Abstract

Heaving bedrock is a geological hazard that is related to expansive soils, but it is more complex in terms of its uplift morphologies, deformation mechanisms, and regional distribution. It is common along Colorado’s Front Range piedmont where steeply dipping sedimentary bedrock containing zones of expansive claystone is encountered near to the ground surface. It occurs in the Pierre Shale and other Upper Cretaceous formations. The heave features associated with heaving bedrock are distinctly linear and are caused by differential swelling and/or rebound movements within the bedrock. Heaving bedrock has caused exceptional damage to houses, roads, and utilities along the Front Range piedmont since suburban-type development began in the early 1970s. Much of this damage may be attributed to the longstanding tendency to assume that the bedrock may be treated, for site-exploration and design purposes, as an expansive soil having essentially uniform properties. This approach ignores the strong heterogeneity that is often present in the bedrock. In particular, drill-hole exploration surveys and drilled pier foundations, which are generally appropriate for expansive soil hazards, have proven to be inappropriate for recognizing and mitigating heaving-bedrock hazards.

This article presents a summary of heaving bedrock as a distinct geological hazard and describes the technological and policy advances that have been made in recent years to promote understanding and effectively mitigate the problem. The Colorado Geological Survey has played a key role in these advances by introducing the term “heaving bedrock” to differentiate the problem from expansive soils; leading stakeholder field trips and conferences; investigating the physical characteristics, mechanics, causes, and distribution of heaving bedrock; publishing the investigation results; assisting county governments in creating new land-use regulations; and reviewing site investigation reports for actual subdivision projects. From this experience, we conclude that a state geological survey must be active in numerous arenas—scientific, practical, and political—to assist effectively in addressing potential hazards that impact the general public.