Subsidence and rock creep in a cross-valley fill

Cross-valley fills composed cf waste rock from mining operations pose environmental concerns related to their long-term stability. A large (2.06×10⁷ m³) phosphate waste dump in southeastern Idaho was monitored from 1981 to 1988 for subsidence and horizontal displacement in conjunction with different surficial rock types and long-term precipitation patterns. Most of the 19 surveyed benchmarks on the dump face moved in the northeast direction during the 7-yr study period, roughly the same orientation as the aspect of the fill face. Average vertical subsidence rate decreased exponentially with time from a high of 0.15 m/yr in 1981–1982 to a fairly stable level of 0.008 m/yr by 1988. Net displacement similarly decreased from 0.19 m/yr shortly after dump construction to 0.023 m/yr by 1988. Both subsidence and net displacement were initially higher in the shale cap on the west side of the chert-filled dump; however, by 1986–1988 subsidence and displacement rates were actually lower in the shale material. Subsidence and net displacement appeared unaffected by temporal precipitation patterns; both subsidence and net displacement rates declined significantly during the very wet snowmelt seasons of 1983 and 1984. The temporal patterns of movement at the dump site suggest that the breakdown of the rock (especially shale) occurs primarily during the first 1.5 yr following dump construction. Periodic saturation of the upper 6 m of spoil occurred in nine of 60 neutron access tubes, normally during the spring snowmelt seasons. Saturation occurred at depths 1.8 m and is thought to be associated with uneven compaction or heterogeneities in the fill. Overall, the dump appeared to be internally well drained with rates of rock creep unaffected by seasonal water inputs.