Multiple challenges

Among the many tunnelling works being completed or getting underway in Latin America and Asia there are a selection of hydro projects that present a range of challenges through combinations of scale, layout and, of course, ground conditions.

In Chile, the La Confluencia project is the upstream of a two-plant cascade scheme and its headrace system involves two long, divergent branch tunnels, each with multiple inlets, and the spread of bores have been excavated by drill and blast. Other projects in the country – such as Angostura and Chacayes – also have interesting tunnelling works.

Elsewhere in Latin America – in Peru and Panama – are further hydro tunnel challenges. In Peru, preparations are underway for Cheves project which calls for a wide variety of tunnel and cavern excavations. Then, in Panama, the Pando-Monte Lirio and El Alto projects, at the upper part of a small hydro cascade, require consecutive headrace tunnels that are not insignificant in themselves and also present a challenge to tunnelling in the uncommon barrier of volcanic lahar rock.

On the other side of the world, where the variable and often difficult ground conditions of the Himalayas are not unknown to hydro schemes, boring is to begin soon with a TBM on the major headrace tunnel for the Kishanganga project in India. The more standard approach to excavation in the region may be drill and blast but the technology available and skill of the international contractor has persuaded that the TBM approach is the best way ahead.

Staying in India, another project in development is Luhri where drill and blast is the order of the day, not least due to tunnelling risk factors. But, unusually, studies have determined that the best approach for its headrace is to excavate twin tunnels.

Chile, Peru

La Confluencia – Chile

Located about 150km south of Chilean capital Santiago, the 155MW La Confluencia project has been developed by a joint venture of Australia’s Pacific Hydro and Norwegian firm SN Power in upper reaches of the Tinguiririca river basin, in the San Fernando area. The special purpose company created to build and operate the project is Tinguiririca Energia.

The higher region of the basin includes the upstream section of the Tinguiririca River and also the tributary of the Portillo/Azufre rivers. The rivers are being tapped by two headrace tunnels arranged in a wide Y-shaped layout, and their lengths are 9.05km and 11.3km, respectively.

Each headrace tunnel has many collector adits that tap smaller rivers and streams. From where they join the flows are conveyed down the relatively short distance to the surface power station, which was inaugurated at the end of October 2010. Commercial operations are to begin in stages, starting in January 2011.

From La Confluencia, the discharged flows pass downstream through a 17.7km long transfer tunnel to the La Higuera power plant, also on the Tinguiririca River, and developed by the same JV partners. La Higuera was brought into service earlier in 2010.

Construction work on La Confluencia was undertaken on a turnkey basis by a JV of Germany’s Hochtief Construction and Chilean group Tecsa.

Separately, SN Power is developing and Hochtief is to work on the Cheves hydro project and tunnels, in Peru (detailed later).

Tunnelling work on La Confluencia commenced in April 2008, after main construction had commenced the previous October, and the original programme was for the underground works to be completed by May 2010 for the scheme to be finished by July 2010. While construction challenges have set back the programme by some months the works are almost complete.

Geology in the area comprises andesitic tuff and breccias with hematitic and arcillic alterations, and unconfined compressive strength (UCS) ranges over 80MPa-120MPa. The main rock classes are III-V whereas class 1-II was also anticipated but was not encountered during excavation. There was groundwater encountered in the excavations, most notably in the El Manzano area.

The tunnels are generally about 6m wide and almost 6m high, with horseshoe-shaped cross sections on inclined parts and semi-circular upper parts on horizontal parts. The Portillo branch tunnel has a section area of almost 33m2 while that of the shorter Tinguiririca tube is slightly larger, at almost 37m2. The 95m high surge shaft has a section of slightly more than 200m2.

Drill and blast excavation was undertaken with Sandvik rigs, including DT720 and DT820 jumbos. Excavators included Caterpillar 313, 416 and JCB, and spoil transport used Gia Industri Haggloader 10HR-B backhoe loaders and also Terex equipment. Support for both primary and secondary lining comprised shotcrete, rock bolts, mesh and also steel frames, as required. The concreting works followed.

Works progressed reasonably well but faced challenges due to ground conditions, and there were also some difficulties with concrete works in the surge shaft.

Consultants on La Confluencia include HT Consult, EDIC, Geocontrol, Norconsult, MWH and Golder Associates. Feasibility studies were performed over 2005-7, and the outline design and site investigation immediately afterwards. Construction began soon after with detailed design continuing into that stage of the scheme’s development.

Angostura, Chacayes – Chile

Other hydro tunnelling work that will be done in Chile in the near future or underway includes excavations for the Angostura and Chacayes projects.

