News Release
| Release Number: | 03-177 |
| Dated: | 10/15/2003 |
| Contact: | Heidi Y. Helwig, 503-808-4510 |
Portland, Ore.-How deep does water infiltrate and saturate the soil within Fern Ridge Dam on the Long Tom River when it rains? That is one of the questions the U.S. Army Corps of Engineers hopes to gain answers for during a 28-day sprinkler test that will begin Friday at this earth-fill dam.
The sprinkler test is one of the Corps' three major efforts to assess the structural stability of the dam since circular depressions were found on its downstream slope in July 2002. The first two efforts included installing piezometers on the downstream face of the dam on two separate occasions-in May 2003 and September 2003. (Piezometers are instruments used to measure the elevation of ground water inside the dam.)
For the sprinkler test, the Corps has installed three separate sprinkler systems on the dam. One system is the study's control system: it is located on a portion of the dam that has never exhibited any unusual seepage and has no visible depressions. The second sprinkler system covers one of the depressions found in February of this year. The third system is on an area of the dam that seeps unusually large amounts of water during heavy rain.
"The goal of this controlled study is to determine the depth of saturation of the dam. Is it deep saturation or just surface saturation? And, what else happens when the dam is this wet-is material being carried out with the water?" said Kristie Hartfeil, the Corps' technical lead for geotechnical studies at Fern Ridge Dam.
The Corps will learn the answers to Hartfeil's questions by reading data generated by 10 piezometers installed at each of the test locations and at various depths of the dam, and by watching external drains for turbid water.
Though Hartfeil admits the test is being done during the start of the traditional rainy season, she said having controlled conditions would produce the best results, adding that relying upon Mother Nature is tricky, at best.
If, after the 28 days of simulated rain, only the top 5 feet of the dam's earthen fill is wet, Corps geotechnical specialists can diminish the problem with a French drain (a trench filled with gravel) or other similar action.
If the lower third or lower half of the downstream part of the dam is fully saturated, however, that could spell trouble, indicating an unstable slope, Hartfeil said. In that case, a more complicated measure would be used, consisting of excavating dirt from the dam to make room for a watertight liner or placing a rock buttress on the downstream dam face. While none of these measures is a permanent fix, they would most likely permit the reservoir to resume normal operation. The Corps is currently operating the reservoir with a 2-1/2-foot restriction.
Determining the level of soil saturation within the dam is only the beginning. While the use of a tiny video camera earlier this year confirmed the Corps' suspicions that the dam's 60-year-old internal drainpipes are significantly deteriorated, the camera was only able to view the 11 short lateral drain pipes, but not the mile-long main drain line. Later this year, though, Hartfeil said the Corps would benefit from a robotic camera device able to travel up to 6,000 feet.
"We can inspect the full the length of the dam's main drainage system that follows the centerline of the dam, providing us a better look at what is going on inside the dam," Hartfeil said. The Corps will use the robotic equipment in December of this year, she said.
The first indication there was a problem with the internal workings of the dam came in July 2002 when Corps personnel noticed a circular depression about 10 feet in diameter on the downstream slope of the dam. In February 2003, two more circular depressions were found directly above drain lines.
In addition to the sprinkler tests and the upcoming use of the robotic camera, the Corps continues to monitor the dam on a daily basis and will perform additional analysis to determine the extent of damage and necessary repair to the dam.
Existing information on the condition of the dam can be found on the Corps' web site at https://www.nwp.usace.army.mil/ under "Top Issues."