Flow behavior of allophane and ferrihydrite under shearing forces

Abstract

Allophane and ferrihydrite, two minerals of short-range order, impart major physical and chemical character to soils derived from weathered volcanic ashes. Undisturbed samples of both minerals, having low dry bulk densities, ~0.3 t m-3, and high natural water contents, ~250% w/w, have been sheared between cone and plate in a rotary-type viscometer. During rotor acceleration, both developed maxima of shear stress that did not re-establish on deceleration. A sudden loss of strength indicated sensitivity to shearing forces. By sieving lumps of aliophane and ferrihydrite, pastes were readily produced which were adjusted to a range of moisture contents and sheared. These pastes developed maxima of sensitivity at ~170% H2O w/w. Below 100% H2O w/w a state of friability, with make and break of particle adhesion, replaced that of laminar flow. Ferrihydrite, in both crumb and paste form, had lower sensitivity than allophane. Pastes of both materials became less sensitive on resting, but their initial strength had not recovered after 2 months. These properties of sensitivity to shear and lack of strength in their disturbed state have implications in soil engineering for trafficability and stability of earthworks.