Robin Chowdhury

Geotechnical Slope Analysis

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Uncertainties in rainfall-induced landslide hazard

The paper addresses the main uncertainties associated with the occurrence of rainfall induced landslides. Spatial variability of site conditions, local geology and rainfall contributes significantly to the uncertainty of landslide hazard. Urban landslide problems require the management of slopes of marginal stability. Assessment of hazard, vulnerability and risk require the use of observational approaches, the analysis of rainfall data and the relationship between rainfall magnitudes on the one hand and slope movement on the other. Details of research carried out in the Illawarra area of New South Wales, Australia are provided. Reference is made to the comprehensive database enabling the determination of landslide frequencies. The concept of antecedent rainfall percentage exceedence time (ARPET) is explained. The use of inferred threshold rainfall magnitudes for real-time prediction and warning is explained. Uncertainties concerned with this approach are explored with particular reference to rainfall distributions in the study area.

Hazard and risk assessment of rainfall - induced landsliding along a railway line

The occurrence of landslides along a transportation route such as a railway line can have severe and even catastrophic consequences. The assessment and management of hazard and risk is, therefore, of paramount importance. This paper introduces an approach for assessing the hazard and risk of rainfall-induced landsliding along a 30 km section of railway track. Careful analysis is made of rainfall data for previous storms, which have led to widespread landsliding. The recurrence interval of a typical landslide triggering rainstorm is estimated. From this the annual probability of widespread landslide occurrence is then calculated. This value combined with information on the frequency of train operations enables the estimation of the annual risk of human casualty. This estimate is then compared with proposed tolerable or acceptable levels. Such a systematic quantitative procedure can facilitate efficient risk management. Moreover, the outcomes of the assessments proposed in this paper can be combined with information and analyses about the relative landslide susceptibility of different section of track based on knowledge of local geology, topography and other factors.

Quantitative landslide hazard and risk assessment: a case study

This paper is concerned with landslide hazard and risk assessment along a railway line south of Sydney in the State of New South Wales, Australia. Attention is focused on a quantitative assessment at a specific site where landslide movements have been triggered by significant rainfall. Although small in magnitude such movements are sufficient to cause train derailment and hence human casualty. Utilizing all available data, the relationship of slope instability to rainfall is first established. Slope stability analyses are carried out to validate the proposed geological-geotechnical model. The concept of landslide triggering critical rainfall magnitude (specific to a given site) is proposed in this paper. Detailed rainfall analysis reported elsewhere (Ko Ko 2001) leads to the conclusion that the most significant period of antecedent cumulative rainfall for this site is 15 days. The historical rainfall data then leads to an estimation of landslide frequency or hazard. The risk to human casualty is then based on an analysis of train operations and their frequency. Different results are obtained for the tourist train service and the freight train service. Some attention is given to tolerable risk criteria proposed elsewhere in the literature.

ArcGIS V.10 Landslide Susceptibility Data Mining Add-in Tool Integrating Data Mining and GIS Techniques to Model Landslide Susceptibility

Landslide susceptibility modeling is an essential early step towards managing landslide risk. A minimum of

Spoil stability considering progressive failure

4.8 million is lost due to landslide related damages every year in Illawara region of Australia. At present, Data mining and knowledge discovery techniques are becoming popular in building landslide susceptibility models due to their enhanced predictive performances. Until now, the lack of tools to undertake data extraction and making the predictions have limited the applicability of this novel technique in landslide model building. This paper discusses the development of the LSDM (Landslide Susceptibility Data Mining) toolbar which was designed to utilize machine learning techniques within a GIS environment by coupling GIS and data mining software (See5) capabilities. The software development kit available with ArcGIS v.10 has been utilized in developing the toolbar add-in. The fundamental tasks; data preparation, model optimizing, derivation of decision trees, predictions and validation are all performed using the individual controls available in the toolbar. This tool automates the entire model building process and in preparation of training data and producing outcomes that are compliant with both national and international Landslide Risk management guidelines.

Risk of slip along discontinuities in a heterogeneous medium

Abstract In spoil piles, shear strength often decreases significantly as a consequence of moisture softening. Moreover, spoil material exhibits characteristic brittleness so that overstressing can also lead to a marked decrease in shear strength. Progressive phenomena are always important in such circumstances and in this paper an approach is outlined to include failure progression within a two-wedge limit equilibrium model. A comprehensive approach is outlined and it is suggested that the factor of safety be obtained as a function of the residual factor which has values between 0 and 1, corresponding to no local failure on the one hand and complete softening of the slip surface on the other. It is shown that the extent of the softening zone can be obtained from simple finite element analysis. The usefulness of this approach for updating spoil stability during its life is highlighted. The significance of the direction or mode of failure progression is also discussed in the paper.

Some practical aspects of spoil pile stability by the two-wedge model

Abstract This paper is concerned with the methodology for the calculation of risk or probability of slip along planar discontinuities within a rock mass. A risk model is presented in which both geometrical and shear strength parameters may be regarded as random variables. However, the paper focuses attention on the randomness of shear strength parameters. Adjacent discontinuities may be dissimilar in respect of the values of shear strength parameters. However, individual shear strength parameters may be correlated and, therefore, the safety margins against slip along individual discontinuities are also correlated. Thus, it is not sufficient to calculate the risk of failure along a discontinuity as an independent event. It is also important to calculate the conditional probability of failure along a discontinuity given that an adjacent discontinuity has slipped. The relevant equations and calculation procedures are outlined and typical results presented.

Geotechnics in the Twenty-First Century, Uncertainties and Other Challenges: With Particular Reference to Landslide Hazard and Risk Assessment

Abstract This paper is concerned with the stability and design of spoil piles associated with strip coal mines. Reference is made to surface mining operations in Australia and to spoil pile failures in which a two-wedge failure mechanism has been inferred from careful observation and analysis. A numerical model for slope stability analysis based on the two-wedge failure mode is introduced. The model enables the estimation of the factor of safety associated with a spoil pile considering shear strength parameters for the main body of the spoil to be different from those of the moisture-softened material at its base. The influence of stabilisation measures on the factor of safety F may be studied without difficulty. The variation of F with independent variations in berm width and coal wedge length is presented for a typical problem. A number of stability charts are presented based on a range of values for the shear strength parameters of spoil and of the moisture-softened material forming the spoil base.

Landsliding in an urban area

This chapter addresses emerging challenges in geotechnics in the context of the significant challenges posed by hazards, both natural and human-induced. The tremendous importance of dealing with uncertainties in an organized and systematic way is highlighted. The chapter includes reflections on responding to the need for multidisciplinary approaches. While the concepts and ideas are pertinent to diverse applications of geotechnics or to the whole of geotechnical engineering, illustrative examples will be limited to research trends in slope stability and landslide management.