The 316MW Angostura project is being developed far south of Santiago, in the Bio-Bio region, by energy utility Colbun. Under construction on the Bio Bio River, the project has a budget of US$657M and Colbun expects the plant to be operational by early 2013.

In mid-2010, Colbun awarded a construction contract for the civil works to a JV of Italian firm Impregilo with local company Fe Grande. Tunnelling works on the scheme will include excavation of diversion tunnels and a large underground powerhouse.

Separately, Fe Grande is working on the 111MW Chacayes hydro project, in a JV with Astaldi to undertake an Engineer, Procurement and Construction (EPC) contract. The project is being developed on the Cachapoal River by Pacific Hydro but with Astaldi, since the Italian contractor became an equity partner in the early stages of the scheme.

Tunnelling work on the run-of-river scheme will include more than 6km of tunnels split into three sections, the main bores being a 2.6km long headrace and the Cipreses transfer tunnel. The full project is to be completed in 2011.

Cheves – Peru

Cheves is being developed on the Huaura River some 130km from the Peruvian capital, Lima, and the peak-load plant will draw on flows in the Huaura-Checras catchments.

Hochtief is getting underway with construction on the 168MW scheme for SN Power, which plans to see the plant operational in 2013. SN Power signed the agreement in 2009 to build the project.

Underground works to be excavated on the Cheves project, says Hochtief, include a powerhouse cavern (60m long by 15.5m wide by 32m high) and the adjacent transformer cavern (27.5m long by 11.2m wide by 14m high).

In addition, the tunnelling will involve excavation of a 2530m long diversion tunnel, a 9882m long headrace with a 894m long adit; a 702m long surge tunnel, two 107m long steel-lined penstocks, and a 3250m long tailrace.

Panama

Located in western Panama, the Pando-Monte Lirio scheme and the El Alto project, immediately downstream, are the first small hydro plants in the Chiquiri Viejo valley cascade. Significant tunnelling works are planned for the run-of-river schemes

More than 16km of tunnel excavation is required to build the three headraces for the projects, and all are to be bored by separate TBMs supplied from a single company, Italian firm Seli, which will also carry out the tunnelling work.

A rarely met rock type dominates the geology in the valley – lahars. This type of rock can be highly variable in properties, not least due to the wide range of methods it can be formed.

Pando-Monte Lirio

The Pando-Monte Lirio plants are being developed on the mid-upper region of Chiquiri Viejo river with the former the upstream project. Pando is to have an installed capacity of 32MW and Monte Lirio will have 52MW, and the developer is Electron Investment which won a 50-year generation concession.

A little over a third of the US$292M budget for the projects is equity with debt financing coming from multilateral sources, such as the Inter-American Development Bank (IADB) – for which Hatch did the project assessment, and the International Finance Corporation (IFC).

Pando is to have a 5161m long headrace, which is to be excavated by a 3.72m diameter TBM of earth pressure balance (EPB) type. The machine is to be launched in April 2011 – two months after a slightly larger (3.92m diameter) shield of the same type has been launched on the 7878m long Monte Lirio headrace drive.

Lahars originate as powerful volcanic slurries with high bulk density – more than 1,400kg/m3 – and the matrix can include widely ranging sizes of debris, from dust and ash to stones and boulders. Once solidified, lahars form into mixed rocks with highly varied properties.

The rock that will dominate the drives has been classified into two types – matrix- and clast-supported, the latter having fewer fines and poorer grading. Groundwater permeability is relatively low but as the tunnel alignments are below the water table the contractor has elected to use EPBMs to deal with the “soil-rock” geology, especially if water-bearing stratum or connate water are encountered.

EPBMs are specialist boring machines used to handle ground conditions where variable characteristics and possibly water are present. They hold back the excavation with a pressurised action, possibly involving conditioning agents, while carefully remove the excavated material in a continuous plug extraction using a sealed auger screw.

Often the EPB tunnelling technology is used in urban environments in river or coastal areas for transport and utility projects. In lahar, though, where the rock is dry and stable it will be able to operate in “open” mode, only having to go to full sealed, or “closed”, mode should there be quite wet conditions, an unstable face or loose ground.

El Alto

El Alto is to be built downstream of Monte Lirio, and is being built by a different developer – a JV of locals companies Hydro Caisan and Panama Power Holdings. The 60MW (24.7MW firm) project is to be commissioned in 2013.

Tunnelling works on the project include a 3240m long headrace tunnel, surge tanks, a 96m high shaft and a 368m long penstock. The headrace tunnel is to be bored using a 6.79m diameter EPB shield. Seli is doing the underground works under an EPC contract, and is to have the TBM manufactured by the beginning of January 2011.

