Infiltration behaviour of structurally unstable soils under ponded and non-ponded conditions

Abstract

The infiltration behaviour of unstable soils under ponded and non-ponded conditions are compared with a stable soil and a sand. When an unstable soil is ponded and the surface layers are at a moisture potential of zero or near zero, the soil structure collapses. As a result, the infiltration rate and the rate of movement of the wetting front are small compared with those of stable porous materials. The moisture content and pore water pressure changes with time indicate three main zones during infiltration into unstable soils, viz: (i) a saturated zone where the original aggregate structure has been destroyed and hydraulic conductivity is low; (ii) a transmission zone where moisture content is not quite constant and where both moisture potential gradient and gravitational potential contribute to the movement of water; (iii) a wetting front. These zones have traditionally been identified with infiltration into stable soils, but in stable soils the saturated zone retains its original structure to a large extent, and the transmission zone is saturated (or is near saturation after allowing for air entrapment), and transmission of water in this zone is by gravitational potential alone. By contrast, the infiltration behaviour of unstable soils with restricted supply rates, which do not develop surface ponding, is similar to that of structurally stable materials under the same restricted supply rate, in that the structure is not destroyed. If the unstable soils were wetted slowly, the aggregates retained their identity when subsequently ponded. This system provided a maximum value of hydraulic conductivity to compare with the restricted supply rates. If the restricted supply rates for the most unstable soil were larger then 1/20 of maximum hydraulic conductivity ponding still occurred, but the incidence of ponding was delayed the smaller the restricted supply rate. In addition, structural deterioration was reduced, and in some circumstances this resulted in greater water storage and greater depth of wetting than with immediate ponding.