properties of rocks are negligible. The stresses affecting the tunnel were determined using kinematic analyses, empirical approaches and numerical analyses. In the TBM and classical excavation methods applied to the squeezing rock in the T26 Tunnel, the compression situations were calculated using the convergence limitation method, empirical and semi-empirical equations for supported and unsupported cases. A maximum accuracy of 75% was achieved in predicting the compression state with empirical and semi-empirical compression equations. In order to increase the compression prediction rate, an empirical equation was created using the Q classification system, the maximum environmental pressure calculated with the k value (σmax), and the maximum stiffness of the support system (koutmax). With the new empirical compression equation, the compression situation in the T26 Tunnel was predicted with 82% accuracy. When the TBM and classical excavation methods applied in the squeezing rock in the T26 Tunnel were evaluated, it was determined that the problems experienced in the excavations with TBM were caused by the insufficient torque and thrust force of the TBM. As a result, excessive excavations occurred. In the classical excavation method, it was determined that daily production speeds decreased to 0.2 m with the support system applied. In order for the tunnel to progress at a certain level, a Hybrid Support System (HDS) has been proposed, which combines the Controlled Deformation Analysis of Rocks and Soils (ADECO R.S.) method with full circular excavation, strong and passive initial support and active final coating application while deformations continue. HDS and applied support models (NATM) were compared with the FLAC3D V2.1 computer program. According to the analysis results, it was observed that lower squeezing occurred in HDS compared to the NATM method. In this context, revisions have been proposed for NATM principles. As a result, it was understood that the problems experienced in the T26 Tunnel were due to inadequate evaluation of geological conditions during tunnel design. In addition, it has been determined that the first choice of excavation in tunnels with a high risk of squeezing should be EPB (Earth Pressure Balanc) type TBM. If the classical excavation method is preferred, it is recommended to use HDS for low strain, sustainable manufacturing speed and rapid adaptation to unexpected conditions.