Latin challenges

Over the last 18 months, three projects in Latin America have shown the range of underground challenges that tunnellers can face in the hydropower sector. The construction hurdles all come from geological challenges and include works in new and existing tunnels. Two of the tunnelling projects are in Panama, and one is in Peru.

In Panama, the geological challenge comes from volcanic lahars, which are rarely encountered by tunnellers. However, three TBMs have been busy in the tricky lahars of the Chiriqui Viejo valley, in the west of the country. Each EPBM has faced its own excavation challenges, but two of the headrace bores have been completed and the third in the cascade system is still underway.

Also in Panama, and not so far away, a tunnel collapse at the existing 120MW Esti plant had to be investigated and fixed. The plant is back in operation following fast-track repairs at the local collapse zones but also strengthening of the lining throughout the headrace.

Farther south, in the Andes mountains of Peru, tunnelling works are well advanced on the Cheves hydropower scheme. Tunnellers have overcome the geological difficulties that prolonged the construction programme, enabling the drill and blast excavation to enter into its final stages.

Panama: Boring in lahars
From upstream to downstream, the sequence of the hydro projects in the upper cascade of the narrow Chiriqui Viejo valley is Pando then Monte Lirio and El Alto. Separately, another project - Bajo Frio - is being developed farther downstream, and there are to be more in the valley.

Construction of the three upper projects has called for approximately 16.4km of bored tunnel excavation in lahar rock. Lahar is typically formed when mudflows, triggered by volcanic events, solidify, and this is the case in west Panama. Therefore, it is not only an uncommon type of strata in tunnelling, it is also an unpredictably variable material.

It was decided to use TBM technology to bore the headrace tunnels in the lahar rock, and caution over the variability, and potential weak sections and groundwater presence, called for the earth pressure balance (EPB) type shields to be adopted.

Even though the projects are neighbouring, they have been developed individually. Despite the headraces not requiring particularly long bores, the tunnels were designed with different tunnel diameters. Therefore, separate, dedicated EPBMs were employed for each continuous headrace drive.

All three EPBMs were designed, fabricated, supplied and operated by Italian manufacturer and contractor Seli. Procurement was done in a short period, and all three shields were to be launched within a tight window in the first half of 2011 and would bore in lahar at the same time. Therefore, the design approach for the EPB machines was the same, to use mixed faced cutterheads and dual operating modes.

Farthest upstream, excavation of the almost 5.17km long (3.0m i.d.) Pando headrace tunnel is still underway. The 3.35km long (5.8m i.d.) El Alto power tunnel was completed in February, and before mid-year the almost 8km long (3.2m i.d.) Monte Lirio was approaching its finish.

Seli has described the conditions encountered as good to poor across the drives: wet then loose ground and inflows harried the advance at Monte Lirio, so much so that at one point the shield was stuck for some months; water, again, was the problem on the El Alto bore where sections of the headrace had to be constructed by boring through uncemented rock; and, on the Pando bore, after enjoying relatively better conditions in the early days, on this drive there would also come a range of tunnelling challenges brought by groundwater and loose lahar.

Launched just after mid-2011, El Alto tunnel took about 18 months to complete. Both the Monte Lirio and Pando bores began in early 2011. By late May this year the Monte Lirio tunnel was about to be finished, though the Pando drive still had about 40%, or about 2km, to go.

Panama: Fixing a headrace
Not so far away from the lahar tunnelling projects is the existing Esti hydropower plant, owned by US energy group AES. The run-of-river plant was built during 2000-3, but in late 2010 it suffered a series of collapses in its 4.8km long headrace tunnel which put it out of action.

However, the station is a key asset in the national generation portfolio, and a rapid recovery of facility was vital.

Initial investigations found the collapse problems in the 9m wide tunnel, built during 2001-2, to be greater that expected. More detailed investigations were undertaken by Norconsult, which also advised on an approach to recover the power tunnel.

The repair work on the headrace was undertaken on a design and build basis by a joint venture of Seli, Lombardi and Obras Subterraneas. The JV signed a contract for the repair works with the client in mid-2011. At this point Seli was already working with TBMs to bore in the lahars of Chiriqui Viejo valley for new tunnels, but the tunnelling works required to be undertaken at Esti were to be quite a different challenge.

Esti's headrace has a D-shaped cross-section and an average area of 67m2. Design of the power tunnel's horizontal and vertical alignments was governed by a series of geological charateristics, explains Seli, including the horizontally layered strata and insitu rock stresses. Further aspects of the alignment design included what excavation equipment was then available, and the required support methods, it further notes.

Seli adds that other factors defining alignment included locations of access adits for the construction work, the location of the surge shaft, and considerations over the side cover against hydraulic fracturing risk.

