Corps of Engineers studies risk of fault under Lake Isabella Dam
October 30, 2009
- The Sacramento District is studying fault activity near the Lake Isabella auxiliary dam.
- Engineers and geologists are digging a series of trenches and boring deep holes to study soil samples.
- Studies revealed that the Kern Canyon fault, which runs beneath the auxiliary dam, is capable of causing large magnitude earthquakes
- Results of the studies will be used to determine what remediation measures the Corps will need to take to keep the dam safe.
LAKE ISABELLA, Calif. - Many dams throughout the world, even recently constructed ones, are located on active faults. One such dam, located at the Corps-managed Lake Isabella, is considered at risk after U.S. Geological Survey (USGS) studies revealed that the Kern Canyon fault, which runs beneath the auxiliary dam, is capable of causing large magnitude earthquakes.
Contractors working for the U.S. Army Corps of Engineers are nearing completion on a study of the fault at Lake Isabella by conducting two geotechnical exploratory actions near the auxiliary dam. Study results will be used as part of the Isabella Dam Safety Assurance Program.
Partnered with the Corps on the trenching and drilling at Isabella are various trenching and drilling contractors and geologists, said Tony Kittner, Isabella Dam project geologist, Sacramento District geotechnical engineering branch, who referred to some of the contractors and geologists as the "best in the world." The USGS is also performing geophysics work on the auxiliary dam and upstream in the delta region below nearby Kernville, Calif.
Workers are nearly half finished digging the roughly 1,500-foot-long seismic trench, which runs 10 to 15 feet deep and 4 feet wide and parallels the auxiliary dam approximately 100-150 feet from the toe (lowest portion) of the dam. Additionally, 21 boreholes 180-300 feet deep were drilled, part of a series of trenches that were dug in the area to study the Kern Canyon fault activity.
"The significance of the drilling is to gather geotechnical information on the dam's foundation materials, ,as well as the material in the embankments the dam is made up of," said Kittner. "We need to determine what the soil properties and characteristics at various locations and depths are and how well they are holding up, what the soil densities are and how compacted they are, plus how much of an earthquake loading they can take.
"We have to assist in evaluating the past activity on the Kern Canyon Fault," Kittner said, "and determine surface fault rupture segments, past displacement direction and amount, and provide estimates of future earthquake magnitude, recurrence interval (frequency of occurrence), and style and amount of offset. C-14 radiocarbon and OSL (optically stimulated) luminescence dating sampling methods are used for determining the age of the material found in and near the fault displacements."
A preliminary screening of possible alternatives and remediation measures for the Isabella Dam Safety Project is scheduled for December using the seismic and seepage evaluations and data collected from the field investigations, said David Serafini, Isabella Dam technical lead. The screening of alternatives will address the seepage, seismic and hydrologic deficiencies for the main dam, auxiliary dam and the spillway, he said. Findings will determine what action the Corps will take, said Kittner and Ronn Rose, geologist, dam safety program manager with the Sacramento District.
"Several measures will likely be evaluated to strengthen the Auxiliary Dam for seismic and seepage concerns," Serafini said. "The remediation of the homogeneous embankment could include the installation of filters or drains, foundation improvement or possible replacement of portions of the embankment. Seismic remedial measures may also be required for the control tower and Borel Canal conduit (channel). It is likely that a combination of all of these will be required."
Rose said results thus far reveal: the fault has been active north and south of Lake Isabella within the past 3,500 years; the fault poses surface rupture and a ground-shaking hazard to Lake Isabella; at least six events have occurred on the fault within the past 35,000 years, and the ruptures have been between 12 and 36 inches with western side up displacement.
Three locations downstream of the dams in the Isabella and Havilah Valley areas were trenched previously from October 2008 through January 2009. Three locations were trenched upstream of the dam above Kernville (Corral Creek, Rincon Springs and Brush Creek sites) also, beginning in May 2009 and completed in early July.
"The current trenching is being performed due to the USGS geophysical results that showed the extent of faulting beneath the auxiliary dam was much greater than what was originally anticipated when the dams were built," Kittner said. "Recent USGS seismic survey results recorded breaks in the bedrock beneath the entire length of the auxiliary dam and the rocks cored in deep drill holes have verified some of those results.
"The faulting and resulting poor condition of the rock found along the entire length of the auxiliary dam is believed to be caused by an older strike-slip fault that has recently reactivated as a normal or reverse fault," Kittner said. "That fault is currently mapped between the right abutment and first 800 feet of the west side of the auxiliary dam.
"Multiple seepage problems and poor foundation materials at the auxiliary dam were driving the DSAC rating for this project until it was learned the auxiliary dam was built on an active fault, a direct result of the fault trenching project," Kittner said. "That drove the risk higher due to earthquake loading and liquefaction, and made it more of a risk for the population living downstream, as well as made the entire project more complex from a dam safety and geotechnical standpoint."
"It is important to note that the Kern Canyon Fault is only one part of the seismic problem," Rose added. "There are multiple seismic sources that could cause strong ground shaking at the dam. However, the Kern Canyon Fault does have the potential to cause a rupture of the dam and this can be dealt with in a remedial design."
Serafini said the Corps will await the test results and then evaluate what measures might be needed to address dam remediation.
"The Corps is in the process of completing the characterization of the seepage and seismic deficiencies for the Isabella Dam Safety Project," he said. "A detailed base line risk analysis of the project is also under way and should be completed in January with the coordination of the U.S. Army Corps of Engineers Flood Risk Management Center. In December, a group of dam safety experts will be developing a short list of possible alternatives and remediation measures for the project."