GEOLOGIC INVESTIGATION OF ROAD CUT SLOPE FAILURES AT LA VETA PASS, COLORADO

Several small slope failures have been identified across a 2.5-mile section south of the La Veta Pass along Hwy 160 in southwestern Colorado. La Veta Pass is the main transportation route into the San Luis Valley, covering major tourist destinations nearby. This section of the highway was constructed in the mid-1960s to augment the old La Veta Pass Road. Examination of historical aerial photographs indicates that these slope failures did not occur prior to the construction on Hwy 160. The slope failures were facilitated by road construction combined with the geologic characteristics of the local exposures of Sangre de Cristo Formation. The Pennsylvanian/Permian Sangre de Cristo Formation is composed mainly of arkosic sandstone, shale and conglomerate. The variation in lithology in terms of more competent sandstones underlain by the weaker shale inclined toward the highway, along with the presence of two prominent joint sets, further facilitated slope failures. Rainfall acts as an additional driving force.

The lithology of the rock units and the attitude of their bedding planes with respect to the highway varies considerably along the 2.5-mile section of Hwy 160. Consequently, the nature of each of the 9 slope failures show considerable disparity. Furthermore, based on evidence obtained from examination of historical aerial photographs, it has come to light that these events have occurred at different times since road construction began. The objective of this study is to map the slope failures on a large scale and evaluate what interacting driving forces triggered each event.