La Quemada Dam Failure, Zacatecas, Mexico

There are still are great number of masonry dams in Mexico that date back to the eighteenth and nineteenth centuries. Some were well designed and built with quality construction materials and are still functioning well to this day. Others were not and are unsafe. This is the case of La Quemada Dam, which failed on July 15, 1991. Located in the state of Zacatecas, this high hazard potential dam was built in 1790. Fourteen meters high, it was constructed on rhyolites. After its construction, it was raised an additional two meters using a wall 1 m in thickness. An exceptionally high flood produced a 1.1 m overtopping of the dam, ultimately causing its failure. Maximum outflow was estimated at 260 m³/s while the spillway capacity was only 20 m³/s. This article describes the mode of failure, based on field observations, laboratory tests and stability analysis using numerical methods. It concludes that the failure was caused by the sliding of the base at the dam's widest section. Safety deficiencies for this type of dam are discussed, along with empirical rules and economically feasible modifications to effectively eliminate them. 6 pp.