Dam Incident: Pacoima, 1994-01-17

Dam Name Pacoima  
Incident Date 1/17/1994  
Country United States  
State/Province California  
Dam Height (m) 111.3  
Uncontrolled Release of the Reservoir No  
Dam Type Arch  
Detailed Dam Type  
Incident Type Seismic  
Incident Description The 1994 Northridge earthquake severely shook Pacoima Dam, a 372-foot-high concrete arch dam, located a short distance away from the epicenter. As it did previously during the 1971 San Fernando Earthquake, this dam, a flood-control facility (which, therefore, had a low reservoir level at the time of occurrence of the earthquake), experienced nearby ground accelerations well above one g, at its left abutment. Indeed, horizontal and vertical peak ground accelerations (PGA) recorded in 1994 near the top of that abutment were 1.76g and over 1.60g, respectively. Downstream records, near the toe of the dam, were only 0.44g (horizontal) and 0.22g (vertical), emphasizing the significance of ridge effects upon amplifying ground motion, and perhaps the influence of the distress previously experienced in 1971 within the left abutment rock mass (USCOLD, 1992). In 1994, the joint between the left abutment concrete thrust block and the left end of Pacoima Dam opened about two inches. The left abutment thrust block also moved 1/2 inch downstream, relative to the crest. The protective gunite cover was severely cracked at both abutments. Post-earthquake surveys indicated a maximum horizontal displacement of about 19 inches at one location on the left abutment, and 14 inches of downward vertical movement of the rock mass at another location. This experience also confirmed that some lift joints did open (CSMIP, 1994). Post-tensioned tendons, installed in 1971 to hold down potentially unstable rock wedges in the upper left abutment, became inoperable for post-tensioning adjustments, due to failed O-rings. They were subsequently repaired and re-stressed. Overall, Pacoima Dam performed satisfactorily during the 1994 Earthquake, as it did before in 1971.  
High Level Failure Mode Cracking/Structural Failure  
Lessons Learned  
Initiating Event Seismic  
Initiating Event Description The 1994 Northridge earthquake severely shook Pacoima Dam, a 372-foot-high concrete arch dam, located a short distance away from the epicenter. As it did previously during the 1971 San Fernando Earthquake, this dam, a flood-control facility (which, therefore, had a low reservoir level at the time of occurrence of the earthquake), experienced nearby ground accelerations well above one g, at its left abutment. Indeed, horizontal and vertical peak ground accelerations (PGA) recorded in 1994 near the top of that abutment were 1.76g and over 1.60g, respectively.  
Structures/Components  
Failure Mode Description A Magnitude 6.7 earthquake caused cracking to some structures of the dam.  
Emergency Action Plan Implemented Unknown  
Consequences Consequence Data Unavailable  
Geographic Coordinates - Decimal Degrees 34.3350, -118.3950  
Normal Reservoir Storage (m3):  
Crest Elevation (m)  
 
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Photographs

Pacoima -

View of cracking damage to the dam after the earthquake

Pacoima -

View of damage to the dam after the earthquake
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