Effects of Mica Content on Engineering Properties of Sand

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Abstract
Sand mixtures with variable weight percentage of mica (Wm) in a quartz matrix were evaluated to isolate the effects of mica content on compacted density, compressibility, shear strength, and bearing capacity. Maximum compacted density decreased linearly with increasing Wm, reflecting the inhibitory effect of platy grains on close‐packing of spheroidal grains. The initial tangent axial compression modulus (Eo), a reciprocal index of undrained compressibility, decreased logarithmically with increasing Wm. The relative effect of Wm on Eo was greater at higher confining pressures. Shear strength (φ), bearing capacity (qoh), and California bearing ratio (CBR) also logarithmically decreased with increasing Wm. Curves for Eo, φ, qoh, and CBR showed shoulders marking diminishing effects of mica additions beyond mica weight percentages of 10 to 15%. A hypothesis that the curvilinear nature of parameter‐vs.‐Wm relationships is due to the sensitivity of these parameters to mica‐quartz interaction at particle contacts is supported by observed linearity between the parameters and mica cleavage face area (MFA). The latter is indexed by mica frequency per 100 grains (Fm) measured by petrographic grain count on a specific size fraction. If the range of mica grain thicknesses is small relative to the range of diameters, then Fm for a single size fraction effectively approximates the MFA for the mica over the whole particle size range. Mica effects on certain engineering properties may be indexed more effectively by Fm than by Wm.

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