Austrian Dam is a 180-foot high embankment dam which was severely damaged during the October 17, 1989 Loma Prieta Earthquake. It is located within California’s Santa Cruz mountains and situated at the convergence of the Sargent Fault located 700 feet northeast of the dam, and the San Andreas Fault 1700 feet southeast of the dam. Built in 1951, the dam has 15 feet of freeboard and impounds a 6200 acre-foot reservoir. The dam’s foundation is a thin-bedded clay shale while the abutments are comprised of a sandstone and clay shale. Both the up and downstream slopes are 2.5:1 near the crest and transition to 3.5:1 near the toes.
The dam is rolled earthfill(GC-GM) and is nearly homogeneous. An attempt was made during construction to place the more impervious materials in the upstream zone. The average percent passing the #200 sieve for up and downstream zones are 25 and 18 %, respectively. Based on isotropically consolidated undrained triaxial tests, average effective strength values of 44 deg friction and zero cohesion and total strength values of 21 deg. Friction and 290 lbs/ft2 cohesion were determined.
Extensional cracks up to 12 centimeters (4.7 inches) wide in the concrete spillway to Austrian Dam, north abutment (from U.S. Geological Survey Open-File Report 90-547)Earthquake Damage
The epicenter of the main shock was sited 7 miles south of Austrian dam. Recorded pga is 0.64g at the Corralitos station, 5 miles east of the epicenter. The earthquake induced cracking is shown in the next page. The cracking and movement are categorized as follows:
(1) down slope movement of the right abutment causing severe damage to the spillway in the form of tension cracks along its entire length
(2) significant cracking and displacement at the embankment and spillway wall contact at the right abutment
(3) down slope movement of the left abutment near the sloping intake tower in the form of shallow surface sliding
(4) 60-foot (18.3m) wide zones of parallel cracks on both the up and downstream slopes parallel to and below the crest
(5) upthrusting at the downstream toe
(6) through going transverse cracking of the crest near the left abutment to a depth of 32 ft (9.7m)
There was significant evidence of compression and upthrusting at the downstream toe. A bridge located near the toe was moved off its foundation and displaced downstream 18 inches (0.46m).
The epicenter of the main shock was sited 7 miles south of Austrian dam. Recorded pga is 0.64g at the Corralitos station, 5 miles east of the epicenter. The earthquake induced cracking is shown in the next page. The cracking and movement are categorized as follows:
(1) down slope movement of the right abutment causing severe damage to the spillway in the form of tension cracks along its entire length
(2) significant cracking and displacement at the embankment and spillway wall contact at the right abutment
(3) down slope movement of the left abutment near the sloping intake tower in the form of shallow surface sliding
(4) 60-foot (18.3m) wide zones of parallel cracks on both the up and downstream slopes parallel to and below the crest
(5) upthrusting at the downstream toe
(6) through going transverse cracking of the crest near the left abutment to a depth of 32 ft (9.7m)
There was significant evidence of compression and upthrusting at the downstream toe. A bridge located near the toe was moved off its foundation and displaced downstream 18 inches (0.46m).
Ariel view of Austrian Dam (Courtesy of David Gutierrez)
Lessons
Importance of soil-structure interaction and transverse cracking
The significant dam safety lesson that this case history taught is that soil-structure interaction and transverse cracking are highly probable modes of failure. It was fortuitous that the reservoir level was as low as it was at the time of the earthquake. The displacements experienced by Austrian Dam demonstrated the potential for an uncontrolled release by either, cracking at the spillway wall/embankment contact, and/or the transverse cracking. The latter was found to extent more than twice the available freeboard.
Mode of failure in homogeneous dam
Deep seated shearing where the embankment behaves as distinct blocks are governed by inertia forces and the undrained strength characteristics of the materials. Care must be taken to choose earthdam material.
Drainage function
Gravel strip drains were placed beneath the downstream pervious zone. But the gravel drains are not completely effective in relieving downstream seepage pressures.
Compaction of fill material is very important.
What could be done better
- Placing a zoned fill with chimney and blanket drains in the crest fill at both embankment ends
- Creating an impervious blanket at the upstream face
- Grouting the cracking or fissures in the spillway and grouting voids beneath the spillway slabs
- Grouting of the rock at the abutment contact with the fill
Resources:
Chris Kramer, Michael D. Lee, Preparedness for Dam Failures in the San Francisco Bay Area, Natural Hazards Review, Vol. 5, No. 1, 2004, ASCE
City of San Jose, Emergency Operations Plan, 2006
I.M. Idriss, Ralph J. Archuleta, Evaluation of Earthquake Ground Motions draft 06.5, 2007, www.ferc.gov/industries/hydropower/safety/guidelines/eng-guide/chap13-draft.pdf -
John Vrymoed, Wallace Lam, Earthquake Performance of Austrian Dam, California during the Loma Prieta Earthquake, Division of Safety of Dams, Department of Water Resources, http://damsafety.water.ca.gov/seismic_austrian.cfm
L.F. Harder, Jr., J.D. Bray, R.L. Volpe, R.L. Volpe, K.V. Rodda, Performance of Earth Dams during the Loma Prieta Earthquake, The Loma Prieta, California, Earthquake of October 17, 1989-Earth Structures and Engineering Characterization of Ground Motion, U.S. Geological Survey Professional Paper 1552-D, 1998
Importance of soil-structure interaction and transverse cracking
The significant dam safety lesson that this case history taught is that soil-structure interaction and transverse cracking are highly probable modes of failure. It was fortuitous that the reservoir level was as low as it was at the time of the earthquake. The displacements experienced by Austrian Dam demonstrated the potential for an uncontrolled release by either, cracking at the spillway wall/embankment contact, and/or the transverse cracking. The latter was found to extent more than twice the available freeboard.
Mode of failure in homogeneous dam
Deep seated shearing where the embankment behaves as distinct blocks are governed by inertia forces and the undrained strength characteristics of the materials. Care must be taken to choose earthdam material.
Drainage function
Gravel strip drains were placed beneath the downstream pervious zone. But the gravel drains are not completely effective in relieving downstream seepage pressures.
Compaction of fill material is very important.
What could be done better
- Placing a zoned fill with chimney and blanket drains in the crest fill at both embankment ends
- Creating an impervious blanket at the upstream face
- Grouting the cracking or fissures in the spillway and grouting voids beneath the spillway slabs
- Grouting of the rock at the abutment contact with the fill
Resources:
Chris Kramer, Michael D. Lee, Preparedness for Dam Failures in the San Francisco Bay Area, Natural Hazards Review, Vol. 5, No. 1, 2004, ASCE
City of San Jose, Emergency Operations Plan, 2006
I.M. Idriss, Ralph J. Archuleta, Evaluation of Earthquake Ground Motions draft 06.5, 2007, www.ferc.gov/industries/hydropower/safety/guidelines/eng-guide/chap13-draft.pdf -
John Vrymoed, Wallace Lam, Earthquake Performance of Austrian Dam, California during the Loma Prieta Earthquake, Division of Safety of Dams, Department of Water Resources, http://damsafety.water.ca.gov/seismic_austrian.cfm
L.F. Harder, Jr., J.D. Bray, R.L. Volpe, R.L. Volpe, K.V. Rodda, Performance of Earth Dams during the Loma Prieta Earthquake, The Loma Prieta, California, Earthquake of October 17, 1989-Earth Structures and Engineering Characterization of Ground Motion, U.S. Geological Survey Professional Paper 1552-D, 1998
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