Pushing projects in Latin America

A number of project are under development in Latin America, and among those with interesting challenges of underground excavation and showing a range of tunnelling methods, are the Cheves hydro project and Olmos water conveyance scheme, in Peru, and the plants on the Chiriqui Viejo upper cascade, in Panama.

The three schemes are also at different stages of construction – early at Cheves, recently completed on the TBM bore for Olmos, and between those the Panama projects are making progress.

Cheves, Peru

Cheves is a 168MW (2x 84MW) Cheves hydroelectric scheme under development north of Lima, Peru, by Norway’s SN Power and scheduled to be commissioned by late 2013. SN Power is undertaking the project through a special purpose company, Empresa de Generacion Electrica Cheves S.A. The high-head plant is expected to generate up to 835GWh of electricity per year.

Project designer is Norwegian consultant Norconsult, and the main contractors and manufacturers are: Constructora Cheves S.A.C., a JV led by Hochtief (65%) with Salfa (25%) and Ingeniero Civiles y Contratistas Generales S.A. (ICCGSA) (10%) to perform the civil works.

Other 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.

Funding for the scheme is being led through the International Finance Corporation (IFC), and includes other banks, such as DnBNOR and Nordea Bamk which have both supported SN Power capital investment before. Funding is also coming from WestLB and Societe Generale.

The Ministry of Energy and Mines (Ministerio de Energia y Minas) is overseeing the hydropower scheme – one of many under development in the country. Others include Quitaracsa (92MW – Suez Energy), Huanza (90MW – Buenaventura), Chaglla (300MW - Odebrecht), Tam40 (1286MW - Odebrecht) and Cerro de Aguila (500MW – Inkia Energy).

The project area, some 200km from Lima, is reached by the Huaura-Sayan-Churin-Oyon highway, running from the coast up the valley with asphalted surface and then gravel road makeup. The local hydrology is typical for a coastal valley of the Andes mountains with little precipitation (less than 300mm) and concentrated in the wet season. Otherwise solar radition is intense and temperatures vary between 10 degrees Centrigrade and 30 degrees Centigrade.

Dominated by underground infrastructure, the Cheves run-of-river scheme includes a transfer tunnel between two dams high in the catchment, a headrace tunnel, powerhouse and transformer caverns, a tailrace tunnel and a long and large access tunnel.

In addition to the two dams – for the Huaura (10m high) intake and diversions structure, and the Checras (25m high) regulation reservoir, respectively – leading to sediment stilling channels before the headrace inlet, the surface civil works include a further small regulating dam downstream of the tailrace, at Picunche (12m high).

Underground details and progress

Along with the challenges of floods and sediment transport, and seismic stability, the prime design challenges also include geology.

The geology in the catchment comprises a varied mix of igneous, sedimentary, metamorphic and some volcanic strata. There are two main formations in the area – Chimu and Casma – which are split across the upper and lower sections of the project.

The Chimu Formation holds a mix of a quartzite, quartzitic sandstone, bituminous shale and coal, and there’s also some volcanic breccia. The Casma Formation has andesitic rocks with some granodiorite, and there is some horngels lower in the scheme.

Overburden varies significantly over the length of the 9.7km long headrace, from approximately 135m to more than 1200m. The powerhouse and transformer caverns are under about 750m of rock.

The longest structure is the headrace, which is in two parts, the steeper grade (14%) starting at the junction with the approximately 700m long surge tunnel. Like all the other tunnels, the headrace is horseshore-shaped in cross section, which changes in areas from 22.6m2 to 30.1m2 for a constant width of 5.5m from the upper to the lower sections of the conduit.

Separate to the headrace, and upstream, the transfer/diversion tunnel between the Huaura and Checras dams is just over 2.4km long (15.9m2 cross section).

The headrace joins the powerhouse cavern (60m long by 32m high by 15.5m wide), and immediately after is the transformer cavern (27.5m by 14m by 11.2m). The tailrace tunnel is just over 3.3km long (24.9m2 cross section).

A total of approximately 500,000 m3 of rock is to be excavated in the project. In terms of volume, the largest underground structures are the headrace, which accounts for almost half of the tunnelling to be done, and then tailrace, transfer/diversion and access tunnels with totals of approximately253,000m3, 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 also has the largest cross section of the tunnels (41m2) based on a height of 7.3m and a width of 6m.

Construction progress

Construction of the surface works on the scheme is progressing well with the dam foundations completed and concrete structures climbing above the general level of the river bed.

Underground works are making advances, too, especially in the tailrace, the access tunnel which is almost one kilometre long, and also the powerhouse cavern where the crown has been completed and bench excavation is well underway.

Adits have been excavated at key areas along the headrace to commence drill and blast excavation on multiple faces, and also enable work to begin opening up the surge tunnel. At the head of the project, though, it is early days on tunnelling for the transfer/diversion tunnel.

By the middle of this year the rapid excavation of the powerhouse and transformer caverns should be completed, enabling quick progress for mechancial and electrical works to get the twin vertical-axis Pelton turbines and generators in place. 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 next year.

Olmos Trans-Andean Tunnel, Peru

With overburden up to 2000m, or approaching twice as much as the maximum at Cheves, the Olmos water transfer tunnel, in Peru, was always going to be a major tunnelling challenge, in addition to the complicated geology of the young mountain range.

The challenges did come, but so, too, did the adaptations and engineering solutions, and just before the end of 2011 the 5.3m main Robbins main beam TBM finally broke through to mark Brazilian contractor Odebrecht’s completion of the 12.5km long bore.

The Olmos Trans-Andean water transfer scheme, which will be more than 20km in total, has been a dream for more than a century. The rains fall on the east side of the mountain range, in the Huancabamba river basin, and the dream was to convey some of the abundance of that wet hydrology across to the parched west, on the Pacific Ocean side, for irrigation. More than 2 billion m3 is to be conveyed annually.

