By this month, Seli will have tested and delivered at its factory in Aprilia, Italy, the second of two very similar EPB TBMs to be launched shortly in Panama on sister drives through lahar, a variable volcanic rock rarely met by tunnellers.

Each machine will be used to construct a headrace tunnel, one for the Monte Lirio hydropower plant and the other for the Pando project, just upstream. The run-of-river projects are on the Chiriqui Viejo River, which runs west in Chiriqui province, close to the border with Costa Rica.

The first of the mixed-face EPBMs was tested last month, and is now being shipped to Central America for a January launch on the 7878-m long headrace for the Monte Lirio project. Then, in February, the second machine is to start boring the 5161m -ong Pando drive.

To all intents the machines are almost twins, except for the very slight difference in diameter—3.92m for the Monte Lirio TBM and 3.72m for the machine on the Pando drive. Aside from the slightly differing inner diameters of 3.2m and 3.0m, the pre-cast concrete tunnel lining, too, is almost the same—1.2m long rings with five-plus-one 250mm-thick segments.

What has brought the similarities in tunnelling equipment and lining is the close geographic relationship between the two projects, which have, in effect, a hydro cascade arrangement, the same uncommon lahar geology, and Seli being appointed the engineering, procurement and construction (EPC) contractor for both headraces. The drives are below groundwater level.

The project clustering extends further—Seli was just last month awarded a separate contract to build the headrace for the El Alto project, the next plant downstream in the Chiriqui Viejo cascade. However, a bigger EPB TBM—6.79m in diameter—is required for that project, which is being developed by a different group, a local joint venture of Hydro Caisan and Panama Power Holdings.

Pando-Monte Lirio scheme
Located in the mid-upper stretch of the river, the Pando-Monte Lirio scheme is one of many hydropower projects planned for the Chiriqui Viejo valley. It is being developed by Electron Investment, which won a 50-year electricity generation concession that was awarded by the state regulator, Autoridad Nacional de los Servicios Publicos (ASEP).

Electron Investment is responsible for the design, construction, operation, maintenance and funding of the projects as well as a transmission link to the Central American grid. Pando (approximate rating 32MW) is to be commissioned in mid-2012 and Monte Lirio (approximately 52MW) by the end of that year. The scheme will have a total installed capacity of 83.3MW (firm capacity of 57.5MW), and is expected to generate 450GWh of electricity per year, which will be split between utility and spot market sales.

The hydropower scheme is estimated to cost USD292M, and a 63:37 debt-to-equity ratio is planned. Funding for the project is coming from multilateral lenders such as the International Finance Corporation (IFC), the arm of the World Bank Group dealing with private projects and the Inter-American Development Bank (IADB). Hatch Mott MacDonald undertook a project assessment for IFC.

Equity is being provided to the scheme by its two sponsors—the Spanish investment firm Inveravante Inversiones Universales, which is owned by entrepreneur Manuel Jove Capellan, and Fundacio Fernando Eleta Almaran, an investment vehicle owned by the local Eleta family. They jointly own Electron Investment, which is a special purpose company established in Panama. The Spanish investor holds 51 per cent, which is to rise to 65 per cent.

Seli was awarded a USD105M engineering, procurement and construction (EPC) contract for the two tunnels by Electron Investment in the third quarter of 2009. It will undertake the work through a subsidiary, Seli Panama, which it established in November last year.

Lahars
Chiquiri Viejo river valley is steep-sided and runs in the geomorphological border formed of detritus from separate collapses, millions of years ago, of the Baru and Colorado volcanoes in the early Tertiary Period and the more recent Quaternary Period. In the project area the geological formations comprise detritus flows, lahars and landslides. The headraces will be bored mainly through lahars.

Lahars, which are common to the Pacific Rim, originate as volcanic slurries, often with significant water content, especially if the gravity flow is produced from an area involving crater or sub-glacial lakes. The powerful slurries can also result from pyroclastic flows becoming mixed with either running or bodies of water, or snow and ICE.

