The US$302M San Francisco hydroelectric project is currently being constructed in Ecuador to relieve the country’s present power deficit. The 230MW project is located entirely underground and includes the construction of a number of large caverns and a 9km long hard rock TBM drive. Amanda Foley recently visited the project site
In the heart of Ecuador, approximately 150km southeast of the country’s capital Quito, the Andean highlands meet the Amazon basin, with breath-taking results. A landscape of lush forested mountains and active volcanoes surrounds the towns and villages of the Tungurahua Province. Here the Rio Pastaza, an ancient Amazon tributary, cuts through the town of Baños, winding its way south through the spectacular ‘Avenue of Waterfalls’ – a series of river confluences that create an area of exceptional natural beauty.
Aside from the river’s significance to the country’s burgeoning tourism industry, the Pastaza also holds the potential to significantly relieve Ecuador’s present electricity deficit. This is being addressed with the construction of the 230MW San Francisco hydroelectric power project. The US$302M run-of-river scheme currently being constructed by the Hiropastaza consortium, consisting of Hidro Agoyan SA (80%) and Construtora Norberto Odebrecht SA (20%), will increase the country’s current power capacity by approximately 12%.
Based on a 30-year concession granted by the recently established Ecuadorian electricity council CONELEC, the project is being financed with a US$243M loan from Brazilian development bank, Banco Nacional de Desenvolvimento Económico e Social (BNDES), and equity contributions of US$59M from Hidropastaza’s shareholders.
The project, which will be located entirely underground, is in a sense an extension of the adjacent Agoyán hydroelectric plant, also owned and operated by Hidro Agoyán SA. Originally, the San Francisco plant was to mirror the traditional configuration of this existing 156MW plant, but the construction of a second dam downstream of the Agoyán’s discharge tunnels would have involved the flooding of a significant section of the Pastaza canyon. As environmental sensitivity and minimal construction impact upon the area were deemed to be key project concerns from the outset however, this idea was quickly discarded as untenable.
Instead, an innovative design was developed to intercept the Agoyán’s discharge tunnels underground and redirect the flows into an interconnection chamber (figure 2). This allows for minor fluctuations in water volumes during operation and feeds the 11.3km long headrace tunnel (9km of which will be driven using a Wirth hard rock TBM).
Following a frustrating start-up delay, when the project’s loan guarantee was temporarily suspended by the Reciprocal Credits and Payments Agreement (CCR) of the Latin American Integration Association (ALADI), Odebrecht, alstom and va-tech commenced their US$286M EPC contract for the construction and electromechanical works in December 2003. As soon as it was given the go-ahead, Odebrecht took possession of the site and mobilised simultaneously at a number of points along the project alignment.
Construction works commenced concurrent with site set-up at the interconnection zone, the main campsite at Adit 4 and at the power house complex. This work was hampered slightly by access problems at river level, posed by the steep inclines of the canyon. Excavation proceeded at an impressive rate however, rock quality along the project being variable, but generally good; ranging from hard Palaeozoic granite to gneiss and schist of the same era.
Fast-tracking at the power house
An access road to the power house complex – which houses two Francis turbines – was promptly built and drill and blast excavation of the 451m long 52m2 section main access tunnel (figure 2) commenced shortly afterwards. The 200MPa granite in this zone is some of the best rock on the project, in respect to foliation and rock mass characteristics, and a three-boom Tamrock Axera jumbo quickly established a daily advance of 7.68m.
Two further access tunnels were soon also underway; the first of these was to assist the construction of the discharge tunnel portal and the second was to enable the excavation of the stopgate chamber. The rock here was less favourable than in the rest of this zone, with the presence of a nearby fault leading to joint discontinuities and block failures in the soffit of the stopgate chamber. This required significant over-excavation back to solid rock and the adoption of systematic rockbolting at 2.5m x 2.5m centres, combined with a 100mm thick layer of Dramix steel fibre reinforced shotcrete.
Originally, excavation of the main 76.2m long x 19.2m wide x 45.4m high power house cavern and its integral control centre had been planned via the escape tunnel. However, as part of a value engineering exercise undertaken at the beginning of construction, the decision was made to raise the level of the cable tunnel from its planned location at the base of the power house and utilise it as access for the cavern’s top-heading, exploiting a short section of the drainage tunnel and access 3 for muck removal. ‘This allowed drill and blast works to begin in July 2004, a month earlier than planned, and provided considerable cost and programme savings,’ explained Gustavo Belitardo, Odebrecht’s engineering manager.
