Oxec II Hydroelectric Project

56 MW greenfield hydropower project in Guatemala

First cellular cofferdam in Guatemala

40% savings in spillway concrete volume

4-month reduction in overall construction schedule

Challenges

A high-magnitude flood design complicated the spillway energy-dissipation design.
A high-magnitude, temporary diversion flood complicated the diversion works.
Unstable ground and a poor-quality foundation has been associated with landslides during excavation.
A large volume of work had to be accomplished within a limited, compressed schedule.

Solutions

An optimized design changed the spillway stilling-basin design to a submerged roller bucket one, resulting in up to 40% savings in the concrete volume of the spillway.
A revised spillway reduced the length of the cofferdam and temporary works.
The original conventional cofferdam was replaced with a cellular alternative, resulting in a much smaller footprint and hence a smaller excavation.

Highlights

The cofferdam eliminated a major phase in diversion, reducing the overall construction schedule by 4 months and achieving a remarkable reduction in bank excavation.
The first of its kind built in Guatemala, the cofferdam has 29 cells, is nearly 300 metres long, and consists of 2000 tons of steel and over 50,000 m3 of fill.
The cofferdam resists fast flows, controls the release of sediment, and exhibited high reliability and safety during its installation and life in service, needing no remedial works after flood events.
The fill material was processed with a view to removal methodology. The maximum size of the aggregate was limited so the cell fill can be mixed with water and pumped off the project site.

Project Numbers

56 MW greenfield hydropower project in Guatemala
200 m long concrete gravity dam, and 41 m high powerhouse
150,000 m³ of concrete construction in less than 30 months
4 spillway radial gates (12 m W x 16.5 m H) to pass an extreme flood of 6144 m³/s
29-cell cofferdam designed for a diversion flood of 2810 m³/s
50,000 m³ of fill for Stage 1 cofferdam
1800 sheet piles constructed in a 4-month period