Nowadays, numerical methods are becoming commonly used in the design of urban tunnel excavations where both displacements and stresses must be predicted realistically. These tunnels may be required to be designed in low overburden where the rock is fractured and weathered. A geotechnical risk management including a back analysis can be implemented to meet budget and schedule aims without scarifying the safety and the quality. This paper firstly, describes the methods to define the tunnel geology and system behaviour and numerical design methods to predict the displacements and the stresses for an NATM Tunnel in fractured rocks. Secondly, it describes a monitoring system and suggests back analysis methods to validate & revise the design itself for implementing a geotechnical risk management system.


Ministry of Transport, Maritime Affairs and Communications of Republic of Turkey, framed a Built-Operate-Transfer (BOT) project that will positively help Istanbul's increasing traffic problems across Bosphorus. The contract was awarded by a special purpose company named Eurasia Tunnel Operation Construction and Investment Inc. (ATAS) which was formed under the leadership of Yapi Merkezi from Turkey and SK E&C from Korea.

The Eurasia Tunnel Project (Istanbul Strait Road Tube Crossing Project), received the Major Tunnel ing Project of the Year at the ITA Tunneling Awards, held for the first time by the International Tunneling and Underground Space Association (ITA).

The major components of the Project are the up- grading/widening of existing roads (63% of project length) and a subsea tunnel under Bosphorus (23% of project length). The rest which includes cut and cover structures and conventionally mined twin tunnels (NATM) which is the subject of this study.

Twin NATM tunnels composes of two, generally parallel tunnels, as it can be seen in Figure 1, each with a width of 11.0 m and a height of 8.0 m, amounting to an excavation area of about 85 m2 ~ 96 m2. The longest tunnel is the east- bound one with a length of 969.0 m, on the other hand the westbound tunnel holds a length of 924.0 m except for a few meters thick fill. The tunnels were excavated within the Trakya Formation bedrock having very weak and weathered rock mass sections. In addition, the twin tunnels were passing under a densely populated district, city water lines, crossing roads and an eye hospital with minimum 8.0 m to maximum 41.0 m overburden.

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