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Effects of Mica Content on Engineering Properties of Sand¹

ABSTRACT

Sand mixtures with variable weight percentage of mica (W_m) 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 W_m, reflecting the inhibitory effect of platy grains on close-packing of spheroidal grains. The initial tangent axial compression modulus (E_o), a reciprocal index of undrained compressibility, decreased logarithmically with increasing W_m. The relative effect of W_m on E_o was greater at higher confining pressures. Shear strength (), bearing capacity (q_oh), and California bearing ratio (CBR) also logarithmically decreased with increasing W_m. Curves for E_o, , q_oh, 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.-W_m 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 (F_m) 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 F_m 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 F_m than by W_m.

¹ Contribution of the Dep. of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.

² Research Associate, Assistant Professor, and Professor, respectively.

Received for publication June 28, 1983. Accepted for publication November 29, 1983.