Robust Single Shield Navigates Turkey’s Hardest Rock

Project Overview

Gaziantep province, in Southeastern Turkey, is an important center of agriculture and trade divided into nine districts. With a population of nearly 1.7 million, the province is overhauling its public transportation with a rail line between the town of Bahçe and the Nurdağı district.  The Bahce-Nurdag Railway Tunnel will consist of two parallel 10.1 km (6.3 mi) tunnels, excavated by both drill and blast (2.9 km/1.8 mi) and TBM (7.2 km/4.5 mi).

Geology

The complex geology of the region is affected by the East Anatolian Fault zone, making ground conditions unpredictable. Mixed ground conditions prevail on the project, and range from interbedded sandstone, quartzite, and mudstone to highly weathered shale and dolomitic limestone.  High water ingress is predicted with several springs directly overhead.

Machine Design

The customized Single Shield TBM was designed with Difficult Ground Solutions (DGS).  Closure gates and an emergency sealing system effectively seal off high-pressure inrushes of water. The system reduces or eliminates the amount of mud or water that can pass through the machine and gives the crew time to safely grout off the water inflows.  During such an event the machine would be stopped until water inflows are reduced to the point that the TBM could begin excavation once again.

Onsite Assembly & Excavation

The Single Shield TBM was assembled at the jobsite using Onsite First Time Assembly (OFTA). The jobsite location about 20 miles (32 km) from the Syrian border complicated shipping of some parts but the assembly was completed and a launch ceremony held in January 2016.

The TBM encountered a very high strength rock mass—perhaps the highest strengths yet encountered in Turkey. Samples of meta-sandstone and meta-mudstone have tested at 223 Mpa UCS.

Breakthrough

On July 24, 2020 the first TBM-driven portion of tunneling was completed. The 8.9 km (5.5 mi) long tunnel that was excavated had some of the hardest and most abrasive rock encountered in Turkey. The TBM achieved up to 456 m (1,500 ft) per month, a result achieved with the help of a Robbins Continuous Conveyor System for muck removal.