This paper introduces a computer program PERISI stands for Preliminary Evaluation of Rainfall-Induced Slope Instability. The program was developed by utilizing the concept of extreme rainfall and unique relationships between rainfall characteristics, hydraulic conductivity, suction, and water content of unsaturated soil to evaluate the minimum suction distribution in soil. The outcome of the program is the factor of safety of soil slope in the critical condition as compared to normal condition

Gofar, Nurly

Lee, Lee Min

English

Universiti Teknologi Malaysia Institutional Repository

Malaysian Journal of Civil Engineering 21(2) : 235-240 (2009)  SHORT NOTES   PERISI: A COMPUTER PROGRAM FOR PRELIMINARY EVALUATION OFRAINFALL-INDUCED SLOPE INSTABILITY Nurly Gofar1, Lee Min Lee2  1Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Malaysia 2Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Malaysia.  Corresponding Author: nurly@utm.my   Abstract: This paper introduces a computer program PERISI stands for Preliminary Evaluation of Rainfall-Induced Slope Instability. The program was developed by utilizing the concept of extreme rainfall and unique relationships between rainfall characteristics, hydraulic conductivity, suction, and water content of unsaturated soil to evaluate the minimum suction distribution in soil. The outcome of the program is the factor of safety of soil slope in the critical condition as compared to normal condition.    Keywords: Hydraulic conductivity, Volumetric water content, Pore water pressure; Slope stability evaluation   1.0 Introduction  The mass instability of soil slopes continue to affect large Malaysian populations each year. The occurrences of the slope failures can be attributed to two major factors: intense and frequent downpours, and natural characteristics of the soil mass. Slope stability analysis based on the assumption of saturated soil behaviour is commonly applied in current practice. The approach, however, is exposed to several weaknesses. For an example, the effect of rainfall infiltration on the soil water content, hence on the stability of slope could not be assessed since the soil is already in saturated condition. The increasing acceptance of unsaturated soil mechanics has highlighted the need to correlate the slope failure with rainfall in order to understand the mechanisms of failure. The unsaturated soil mechanics could be used to explain the mechanisms of rainfall-induced slope failure (Li et al., 2005). The potential mechanisms of rainfall-induced slope failure include the development of perched water table, loss in soil suction, and rise of groundwater table. Previous Malaysian Journal of Civil Engineering 21(2) : 235-240 (2009) 236   researches found that more than 80% of the rainfall-induced slope failures in tropical regions were due to the loss in soil suction (Au, 1998).  The application of unsaturated soil mechanics, however, is still not a common practice owing to a number of uncertainties associated with the stability analysis including the intensity and duration of rainfall, the minimum soil suction in the slope, and the depth of wetting front. The research gaps have initiated the need for development of simple model which is able to integrate the unsaturated soil mechanics to slope stability analysis (Gofar and Lee, 2008a). This paper is aimed to introduce a computer program (PERISI) for preliminary evaluation of rainfall-induced slope instability. The program was developed based on the concept of extreme rainfall and unsaturated soil mechanics.    2.0 Development of PERISI  Gofar and Lee (2008a) have demonstrated a unique relationship between extreme rainfall condition and soil characteristics. The extreme rainfall, commonly referred as Intensity-Duration-Frequency (IDF) curve, indicates the highest possible rainfall intensity to occur at a location for a given duration and return period. Long duration (i.e. 30 days) IDF curve is required for slope stability evaluation to cater for the effect of antecedent rainfall (Lam and Leung, 1995; Rahardjo et.al., 2001). The unsaturated soil characteristics are presented by soil water characteristic curve (SWCC), and hydraulic conductivity function.  Through such correlations, the minimum suction and water content in soil under extreme rainfall of any duration can be predicted. The minimum suction is an important input parameter in the computation of unsaturated soil shear strength, while the water content is essential for the estimation of wetting front depth based on the water balance theory. Besides, the slope geometry and shear strength properties of soil were also taken into account in the analysis since these parameters are the essential contributing factors in any slope stability analysis.   