Development of evaluation system for groundwater level in relation to slope stability forecasting
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Date
2016-04-01
Authors
Ng Soon Min
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Abstract
The aim of this study focused on developing a new evaluation system of groundwater level for slope stability forecasting by conducting a case study at a cut slope in Precinct 9, Putrajaya, Malaysia. This system involved the development of subsurface conceptual model, development of groundwater table fluctuation model, stability assessment and investigation of unsaturated soil behaviour which incorporates the response of pore air pressure. Subsurface characterization of the study area was carried out by using the integration of borehole drilling, electrical resistivity survey and seismic refraction survey. Three dimensional (3D) models of stratigraphy, electrical resistivity and seismic velocity were proven to be effective in identifying the potential failure zone that coincides with the past slope failure zone. The potential failure zone that consist of silt soil was identified to have high water content with electrical resistivity ranges from 10Ωm to 300Ωm and intermediate P-waves velocity (vp) of 500m/s to 1000m/s. Subsequently, the multi tank model and numerical analysis was developed to determine the groundwater table fluctuations and pore water pressure distributions of the slope. The multi tank model was able to produce a root mean square error (RMSE) of 0.156 and 0.169 for the calibration model using year 2011 data and prediction model using year 2012 data respectively. The factor of safety (FOS) of the slope throughout the year 2012 varies according to the fluctuations of groundwater table analyzed by using both limit equilibrium method (LEM) and finite element method (FEM). Furthermore, the FOS for the slope model assigned with multi tank model groundwater table produced approximately 1% error compared to the slope model assigned with the observed groundwater table. Therefore, this outcome proven that the developed evaluation system can be utilized as an assessment tool for slope stability forecasting. In addition, the component of pore air pressure which is often neglected in unsaturated soil behaviour during rainfall infiltration was also investigated through one dimensional (1D) soil column numerical simulation and physical modelling. It was found that the development of pore air pressure can be influenced by soil type, rainfall intensity, and initial water content. Both results of numerical simulation and laboratory experiment show that the soil sample from site with rainfall intensity of 26751.6mm/hr and initial water content of 21% produced the maximum pore air pressure. Finer particle size with higher rainfall intensity and initial water content lead to the development of confined pore air as the rainwater cannot replace the pore air smoothly subsequently resulting in pore air pressure build up.