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Cover Photo: Surface view of a near-vertical bentonite layer in the Upper Cretaceous Pierre Shale in Jefferson County, Colorado. The layer heaved with a differential displacement of 8 cm within 24 hours after a rainstorm at this construction area. Note the hump in the fence aligned with the trend of the bentonite layer. Heaving bedrock damage is occuring in the 2.5-year-old subdivision in the background. Photograph by David C. Noe.
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FIGURE 1: Index map ofthe Front Range piedmont area, Colorado (modified from Noe and Dodson, 1995). Heaving-bedrock damage is most pronounced in Jefferson, Douglas, and El Paso counties within 1.6 to 4.8 km (1 to 3 miles) of the mountain front.
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FIGURE 2: Photograph of steeply dipping Pierre Shale in Jefferson County, Colorado. Steepling dipping bedrock is especially prone to differential heaving because of the large number of beds interescting the ground surface, with each bed having different composition and engineering properties.
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FIGURE 3: Photograph of parallel, linear heave features associated with heaving bedrock. Heaving bedrock has caused extensive damage in this neighborhood, which overlies near-surface Pierre shale.
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FIGURE 4: Stratigraphic column of Upper Cretaceous and younger rocks in the greater Denver area (modified from LeRoy, 1955). Heaving bedrock occurs in all of the Upper Cretaceous formations where they are steeply dipping, on the flank of the Front Range uplift. The Pierre Shale is the most important formation with respect to heaving bedrock occurrence and damage.
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FIGURE 5: SWELLING SOILS Block diagrarn showing a widely used, general model for expansive soils (modified Forn Noe and Dodson, 1995). The model assumes that horizontally bedded clay soil or bedrock has unilorm composition. Hydration and expansion, resulting in uniform, vertical uplift of the ground surface (vertical arrows), occur within the near-surface zone of moisture change where naturally dry soils are wetted. The soils or bedrock beneath this zone remain at constant moisture and are therefore unaffected by wetting.
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FIGURE 6: Block diagrams showing different types of heave features associated with heaving bedrock (modified form Noe and Dodson, 1995).
(A) Near-symmetrical heave features formed by differential swelling and/or rebound of individual bedrock layers, each having a different swell potential. This type of heaving results in straight-crested, longitudinal uplift of the ground surface, oriented parallel to bedding strike.
(B) Strongly asymmetrical heave features formed by thrust-like, shear-slip movement along bedding planes or fracture surfaces. The bedding-plane features are straight crested and are oriented parallel to bedding strike, whereas the fracture-plane features have curvilinear crests and may not necessarily be oriented parallel to bedding strike.
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FIGURE 7: Assymetrical heave feature in a graded cut caused by sudden, thrust-like heaving of the bedrock block on the right. This feature formed along a preexisting fracture plane. More than 30 cm of prior Holocene displacement was evident along the plane (at a point in the cut wall marked by the wooden stake) when the cut was first exposed. The heaving episode shown here occured within 24 hrs after a rainstorm and resulted in another 8 cm of total displacement.
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FIGURE 8: Map of Douglas County showing areas of steeply dipping bedrock and expansive soils/bedrock (from Noe and Dodson, 1995). The Dipping Bedrock Overlay District is defined as the overlapping portion of the two areas.
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FIGURE 9: This trench has exposed dipping beds of bentonite (dark layers) at a high school site. Earlier drillhole surveys had not encountered the bentonite. The foundation design for the school was subsequently changed from a drilled-pier system to an overexcavation design.
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