Drill and blast excavation is making steady progress in the early phase of underground works on the Cheves hydropower project, under construction in the Andes mountains of Peru. The contractor, Hochtief, leading JV Constructora Cheves, has excavated the adits and access tunnels at key areas of the 9.7km-long headrace, including the surge tunnel, the powerhouse where the crown has already been opened up. It is blasting from the lower section of the 3.3km-long tailrace as well as both ends of the transfer conduit between reservoir high in the catchment area.
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The scheme is being developed by Norwegian power group SN Power, and work began in late 2010, a year after the agreement was signed with the government to build the scheme. With a scheduled start-up shortly before the end of 2013, the power plant will have been brought into operation within three years of work commencing on site.
By the middle of this year the rapid excavation of the powerhouse and transformer caverns should be complete, enabling fast progress for mechancial and electrical installation. Downstream of the caverns, the tailrace tube is due to be finished by early 2013 and, upstream, the transfer and headrace tunnels are scheduled to be completed by the middle of 2013. A total volume of some 500,000m3 of rock is to be excavated for the project.
SN Power and Hochtief/Salfa worked together in Chile recently on the first phase of the La Confluencia hydro scheme, which has comparable installed power capacity and also called for signficant tunnelling
work to be undertaken by drill and blast.
Preparing for Cheves
Peru has seen a number of hydropower projects with underground works built in recent decades, and alongside Cheves others currently under construction or in preparation include Quitaracsa (92MW – Suez Energy), Huanza (90MW – Buenaventura), Chaglla (300MW – Odebrecht), Tam40 (1,286MW – Odebrecht) and Cerro de Aguila (500MW – Inkia Energy). Cheves, on the Huaura River some 200km north of the capital, Lima, is the most advanced of the schemes currently under construction.
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By GlobalDataA high-head scheme, the 168MW plant is expected to generate up to 835GWh of electricity annually. The powerhouse cavern, one of the main structures on the scheme in Oyon province, Lima department, will hold two 84MW generating units. While primarily a power scheme, the run-of-river project will see river regulation during the dry season.
SN Power, which wholly-owns the scheme, is developing the project through the special purpose company Empresa de Generacion Electrica Cheves.
Designer on the project is Norwegian consultant Norconsult. The main contractor and manufacturer is: Constructora Cheves a JV led by Hochtief (65 per cent) with Salfa (25 per cent) and Ingeniero Civiles y Contratistas Generales (ICCG) (10 per cent) to perform the civil works.
Hochtief and Salfa worked together on the underground works for La Confluencia, developed by SN Power with Australiabased Pacific Hydro. Tecsa was the JV partner and last year was acquired by Salfa.
Other key companies, supplying the scheme, are Rainpower, ABB and Jeumont Electric for the electro-mechanical works; Abengoa Peru for the 75km long, 220kV transmission line; and, Cempro Tech for the hydraulic steel works.
The Government department overseeing the scheme is the Ministry of Energy and Mines (Ministerio de Energia y Minas).
Funding has come through the International Finance Corporation (IFC), which is part of the World Bank Group, and other banks, such as DnB NOR and Nordea Bank which have both supported SN Power capital investments before, and also West LB and Societe Generale. They are putting up USD 250M, which is the bulk of the funding as long-term debt finance.
Project layout
While the scheme features three dams, they are small and the underground works dominate the infrastructure, having a total of approximately 18km of tunnels.
In layout, the uppermost of the tunnels on the scheme is the 2,580m-long, 15.9m2 horseshoe-shaped cross section transfer conduit between the Huaura intake on the main river to the lower, Checras reservoir on a tributary.
After passing through settlement channels to reduce the sediment load, the flow enters the 9,693m-long headrace tunnel, which has an upper section at a grade of two per cent and a lower section at 14 per cent grade.
At a constant width of 5.5m, the upper and lower sections have heights of 4.5m and 6m respectively, with their horseshoeshaped cross sections having areas of 22.6m2 and 30.1m2 respectively.
