Located in the Tinguiririca Valley in the foothills of the Chilean Andes, the 158MW La Confluencia is a run-of-river project involving the design and construction of a powerhouse to install two turbines, approximately 19km of tunnel and two river diversions. The project – which is expected to cost US$350M – is being developed by Tinguiririca Energia, an electricity operating company made up of the Australian utility Pacific Hydro and SN Power, a Norwegian venture of utility Statkraft and the Norfund Power Invest AS fund. Main contractor on the project is Hochtief-Tecsa Joint Venture.

La Confluencia is upstream of the La Higuera project, which is also being developed by the Tinguiririca Energia. Although being developed entirely separately of each other, the two projects are designed to operate in cascade, and both can operate independently in the event of closure of either plant. It is expected that both projects will generate approximately 1400GWh annually.

La Confluencia is located on the Tinguiririca, Portillo and Azufre rivers and consists of intakes and conveyance systems on two branches diverting flows to a surface powerhouse.

The Portillo branch comprises a low weir and spillway on the Portillo River at 1465m asl. Water will pass through a desander and short open channel before entering an 11km low-pressure tunnel that runs to the surge chamber above the powerhouse at the confluence of the Azufre and Tinguiririca rivers.

The Tinguiririca branch consists of a low diversion weir and spillway across the Tinguiririca River at 1450m asl that will divert partial flows through a desander and short open channel to a regulation pondage of 1.2Mm3 live storage capacity.

Water from this will be taken via a 9.3km low-pressure tunnel that joins the surge chamber above the powerhouse.

Both the Tinguiririca and Portillo branch tunnels will terminate at a concrete lined vertical shaft dropping to the open-air powerhouse via a concrete and steel lined high-pressure tunnel. Hochtief-Tecsa JV plant manager Otto Krahan says that tunnelling forms the dominant part of the works.

As the Andes are characterised by uplifted sedimentary, volcanogenic and intrusive units of highly variable nature, it has not been possible to predict with certainty the rock conditions that the contractor will find once tunnelling begins. Nonetheless, fast progress is necessary and as there will be up to 10 active fronts in the two tunnels during the greater part of the construction period, the tunnels will require intensive management.

Tunnel excavation

Six Sandvik DT series tunnelling jumbos will be used to excavate the project tunnels. The jumbos will work together with three Sandvik DC300 drilling rigs that are already on site.

The Sandvik drilling rigs and jumbos have been purchased by the JV from Sandvik Chile in Santiago, and will be serviced on the site by a team of Sandvik personnel who will be based there for the duration of the project. The three Sandvik DC300 hydraulic, self-propelled drilling units have so far been used for stabilising the rock face for the powerhouse excavation, where they have been drilling holes for the benching.

The DC300 machines, specifically designed for operations such as benching, are drilling 3.5m deep holes using Sandvik R32 bench drilling bits, sized at 38mm diameter and 64mm diameter, representing the lower and upper range of drilling that the DC300 can undertake.

Blasting is being carried out daily, and the stabilisation will be complete by the time the Sandvik jumbos begin tunnelling.

The six jumbos are all Sandvik DT 720C units, and will be working on the two main tunnels, the surge chamber, and other tunnelling work on the project. Built on diesel-driven carriers, they are electro-hydraulically powered and equipped with two booms designed for extremely fast rock drilling across a maximum cross-section of 70m2, and will be working at a maximum height of 7m and width of 11m.

The jumbos will mainly be equipped with Sandvik R32 drilling tools of 45mm diameter, fitted with nine-button RT300 bits that will be sharpened in the site workshop by Sandvik personnel; extending the life cycle of each bit

In each of the two main tunnels, the DC300 rigs will follow the jumbos to carry out the reinforcement drilling, using a Sandvik R32 drilling tool of 51mm diameter for the central hole on each section and an R32 of 38mm for the surrounding holes. Once the reinforcement is done, the tunnel will be lined with shotcrete.

Otto Krahan says that despite the uncertainties relating to the type of rock that will be encountered, the projected rate of tunnel advance will be 9m per day, to be achieved in two cycles of drilling, blasting, cleaning and reinforcing per 24-hour period. The construction period for La Confluencia is expected to be three years, depending largely upon the rock conditions encountered by the tunnellers.