TENSILE-STRENGTH OF DRY, REMOLDED SOILS AS AFFECTED BY PROPERTIES OF THE CLAY FRACTION

Changes in tensile strength with clay type, clay particle size, and amounts of spontaneously and mechanically dispersible clays were assessed for dry, remoulded samples of eight Australian Vertisols and Alfisols varying in clay mineralogy. The effects of different clay content and particle size on tensile strength were also measured. The average particle sizes of whole clay, and of spontaneously and mechanically dispersible clays were also determined. Tensile strength of soils was influenced by the type and amount of clay present, clay particle size, and amount of dispersible clay. Soil strength increased with increasing clay content. When the clay content increased beyond 20%, there was a dramatic increase in soil strength indicating the possible role of particle arrangement. Correlations between different clay types, clay particle sizes, cation exchange capacity and tensile strength clearly illustrated that the sensitivity of tensile strength to these factors was greatest in soils dominated by smectite, followed by illitic and then kaolinitic soils. Tensile strength of soils was positively and significantly correlated with both spontaneously and mechanically dispersible clays, A significant negative correlation was obtained between clay particle size and tensile strength for illitic soils, whereas the same linear relationship was not significant for smectitic soils. However, a significant negative exponential regression was obtained between the cube of clay particle size and tensile strength when the results of all soil samples were pooled together. Transmission electron micrographs (TEM) showed a wider clay particle size distribution in smectites than in illites, Soils with the highest amounts of fine clay and the widest clay particle size distribution had higher tensile strengths. The size of spontaneously dispersed clay particles was smaller than that of mechanically dispersed clay. No difference in clay particle size was found between spontaneously and mechanically dispersible clays from mixtures of a fine clay with a basic sand and silt matrix, Regression, collinearity diagnostics and principal component analyses were used to analyse the data. A high correlation was found between the cation exchange capacity (CEC) of clays and soil strength. The principal component analysis indicated that spontaneously dispersible clay, percentage of randomly interstratified minerals (RIM) and CEC were important factors in predicting the strength of remoulded soils.