PRELIMINARY EVALUATION OF EARTHQUAKE HAZARDS ALONG TRANSPORTATION ROUTES IN COLORADO

Recent research by the Colorado Geological Survey and others has raised the level of awareness of earthquake potential in Colorado. However, little work has been done to evaluate the vulnerability of transportation routes to earthquake hazards. A preliminary study has been completed to assess the potential for earthquake related damage to roadways, and to identify critical areas for more detailed study. The hazards that could deny roadway use include landslides, liquefaction, and flooding.

Landslide hazards were evaluated for typical clayey soils (phi=15, c=450 psf) on slopes or embankments of typical steepness (3:1 or 4:1). Slope failure can be anticipated for earthquake accelerations on the order of 20%g, which could occur throughout a significant portion of west-central Colorado (using a 2% probability of exceedance level in 50 years).

Liquefaction can be expected for typical sandy saturated soils with corrected SPT values in the range of 12-16 exposed to 20%g, or SPT values of 7-11 exposed to 10%g. These soils would include young Quaternary sands and gravels in river valleys. River valleys that may be exposed to the required level of earthquake shaking include the Eagle, Colorado, Gunnison, Uncompahgre, Animas, Los Pinos, and Upper Rio Grande. Potentially affected communities include Delta, Montrose, Ridgway, Durango, Gunnison, Creede, Del Norte, and the San Luis Valley (particularly the northern and southern ends).

Flooding could result from canal or levee breaks (such as the recent one in Grand Junction) or from catastrophic dam failure caused by earthquake shaking. Sixteen reservoirs are located within 50 miles upstream of communities, and within the region expecting 20%g.