Eng Choon Leong

Controlling parameters for rainfall-induced landslides

Slope instability due to rainfall is a common geotechnical problem in tropical and subtropical areas. The numerical analysis of this problem requires the definition of various hydrological characteristics of the soil in addition to the climatic information. This work attempts to identify the influence of some of those parameters on the stability of an unsaturated soil slope during rainfall. The results of the analysis show that the ratio between the saturated coefficient of permeability with respect to water and the rainfall patterns can significantly influence the seepage pattern within an unsaturated soil slope.

INFILTRATION EFFECTS ON STABILITY OF A RESIDUAL SOIL SLOPE

Abstract Many slope stability studies have indicated that the infiltration of rainwater into a slope decreases the stability of the slope. However, the difficulty of quantifying the effect of rainwater infiltration on slope stability still exists. It would be advantageous to know what percentage of rainfall enters a slope as infiltration and how much this infiltration decreases the stability of a slope. Numerical models were used to study how infiltration into a slope varied with respect to rainfall intensity and how this infiltration affected the stability of the slope. A numerical study revealed that the amount of infiltration was highest at the crest of a slope. A case study revealed that it was difficult to quantify the amount of infiltration occurring in a slope with the numerical model. The numerical model was, however, able to illustrate the effect of infiltration on slope stability through the combined use of seepage and slope stability analyses.

The effect of antecedent rainfall on slope stability

AbstractA case study is presented in order to identify the effect of antecedent rainfall on slope stability for Singapore. A storm in February 1995 (during which 95 mm of rain fell in 2

Factors affecting the filter paper method for total and matric suction measurements

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Effects of Groundwater Table Position and Soil Properties on Stability of Slope during Rainfall

h) caused more than twenty shallow landslides on the Nanyang Technological University Campus. Details of the location, size and morphology of the landslides are presented. The antecedent rainfall during the five days preceding the event was significant in causing these landslides since other rainfall events of similar magnitude (but with less antecedent rainfall) did not cause landslides. To further understand the effect of antecedent rainfall, numerical modelling of one of the slope failures is presented. The changes in pore-water pressure due to different rainfall patterns were simulated and these were used to calculate the changes in factor of safety of the slope. The results demonstrate that antecedent rainfall does play an important role in slope stability.

Relationship between porosimetry measurement and soil—water characteristic curve for an unsaturated residual soil

The use of bender element to measure shear wave velocity is popular due to its simplicity. However, there are still some uncertainties in the interpretation of the bender element test. In this paper, the use of bender elements in the determination of shear wave velocities of dry, unsaturated, and saturated soil specimens is examined with respect to the characteristics of the bender elements, waveform, magnitude, and frequency of the applied voltage to the transmitter bender element and method of travel time determination. The interpretation of the bender element test can be improved if two performance criteria are adopted: (1) a signal-to-noise ratio of at least 4 dB for the receiver signal, and (2) a wave path length to wavelength ratio of at least 3.33. Procedures to estimate the strain level associated with the shear wave velocity measurement using bender element are also described.

Instrumentation of an Unsaturated Residual Soil Slope

The filter paper method is used as an indirect means of measuring soil suctions. The advantages of the method include its simplicity, its low cost, and its ability to measure a wide range of suctions. The filter paper method has also been used in the field to measure soil suctions. However, the simplicity of the filter paper method has led to an inadequate understanding of the method and, therefore, improper usage. Recent findings have highlighted the need for more precautions in the use of the filter paper method for suction measurements. This paper presents a literature review of the factors affecting the accuracy of the filter paper method and identifies gaps in the current knowledge of the method. An experimental study was also conducted to clarify some of the factors affecting the response of the two most commonly used filter papers for suction measurements. Whatman No. 42 and Schleicher and Schuell No. 589. Calibration curves of the filter papers show that the water content-suction relationships are different for total and matric suctions. The total suction calibration curve is not very sensitive to applied suctions less than 1000 kPa. The calibration curves obtained for total suction are different from those recommended in ASTM D 5298-94, suggesting that the ASTM D 5298-94 equations should not be used for total suction. Instead of existing bilinear equations, new equations are proposed to describe the total and matric suction calibration curves. All the equations were evaluated using the filter paper method to measure the suction of soil specimens of known matric suctions.

Effectiveness of horizontal drains for slope stability

Rainfall-induced slope failure occurs in many parts of the world, especially in the tropics. Many rainfall-induced slope failures have been attributed to antecedent rainfalls. Although it has been identified as a cause of rainfall-induced slope failure, the pattern or distribution of the antecedent rainfall has not received adequate attention. In this study, parametric studies were performed by using three typical rainfall patterns, identified by analysis of available rainfall data for Singapore and two different soil types to represent high- and low-conductivity residual soils of Singapore. Antecedent rainfall patterns were applied on soil slopes and a transient seepage analysis was conducted. The computed pore-water pressures were used in stability analyses to calculate the safety factor of the slope. Results indicated that antecedent rainfall affected the stability of both high-conductivity (HC) and low-conductivity (LC) soil slopes. However, the stability of the LC soil slope was more significantly af...