This paper presents a study of the hydraulic response of an infinite unsaturated slope exposed to a perturbation of the ordinary seasonal climatic cycle. The ground flow is modelled via a simplified one-dimensional finite difference scheme by decomposing the two-dimensional slope seepage into antisymmetric and symmetric parts. The numerical scheme incorporates two distinct hysteretic and non-hysteretic soil water retention laws, whose parameters have been selected after a preliminary sensitivity analysis. Results indicate that, in the hysteretic case, the memory of the perturbation takes a long time to fade, and the ordinary soil saturation cycle is only restored after several years of normal weather. Instead, in the non-hysteretic case, the recovery of the ordinary saturation regime is almost immediate after the perturbation. In contrast with the markedly different predictions of degree of saturation, both hysteretic and non-hysteretic slope models predict virtually identical evolutions of negative pore water pressures, with an almost immediate restoration of the ordinary cycle after the perturbation.

Analytical expressions are presented for the determination of the pore air and pore water pressure parameters in unsaturated soils under undrained conditions. The kinematic constraints governing the volumetric alterations of the soil skeleton and fluid phases, in conjunction with the effective stress principle, are utilised to derive the pore pressure parameters. The effect of hydraulic hysteresis on the effective stress parameter and the soil water characteristic curve is considered in the solutions. The expressions derived are validated against the experimental data available in the existing literature. It is demonstrated that: - with an increase in the degree of saturation of the soil, both pore pressure parameters increase and approach unity; - the change in the pore water pressure in an unsaturated soil under undrained compression is greater than the change in the pore air pressure; - the pore pressure parameters are strongly dependent on the compressibility of the soil skeleton; - the hydraulic hysteresis has a substantial influence on the pore water pressure parameter; - the pore air pressure parameter is slightly affected by the hydraulic hysteresis and it is mostly influenced by the compressibility of the air phase; - an increase in the degree of saturation leads to a reduction in the instantaneous settlement; and - the hydraulic state has a significant impact on the instantaneous settlement of unsaturated soil.

The ink-bottle effect, resulting from the non-uniformity of pore structure, plays a significant role in the hydraulic behaviour of soils. In this paper, a series of laboratory tests including standard mercury intrusion porosimetry (MIP), intrusion-extrusion cyclic MIP (IEC-MIP), and scanning electron microscopy (SEM) were conducted on two soils (a Nanyang expansive clay and a Kaolin mixture) to examine the evolution pattern of ink-bottle effect in soils with different initial void ratios. To evaluate the impact of such evolution on the soil-water retention behaviour, the axis translation and vapour equilibrium techniques were utilised to measure the water retention curve of the tested soils (SWRC). A key part of the analysis involved a separation between pore bodies (or inkbottle pores) and pore throats to facilitate a more thorough examination of the pore structure. The test results indicated that the reduction in soil volume during compression is mainly attributed to the reduction in pore body size, whereas the pore throat size remains relatively constant. Therefore, the ink-bottle effect would weaken with a decreasing initial void ratio, which will reduce the hysteresis potential of soils, manifesting as a contraction of the SWRC hysteresis loop. Additionally, under a fixed void ratio but with decreasing pore size, the ink-bottle effect will also weaken, which will reduce the distance between the drying and wetting curves with increasing matric suction. Finally, the mechanisms regarding how the evolution of ink-bottle effect during soil deformation influences water retention hysteresis were addressed.