Page:The Failure of the Church Rock Tailings Dam.pdf/2

 Symposium on Uranium Mill Tailings Management, Fort Collins, Colorado, November 24-25, 1980 Geotechnical Engineering Program, Civil Engineering Department, Colorado State University

FAILURE OF THE CHURCH ROCK TAILINGS DAM

by

John D. Nelson and Joseph D. Kane

INTRODUCTION

On July 16, 1979, the Church Reck Tailings Dam failed. After the failure, United Nuclear Corporation retained two consultants to conduct independent investigations to determine the cause of the failure. At the same time, several Federal and State agencies reviewed the data and the reports by the owner's consultants. The authors of this paper were responsible for the technical review on behalf of the U.S. Nuclear Regulatory Commission.

The general nature of the failure as reported by the owner's consultants is generally agreed upon. The probable cause of failure as reported by them is presented in the following section of this paper. During the review, a number of factors became evident that may have significant influence on the design of earth dams for the retention of uranium mill tailings.

PROBABLE CAUSE OF THE BREACH

The probable cause of failure presented by each of the owner's consultants (Ref. 1, 2) was nearly the same and will be summarized below.

The embankment was about 35 ft. high and was constructed on a relatively deep deposit of clayey, silty sand. Certain soils were collapsible and some laboratory tests indicated collapse in excess of 10% upon wetting. The impoundment was not lined, and seepage into the foundation could occur readily. In addition, the fact that borrow for embankment fill was excavated just upstream of the tailings dam would have increased the seepage flow and rate of wetting of the foundation soils. A wetting front would have advanced downward from the impoundment with lateral spreading into the area beneath the embankment. Along the southern half of the embankment, approximately 3 feet of settlement had been observed since the beginning of operations in 1977.

As a result of the large settlement, differential movement of the embankment would be expected. Both longitudinal and transverse cracks had been observed in the embankment prior to the failure, and were