Geotextile tube coffer dam
By Sören Schmidt
The problem, the plan…
The Angostura project is owned by Colbún, one of the largest energy producers in Chile. Angostura is a hydroelectric dam project with a capacity of 350mw. The dam is situated in the Bio-Bio River, a few kilometers downstream from two other Bio-Bio hydroelectric facilities.
In this region, the Bio-Bio has a flow ranging from 400 to 1,200+ m³/sec. The river was diverted through a manmade bypass channel. A second smaller bypass channel was also built closer to the dam.
The main tunnel, 15m wide and 20m high, returns the water to the path of the river right after the dam. The project is in a dam structure with three turbines inside a cavern. Colbún awarded the project to Impregilo S.p.A., an international construction company based in Italy.
When the original project was completed, a design was prepared for the third turbine outflow to use the last 50m of the bypass tunnel. That meant that the connection of the two tunnels had to be blocked with a 7-m, high-strength concrete wall that would be removed before hydroelectric operations started. At the time, there were not considerations regarding work inside the tunnel and that its level of water would be about 4–6m. When it was realized that elimination of the water inside the tunnel was required, a geotextile tube installer was contacted.
The challenge of this project was to build, in a maximum of 15 days, a geotextile tube pyramid at least 7.5m high and a maximum 12m wide to leave an operations space in front of the mouth of the tunnel. This space was needed to provide an opening between the mouth of the tunnel and the geotextile tube containers to deposit equipment and the crew needed to go into the tunnel to seal it. It was estimated that 5m would be sufficient.
Fill materials would be available at a rate of 20m³/hr, water at a rate of 200m³/hr. At the time of the geotextile tube pyramid installation, the water depth would range from 0–6.5m without current or turbulence.
…and here is what really happened
The water was extremely turbulent and there was a circular current created by the river’s proximity. Because of the opening of the secondary bypass, the violent flow destroyed part of the channel’s wall where the operators and diving equipment were to be installed. The access road used to go to the jobsite was also destroyed. In addition, the violent flow deposited large rocks in the middle of the river, which then increased the level of the water, forcing the need to make the geotextile tube coffer dam 1m higher (see photos, left).
The promised four hours of zero water never happened, so a submarine inspection was done, revealing only 15m of flat surface suitable to install the geotextile tube coffer dam. This required that the geotextile tube pyramid be built closer to the tunnel where the channel was narrower. This also meant that the geotextile tube containers would have to be longer than originally planned.
The necessity to work only at low water levels for safety reasons reduced the working hours to four on some days and none on other days. The diving support equipment had to be installed and removed every day, an operation that took two hours to install and 30 minutes to remove.
The original plans to install the geotextile tube containers had to be changed and a steel frame had to be built to hold the containers in position as sand was poured in (see photo, far left).
The frame had to be built smaller than the width of the channel and smaller than the containers, so the geotextile tubes were rolled around the frame. When the tips of the containers were released, the current twisted them and they had to be tied with ropes (see photo, left bottom).
After two weeks, when the jobs inside the tunnel were finished, the coffer dam was removed. This was accomplished by cutting the geotextile tubes from the top and allowing the water flow to wash away the sand (p. 26, bottom photo). A crane was used to remove the geotextile pieces once the sand was released from the tubes .