Rainfall induced transient pressure wave mechanisms and pore water pressure dynamics in tailings

Tailings Storage Facilities are some of the largest engineered structures responsible for the containment of mining waste, yet some physical/hydrological processes causing slope instability are still poorly understood. Previous studies conducted on the causes of failure of TSFs indicate rainfall-induced slope instability as the main trigger. However, the generation of certain physical/hydrological processes and the behaviour of soil hydraulic properties in the vadose zone when exposed to high intensity rainfall events, have only been considered on natural hillslopes.
The purpose of this study is to investigate one such process resulting from high rainfall intensities on partially saturated conditions. In particular, it is hypothesised in this work that the wetting front advance from high rainfall intensities, anticipated under the force of climate change, may cause transient air pressure waves to change pore water suction and elevate the phreatic surface, subsequently impacting on the effective stress. The mechanisms that lead to the mobilisation of pre-event water through transient air pressure waves are known as Groundwater Ridging and the Lisse Effect. In this study, it was hypothesized that these phenomena contribute significantly to slope instability. In order to quantify the processes and predict these phenomena and their resultant impact on stability, specific experiments were set up. Laboratory experimentation included a large leak-proof column (600 mm ID x 3,0 m tall) filled up to 2,65 m with tailings at in-situ dry bulk density of 1 675 kg/m³. The PVC column was instrumented with seven data ports, vertically spaced down the column. Each port consisted of a time domain reflectometry probe to measure volumetric water content, a mini tensiometer to sense pore water suction and a pore air pressure probe. An external manometer tube measured the phreatic surface created near the base. The experimental setup allowed for the application of artificial rainfall at different intensities and controlled boundary conditions, during automatic logging of the hydraulic state variables, recorded at twenty-second intervals. Recorded observations were used to determine phreatic surface dynamics, the soil moisture profile and general pore water/air pressure responses. The experiment was specifically designed to differentiate between pore water migration due saturated/unsaturated Darcy flow, from pore water responses to pneumatic pressure transfers. This study also examined the anticipated impact of climate change on rainfall parameters and quantified geotechnical characteristics of the tailings porous medium under variably saturated states. The results demonstrated that after the application of water to the soil surface, pore air pressure ahead of the wetting front increased, causing higher water content levels and a decrease in pore water suction. This led to the conclusion that high intensity rainfall events generate transient air pressure waves in tailings porous medium, thereby inducing the transmission of pore air pressure head to a potential failure plane. Since air pressure in the unsaturated zone affects pore water pressure, water content and subsequently shear strength, it also contributes indirectly to slope instability.