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

Abstract

Abstract--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.

Steeply Dipping Heaving Bedrock, Colorado: Part 1 Heave Features and Physical Geological Framework

“…Heavefeature mapping (Weakly, 1989;Dodson, 1996;Gill et al, 1996;and Noe, 2003b) shows that individual segments may sometimes line up with each other and are probably continuous features. The longest from Weakly's maps are up to 1,067 m (3,500 ft) in total length, although most are on the order of 305-m (1,000-ft) long or less.…”

Mapping clay minerals in an open-pit mine using hyperspectral and LiDAR data

“…Shales with large amounts of clay also represent lines of stratigraphical weakness along which landslides can occur [Hutchinson, 1961;Cornforth, 2005;Hancox, 2008]. Smectite group clays in particular can undergo large changes in volume though swelling which can cause localised instability and ground heave [Gill et al, 1996;Goetz et al, 2001]. Many techniques have been developed to classify minerals on the basis of their entire spectral curve in the ShortWave InfraRed (SWIR) where diagnostic absorption features of many minerals are located.…”

Mapping Layers of Clay in a Vertical Geological Surface Using Hyperspectral Imagery: Variability in Parameters of SWIR Absorption Features under Different Conditions of Illumination

“…Clays, by attracting water into their layered structure, can undergo an expansion of up to four times their volume when transitioning from a dry to a saturated state [8]. Smectite-group clays have the greatest potential to swell and therefore present the greatest capacity for failure because they undergo repeated successive phases of expansion and contraction, causing localized disturbance and ground heave [8][9][10][11]. Identifying the types and abundance of clay minerals in these layers provides information that can be incorporated into assessments of risks or models of potential slope failure in road cuttings or in open pit mines, e.g., [12].…”