Effect of hysteresis on the stability of residual soil slope

Soil-water characteristic curve (SWCC) is an essential parameter in unsaturated soil mechanics.Matric suction in unsaturated residual soils changes with varying climatic conditions associated with cyclic drying-wetting conditions that result in hysteresis in the SWCC.The soil mechanical behaviour under wetting is crucial since numerous rainfall-induced slope failures occur during the wetting process.However,many slope stability analyses were carried out using drying SWCC.Consequently,the factor of safety (FoS) calculation may not represent the actual field condition.This paper presents the effect of hysteresis in SWCC on the stability of unsaturated residual soil slopes from Bukit Timah Granite in Singapore.The study focused on the analyses considering the differences in pore-water pressure and water content variations under drying and wetting conditions by performing numerical seepage and stability analyses.Each analysis was carried out on a slope subjected to dry and rainy periods under three different conditions:i) using only drying SWCC;ii) using only wetting SWCC;iii) using combined drying and wetting SWCCs.The results indicated that the FoS variations obtained from the numerical analyses incorporating combined SWCC matched more closely those obtained by incorporating only wetting SWCC than those obtained by incorporating only drying SWCC,regardless of the wetting or drying processes that the soil experienced.Moreover,the numerical analyses under high rainfall intensity by incorporating only wetting SWCC gave a more conservative FoS as compared to those obtained by incorporating combined SWCC.Numerical analyses incorporating only drying SWCC gave the most conservative FoS regardless of rainfall intensity.

Effect of variations in rainfall intensity on slope stability in Singapore

Numerous scientific evidence has given credence to the true existence and deleterious impacts of climate change.One aspect of climate change is the variations in rainfall patterns,which affect the flux boundary condition across ground surface.A possible disastrous consequence of this change is the occurrence of rainfall-induced slope failures.This paper aims to investigate the variations in rainfall patterns in Singapore and its effect on slope stability.Singapore's historical rainfall data from Seletar and Paya Lebar weather stations for the period of 1985–2009 were obtained and analysed by duration using linear regression.A general increasing trend was observed in both weather stations,with a possible shift to longer duration rainfall events,despite being statistically insignificant according to the Mann-Kendall test.Using the derived trends,projected rainfall intensities in 2050 and 2100 were used in the seepage and slope stability analyses performed on a typical residual soil slope in Singapore.A significant reduction in factor of safety was observed in the next 50 years,with only a marginal decrease in factor of safety in the subsequent 50 years.This indicates a possible detrimental effect of variations in rainfall patterns on slope stability in Singapore,especially in the next 50 years.The statistical analyses on rainfall data from Seletar and Paya Lebar weather stations for the period of 1985–2009 indicated that rainfall intensity tend to increase over the years,with a possible shift to longer duration rainfall events in the future.The stability analyses showed a significant decrease in factor of safety from 2003 to 2050 due to increase in rainfall intensity,suggesting that a climate change might have existed beyond 2009 with possibly detrimental effects to slope stability.