CIRES collaboration uses imaging to find expansive soils
By Annette Varani
The steeply dipping Pierre Shale along the Front Range of Colorado provides a major challenge to geotechnical engineers and builders because of the heterogeneous nature of the expansive soil characteristics. Here, researchers have opened the trench perpendicular to strike to expose the vertical layers visible in the trench walls, which are composed of different soil types. Top, Prof. Harold Olsen, Colorado School of Mines; sitting background CIRES undergrad John Rose, 2 standing foreground, David Noe of the Colorado Geological Survey and Colorado School of Mines masters student Jessica Humble; foreground standing, unknown. |
In early summer 2002 Professor Alexander Goetz of CIRES, with colleagues from the Colorado School of Mines, the Colorado Geological Survey and the U.S. Geological Survey, exposed 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, was undertaken to 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 swellingand-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.
Researchers position a section of grid against the trench wall, so readings could be taken at 5-centimeter intervals. Left to right: John Rose, CSES staff Eric Johnson, CSES masters student Angel Gutierrez. |
"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 advised by members of the Colorado Association of Geotechnical Engineers.
The researchers and crewmembers used a backpack spectrometer designed by the Boulder firm Analytical Spectral Devices, Inc. The team wire-gridded a 15-foot-deep trench wall.
The spectral image of the trench is made up of 29,000 individual spectra on 5-centimeter centers, surveyed into their exact location in the trench. The bands of color trending toward the upper left are individual beds of differing clay composition as well as calcite, gypsum and limonite, an iron mineral, all within the nearly vertical, 65 MY old Pierre Shale. |
"We took point measurements on the ground every 5 centimeters through
the grid," Goetz said. "We brought 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 took over 50,000 measurements across the wall in the Pierre
Shale, from which we developed a 29,000-wavelength-band image to help 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.
|
