A diagrammed cross section of the breach from the report commissioned by the NRC. The "point" in the bedrock that UNC said acted as a fulcrum in the dam's breach is visible beneath the embankment. Extent of seepage-impacted groundwater, weakening South Cell wall foundations The dam formed the southern wall of one of the<unk> 's three holding ponds, which were used to evaporate tailings solution until the remaining solid waste could be buried. From 1967 to 1982, the<unk>  produced an average of of tailings every day, for a total of . The high embankment was constructed on a deposit of collapsible clayey, silty sand, deep. United Nuclear used a new design, recommended by the Nuclear Regulatory Commission, that used earth rather than tailings themselves as building material. The holding pond was not lined, a violation of the<unk> <unk>  Tailings Radiation Control Act of 1978. This allowed tailings solution to seep into the ground, weakening the foundation of the dam and contaminating the groundwater. Horizontal and vertical cracks formed along the southern part of the embankment, allowing the acidic tailings solution to penetrate and weaken the embankment. A sand beach was constructed to protect the face of the embankment from the tailings solution, but it was not properly maintained. The liquid in the holding pond eventually rose higher than the dam's designed limit, past the point where the sand beach could protect the dam. The United States Army Corps of Engineers concluded in its report to Governor Bruce King of New Mexico that the principal cause of failure was differential settlement of the foundation beneath the dam wall, and the report commissioned by the Nuclear Regulatory Commission corroborated this conclusion. Critical variations in tailings pond operation practice from approved procedures contributed further to the dam failure. United Nuclear's Chief Operating Officer, J. David Hann, blamed the failure of dam on the pointed shape of the bedrock beneath the embankment, which he said acted as a fulcrum and weakened the dam. In December 1977, independent consultants spotted cracks in the dam wall. Three months later, United Nuclear sealed the cracks with bentonite and kerosene slurry but took little or no other action, despite the consultant's urging for regular inspections of the dam. Further cracking was noted in October 1978. Neither the facility owner nor the State Engineer were formally notified of the cracks, though Arizona representative Morris K. Udall testified before Congress that at least three federal and state agencies had "ample opportunity" to predict that the dam's failure was likely. At the same Congressional hearing, the United States Army Corps of Engineers testified that had the dam been built according to legal specifications, the failure would not have occurred.
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