On September 18, 2018, a Robbins mega-sized slurry machine, measuring 13.7 m (44.8 ft) in diameter, made its first cut into hard rock. The epic launch at an urban jobsite was made possible by Onsite First Time Assembly (OFTA) of the TBM in Japan for the Hiroshima Expressway Line 5 project. The contractor, a joint venture of Obayashi-Taisei-Kosei, had a strict timeline of eight months to adhere to when it came to machine assembly. “This deadline was very important. After assembling the TBM, I think OFTA was appropriate for this project,” said Mr. Ryota Akai, Deputy Project Manager for the Obayashi JV. 

Due to the project location there were also restrictions on delivering the TBM—in order to meet controlled transportation limits within the city, the TBM had to be divided into small transportable weights and sizes, then assembled in a small jobsite measuring just 30 m (100 ft) wide x 60 m (200 ft) long. The 2,400 metric ton (2,650 US ton) machine will bore 1.4 km (0.9 mi) of the 1.8 km (1.1 mi) long tunnel that, once completed, will significantly improve traffic conditions in Hiroshima. 

The massive machine is the country’s first foreign-made large diameter Slurry TBM to excavate hard rock in Japan. “There is a lot of hard rock in Hiroshima,” said Mr. Akai, “and Robbins has a lot of experience boring hard rock.” The machine is expected to encounter granite with rock strengths up to 130 MPa (19,000 psi) UCS. Those involved in the project are excited to see what effect this will have on how Slurry TBMs are used in the future. “The development of this TBM is a milestone,” said Mr. Kiyomi Sasaki, General Manager of Robbins Japan, “it will lead to new tunnel applications worldwide.”

 
Image 1: Robbins crews assisted the contractor during Onsite First Time Assembly (OFTA) of the massive Slurry TBM in a jobsite measuring just 30 m (100 ft) wide x 60 m (200 ft) long.

The design of the Slurry machine is robust in anticipation of potentially abrasive rock conditions and water pressures up to 13 bars. “The Robbins machine is very tough, for example the weight is very heavy. The cutterhead, both its material and structure, are very tough. It will not break in hard rock,” said Mr. Akai.

In preparation for the conditions, the machine was designed for 20-bar water pressure. The robust cutterhead was fitted with 20-inch and 17-inch diameter pressure compensating cutters, which utilize a patented design to effectively operate under high pressure. The joint venture intends to change the disc cutters an estimated 10 times during the bore as part of the machine’s maintenance.

Throughout the assembly and launch process the joint venture crew worked with Robbins Supervisors who assisted and provided guidance. “Robbins crews have a lot of experience; they help us every day despite the language barriers. I appreciate it,” said Mr. Akai. 

The new Expressway Line 5 tunnel will directly connect Hiroshima’s urban area with a major national highway network and is expected to improve access to Hiroshima Airport. Tunnel completion is planned for 2020.

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