INCIPIENT FAILURE OF SOIL AGGREGATES

Abstract

SummaryA comparison of the rate of wetting of aggregates taken from a virgin and continuously cultivated Red‐brown Earth has been made. The aggregates were placed on a glass sinter which was connected to a horizontal capillary. The rate of water uptake was obtained from the rate of meniscus travel as water was adsorbed from the sinter by the aggregates.When air‐dry (P/P0≊ 0·50) aggregates were wetted at 2 and 10 cm. water suction the cultivated aggregates wetted more rapidly than the virgin aggregates. At a suction of 30 cm. both groups of aggregates exhibited almost identical rates of wetting. The difference in the rate of wetting could not be attributed to differences in pore structure since both groups of aggregates had almost identical total porosity and clay content and furthermore showed extremely similar rates of wetting when wetted at a low suction with a non‐polar liquid. When aggregates from a virgin and cultivated soil were wetted slowly the pF‐water content relationship was very similar; it was therefore concluded that organic matter did not give rise to a finite contact angle for the water advancing into the virgin aggregates.Because the cultivated aggregates when wetted at 30 cm. and then at 2 cm. ion did suctnot slake when placed in water, whereas cultivated aggregates wetted directly to 2 cm. suction from the air‐dry condition slaked when placed in water, the phenomenon is described as incipient failure. When the cultivated aggregates are wetted at 2 cm. suction the work done by the rapid wetting causes planes of failure to be set up and the soil moves from an over‐consolidated condition to that represented by the n o d or virgin consolidation curve (Croney and Coleman, 1954). It is proposed that the organic matter in the virgin aggregates strengthens the pores against failure and so this material retains its initial pore structure. Mechanical strength measurements on virgin and cultivated aggregates support this hypothesis.