By rosebell
With 13.67m-diameter a Robbins slurry TBM has broken through on the Hiroshima Expressway Line 5 and completed the 1.4 km section on April 30 after boring through challenging geology.
As the first time that a foreign-made large-diameter slurry TBM had been used to dig hard rock in Japan, this 2,400-tonne TBM was deployed to excavate beneath Hiroshima’s dense urban landscape for the Obayashi-Taisei-Kosei joint venture. Linking the city’s urban centre with a major national highway network, this tunnel improves access to Hiroshima Airport.
According to Obayashi deputy director Tetsuya Taniguchi: “While there is little track record of large-diameter hard rock drilling with TBMs in Japan, Robbins’ know-how and on-site support for disc cutters, which have a long track record in hard rock drilling, were excellent. We have been able to obtain great results and technical knowledge for future large-diameter slurry hard rock drilling work.”
Considering that the region’s challenging geology comprises of granite formations with unconfined compressive strengths of up to 130MPa (19,000psi), the TBM was designed to handle this and the cutterhead and other components were built to withstand these abrasive rock conditions as well as high water pressure.
The location of assembling this TBM was the confined jobsite, only 30x60m, using Onsite First Time Assembly (OFTA), that enabled the team to meet a strict timeline of eight months for machine assembly.
Very massive, hard granite were the expected general geology and while some fracture zones were expected, crews conducted horizontal boring in front of the face every 50m but these fracture zones were not found.
Taniguchi added: “In addition, there was a section of low cover directly beneath a residential area, but we were able to pass through it safely by taking the appropriate measures to mitigate environmental impacts such as vibration, noise and ground surface displacement.”
Robbins says: ” The success of the project proves the effectiveness of slurry TBMs in challenging hard rock conditions. The completion of the tunnel for the project owner, Hiroshima Expressway Public Corporation, is also a boon to the region, supporting its growing economy and tourist sector.”
“From the complex machine launch to breakthrough, we are proud of this tunnel being completed for the city of Hiroshima and congratulate the Obayashi-Taisei-Kosei JV. It will make a huge positive impact for the city,” said Robbins chief engineer Shinichi Konda.
