Factors Contributing to Rockfall Occurrence and the Associated Risk in Rockville, Utah

Abstract

The east-west trending, south-facing slope in Rockville, Washington County, Utah is well known for experiencing large-size rockfalls, including the fatal rockfall of December 2013 that destroyed a home and killed two people. The stratigraphic units present in the area include the well-indurated, Upper Triassic, Shinarump Conglomerate Member (cliff-forming unit) of the Chinle Formation that caps the less-resistant, Lower Triassic, Moenkopi Formation (slope-forming unit). We performed a detailed study to investigate the factors that contribute to rockfall occurrence and the associated risk in the Rockville area. To accomplish this objective, we selected four representative sites along the slope and a few supplemental sites for additional data. The results of our study indicate that the main factors contributing to rockfall occurrence include unfavorable orientation of discontinuities, differential weathering resulting in undercutting, and regional climate (infrequent, high precipitation events; freeze-thaw cycles) whereas those contributing to rockfall risk include high slopes, presence of gullies that channelize rockfall debris, and proximity of homes to the toe of the slope. The results also show that the western portion of the study area poses the highest risk due to the presence of higher slopes, larger exposure of the Shnabkaib Member (the weaker rock unit) resulting in undercutting, higher number of deep gullies, and the higher number of residences closer to the slope toe.