The end of the tunnel finally met the light last Friday.
After eighteen months — five morethan originally planned due to a pair of roof collapses — the $11.5 million WIRTH-built tunnel boring machine broke through after drilling an 11.2-kilometer tunnel through a more than 2,400-meter-high mountain in Central Ecuador.
Photo:San Fransico Project JV
|After 18 months, TBM holes through|
The tunnel, 7.0 meters in diameter, is part of the $314.6-million San Francisco Hydroelectric Project being constructed by an Odebrecht-led consortium. The completed tunnel will connect the existing 156-MW Agoyán powerplant to the new 230 MW hydroelectric powerplant, both on the Pastaza River near the resort town of Baños.
The TBM began excavating in March of 2005 and was expected to complete the tunnel April of this year. Its progress was halted when it struck a fault on Nov. 8 of last year.
Despite the delay, Odebrecht officials say the project is nine months ahead of schedule. Currently they are on track to complete the work by mid-December and the first of the two turbines is scheduled to go online in February with the second following three months later, said Project Director Afranio de Oliveria.
Due to the relative depth of the excavation — at its deepest point, the TBM was boring under more than a half mile of material — the project posed unique problems for the company.
Surface studies identified at least three major faults in layers of granite, gneiss and schist, but their specific location could not be determined since the depth of the tunnel made it impossible to obtain core samples, Olivera said. Probe drills and limited core samples in front of the machine but on a limited basis in order not to greatly affect the drilling schedule, he said.
The incident occurred at one of the deepest portions of the tunnel, more than 900 meters (3,000 feet) below the surface. Water held in the loose rock of the fault hit the TBM with more than 90 bars of pressure bending the outer shield and knocking it off track. Once the liquid had drained from the fault the material then collapsed onto the machine.
To rescue the TBM, engineers dug a smaller tunnel above it and stabilized the loose material with foam and resins. The machine began digging again on Feb. 8 but, days later, hit a second fault. It did not resume normal excavation until mid-April.
The TBM, built in Germany by WIRTH in 1999, excavated Italy's 9.3-km Ginori Tunnel. Italian engineer SELI SPA rebuilt it to Odebrecht's specifications for the San Francisco project. Under regular drilling conditions, it averaged between 21 and 29 meters per day depending on the material it encountered.