Jan 162017

On solid ground — that’s how many of us think of good old, stable earth. So it’s disconcerting when the ground moves out from under us in any way.

Because of our environment, history, and geology, Colorado has conditions where ground movements can costs millions of dollars in annual property damage from repair and remediation, litigation, required investigations, and mitigation. There has been recent attention to swelling clay soils and heaving claystone bedrock, and the media has helped publicize these problems, which are predominant along the Front Range. But that’s only half the story. Geologic hazards in Colorado also include ground that sinks. Ground subsidence and soil settlement pose significant hazards in Colorado in many areas throughout the state. A variety of causes, some human-made and others inherent to the geology and geomorphology of Colorado, cause these sinking problems.

Coal and hard rock mining were important parts of the early development of Colorado and continue to play a role in the state’s economy. Besides mineral wealth, mining also left a legacy of undermined lands that are susceptible to ground subsidence because the old underground mine workings are caving in and cavities are propagating to the surface. Here at the CGS, we keep a state coal mine subsidence library complete with maps of historic coal mines and subsidence risk investigations and assessments. Today’s growth pressures are accelerating development in or near undermined areas. An important point to mention here is that normal homeowners insurance does not cover ground movements at all, whether they are human caused or natural soil or bedrock hazards.

A sinkhole near Carbondale initiated by hydro-compaction of surficial deposits and enlarged by piping into a void in the Eagle Valley Evaporite.

A sinkhole near Carbondale initiated by hydro-compaction of surficial deposits and enlarged by piping into a void in the Eagle Valley Evaporite.

Ground subsidence hazards also occur where evaporitic bedrock (gypsum, anhydrite, and rock salt) dissolves. Subsidence sags and ground downwarping, caverns and open fissures, ground seepage and streams flowing from bedrock, and various types of sinkholes, are landforms collectively called karst morphology. The CGS is doing exciting and important scientific work in evaporitic bedrock areas of Colorado where these landforms exist. The research, which shows thousands of feet of regional collapse, salt neotectonics, diapiric piercement structures, and cubic miles of evaporite minerals dissolved and washed down the Colorado River over hundreds of thousands of years, illustrates how fascinating our state’s geology is.

Collapsible soil is yet another different and difficult geologic phenomenon in Colorado. In areas of collapsible soils, soil settlement sometimes occurs very rapidly. A wide variety of soil types, including hydrocompactive, piping, dispersive, and gypsiferous soils, can be a major concern where development and structures are founded on them. This problem exists mostly in western and southwestern Colorado. Unlike expansive soils that swell when water is added, these soils collapse and settle when they become wetted, usually from human activities, at a saturation level not naturally occurring in the semi-arid climate. The CGS continues to study the occurrences of collapsible soils and has published both an award-winning definitive statewide study and a regional susceptibility map series.

If you are a geologist in Colorado, the variety and complexity of geologic conditions in our state will continuously fascinate you. If you are a homeowner, home-builder, or community planner, the geology may seem more vexing than fascinating. Through comprehensive research, the geoscientists at the CGS are taking on these problems and providing solid information that supports solutions.

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