While active on the substantial concentration of TBM work in the lahars for small hydro in Panama, Seli is also preparing to undertake a major tunnelling challenge also in the water power sector but on the other side of the world for the Kishanganga project, in India. On that project the geological challenge includes squeezing rock.

India

Kishanganga

The 330MW Kishanganga project is being developed by NHPC Ltd (formerly National Hydroelectric Power Corporation) in Jammu & Kashmir (J&K) province, and is due to be operational in 2016. NHPC took over development of the planned project from the state government.

Key features of the project include a dam on the Kishanganga River, which is a tributary of the Jhelum, and significant underground works – a diversion tunnel, headrace, penstock, surge shaft, powerhouse and tailrace. The major tunnelling works will convey water southwards from the Kishanganga catchment to a neighbouring basin that will supply the plant being built near Bonar Nala River, which drains into Wular lake, north west of Srinagar.

Civil works construction is being undertaken by Indian contractor Hindustan Construction Co (HCC) in a 98:2 JV with consultant Halcrow. The underground works are to be completed by early 2014.

The headrace is approximately 24km long, and more than half is to be excavated by TBM – an uncommon choice for the Himalayas where squeezing rock and fault zones have presented a number of problems for the tunnelling method, such as at Dulhasti and Parbati II hydro projects, in India, and costs are sometimes also a concern.

For such a major headrace tunnel at Kishanganga, therefore, the choice of excavation system is a major strategic assessment and exercise in risk management. That a TBM has been picked to excavate more than 14.6km of the tunnel is, therefore, an interesting choice to say the least. But given that the tunnelling subcontractor, Seli, is both manufacturer and has crews with experience of difficult conditions including high overburden pressures and fissure zones it will be an important point in TBM engineering in the Himalayas to learn how the machine functions in the long headrace drive.

Geology along the alignment is mostly Panjal volcani basalt, andesite and intrusive granite/granodiorite which may have some association with hydro-geothermal activity at faults and discontinuities. In addition, the ground conditions are expected to include siltstones and sandstones affected by tectonics, and there could be shales and phylites. Cover on the headrace will typically range over 750m-1000m but will reach up to 1400m.

A 6.18m double-shield TBM with 19” discs is being supplied with advanced systems designed to help counter pressures and movements due to high overburden, and include: high conicity for continuous over-excavation by 100mm, which will help the shield to manoeuvre in squeezing ground; arrays of holes for probing ahead; and, not having to stop excavation and risk being locked while at standstill while undertaking ground conditioning ahead of the drive, and also contact injection during lining operations.

In addition, extra cutterhead motors have been added during design, taking the power to 2520kW (8 x 315kW), to give sufficient torque should there be stoppages and restarts in difficult ground conditions. Further, a dozen auxiliary cylinders are fitted to advance the rear shield in squeezing ground.

The tunnels will have a finished diameter of 5.2m, and the lining will comprise 350mm thick hexagonal concrete segments.

TBM and backup equipment is being moved to site by HCC, and assembly is to commence in January 2011. However, if Kishanganga is seen as a long tunnelling challenge then there is another, even longer, and also in India – for the Luhri project, in Himachal Pradesh

Luhri

Luhri project is being developed by Satluj Jal Vidyut Nigam (SJVN), which signed a Memorandum of Understanding (MoU) with the state government for the 775MW scheme in 2004. As the design developed, the dam site was moved on the Satluj River, adding head and power but also more than doubling the length of the initially proposed headrace tunnel to 38.1km.

Studies were undertaken to assess the large-scale tunnelling options for cost and programme risk during construction, and analyses examined the diameter as well as the number of tunnels. The work was undertaken by consultant Mott MacDonald, and the investigation looked at three key factors to establish the best solution – minimising head loss, operational needs of the scheme, and construction risk.

Focusing on construction risk included the region’s often challenging geology, possible flexibility to modify the alignment for excavation by different methods, if cover could be reduced and equipment costs – cheaper drill or blast or two, potentially four, TBMs.

Geology along the alignment includes gneiss, schist, quartzites, phyllites, shale and limestone, and also the strata are folded and there is a major thrust fault presenting potentially squeezing ground and blocky stretches – and the middle of the tunnel passes twice through this zone.

In the end, the prime options were for a 11.75m diameter single tunnel or twin 9m diameter tubes, it being clear that double tunnels would provide much greater operational flexibility, especially over such lengths.

Other underground works on the Luhri project include the powerhouse.