Effects of pore water and pore air pressure on the slope failure mechanisms due to rainfall in centrifuge investigation

Abstract Background The slope failures triggered by heavy rainfall are challenging to predict. However, their severe impact and potential for extensive damage emphasize the urgent need for effective strategies to mitigate these risks. This study aimed to investigate the effects of pore water pressures (PWPs) and pore air pressures (PAPs) on slope failure during heavy rainfall under centrifuge condition. Three centrifuge rainfall model tests were conducted with varying rainfall intensities ( I ) and relative densities ( D r ). In this experiment, the slope model was subjected to a centrifugal acceleration of 30 G. Results In all cases, slope failures started at the slope toes when the PWPs became positive and phreatic surfaces appeared on the slope surface. In the Toyoura sand case, the PAPs increased slightly at the slope crest, hardly changed at the slope toe, and oscillated at the slope shoulder due to rainwater infiltration. The entire slope failure was observed in this case. In the Silica sand cases, only localized slope toe failures were observed. The PAPs slightly oscillated with small changes in the silica sand case with a relative density of D r = 50%. However, in the case with a D r = 25%, the PAPs at the slope toe significantly increased compared to the other parts, and the oscillations were comparatively large. After the first failure occurred at the slope toe in the silica sand case with a D r = 25%, cracks and slip lines appeared at the slope crest. Conclusions The three cases illustrated the complex relationship between soil properties, rainfall intensity, and the dynamics of pore water and pore air pressure on slope stability during heavy rainfall. From the behaviour of PAPs and PWPs in all cases, it was found that the transition from air to water occurred smoothly, and the isolation of air was not observed even during heavy rainfall. Changes in PAPs were far smaller than those in PWPs, indicating a smaller impact on slope stability than PWPs.