The Utility of Aggregate Processing Fines in the Rehabilitation of Dolomite Quarries

It is economically and environmentally desirable to use aggregate processing fines (pond fines), a by-product
of dolomite crushing/washing operations, in the rehabilitation plans of the aggregate quarry where they are
produced. A study was undertaken to determine if the physical and mechanical properties of these
dominantly silt-sized materials also made them suitable as embankment material from a geotechnical
perspective. The after-use proposed for the dolomite quarry investigated in this study (Milton quarry,
Ontario) was to be a naturalized area surrounding a large water reservoir, and pond fines were being used to
construct sloped embankments at the base of the vertical perimeter quarry walls for this purpose. The
specific objectives were i) to measure and analyze the physical and mechanical properties of the pond fines
that are relevant to their usage in site rehabilitation (earthen embankment), and ii) to predict the long term
stability of the embankments given current emplacement practices and a scenario of partial submergence by
a reservoir. Shear strength, measured by direct shear and drop cone penetrometer methods, increased with
higher clay-sized particle content and higher dry bulk density, with lower sand content, and it peaked at soil
water contents (4 to 9%kg/kg) well below the measured field capacity of about 16%kg/kg. Factors of safety
calculated for slope stability (modified Bishop method of analysis) decreased with decreasing shear strength
(cohesion, angle of shearing resistance), with increasing slope gradient, and with the introduction of a water
table. Nevertheless, the factors of safety were acceptably high for all scenarios tested (> 1.8), including
scenarios using the maximum measured slope gradient of 37%. Increasing the dry bulk density by 0.1 g/cm*
over the mean density measured in situ (i.e., to about 87% of the Standard Proctor maximum dry density)
did not improve the calculated factors of safety for slope stability enough to warrant increased mechanical
compaction of the pond fines during embankment construction.