Two Axera T08 jumbos were employed at the power house complex, one for the tunnels and the other working on the main cavern. Excavation of the cavern was initially conducted from the cable tunnel, retaining a series of ramps for mucking, until the main access tunnel was reached. Subsequent headings from the penstock bifurcation and surge tunnels were later also utilised.
Support consisted of systematic 6m long, 25mm diameter, rockbolts drilled at 2.5m x 2.5m centres in the arch and 3m x 3m centres in the walls, followed by an 80mm thick steel fibre reinforced shotcrete lining. Drains were installed at 5m x 5m centres and a suspended corrugated-steel ceiling fitted in order to redirect water away from the activities below.
The power house was excavated in just six months and in early April this year, all drill and blast works at the complex had been completed and concreting of the main access tunnel and power house cavern was well underway.
The 170m deep Penstock shaft was also progressing well. Having blasted a 3.2m diameter pilot shaft using an Alimak Raise Climber, workers were widening down with a Galloway stage in order to achieve a finished diameter of 7m.
At the Interconnection zone, site access posed a serious challenge, the canyon’s steep incline making a portal heading almost impossible. However, an innovative strategy was formulated to gain early access by utilising the Agoyán plant’s existing maintenance shaft and gantry crane. ‘All the machinery and equipment was dismantled and lowered piece by piece down the Agoyán shaft,’ said Belitardo. ‘Everything was then reassembled again at the bottom.’ Following the installation of blast protection measures, Hidro Agoyán granted Odebrecht permission to begin drilling from within the operational power plant.
The configuration of the Interconnection Zone reflects the simplicity of the project concept adopted. Agoyán’s discharge flows (116m3/sec) are intercepted and diverted through the 21,250m3 interconnection chamber, which then feeds into the 7.5m wide (54m2 section) 11.3km long headrace tunnel. A series of stopgates allows for different operational flow modes, depending upon the Pastaza’s seasonal volumes.
Having gained access to the interconnection chamber and the surface, excavation advanced rapidly. This progress has since been greatly assisted by the construction of a large plant and equipment transporter, in October 2004, down the steep incline of the bank. The vast majority of drill and blast operations for the interconnection zone, including the chamber and the discharge tunnel, have now been completed.
Adit 4 and the campsite
The project’s main campsite is located on a natural plateau, a short distance upstream of the power house complex. This is also the location of Adit 4, the main construction heading for the headrace tunnel. In the original bid documents four construction adits were planned, but only one has actually been constructed; in order to launch the TBM and drill and blast the downstream headrace tunnel, which ends at the 300m long upper surge tunnel and the penstock.
As soon as Odebrecht gained possession of the land in January 2004, site set-up commenced immediately, concurrent with portal construction and drill and blast works for the 648m long adit.
The small campsite accommodates all the project’s workshops and equipment stores, as well as the segment production yard and a muck storage facility. ‘Room was extremely tight, especially in the beginning when new machinery and equipment was arriving everyday,’ remarked production manager Afranio Oliveira. ‘However, as more and more of this moved into the tunnel, things became easier.’
Another Tamrock Axera T08 jumbo was mobilised on Adit 4 and the headrace, equipped with a TCAD positioning system to assist drilling the 99 x 45mm diameter holes for each advance. Rounds were drilled to a 5.10m depth and pulled with a 93% efficiency, to give an advance of 4.74m. Imported IBEMUX emulsion explosives, with a specific density of 1.10kg/cm3, were used as local explosives were quickly found to be unreliable. Rounds had a powder factor of 2.33kg/m3, with a total of 585kg of explosives used for each blast. Working three shifts per day, six days a week, an average daily advance of 7.66m was set for the two headings and by January this year all the drill and blast works had been completed.
For all the drill and blast works on the project Caterpillar 972G wheel loaders, with side tipping buckets, and Volvo A25 trucks have been employed to remove excavated material and transport it to the disposal areas at each of the three work sites. Here the rock is crushed and either employed as aggregate for concrete production or used to carefully re-landscape the left bank of the river. Once the project is completed, these areas will be re-planted and, due to the extremely fertile nature of the area, are expected to re-establish themselves within a year.
