SHEARING CHARACTERISTICS OF AN ALLOPHANE SOIL

Proto-imogolite allophane from a buried volcanic ash layer has been subjected to shearing forces in the gap between a cone-shaped rotor and the stator plate of a programmable viscometer. Its strength, as measured by shear stresses developedunder increasing rates of shear, decreased rapidly during shearing. Plots of shear stress against shear rate showed hysteresis between rotor acceleration and subsequent deceleration. Resting a sheared allophane in the rotor gap for up to 1 week at constant moisture, produced only a slight recovery of strength. Scanning electron micrograph of freeze-dried allophane revealed a cellular mesh structure that coalesced at surfaces. Shearing action changed this structure to globular clusters of adhering particles. Drying allophane from its natural moisture content of 250% to 100% H2O (w/w) enhanced strength over a range of shear rates but further drying induced a substantial loss of strength at higher rates of shear. Drying to 60% H2O (w/w) coalesced the mesh structure to form adhering globular particles but at 0% H2O these allophane particles acquired sharp edges. Rewetting after drying to 60% H2O (w/w) enabled shear strength to recover at low rates of shear but after drying to 0% H2O the rewet allophane had little strength.