At the junctions of the lower and upper tunnels will be an offshoot stub tunnel – the approximately 700m-long (30.1m2 cross section) surge chamber, also with a 14 per cent slope.
The bottom end of the headrace tunnel splits into two short penstock tubes, taking the flow to the generating units in the powerhouse cavern (60m long by 32m high by 15.5m wide). The slightly smaller transformer cavern (27.5m by 14m by 11.2m) is immediately adjacent, and the large access tunnel reaches both.
Downstream, the flow is discharged into the 3,312m long (24.9m2 cross section) tailrace tunnel.
In terms of volume, the largest underground structures are the headrace tunnel, which accounts for almost half of the tunnelling to be done, and then the tailrace, transfer/diversion and access tunnels with totals of approximately 253,000m3, 81,000m3, 41,000m3 and 39,000m3, respectively, based on minimum cross sections.
Aside from the powerhouse and transformer caverns (24,850m3 and 4,400m3), the access tunnel holds the largest cross section of all the tunnels, at 41m2 based on a height of 7.3m and a width of 6m.
Design and geology
The prime design challenges for Cheves were around geology, floods and sediment transport and seismic stability, says a Norconsult spokesman.
Geology in the area is extremely varied with sedmintary, igneous, volcanic and contact metamorphosis, and there is also some risk of methane gas or hot water being encountered. The areas have two main formations – Chimu and Casma – split across the upper and lower portions of the hydro scheme.
The Chimu formation at the transfer tunnel and upper section of the headrace tunnel is a mix of quartzite, quartzitic sandstone, bituminous shale, coal in the Chimu Formation and volcanic breccia.
The mid-to-lower headrace area is in the area of the Casma Formation with andesitic rocks and some granodiorite, and the latter for along the remainder of the alignment.
The rock in the powerhouse complex is grandiorite. Some may also be met in the tailrace excavation but the rock is mostly andesite and some hornfels.
Overburden varies significantly over the length of the headrace, from approximately 135m to more than 1,200m in the lower portion of the tunnel, up from the powerhouse and transformer caverns, which are themselves under about 750m of rock cover.
Tunnelling
Excavation is underway at a number of faces on the project – in the transfer tunnel, the upper and lower headrace, the powerhouse cavern and tailrace. The access tunnel to the powerhouse cavern passes through the transformer site and has been completed. The tunnels are being supported with shotcrete and bolts.
At the lower end of the scheme, more than a third of the tailrace has completed, tunnelling from the downstream end.
Coming in through the 953m-long main access tunnel, and a further 143m to cut through the transfer cavern space and gap to the powerhouse cavern, the contractor has excavated more than 5,600m3 for the crown (96m2) of the main hall. In addition, a number of bypass tunnels around the caverns have been constructed.
High in the catchment area, excavation of the transfer tunnel is being blasted from the lower end while the first short section for the portal at the upper end has already been created.
Two adits with lengths of 152m and 893m respectively have been completed in the upper section of the headrace and early advances are underway in the main tunnel starting from both adits.
The 152m Checras HRT adit has allowed faces to be advanced upstream, towards the inlet end of the headrace, and downstream to complete just over a third of the planned drive for that section.
The 893m adit one has also allowed excavation to proceed in both directions, including reaching the site for the offshoot surge tunnel to enable its excavation to begin in earnest.
Equipment being used for the tunnelling work includes seven Sandvik DT 720 twin boom jumbos, and also from the manufacturer a DC 301 rubber-tyred bench drill. There are four Gia Haggloader HR 10 loaders and three ITC 312 excavator/loaders and five CAT 938H/950H wheel loaders.
On the underground works there are also four Terex TW 110 mobile excavators and a pair of CAT 330D crawler excavators, plus 10 Manitou MT 1030 telescopic handlers and 20 DUX/Paus DT 20 dumpers.
The main tunnel equipment is completed with a half dozen Semmco Alpha 20 shotcrete robots, 16 MB Actros 3336 rear tippers and 17 Dieci L 4700 truck mixers.
