Correlations Between Plasticity Angle and Engineering Properties of Volcanic Ash Soils

Abstract

ABSTRACT The presence of allophane and halloysite minerals impart special properties to volcanic ash soils including the absence of any correlation between the engineering properties and index properties for these soils. The plasticity angle has been suggested as a measure of the allophane content of volcanic ash soils by soil scientists and (plasticity angle) is defined by the angle, relative to the A-line intercept, where the Atterberg limits of the volcanic ash soils occur on the Casagrande plasticity chart. This technical note examines the correlations between plasticity angle and engineering properties of volcanic ash soils. The Atterberg limits and engineering property data for volcanic ash soils were taken from literature. The results of the study suggest that the plasticity angle is a good indicator of the shear strength (represented by φ'peak and φ'residual values) and compressibility (represented by Cc/(1+eo)) properties of allophanic and halloysitic soils from quite different geographical regions. The strong relationships between plasticity angle and engineering properties is attributed to the dependence of the plasticity angle as well as that of the engineering properties on the allophane content of the volcanic ash soils.