Hard rock tbm drive
A big issue during the project start-up period was the Baños to Puyo road, which is used to access the various work sites. Until December 2004, a separate government funded project to upgrade the route, which extends from the Andes into the Amazon basin, remained only partly completed. The original winding, single-lane road runs along the steep mountain edge of the left bank of the Pastaza river, with few opportunities for vehicles to pass. This could make the short, now 15 minute, journey from Baños (where the project offices are located) to the campsite an arduous journey of up to 11/2 hours. As the project’s TBM delivery date approached, completion of the road became critical. ‘We were more than a little anxious, as there was no way the TBM could have been transported along the old road,’ engineer Carlos Guimaraes explained. ‘However, the road was finally opened, just weeks before the TBM delivery on New Year’s Eve.’
The hard rock TBM, which has a total installed power of 2100kW and a maximum cutterhead torque of 12,500kN, was supplied by SELI. The firm purchased the 6.3m diameter machine from Wirth and adapted it to 7.04m for the project. The cutterhead was also enlarged and redesigned and is now dressed with 50 face cutters and three over cutters. On average each dressing is expected to last 170m in the granite and 460m in the schist. TBM assembly was completed on 15 January this year and, after moving the machine through Adit 4, test excavation began a month later. At the time of site visit, the TBM had travelled a distance of 90m and during this start-up phase progress rates of 7m per day were being achieved. However, once true excavation begins a daily advance rate of 25m is expected.
For the majority of the 9km drive rock quality is fairly good and thus lining will be limited to pre-cast invert segments only. However, a total of 605 full segment rings will be required in a number of potentially difficult fault and river crossing zones, such as the Blanco, Verde and Machay rivers (figure 2).
The TBM’s backup is also equipped with two hydraulic rockbolting drills, an installation platform and a Sika shotcrete robot, in order to deal with a wide range of support requirements; from spot bolting to systematic rockbolting and a 100mm thick layer of steel fibre reinforced shotcrete. The segments were manufactured by Odebrecht in a factory at the main campsite using moulds supplied by Bernold Ceresola of Switzerland. Each 6.4m i.d. ring consists of six segments and is reinforced with an average 93kg of steel rebar/m3 for demoulding, transport, TBM thrust forces and ground pressures. The segments are delivered to the TBM via SELI supplied rail cars, driven by a Schöma loco. The system also provides cars for the supply of peagravel and mortar for annulus backfilling, as well as for mucking, personnel transport and maintenance.
Working 24hrs a day, seven days a week, the TBM is due to complete its 9km long drive in April 2006. In the meantime, a number of challenges are anticipated. Geological site investigation has indicated a number of faults along the alignment. Of these, three major faults at PK 7+000, PK 3+000 and PK 1+000 (which intersects the bed of the Rio Blanco), are predicted to be the worst. When the TBM reaches these areas systematic probe drilling, grouting, and if required core drilling, will be performed from within the TBM’s the gripper shield.
Positive attitude for success
Much of the success already achieved on the project can be attributed to the positive and forward thinking attitude adopted by Odebrecht and its employees. A strong emphasis on sensitivity to the local environment, value engineering, teamwork and investment in employee development, is prevalent. An example of this can be seen in the use of labour on the project. Over 1500 locals are employed on the scheme, providing a close working relationship with the community. ‘A special division was set up within Odebrecht to oversee training of these men and the establishment of each new work activity,’ explained administration manager Eduardo Barbosa. ‘When each activity comes to an end the workers are moved on to the next activity and re-trained.’ This will result in miners who will be able to gain work on future projects as experienced multi-skilled personnel.
Amanda Foley is Deputy Editor of our sister publication Tunnels & Tunnelling International. For further information on the journal, visit www.tunnelsonline.info or email firstname.lastname@example.org
Amanda would like to thank Odebrecht’s project manager, Jose Santos, for organising the site visit. Thanks also go to Gustavo Belitardo, Carlos Guimaraes and Eduardo Barbosa for their hospitality and assistance in preparing this article.
|Pilot role in privatisation|
Hydro power is one of Ecuador’s four main natural resources, besides oil, fish and timber. However, the country currently has a significant electricity deficit (300MW/year in 1998, with an estimated demand growth of 150MW/year since). This situation was largely addressed by the implementation of a new electricity law in the late 1990s, which saw state owned electricity monopoly INECEL dismantled and its assets transferred to new authority CONELEC. Since then the government has aimed to boost investment and competition through privatisation and by promoting the construction of new private projects. The establishment of a wholesale electricity market in 1999, operated by the non-profit organisation CENACE, has enhanced the foundation of this new competitive environment.
|Project fact file|