Working to a tight schedule for the recovery works, the JV used the original access adits to remove the collapse material from the tunnel and repair the headrace. Sprayed concrete lining was used in the collapse areas, with specific support solutions employed in geologically sensitive areas, says Seli. Then, the entire headrace was lined through with cast insitu, fibre-reinforced concrete.

With the recovery works completed in just under a year, Esti's two 60MW Francis units were brought back onto operation just over a year ago, in June 2012. The station is designed to operate under a head of 122m and generate 650GWh of electricity per year.

Esti is held and operated through the group's partly-owned local subsidiary AES Panama. It was built under an EPC contract by Consocio Esti - a group comprising Skanska International Civil Engineering, GE Energy (Sweden), Alston Power Generation and SwedPower International.

In addition to the headrace tunnel, other key infrastructure at Esti include the 25m high Chiriqui dam, a 6.5km long canal, the 60m high Barrigon dam and the Canjilones powerhouse.

Peru: Tough ground at Cheves
In Peru, despite steady early progress on underground excavations for the 168MW Cheves hydropower project, geological challenges were to arise which meant tunnelling would take longer to complete. The project is being developed by Norway's SN Power, and the contractor is the Hochtief-led JV, Constructora Cheves.

The infrastructure assets at Cheves are heavily dominated by underground works. In addition to cavern construction, the project calls for approximately 19.1km of tunnels, almost 70% of which are accounted for by the 9.8km long headrace and 3.4km long tailrace, respectively.

Geological conditions are varied - sedimentary, igneous and volcanic rocks, and zones altered through contact metamorphosis. Overburden varies up to more than 1200m, while the caverns are approximately 750m below ground. In the pre-construction phase, concerns around geology also included risks of encountering methane gas or hot water.

Other, non-geological concerns, which were further design challenges, included flooding risk and sediment transport.
Construction began in late 2010 and a total volume of approximately 500,000m³ of rock was to be excavated. The plant was scheduled to start generating before the end of this year.

Tunnelling pushed ahead on numerous headings. By early in the second quarter of 2012, progress was being made from both ends of the 2.46km long transfer tunnel; key areas of the headrace were being blasted, including the surge tunnel; and, a third of the tailrace, at the lower section, had been excavated. The 6,598m³ crown of the powerhouse cavern had been opened up.However, many of the underground fronts then encountered adverse geological conditions, affecting progress.

The adverse geological conditions encountered relate to fault zones, rock stress, water inflow, and swelling clay. There ave also been additional impact due to stoppages by the communities, and also flood events.

Tunnelling progress on the different sections of the underground excavations were as follows, in sections listed from upstream to downstream, by June (Week 130 of the contract):

* Transfer Tunnel - approximately 15%, or a little over 400m, remained to be blasted. Two faces are closing the gap.

* Headrace Tunnel (upper end) - 1755m, or just under a third, of the almost 5.68km long upper section of the power tunnel was left to finish. Two faces are closing the gap.

* Surge Tunnel - blasting of the 696m long tunnel was already completed.

* Headrace Tunnel (lower end) - 1331m, or just under a third, of the approximately 4.13km long lower section of the power tunnel had yet to be excavated. Two faces are closing the gap.

* Powerhouse Cavern - already had been fully excavated to a volume of almost 27,000 m3, and concreting works were getting underway, with 6% done. The cavern was excavated with the crown and six benches, which in excavation sequence were Benches I & II (6669 m3), Benches III & IV (7564 m3), Bench V (4345 m3) and Bench VI (1683 m3..

* Transformer Cavern - already excavated to a volume of 4283 m3.

* Access Tunnel - the 954m long tunnel was completed early in construction.

* Tailrace Tunnel - the 3.4km long tunnel was completed in two drives of 895m and 2.46km lengths, respectively.

Overall, a total of almost 15.6km, of approximately 82%, of the tunnel excavation (excaluding caverns) had been completed by June. A little over 3.5km of tunnel remain to be blasted.

Key equipment for the tunnelling works includes: seven Sandvik DT 720 twin boom jumbos; a Sandvik DC 301 bench drill; four Gia Haggloader HR 10 loaders; three ITC 312 excavator/loaders; and, five CAT 938H/950H wheel loaders; four Terex TW 110 mobile excavators; two CAT 330D crawler excavators; 10 Manitou MT 1030 telescopic handlers; 20 DUX/Paus DT 20 dumpers; six Semmco Alpha 20 shotcrete robots; 16 MB Actros 3336 reartippers; and, 17 Dieci L 4700 truck mixers.

Other key concreting works on the project, on the surface, are for the projects three dams, all of which are well advanced. More than 80% of the concreting work on both the Checras and Huaura dams, at either end of the transfer tunnel, has been done; and, two-thirds of concreting is finished on Picunche dam, downstream of the tailrace outlet.