Field measurements and modelling of climate-driven hydraulic flux, water content, and suction in a loess slope

Rainfall-triggered shallow slope failures are common in New Zealand loess. While often unsaturated, rainfall infiltration reduces suction in the loess as the water content increases, resulting in reduction in shear strength and increased potential for slope instability. Here the hydraulic response of an in situ loess slope to natural rainfall and evapotranspiration is examined using long-term field instrumentation. Data from a 19-month monitoring period show that the loess’ hydraulic behaviour responds to change in seasonal climatic conditions, highlighting the relationship between moisture content and suction with evapotranspiration and infiltration. To simulate the field response, a one-dimensional steady infiltration/evapotranspiration model has been developed, exploiting the unique power laws which interrelate water content, suction and hydraulic conductivity which arise from a loess’s fractal particle and pore size distributions. Hydraulic hysteresis is also accounted for. Closed form expressions are derived for the subsurface suction profile, from which profiles of moisture content, hydraulic conductivity, and the suction’s contribution to strength may be obtained. Simulated subsurface water contents are in good agreement with field observations. Furthermore, fluxes across the loess surface used to obtain the modelled quantities are in general agreement with local climatic conditions and their variations during the monitoring period. Modelled and measured data are strongly correlated at shallow depths, having a Pearson correlation coefficient of 0.53.