Alto Maipo Chile granted notice to proceed 16 Dec 2013
Shani Wallis, TunnelTalk
Finalized funding arrangements for a capital investment of some US$2,050 billion has secured a notice to proceed on construction of the Alto Maipo hydro scheme in Chile by its two appointed civil construction contractors Strabag SpA of Austria with its Chilean subsidiary Strabag AG, and Constructor Nuevo Maipo SpA, the consortium of Hochtief AG of Germany and CMC Di Ravenna of Italy.
Fig 1. Plan of the Alto Maipo project in the high Andes

Fig 1. Plan of the Alto Maipo project in the high Andes

Between them the international contractors with their lead civil works designer Geodata of Italy, will excavate more than 66km of TBM and drill+blast tunnelling to deliver water to two underground run-of-the-river hydroelectric plants that will contribute approximately 531MW to Chile's Central Interconnected System (SIC) electricity supply grid.
The notice to proceed was confirmed by the AES Gener SA Board of Directors on 11 December 2013 with closure of negotiations that secures debt financing of up to US$1.217 billion from a syndicate of nine banks with the remainder funded by equity contributions from AES Gener, the Chilean company which is 71% controlled of the AES Corporation parent company of the USA, and from Antofagasta Minerals SA (AMSA), Chile's fourth largest copper producer, and in accordance with their respective 60:40% ownership participation. The scheme is designed to generate an annual average of approximately 2,320GWh of electricity, and supply of up to 780GWh/year for a term of 20 years to Minera Los Pelambres, a mine managed and controlled by AMSA. The syndicate of banks agreeing to provide project funding via a finance credit facility comprises the Overseas Private Investment Corporation (OPIC), Inter-American Development Bank (IOB), International Finance Corporation (IFC), Corpbanca, Banco de Credito e Inversiones, Banco ltau Chile, Banco del Estado de Chile, KfW Ipex-Bank GmbH and DNB Bank ASA.
The project's principal civil works contractors, Strabag and Hochtief/CMC JV, are joined by the Brazilian and Chilean subsidiaries of Voith Hydro of Germany for supply and installation of the electromechanical equipment in two underground powerhouses. Turbines for a capacity of 275MW and 256MW respectively will be housed in the powerhouse caverns at Alfalfal II and Las Lajas. Design of the underground works for the owner is completed by consultants Skava of Chile, Amberg of Switzerland and Multiconsult of Norway.
In a statement, CEO Luis Felipe Ceron of AES Gener noted Alto Maipo "is a key initiative for the country's development, and its start of operation, in about 2018, will come at a time when generation of electricity in Chile will be experiencing a critical supply shortage against demand." He noted that the 50km proximity of the Alto Maipo scheme to the southeast of the capital city Santiago, will contribute to the reliability of supply for Chile's principal consumption center.
All environmental and regulatory permits have been obtained for the private enterprise investment, which will require only 17km of new transmission lines to connect to the area's existing AES Gener transmission lines.
Scope of construction contracts
The two civil construction contracts cover excavation of a total 66km of tunneling plus access tunnels in the high Maipo River basin in the Chilean Andes and under an overburden of up to 2,000m (Table 1).
Table 1. The project's main tunnel lengths and their cross sections
Main tunnels Length (km) Section (m2)
Las Lajas tailrace tunnel 13 37.4
Las Lajas headrace tunnel 1 6 21
Las Lajas headrace tunnel 2 3.5 30
Alfafal II tailrace tunnel 3.5 20.1
Alfalfal II headrace tunnel 14.5 15.9
El Volcan diversion tunnel 14 14.0
Alfalfal II access tunnel 2.5 35.1
Las Lajas access tunnel 2 35.1
Strabag is awarded a US$490 million contract to excavate 46.5km or two thirds of the project's underground watercourse and powerhouse works through the mountains and in the valleys of the Colorado and Maipo River basins to the north. The Hochtief/CMC JV has the US$279 million contract to excavate 18.5km of southern tunnels in the Volcán and Yeso valleys.
Both drill+blast and TBM excavation is specified by the owner's consultants for the tunnelling works. TBM is specified for approximately 50% of the tunnel lengths as it is considered faster than drill+blast for the long single heading drives. Using an anticipated four TBMs - three by Strabag and one by the Hochtief/CMC JV - provides the possibility of completing all tunnel excavation in just 4.5 years from notice to proceed.
For its contract Hochtief/CMC has purchased a new 4.1m diameter High Performance Robbins gripper machine to excavate about 7km or half of the 14km Volcan diversion tunnel, working from the downstream end, before being recovered and reassembled to complete about 3km of the Alfalfal II headrace tunnel.
For its contract Strabag has ordered a 6.9m diameter, open gripper machine from Herrenknecht to excavate the majority of the 12.9km Las Lajas tailrace tunnel, or about 10km, working from the downstream end. Another open gripper rock TBM of 4.5m diameter is on provisional order from Herrenknecht to excavate approximately two-thirds of the 14.5km Alfalfal II headrace tunnel, working also from the downstream, for a total bored distance of up to 10km.
The third TBM for the Strabag contract was to be a 3.4m diameter, open gripper machine to excavate about 2km of the Alfalfal II pressure shaft and surge shaft and on an inclination of 45 degrees. Strabag however is said to be preparing an alternative for this TBM drive as part of its means and methods statement for consideration by the client.
Alignment traverses high-altitude Andes

Alignment traverses high-altitude Andes

For both Hochtief/CMC and Strabag, Geodata of Italy, under the leadership of Project Manager Davorin Kolic, has the assignment to provide design assistance of underground civil works of the two hydroelectric plants of Alfalfal II and Las Lajas, and for a total length of about 46.5km of both drill+blast and TBM tunnelling. Excavation is to be mainly through volcanic rock deposits that are expected to be highly abrasive on drill bits and cutters.
As a run-of-the-river installation, the Alfalfal II turbines will be fed by water from five intakes. The intakes La Engorda, Colina, Las Placas and El Morado will convey water through a concrete pipeline of about 3.6km long to a the El Volcan desander, and from the desander, through the 14km long El Volcan tunnel to a junction shaft, where, through a dedicated adduction of about 1.5km long, it joins water collected by the El Yeso intake. The shaft is connected to the Alfalfal II headrace tunnel through the 3.1m diameter steel conduit El Yeso siphon of about 5.2km long. These waters and limited to a design flow volume of 27m3/sec and associated with a gross head of approximately 1,160m to operate two generator turbines for a capacity of about 264MW.
The Las Lajas plant will use the flow volumes discharged by the Alfalfal II plant and the existing Alfalfal I plant, as well as the flow currently used by the Maitenes plant, conducting them through a headrace tunnel to the underground powerhouse. The maximum total flow of 65m3/s associated with a gross head of 483m permits the operation of two generating units with total capacity of about 267MW.
With the December 2013 notice to proceed, the two new power generating plants are due to be in operation in 2018.
Four TBMs for high altitude Andean hydro - TunnelTalk, November 2012
Extreme tunnel planned for Andes rail link - TunnelTalk, November 2012
Geotechnical concerns for deep mountain drives - TunnelTalk, May 2012
Andes highway link a priority for Chile-Argentina-Brazil - TunnelTalk, December 2011
TBM excavation conquers Peruvian Andes - TunnelTalk, January 2012

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