The 531MW Alto Maipo hydroelectric power project is being developed by AES Gener and Antofagasta Minerals. Image courtesy of ATB Group.
The Alto Maipo project comprises the Alfalfal II (264MW) and Las Lajas (267MW) power plants. Image courtesy of ATB Group.
The Alto Maipo hydroelectric power project is located in the San José de Maipo municipality in Chile. Image courtesy of ATB Group.

The 531MW Alto Maipo hydroelectric power project involves two run-of-the-river power plants, namely Alfalfal II and Las Lajas that are currently under construction on the Maipo River, in the Cordillera province of Chile.

The project is being developed through a special purpose company Alto Maipo comprising the Chilean subsidiary of AES Corporation, AES Gener (60%), and Antofagasta Minerals (40%).

The construction works on the £866.24m ($1.375bn) project were started in the fourth quarter of 2013 with commissioning expected in the second half of 2021.

At full capacity, the Alto Maipo power project is expected to generate approximately 2,350 Giga-watt hours (GWh) of electricity annually.

Location and site details

The Alto Maipo hydroelectric power project is located in the Andes, on the Maipo River, in the San José de Maipo district, Cordillera province, Chile, approximately 50km south of Santiago.

The project site lies near the load centre of Chile’s power grid. The Alfalfal II plant is situated in the Colorado River sub-basin, downstream of the existing Alfalfal I power plant, while the Las Lajas power plant lies on the southern bank of the Colorado River in the El Sauce sector.

Alto Maipo hydroelectric power project make-up

The Alto Maipo hydroelectric power project consists of the Alfalfal II and the Las Lajas power plants of 264MW and 267MW capacities respectively. Each plant will have an underground powerhouse equipped with two Pelton-type turbines.

The Alfalfal II plant will receive the water flow for power generation through a 600m-high and 2.4m-diameter penstock, while the Las Lajas plant will be transferred water through two 160m-high and 3.7m-diameter penstocks.

The project also involves the construction of 67km of tunnels, four inverted siphons to cross streams, two forebays, and associated facilities, as well as 31km of new access roads, four new bridges, and 17km of 110/220kV power transmission lines.

The surface civil works include the construction of water intake structures, delivery canals, and new bridges over Colorado and the Yeso Rivers.

As the project does not involve dam construction, the majority of the construction works are limited to two underground powerhouses, water abduction systems, and associated infrastructure. A 4.2t Liebherr 53K self-erecting tower crane, along with an 85 EC-B 5b flat-top crane was deployed for the installation activities.

Water flow for the plants

The Alfalfal II power plant of the Alto Maipo hydroelectric power project will utilise water from the upper Volcán and Yeso River basins.

The Las Lajas plant will use the water discharged from the Alfalfal II power plant, along with the contributions from the intermediate basin of the Colorado River and the Aucayes stream sub-basin.

Power evacuation

The electricity generated by the Alto Maipo hydroelectric power station will be evacuated through 200kV power transmission lines connecting to Chile’s Central Interconnected Grid (SIC).


The Alto Maipo hydroelectric project is being financed through a credit facility of £731.47m ($1.2bn) provided by a group of banks, including KfW IPEX-Bank, Overseas Private Investment Corporation (OPIC), Inter-American Development Bank (IADB), International Finance Corporation (IFC), Banco del Estado de Chile, DNB bank, Corpbanca Bank, Banco de Crédito e Inversiones (BCI), and Banco Itaú.

Contractors involved

Strabag was contracted for the design and construction works of tunnels, caverns, earthworks, along with the installation of hydraulic steelworks, while Voith was placed an order for the powerhouse construction and the supply of turbines and generators in November 2012.

Tractebel Engie was contracted for the basic and detailed design of civil, electrical, and auxiliary systems utilising Building Information Modelling (BIM) technology.

The Hochtief Nuevo Maipo consortium comprising CMC Group and Hochtief was subcontracted by Strabag for the construction works of tunnels upstream of the shafts, while Socolco was subcontracted by Voith for the installation of electromechanical equipment in the powerhouses.

Inprocon was engaged by Strabag for services related to design coordination, cost controlling, schedule controlling, and the coordination of subcontractors.

ATB Riva Calzoni was subcontracted by Strabag for the manufacturing and supply of S690 QL high-strength steel penstocks.

Gall Zeidler Consultants has been engaged for the engineering review services, while Norconsult was contracted for basic engineering services during the feasibility study, conceptual and basic design, as well as for the tender documents for the underground works and electro-mechanical installation works.

Pöyry was subcontracted for the design and preparation of tender documents, along with the review of constructability, while Geodata was hired for the detailed design and construction follow-up services.

Edilmac provided a raise bore machine RBR 400 VF, along with the drill rods and support equipment for the Las Lajas plant construction, while Liebherr delivered tower cranes for the construction and installation works of the Alto Maipo hydroelectric power project.

Alto Maipo hydroelectric power project background

An environmental impact assessment (EIA) study for the Alto Maipo hydroelectric project was submitted in May 2008, while the Metropolitan Region Environmental Commission of Chile granted an environmental permit for the project in March 2009.

Antofagasta Minerals, a subsidiary of the Minera Los Pelambres, acquired a 40% ownership in the Alto Maipo hydroelectric project from AES Gener in January 2017.