Probabilistic back analysis of a cut slope: A case study of slope failure event in precinct 9, Putrajaya, Malaysia

This paper presents a case study of slope failure in Precinct 9, Putrajaya, Malaysia in which back analysis was performed to investigate the main causes of slope failure. The catastrophic slope failure incident has caused 23 cars to be buried and evacuation of 1,000 people from their houses. Recorded data showed that it had been raining heavily with a total cumulative rainfall of 210 mm prior to the slope failure occurrence. Site investigation that comprised of soil boring and geophysical electrical resistivity survey was initially carried out to obtain information such as subsurface characteristics and groundwater condition. Relevant information such as shear strength parameters, soil profile and groundwater level will be then utilized for the modeling and analysis. Sensitivity analysis suggested that the slope failure is most likely to be caused by the fluctuations of groundwater level that increased the pore water pressure thereby reducing the shear strength. Due to the high degree of uncertainty in the slope stability parameters to be analyzed, probabilistic method was implemented in this study. Two approaches namely the limit equilibrium and shear strength reduction finite element methods were used to perform the stability analysis and the results from both analyses were compared. For finite element analysis, the principle of Point Estimate Method (PEM) was adopted where solutions were computed at various estimation points and then combined with proper weighting in order to get an approximation of the distribution of the output variables. Results from the back analysis may then be utilized for remedial works and redesign of the failed slope.