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Geologic Control of Severe Expansive Clay Damage to a Subdivision in the Pierre Shale, Southwest Denver Metropolitan Area, Colorado

Published online by Cambridge University Press:  28 February 2024

J. D. Gill ,
M. W. West ,
D. C. Noe ,
H. W. Olsen  and
D. K. McCarty
J. D. Gill
Affiliation:
Michael W. West and Associates, P.O. Box 555, Morrison, CO 80465-0555
M. W. West
Affiliation:
Michael W. West and Associates, P.O. Box 555, Morrison, CO 80465-0555
D. C. Noe
Affiliation:
Colorado Geological Survey, Denver, CO
H. W. Olsen
Affiliation:
Colorado School of Mines, Golden, CO
D. K. McCarty
Affiliation:
Montana State University, Boseman, MT
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Abstract

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Shortly after construction of a subdivision in the southwest Denver metropolitan area in 1986, a portion of the subdivision built directly on steeply-dipping strata of the Pierre Shale began experiencing damaging differential movements, causing house foundations to fail and pavements to warp and crack. This formation is a Late Cretaceous marine clay-shale composed predominantly of fluvial mixed-layer illite/smectite and quartz. During deposition of the shale, periodic and explosive volcanism generated thin beds of bentonite, consisting initially of volcanic ash and subsequently altered to nearly pure smectite. Some of these bentonite beds were exposed in a trench adjacent to the subdivision and perpendicular to the strike of the steeply-dipping strata. The thickest bentonite beds correlated well with linear heave features that these beds parallel the bedrock strike throughout the subdivision were mapped via severely deformed pavements. Mineralogical data show the bentonite bed that correlates with the worst damage within the subdivision consists of about 62% smectite by weight with mixed-layer illite/smectite expandability of 92%. By comparison, a sample of the typical silty claystone, which is fluvial mixed-layer illite/smectite mixed with detrital quartz from the adjacent strata, had about 23% smectite by weight with 70% to 90% illite/smectite expandability. Geotechnical tests for swell potential show that samples of 2 bentonite beds swelled 39% to 43% compared to 2% to 8% for samples of the typical silty claystone. It is proposed that differential swell resulting from stratigraphically-controlled differences in clay mineralogy and grain-size is the primary factor controlling extreme damage for this geologic setting.

Keywords

BentonitesExpansive claysPierre ShaleSmectites
Type
Research Article
Information
Clays and Clay Minerals , Volume 44 , Issue 4 , August 1996 , pp. 530 - 539
Copyright
Copyright © 1996, The Clay Minerals Society

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