U. of Colorado collaboration will test use of imaging to find expansive soils

May 30, 2002

Professor Alexander Goetz, a scientist with CU-Boulder's Cooperative Institute for Research in Environmental Sciences with colleagues from the Colorado School of Mines, the Colorado Geological Survey and the U. S. Geological Survey, will expose a 100-foot section of vertically layered sediments for a study of one of the worst construction hazards along the Front Range.

The work, performed in a known problem-area of the Pierre Shale formation, will compare and correlate a new hyperspectral sampling technique for identifying bentonite and other clay soils with standard core sampling methods. Bentonite and some other clay soils expand in contact with water.

"Swelling soils cause slow-acting damage," said Goetz. "It's an insidious problem. We want to develop a more comprehensive and less costly method of surveying construction sites that will help prevent or greatly reduce the remediation required."

Damage from expanding soils costs billions worldwide each year, more than all other natural hazards combined, Goetz said. The gradual damage is less dramatic than a tornado or flood, but the relentless push of clay bedrock into basements, roads, pipelines and utilities is a homeowner's - and taxpayer's - worst nightmare, because in many cases the havoc wrought by expanding soils is uninsured.

Depending on soil moisture, the swelling-and-shrinking process may not manifest its full extent for a decade or longer after new construction. But the structural displacement that swelling ground causes often leaves homeowners stranded with big-ticket damages for buckled foundations and floors.

Expansive clays are the weathered remains of volcanic eruptions that occurred millions of years ago. In Colorado, layered deposits were raised nearly vertical with the uplift of the Rocky Mountains. For residential construction, typically one borehole is required for each home built.

"Many of the structures built near the mountain front over the last two decades have had to be remediated because the problem clays were not adequately identified," Goetz said. That's because bentonite and similar sediments are usually less than a foot thick where they occur.

"By eye you can't always tell the difference between bentonite and less expansive clays," he said. If a borehole taken between bentonite layers is sampled, it may not represent the actual hazard within the site.

In standard testing, cores are moistened and displacement measured using a device called an oedometer. Tests take a week to complete and cost about $200.

"Hyperspectral sampling is much, much quicker - you can survey a complete building site or road bed at a much higher density than by methods currently in use," Goetz said. "We hope it can be used to augment and expand the coverage of measurements made by standard techniques."

The new method evolved from analysis techniques and technologies developed for NASA space programs.

"Using satellites and aircraft, we can detect surface composition using hyperspectral imaging. Clays have characteristic absorption of light that has to do with their atomic structure," Goetz said.

The researchers include Goetz, who is the director of the Center for the Study of Earth from Space at CIRES, Harold Olsen of the Colorado School of Mines, David Noe of the Colorado Geological Survey and Dennis Eberl of the USGS. The group is being advised by members of the Colorado Association of Geotechnical Engineers.

The researchers and crewmembers will use a backpack spectrometer designed by the Boulder firm Analytical Spectral Devices Inc.

The team will wire-grid a 20-foot-deep trench wall.

"We'll be making point measurements on the ground, taken every 5 centimeters through the grid," Goetz said. "We can bring to bear all the techniques we use to analyze space-borne images. The nice thing is that we don't have to look through the atmosphere. These techniques would not have been developed without our interest in viewing earth from space.

"In all, we'll take over 50 thousand measurements across the wall in the Pierre Shale, from which we'll develop a 2,150-wavelength-band image and identify where to sample."

Results will be compared to those achieved using conventional sampling methods. "Ultimately, we're testing how good our measurements are," Goetz said.
Alexander Goetz, (303) 492-5086
Annette Varani, (303) 492-5952
Jim Scott, (303) 492-3114

University of Colorado at Boulder
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