Three Mile Dam Preliminary Slope Stability Analysis

摘要

The Three Mile Dam is a privately owned high hazard embankment dam located northwest of Helena, Montana. The height of the dam is 63 feet and the crest of the dam is 450 feet long. The Three Mile Dam was originally a railroad embankment built in the late 1800s. Around 1927, long after the railroad had been abandoned, the embankment over Three Mile creek was converted to a dam for the purpose of irrigation. Static and seismic analysis was performed on the upstream and downstream slopes of the dam to determine the dam's stability.rnThe stability analysis was done using the computer software SLIDE 5.0 from the Rocscience package. The embankment was tested using the Bishop method of slices. Given that minimal subsurface data are available on the dam it was hard to determine subsurface parameters to use in the SLIDE program. The dam is built on a shale bed dipping to the southwest. The geological materials of the dam had to be determined using soil samples from the surface of the dam. Field developed cross-sections were also used in the determining the slope stability. The embankment stability was calculated at high and low reservoir conditions. At low reservoir static conditions both the upstream and downstream faces of the dam were stable. However, at high reservoir conditions the downstream face of the dam was determined to be potentially unstable as shown in Figures 4 and 5. Seismic conditions were based upon peak acceleration of seismic shaking in the area. Models run on the stability under these conditions also displayed instability in the embankment on both the upstream and downstream sides.rnMany assumptions had to be made, in computing the slope stability of the dam, due to insufficient data. More investigations are necessary to get a more accurate picture of the stability of this dam. Information on the subsurface geology in the dam is crucial to getting more accurate numbers on the factors of safety. Knowing the subsurface geology will also help in seepage analysis. More data would allow for less conservative assumptions and could potentially prove that the dam is stable under current conditions.