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United States Department of Agriculture

Agricultural Research Service

Title: Geotechnical Characteristics of Fractured Glacial Till: Hydraulic Conductivity, Consolidation, and Shear Strength

Author
item Allred, Barry

Submitted to: Ohio Journal of Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 15, 2000
Publication Date: June 1, 2000

Interpretive Summary: Fractured glacial tills are common throughout the midwestern United States. Fracture characteristics such as size of openings and the spacing between them both influence overall engineering behavior. Consequently, correct geotechnical design decisions in places such as Ohio often require careful consideration of fracture impacts on hydraulic and mechanical properties of fglacial till. Most important are the effects on saturated hydraulic conductivity, consolidation potential, and shear strength. Saturated Hydraulic Conductivity: Fractures increase the overall hydraulic conductivity of glacial till, in some cases by two or more orders of magnitude. Consolidation: Settlement occurs at a faster rate when fractures are present. If fractures are open, a modest increase in total settlement is possible. Shear Strength: Glacial till fractures decrease overall shear strength. After excavation or erosion of surface material, stress release and water infiltration lead to further reductions in overal shear strength. This process is called softening and can take years to complete. Once failure occurs, there is another substantial drop in shear strength to the residual value. Because fractured glacial tills are so widespread throughout the northern hemisphere, more investigation of their properties is warranted, particularly with regard to long term shear strength conditions in natural and constructed slopes. Only through increased understanding can the engineering problems associated with fractured glacial tills be adequately addressed.

Technical Abstract: A literature survey was conducted and fracture influences on engineering behavior of glacial till are summarized, specifically with regard to saturated hydraulic conductivity, consolidation potential, and shear strength. Saturated hydraulic conductivity is increased by fractures, in some cases by two or more orders of magnitude. This in turn results in larger values for the coefficient of consolidation, governing the time rat of consolidation. A larger coefficent of consolidation corresponds to faster settlement. Modest increases in total settlement occur only if fractures are open. Fractures also have the overall effect of reducing shear strength. Upon removal of surface material by excavation or erosion, stress release and water infiltration lead to further decreases in shear strength. This strength loss process, called softening, is due mostly to a decrease in effective cohesion and usually takes years to complete. Once failure occurs, there is another substantial drop in shear strength to a residual value. This residual strength is a result of realignment of particles along the failure plane during shear, which decreases the effective angle of internal friction. The fracture impact magnitude on glacial till saturated hydraulic conductivity, consolidation potential, and shear strength is determined largely by aperture and spacing characteristics. As the number and/or size of fractures increase, changes in these geotechnical properties become more pronounced.

Last Modified: 10/21/2014
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