Abstract:
What are the geologic risks present on a large, active slope failure and how does understanding those hazards aid in risk assessment and mitigation programing? In the 1960’s, the NORAD installation was built inside of Cheyenne Mountain during the Cold War to provide for our Nation’s defense. The complex is surrounded by steep slopes, both upward and downward. The slopes generally consist of either colluvium outwash or shear granite faces along the front-range near Colorado Springs. The geometry of the colluvial fill deposit has led many researching the area to conclude that the slope was formed from a large historic bedrock landslide event, perhaps coinciding with an earthquake event. The stability of the area is impacted by concentrated run off events, resulting in erosion and slope movements which have caused significant damage to structures in the general area of the installation. In September 2013, after a period of heavy rain, a landslide occurred that blocked the main entrance to the mountain, preventing staff from entering or exiting through the main gate. Recently, the Air Force undertook a study to evaluate the potential risks the facility faced as they relate to slope instability. SCI performed a thorough exploration program that included a drilling program using ODEX and Sonic drilling methods, with supplemental geophysical testing, to determine soil layer and behavior profiles for the colluvium and shale bedrock. The geophysical methods included ReMi, Seismic Refraction, Electric Resistivity Testing (ERT), and Multichannel Analysis of Surface Waves (MASW). Advanced triaxial testing was conducted to better predict the strength envelop from the samples of Pierre Shale collected. The results of the study are being used by the Air Force to help prioritize programming to address the risks.