Figure 1 shows the IDF curve developed by Gofar and Lee (2008a) for slope stability analysis in Peninsular Malaysia, while Figures 2 and 3 represent the SWCC and hydraulic conductivity functions of typical soil. The unique relationship between extreme rainfall condition and soil characteristics as adopted by PERISI model is shown in Figure 4. Figure 5 shows a view of the PERISI interface. Upon assigning the necessary input data, the program could be used to compute the suction envelope that represents the minimum suction distribution in the soil as the result of several extreme rainfall conditions. The extreme rainfall conditions include various durations of antecedent rainfall, combination of antecedent rainfall and major rainfall, as well as their suction Malaysian Journal of Civil Engineering 21(2) : 235-240 (2009) 237   redistribution. Based on the suction envelope, the factor of safety of the slope is computed using the shear strength of unsaturated soil, as illustrated in Figure 6. The verification of the model were presented in Gofar and Lee (2008b), Gofar et al. (2009) and Lee et al. (2009)  1.00E-071.00E-061.00E-051.00E-040 5 10 15 20 25 30Rainfall Intensity (m/s)Duration (Day)Kuala LumpurJohor BahruPenangKuantanKota Bharu Figure 1: Ten year return period of extreme rainfall in Peninsular Malaysia                Figure 2:Typical SWCC curve for soil       Figure 3:Typical hydraulic conductivity         curve for soil  Malaysian Journal of Civil Engineering 21(2) : 235-240 (2009) 238     Figure 4: Unique relationships between IDF curve, hydraulic conductivity function, and SWCC adopted by PERISI     Figure 5:  Interface of PERISI: Input and Output  Malaysian Journal of Civil Engineering 21(2) : 235-240 (2009) 239     Figure 6: Relative Factor of Safety between extreme rainfall condition and normal condition as compueter by PERISI   3.0 Conclusions  A computer program (PERISI) has been developed based on a unique relationship between extreme rainfall condition and soil characteristics. The  software was not designed to predict the ultimate factor of safety of the slope, but to compute the relative factor of safety between extreme rainfall and normal conditions. This feature is important to evaluate the stability of the slope in response to the rainfall infiltration. PERISI offers several advantages as follows: i. The rainfall characteristics of specified location, and unsaturated soil properties are integrated into slope stability analysis, hence is capable to produce a more comprehensive analysis results. ii. The suction envelope that represents the worst suction distribution in the soil slope for the extreme rainfall of specified return period can be predicted. This suction envelope is useful to promote the application of unsaturated soil mechanics in slope stability analysis. iii. The effect of extreme rainfall infiltration on the slope stability can be assessed through the comparisons of factor of safety between normal and extreme rainfall conditions. iv. The potential depth of failure plane can be predicted. v. The analysis results have been verified through numerical simulations, laboratory tests, and field monitoring. Malaysian Journal of Civil Engineering 21(2) : 235-240 (2009) 240   Acknowledgement:  PERISI- A Tool for Preliminary Evaluation of Rainfall Induced Instability is a Copyrighted Computer Program under Universiti Teknologi Malaysia in 2008.   References  Au, S.W.C. (1998). Rain-Induced Slope Instability in Hong Kong. Engineering Geology. 51: 1–36. Brisson, P., Garga, V.K. and Vanapalli, S.K. (2002) Determination of Unsaturated Flow Characteristics of Nickel Mine Tailings. 55th Canadian Geotechnical Conference, Niagara, Canada, October 2002. Fredlund, D. G. and Xing, A. (1994). Equations for the Soil-Water Characteristic Curve. Canadian Geotechical Journal. 31: 521–532. Fredlund, D. G., Morgenstern, N. R., and Widger, R. A. (1978). The Shear Strength of Unsaturated Soil. Canadian Geotechical Journal. 15: 313–321. Fredlund, D.G. and Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils. New York: John Wiley & Sons, Inc. Fredlund, D.G., Xing, A. and Huang, S. (1994). Predicting the Permeability Function for Unsaturated Soils Using the Soil-Water Character Curve. Canadian Geotechnical Journal. 31(3): 533-546 Gofar, N. and Lee, M.L. (2008). Extreme Rainfall Characteristics for Surface Slope Stability in the Malaysian Peninsular. Journal of Assessment and Management of Risk for Engineered Systems and Geohazards (Georisk), Taylor and Francis. 2(2): 65-78. Gofar, N. and Lee, M.L. (2008). Integration of Extreme Rainfall in the Evaluation of Slope Stability.  Proceedings of the 12th  ISGE Annual Science Meeting, Bandung, Indonesia:p.V1-V6 Gofar, N., Lee, M.L. and Kassim A. (2009). Extreme Rainfall Analysis for Slope Stability Evaluation. in book: Prediction and Simulation Methods for Geohazard Mitigation (Oka, Murakami and Kimoto, eds) CRC Press  Lam, C.C. and Leung, Y.K. (1995) Extreme Rainfall Statistics and Design Rainstorm Profiles at Selected Locations in Hong Kong. Technical Note No. 86. Royal Observatory. Hong Kong. Lee M.L. Gofar, N. and Raharjo, H. A Simple Model for Preliminary Evaluation of Rainfall-Induced Slope Instability. Engineering Geology  Elsevier 108(3-4)272-282 Li, A.G., Tham, L.G., Yue, G.Q., Lee, C.F. and Law, K.T. (2005). Comparison of Field and Laboratory Soil-Water Characteristic Curves. Journal of Geotechnical and Geoenvironmental Engineering, ASCE. 131(9): 1176-1180.  Rahardjo, H., Li X. W., Toll D. G. and Leong E. C. (2001). The Effect of Antecedent Rainfall on Slope Stability. Journal of Geotechnical and Geological Engineering. Netherlands. 19: 371-399.  

PERISI: a computer program for preliminary evaluation of rainfall-induced slope instability

http://eprints.utm.my/21044/1/NurlyGofar2009_PerisiaComputerProgramforPreliminaryEvaluation.pdf

PERISI: a computer program for preliminary evaluation of rainfall-induced slope instability

Abstract

Similar works

Full text

Available Versions

Universiti Teknologi Malaysia Institutional Repository