By Gerardo Marti
South-central Chile is blessed with mountains, rivers, an abundance of rain, and usually a predictable annual snowmelt runoff. All of these natural environment and climate conditions allow for the inexpensive sustainable hydroelectric generation of electricity. With Chile’s burgeoning economy and its mining sector growth, the demand for electricity is increasing to a point that will soon exceed the current generation capacity. Therefore, there is a need to rapidly initiate construction of hydroelectric projects.
In 2012, Constructora Angostura Ltda., awarded a contract to Italian contractor Salini Impregilo S.p.A. to build the Central Angostura 350-megawatt hydroelectric dam. The dam, constructed across the Bio-Bio River, is located about 20km south of Santa Barbara in Chile’s Araucania Region. There are two other operating hydroelectric facilities upstream from this point of the river.
The original plans
To construct the dam across the Bio-Bio River, two bypass tunnels were bored into the mountain on the east side of the river (see diagram). The main bypass tunnel was 400m long by 15m high and 20m wide, starting 120m above the location of the dam and exiting into the river channel 180m below the dam.
A second bypass tunnel was for backup, used when the main tunnel would be closed. The main tunnel was designed to carry the total flow of the river during maximum season volume. When the project was completed, this tunnel would become the discharge tunnel for the water driving the No. 3 turbine.
A 7m-thick concrete plug located 50m inside the tunnel from the exit channel separated the tunnel from the turbine cavern. This plug was removed to bring the turbines online. As completion of the project entered the final phase, the upstream entrance gate to the main bypass tunnel was closed and the flow of the Bio-Bio River was diverted through the second bypass tunnel.
To meet the project’s environmental regulations, the water flow in the river downstream of the dam was required to be maintained at a minimum of 550m³/sec. However, at this volume of discharge, the water level in the river channel backed up into the main tunnel to a depth of 4m, preventing workers from entering the tunnel to remove the concrete plug.
These high water level conditions required the construction of a coffer dam across the main bypass tunnel exit channel to seal off the tunnel interior from the river water. This would allow the contractor to pump the water from the tunnel and to remove the concrete plug in safe, dry conditions.
The contractor’s challenge was to select a coffer dam technology installed in water depths that fluctuated from 4–5m deep and would experience extremely turbulent conditions. Also, the access to the exit channel was inaccessible for construction equipment because it was at the minimum 30m below any access road or working platform. The coffer dam had to be installed quickly and it had to be removed without the use of heavy construction machinery.
Geotextile tube coffer dams
Considering these challenges, a number of coffer dam design proposals were reviewed. The geotextile tube plan consisted of six tubes installed in a 3–2–1 pyramid structure within the exit channel. The proposal was for the unfilled geotextile tube units to be filled in-place underwater with a sand/water slurry, allowing the tubes to fit the contour of the tunnel exit channel’s uneven bottom and rock sides as they were being filled (see diagrams, above).
The high-strength woven engineered textile that formed the tube containers allowed the water to permeate through the surface, retaining the sand inside the structure. The results were geotextile tube containers with an in-place mass of more than 300 tons.
Installation mobilized in August 2013 and started the first layer of construction of the coffer dam. Unfilled geotextile tube units, 10m in circumference by 25m long, were secured within a steel positioning frame at the working platform site located 30m above the tunnel exit.
Quick connection couplings were installed in each tubes’ fill port to allow divers to connect the sand/slurry line once the frame with the tube was lowered into position below the water. A 50-ton crane lifted and lowered the frame with the tubes into place in the 4m-deep water.
The positioning frame had riser poles that extended above the water level to identify its position below the water. Divers in the water assisted in positioning the frame and connecting the sand slurry line.
A 5m³ steel sand/slurry box was positioned on an adjacent working platform at a point 35m above the water level. Sand was dumped into the box with a front-end loader as 4m³/min of water was being pumped into the steel box to slurry the sand to flow down the 100m of slurry line connected to the geotextile tube fill ports.
Each tube level of the coffer dam had a different circumference and length. Lower level tubes were 10m in circumference and 25m long; upper level tubes were 13.7m circumference × 30m long. The sand fill volume varied from 165m³ for the lower level tubes to 300m³ for the upper level tubes.
But … what really happened
After the start of the first level of sand-filled geotextile tube units for the coffer dam during the last week in August 2013, site conditions changed dramatically because of unseasonably high rainfall and heavy snowmelt runoff. Discharge from the dam and secondary bypass tunnel now reached 1,400m³/sec and the water level was now 6m deep inside the tunnel.
The turbulence at the location of the coffer dam was extreme whitewater conditions (see above photos). This required the design height and mass of the geotextile tube coffer dam to be increased for the safety of the workers who would be working inside the tunnel to remove the concrete plug. Therefore, the coffer dam was changed to a 4–3–2–1 structure with a total height of 8m.
Work progressed for the next few weeks as the tube installers had to modify the installation techniques with the ever-changing conditions while remaining on schedule. To ensure the divers’ safety in the water, the installation operation had to be halted several times because of the high turbulence in the tunnel exit channel due to the discharge into the river channel below the Angostura Dam.
After four weeks of work, the geotextile tube coffer dam installation was completed at a height of 8m. For additional safety against overtopping, two layers of steel rebar cages lined with a nonwoven geotextile were placed on top of the structure and filled with sand, achieving a total structure height of 9.5m.
Immediately after completing the coffer dam structure, three 8-in. pumps were installed inside the tunnel and they pumped water out and into the river until the tunnel was dry and safe for the removal of the concrete plug to begin (see photos above).
After two weeks of continuous work inside the tunnel, the concrete plug was removed with jackhammers and the removal process of the coffer dam began. Workers were lowered in a steel safety cage to cut open the geotextile tubes, and the flow from the river washed the sand downstream (see photo below).