Future phases of the scheme have been envisaged to have more tunnels built, this time for hydropower.

Geology consists of quartz porphyry, andesite, dacite, tuff, schist and pyroclastic breccias, and the UCS ranges from 60MPa-225MPa. There are numberous fault lines cross the alignment, including two that were known to be at least 50m wide.

The overburden would also present risk of squeezing conditions for a TBM – previously, in the 1950s, drill and blast had been tried. The depth of the tunnel would also presented issues of high temperatures and the need for significant cooling and ventilation.

In 2004 a 20-year build-operate concession was awarded to Concesionaria Trasvace Olmos S.A. to develop the scheme. Odebrecht undeertook the tunnel works and the TBM from Robbins was launched in March 2007.

However, despite having advanced 6km by the second half of 2008, and an finish hoped for before mid-2009, the tunnelling challenge became much harder with rock bursts and would take another two and a half years. Ultimately, there would be more than 16,000 recorded rock bursting events, about 17% of which were classified as severe.

Rock burst challenges

The severity of the tunnelling challenge was unforseen and led to in-tunnel modifications of the TBM.

As rock bursting became increasing worse, the roof shield fingers were removed and instead the machine was fitted with the a net of closley spaced steel slats anchored into to roof that is called the McNally Support System, which was manufacturerd by Robbins under licence from C&M McNally.

The McNally Support System holds back the loose and unstable rock, forming an effective safety umbrella under which the crews can work.

Other changes, says Robbins, included reinforcements to the cutterhead, and relocating the platforms and operator’s cab. Further measures to help counter the problem of rock bursting included pre-drilling and sequential boring to a regime set by Odebrecht. During a TBM push forward, workers had to leave the area behind the cutterhead and stay back at elast 40m from the face for at least half an hour, which allowed time for stress release and rock deformations to take place.

Advance rates varied between 35m in good ground conditions to as low as only 50cm in the most difficult stretches of tunnelling.

“I am proud to have an extraordinary working team – despite all of the difficulties and the challenges they never lost confidence,” says Hiroshi Handa, Odebrecht’s production manager. “The most important thing is that the designer, TBM manufacturer and contractor worked together to make the necessary adjustments to the TBM.”

Chiriqui Viejo cascade, Panama

In the three hydro projects that consitute the upper cascade of the Chriqui Viejo valley developments, in Panama, tunnelling is pushing ahead through volcanic lahar rock which has proved to be as varied, and challenging, as anticipated.

Electron Investment awarded an engineering, procurement and construction (EPC) contract for the Pando and Monte Lirio underground works, in late 2009, to Seli. A total of 13.1km of main tunnel is to be built for the projects, which are to be completed by the end of 2013. EPC contractor for the main civil works is Cobra.

Shortly after, Seli won the tunnelling works contract for the El Alto project, which is being developed just downstream in the tight upper valley by a JV of Hydro Caisan and Panama Power Holdings. The scheme is also scheduled to be commissioned next year.

Common to all three projects is the challenge of topography, bringing the need to bore headraces through lahar rocks, which derive mainly from volcanic slurries and mudflows. Consequently, their characteristics are markedly varied.

At Chiriqui Viejo valley, two classes were identified prior to the commence of tunnelling: matrix- and clast-supported, the former being poor in fines and less well graded. However, no information was available to help class in terms of cohesiveness. But the rocks showed uniform residual and peak friction. Permeability was seen as low, but the varied nature of the material meant there probably remained groundwater risk.

Monte Lirio

The Monte Liro tunnel is almost 7.9km long with a finished diameter of 3.2m. The tunnel is being bored by a 3.92m diameter earth pressure balance machine (EPBM) with 17” discs as well as rippers on the cutterhead.

Tunnelling had a good start in early 2011 but soon met with difficult wet ground conditions. The EPBM met loose lahar and there were large water inflows, which brought the machine to a halt. A short rescue heading was built over the machine to free the shield, and the cutterhead was found to be trapped by a large, uncemented boulder.

After four months of standstill the drive was able to continue late last year. By the beginning of this year the EPBM had constructed 1550m, or approximately one-fifth, of the headrace, it adds.

The headraces at Monte Lirio and Pando have precast concrete linings comprising 1.2m long rings, and muckong out is being performed by locos and wagons.

Pando

For the Pando project, Seli supplied a 3.72m diameter but otherwise virtually identical EPBM. The TBM is driving an almost 5.2km long headrace of 3.0m i.d.

The TBM was launched in early 2011 and the lahar proved good, with a cemented matrix that enabled good progress. By October, the machine had built approximately 1600m of tunnel in five months ‘with relative ease’.

But a problem quite similar to that experienced at Monte Lirio was met at the end of the year when the machine had bored almost 1900m, or just over a third, of the headrace. Recovery work has included consolidating the face and lowering pressure by draining the groundwater.

Where the ground has been found to be poor, the EPBM has advanced in closed mode with full face pressure.

El Alto

On the El Alto hydro projects, the main underground structures are the 3.24km long headrace and surge tanks, a 96m high shaft and the penstock which is slightly greater than 330m long.

Headrace excavation began in the third quarter of last year with a 6.79m diameter EPBM supplied by Seli. Early in the drive, the machine advanced 350m in three months, which includes the learning curve as well as necessary pauses for installation of a continuous conveyor belt for spoil removal.

However, not long after the Seli crew driving the machine had a challenging time with loose lahar. Seli found the particular lahar to be made of rounded blocks of uncemented rock and there was ‘considerable water pressure’.

However, it notes further that otherwise there have been no major probelms on the drive. By the beginning of this year the machine had constructed 970m, or approaching a third of the length of the headrace.