Coffer dam for hydroelectric power station
TenCate Geosynthetics USA has received an Outstanding Achievement Award for its contribution to Central Hidroelectrica Angostura in Chile. TenCate supplied TenCate Geotube® units (length 30 metres, diameter 10 metres) for the construction of a coffer dam.
With a capacity of 350 megawatts, Central Hidroelectrica Angostura in the Bio-Bio River south of Santa Barbara is the largest hydroelectric power station in Chile.
To complete the building of a dam for the power station, it was necessary to construct a (temporary) coffer dam at the downstream exit of a primary river bypass tunnel 11 metres high by 25 metres wide. This dam was to prevent river water flowing into the tunnel when work was in progress. The builders could then remove a 7-metre-thick concrete plug.
Because of the force of the river and water depths ranging from 4 to 7 metres, as well as the restricted use of heavy equipment and the minimum flow requirements, a robust and cost-efficient solution was demanded. The choice fell on TenCate Geotube® technology.
A coffer dam is a temporary closure across a body of water that allows the enclosed area to be pumped out to create a dry working environment. The coffer dam in the Bio-Bio River had to be able to withstand the pressure and turbulence of the river. After it had been pumped out, the work tunnel had to provide the builders with dry safe working conditions.
In addition, the installation and ultimate removal of the coffer dam had to take place without interrupting the river flow or the work on the hydroelectric dam.
Furthermore, it was important to ensure that the river across which the dam is being built should retain a constant minimum flow downstream during and after the construction. This refers to the average quantity of water (in m3) that a river discharges per unit of time, and in this case a minimum flow between 300 and 1000 m3 per second was applicable. To achieve this, primary and secondary bypass tunnels had to be bored via the rock formation to the mountain side of the river.
To complicate matters even further, the exit channel in question was 35 metres below any access road or working platform and was inaccessible for construction machinery. In short, it had to be possible to install and remove the coffer dam without using heavy equipment.
After weighing up all the requirements, the contractor selected the coffer dam based on TenCate Geotube® technology, as proposed by the company Novatech of Santiago, Chile. A number of arguments decided the issue: the design flexibility facilitating adaptation to changing water levels, the high safety factor, the relatively easy installation, the limited time involved in installation and dismantling, and the lower costs in comparison with alternatives.
Although the original design was based on six filled tubes and a height of 8 metres, ultimately 10 tubes were stacked in pyramid form, reaching a height of 9 metres. As soon as the coffer dam was installed, the builders could remove the thick concrete plug with pneumatic drills. A start could then be made on removing the tubes.