An evaluation of using different excavation methods related to specific geologic conditions in the Istanbul area

Mechanical excavation with tunnel boring machines is the most widely used excavation method in Metro tunnels in Istanbul. However, drilling and blasting, impact hammers, and roadheaders are also used in some cases, especially in metro station tunnels (platform tunnels). It is strongly emphasized that the prediction of the performance of the tunnelling machine plays an important role in the time scheduling for job termination and in the economy of tunnelling projects. In the past, the accumulated data in Istanbul Metro Tunnel drives served as a sound basis to model the performance prediction models of the excavation machines. This study gives a unique opportunity to compare; TBM’s, impact hammers, and roadheaders performances with conventional drilling and blasting methods in a selected Metro Project. Although blasting in urban areas is restricted due to vibrations and difficulties in the transport of blasting agents, in extreme conditions its use becomes a necessity. Road header is proven to be more efficient than impact hammer in terms of machine utilization time and production rate. However, high pick consumption makes it a necessity to use impact hammers in similar conditions. An attempt is made to modify the equation used in the past to predict the net breaking rate of impact hammers and it is proved that a reduction factor in the breaking rate taking into account the thickness of the dykes improved the validity of the proposed model. One important point emerging from this paper is that the dip and strike of the joints of 45° in the direction of cutting of the road headers are found to be in favor of the cutting rate of roadheaders given higher values than predicted performance values. The dip and strike of the joints having an angle of 45° in favor of the cutting direction improves the net cutting rate of roadheaders.