Edmonton, Alberta, has extensive areas of lacustrine silt and clays, formed during the last period of glaciation. The upper few metres of these sediments are typically relatively dry and stable. Where shallow groundwater is present or deeper cuts are performed for new roadway construction, the silts and clays are weak and saturated, exhibiting severe frost heave in winter along with loss of strength upon thawing. The use of polystyrene insulation has been used to insulate the subgrade and mitigate the potential for frost heave of subgrade and associated damage of overlying pavement structures. Construction of several roadway projects in these lacustrine silts and clays have used an insulating layer of bottom ash in lieu of polystyrene insulation. Where grades are flat and drainage is poor, it can be a challenge to construct a roadway section bearing on the saturated silts as they quickly lose strength with construction equipment traffic and provide an unstable platform for layout of the insulation sheets. Bottom ash has proven to be an effective stabilization layer in addition to providing insulating properties as it drains well and has angular particle sizes which provide a stable working platform when placed on top of the exposed silt. This paper presents case histories of several roadway projects in Edmonton comparing the design, cost and performance of roadway projects constructed with polystyrene, bottom ash and alternate frost heave mitigation strategies. Thermal modeling was used to evaluate the insulating properties of bottom ash and proposed alternate subgrade concepts. Advantages and limitations of each frost heave mitigation strategy are presented to provide a basis for comparison and to aid in selection for design of new roadway sections in similar environments. Pavement design considerations are also discussed.
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