Ultra-wide iron ore tunnel in full swing

Work is progressing well on the record-breaking 20.5m-wide Porto Sudeste tunnel in Brazil. With enough capacity to accommodate four conveyor lines, the 20.5m-wide x 11m-high tunnel looks set to become the widest tunnel of its type.

Designed to convey iron ore to dockside ship loaders in the new Porto Sudeste, the 1.7km tunnel is located in Sepetiba Bay, some 80km southwest of Rio de Janeiro. The tunnel will convey iron ore to the port from two storage stockpiles utilising a 13km-long belt circuit with an ore-carrying capacity of 12,000t/h, 

Main contractor ARG-Civil Port JV appointed Brazilian tunnelling specialist J Dantas to excavate the 211m2 cross-section tunnel. Drill and blast will be used to create a 118m2 top bench (scheduled for completion by August 2011), followed three months later with completion of the lower bench.

Four Sandvik jumbo drill rigs are being used, including a recently delivered DT820. Operating with two twin-boom jumbos on each face, the four booms are drilling 176 holes in the top bench, using 4.26m drill rods at one face and 4.87m drill rods at the second face.

According to project manager Antonio Vitor de Souza Sobrinho, the contractor has faced difficult rock conditions comprising very decomposed and cracked granite. “At best, it is Class 2 and 3, and requires extensive shotcreting and rockbolting,” he said.
He added: “Generally for Class 2, the shotcrete thickness is 75mm, increasing to 150mm for Class 3, 200mm for Class 4 and where we pass through Class 5, 300mm is applied.”

Rockbolting is also carried out extensively using resin-type bolts to a depth of 4.5m, with spacing dependant on the granite conditions. In Class 2, for example, a 2m x 2.23m pattern is drilled, reducing to 1.5m x 1.7m for Class 3, and 1.3m x 1.3m for Class 4.

“Despite problems with the rock conditions, we are currently running ahead of schedule, thanks in part to the performance of the Sandvik jumbos and drill tools,” said engineer Antonio Vitor. “We had anticipated falling behind on a section of Class 5 granite at the Port portal (exit) which was originally estimated to be 20m-long, but was actually a 52m stretch and it all required the installation of steel arches.”

When complete, the new facilities will double owner LLX Logistica’s iron ore shipping capacity to 100Mt/yr, which in turn will allow increased production of the MMX-owned Serra Azul ore mines in Minas Gerais.