The second of two Herrenknecht TBMs is through to complete mechanized tunnelling on the headrace tunnel at the Coca Codo Sinclair hydropower scheme in Ecuador, South America. The 24.8km headrace, under construction by contractor Sinohydro Corporation, is claimed as the longest tunnel in the country.
The run-of-river scheme is dominated by underground works to tap the waters at the confluence of the Quijos and Salado rivers, 150km east of the capital, Quito. Sinohydro used the 9.04m diameter double shield TBMs to bore the majority of the headrace tunnel, which cuts through the mountain at a long river bend.
The Coca Codo Sinclair scheme exploits the large head across the river bend to convey water from the rivers down to a compensating reservoir, located near the underground powerhouse. Geology along the alignment comprises basalt, andesitic lava, rhyolite, ignimbrite, tuff, sandstone, siltstone, slate and volcanic rock.
The two TBMs were launched in late 2012 to bore a total of 23.4km, split into separate drives of 13.8km and 9.6km. The longer drive was the first to hole through, in February this year (2015), with the second TBM completing its bore earlier this month (April).
The tunnel is lined to an 8.2m i.d. with precast concrete segments cast in moulds supplied by Herrenknecht Formwork.
The headrace works also include 1.4km of drill+blast excavation within the main alignment. With a total tunnel length of 24.8km, this is claimed as the longest tunnel in Ecuador, according to the state-owned developer, Empresa Publica Estrategica Hidroelectrica Coca Codo Sinclair EP (Cocasinclair).
Other key underground structures on the large hydropower scheme are the powerhouse and transformer caverns, the high-pressure penstock tunnels, the short tailrace tunnel, and various access tunnels. The two penstock tunnels from the compensating reservoir each have horizontal and shaft sections and are 1.9km long. The machine hall cavern is 212m long x 47m high x 26m wide and will house eight turbines.
Sinohydro is building Coca Codo Sinclair under an engineering, procurement and construction (EPC) contract with Cocasinclair. The developer expects the 1,500 MW hydropower scheme to generate at least 8,600 GWh of electricity annually. The majority 85% of funding for the project is being provided by the Export-Import Bank of China. The balance is supported, in part, by revenues expected from the sale of carbon credits for generating clean power.