PACOIMA : Safety of Dams Questioned in Earthquake’s Aftermath

Pacoima Dam survived a pounding by the Northridge earthquake, but the huge ground motions recorded there Jan. 17 should cause dam builders and operators in seismic zones worldwide to re-examine the safety of their structures, a dam expert said this week.

Appearing in Burbank before the state Seismic Safety Commission, Caltech engineering professor John F. Hall said he believed the 365-foot-tall dam in the San Gabriel Mountains, which will require costly repairs, might not “have remained intact” if the quake had struck on one of the rare occasions when its reservoir was full.

But Vernon H. Persson, state dam safety chief, said he believed the massive impoundment would not “have failed even if full.”

Although noting that area dams came through the quake without a single failure, commissioners and earthquake experts marveled at the enormous ground accelerations measured at Pacoima Dam, which reached a peak level of twice the force of gravity.

When “people around the world involved in dam engineering see the records, they’re just going to be amazed,” Hall said in an interview.

Experts say the Northridge quake caused sideways and vertical shaking far stronger than would be expected in a 6.8 quake.