ABSTRACT: In Istanbul, where population is rapidly increasing, underground engineering structures such as subways and tunnels are being constructed to minimize disruptions in transportation. Çamlıca Hill Access Tunnels are constructed to access road transportation to Çamlıca Mosque. Tunnels are excavated in the Kurtköy, Aydos and Gözdağ formations in Istanbul Paleozoic sequence and in the Cretaceous dykes that intrude in to these units. Within the scope of this study, excavation and support works and geo-engineering problems of T2 Tunnel (km:0+795 - 1+066) and Approach Tunnel (km:0+275 - 0+308) belonging to Çamlıca Hill Access Tunnels are studied to be evaluated together with numerical analyses. During tunneling works, additional improvement works have been done especially due to inadequate excavation and support systems. However, it has been determined that proper stability conditions can not be achieved in the tunnels. In addition, generally overbreak, deformations and water inflow are other geo-engineering problems. In the evaluation of these problems affecting the rate of advance and cost, the geological characteristics of the region, the rock properties of the units encountered during the tunnel excavation, rock mass characteristics, excavation and support applications, and current tunnel observations are discussed together. The rock material and rock mass parameters obtained in the study were evaluated in two dimensional numerical analysis based on Hoek-Brown failure criterion and finite element method (Rocscience © Phase 2D software). Longitudinal geological models that were prepared for the examined sections of the tunnel, elastic-plastic zone boundaries and thickness were determined, geological boundary conditions that caused abovementioned problems have been tried to describe. Together with the evaluation of the laboratory data, observations in tunnels and numerical analyses, rock environment that moderate-frequently jointed clayey sandstone and arkosic sandstone with highly and completely weathered (V-VI) levels and especially weak zones (shear, faults etc.) were determined the highest overbreak potential area. In addition it has been revealed overbreak potential is low at rock environment that is slight-moderately fractured, slight-moderately (II-III) weathered arkosic sandstones and quarz arenite without any structural component (fault, shear zone etc.).