Phreatic explosions, or steam blasts, from volcanic activity can also trigger the soil and rock mass to merge with water. Other mechanisms include land shifts or avalanche collapses, from seismic activity. However, in tropical regions there can also be lahars generated by heavy rain falling on recently ejected pyroclastic debris on the slopes of a volcano.

The mudflows can have high bulk density, such as more than 1400kg/m3 or almost two-thirds the density of normal concrete and the matrix can include a wide range of debris, from dust, ash and fragments to stones and boulders, some of which can be several metres wide.

The heterogeneous, unsteady mass can run on a laminar boundary layer at the base, which reduces the resistance to flow and leads to the slurry sliding great distances to wreak widespread damage along the valley channels that they follow, and there can be significant loss of life. The boundary can also keep large debris, such as boulders, within the fine-grained matrix, despite gravity.

Once solidified, the flows form mixed rocks of highly variable properties. The boundary layer from the base of the flow can often be seen in the hardened mass as a fine-grained band.

Tunnelling
Excavating the Pando and Monte Lirio tunnels will be a big challenge due to the complex and variable characteristics of the dominant lahar rock, says Seli. While uncommon in tunnelling, the contractor has had marginal experience with the rock type, next door in Costa Rica, on the La Joya project.

From site visits at Pando-Monte Lirio, Seli says, the lahar formation has been classified into two types—matrix-supported or clast-supported—the difference being the relative percentage of fines and grading, there being more content and better grading for the former. Typical classification of lahars into either cohesive or non-cohesive does not appear to be appropriate, it notes.

Seli adds that laboratory tests showed surprisingly uniform residual and peak frictional characteristics, such as the normal stress range (up to 8MPa) having coefficients of residual internal friction about 0.62MPa. Peak friction values for pre-loaded materials could be up to 0.76MPa-0.80MPa.

In terms of groundwater flow, permeability is generally low in the matrix-supported lahars. However, where there are diminished fines then there could be a degree of water flow, and also with water-bearing strata or connate water.

Seli says that, in its opinion, only an EPB machine would give reliability in this kind of ‘soil-rock’ formation, especially as the alignments are below the water table.

The machines are equipped with 17-in. discs and rippers. The maximum cutterhead thrust is 13 560kN and the cutterhead speed is up to 4 rev/min. The drive power for the cutterhead is 600kW (4 x 150kW).

The TBMs are designed to operate in both closed and open modes without modifying the machine arrangement. The EPBMs are expected to operate in open mode in the presence of rock mass, including highly dense lahar and ancient lava flows, says Seli, adding that closed mode would be used in case of an unstable face or loose ground.

In principle though, there will be no probing of the ground ahead of the machines and nor, therefore, is ground treatment to be called upon. Spoil is to be removed by loco-hauled transport.

In line with local regulations, Seli plans to work a three-shift daily pattern for each drive. At each project site the contractor should have about 80 workers, it says.

The Italian firm is supplying moulds to Euroconcretos, in Costa Rica, to cast the concrete segments. The majority of the tunnels will have the single, segmental rings, except for near the outlets that will also have an inner lining of fibreglass pipe due to high water pressure but relatively small overburden, meaning the concrete lining is not sufficient.

El Alto
To be built immediately downstream of Monte Lirio, the El Alto project calls for construction of a 3.24km-long headrace tunnel plus surge tanks, a 96m shaft and 368m-long penstock.

Seli was awarded a USD46M EPC contract to execute the underground works for the project, which is to be commissioned in 2013.

Hydro Caisan, part of the jv developer, has a 50-year concession on the 60-MW (24.7MW firm) project. It is majority-owned by Netherlands-based Tahal Group International, held by the Kardan Group.


Location of the neighbouring Pando and Monte Lirio hydro projects in western Panama and the layout of their headrace tunnels Seli is supplying two EPB TBMs under an EPC contract to drive the Pando and Monte Lirio headrace tunnels. Excavation starts in January The EPB TBMs will drive mostly through lahars, which are uncommon in tunnelling and are variable, solidified former slurries generated by volcanic activity