The dam’s construction history prov

The dam’s construction history provides insight as to the possible reasons for the failure. The dam was constructed using the upstream construction method over a period of 37 years in 10 raises. No new raisings were constructed after 2013, and tailings disposal ceased in July 2016. The Starter Dam contained features that impeded drainage through the toe. No significant internal drainage was installed during the construction of later raisings, other than small drainage blankets below some of the later raisings, and chimney drains in some of the upper raisings. The drainage blankets and chimney drains in the later raisings were in response to observed seepage from the dam face above the toe during construction. The initial design of the dam established a relatively steep slope. After the Third Raising, a setback was constructed to straighten the alignment of the dam crest. The setback reduced the overall slope of the dam but moved the upper portion of the dam closer to the pond and also closer to the future internal water level. Pre-failure aerial and satellite images show that, at times during the life of the dam, water was close to the crest of the dam, resulting in weak tailings close to the crest and interbedded layers of fine and coarse tailings within the dam.The setback also moved the upper portion of the dam over weaker, finer-grained tailings.