Abstract

In December 2013, a rockfall in the town of Rockville, Utah, released an estimated 2,700 tons (2,450 tonnes) of rock from a 400-ft (122-m) high slope; the rock struck a house at the base of the slope, resulting in two fatalities. We performed detailed field and laboratory investigations to (1) identify the modes of failure and factors contributing to rockfalls along the east-west trending, south-facing slope where it passes through the town; (2) identify sections of the slope that pose the highest hazard for future property damage or injury; and (3) suggest potential remedial measures. Field investigations included mapping discontinuities, establishing stratigraphy, measuring slope geometry, and evaluating potential failure modes at four selected sites. Laboratory investigations included determining dry density, friction angle, and slake durability index of rock samples. Using the Dips software, we determined the principal joint sets and performed a kinematic analysis. The maximum rollout distances for rock blocks of various sizes were determined for each of the study sites using the RocFall software. Results of the kinematic analysis and field observations indicate that wedge, plane, and toppling failures are possible within the Shinarump Conglomerate Member of the Chinle Formation and the Upper Red Member of the Moenkopi Formation along the entire slope. Based on the results of the study, we developed a rockfall-hazard map that indicates that the western portion of the town faces the highest hazard from potential rockfalls. The most feasible future remedial measure is not to build close to the hazardous slopes. Other possible remedial measures include removing loose rock blocks, installing rock anchors, and using drapery mesh.

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