Doctor of Philosophy
Department of Civil, Mining and Environmental Engineering
Ko Ko, Chit, Landslide hazard and risk assessment along a railway line, Doctor of Philosophy thesis, Department of Civil, Mining and Environmental Engineering, University of Wollongong, 2001. http://ro.uow.edu.au/theses/1286
Various methods of landslide hazard assessment and zonation, have been developed in the past. Some research has also progressed towards the assessment of consequence and risk related to slope instability. This thesis is a contribution towards the development of rational and systematic approaches, qualitative and quantitative, for the assessment of landslide hazard and risk. Slope stability and the occurrence of landslides is an interdisciplinary area of study and inputs may be required from some or all of the following discipline areas: geomechanics, engineering geology, geomorphology, hydrology, hydrogeology, statistics and probability theory, and decision science.
In order to assess landslide hazard, it is very useful but not sufficient to carry out slope stability analyses. Attention has to be given to the basic causes and mechanisms as well as to the triggering agents. Although the factors which influence the occurrence of landslides are now well known and mechanisms of failure are recognised, prediction of future events is difficult. In addition to analytical and experimental methods, observational approaches are very important for understanding the processes and mechanisms and for assessing the hazard. Systematic use of observational data, such as those from inclinometers, piezometers and rainfall stations has been demonstrated in this thesis.
The current project focuses on landslide hazard in the urban context and, more specifically along transportation routes such as railway lines in hilly areas. A three-level strategy (Type I, Type II, Type III analyses) which is also described as multi-level or integrated hazard and risk assessment has been proposed, developed and successfully used along the Unanderra to Moss Vale Railway Line constituting a 32.5km of track length. Data are often not sufficient or of the highest quality to carry out quantitative studies. For many years, only qualitative, site-specific approaches (Type I) based on minimum site investigation have been used in practice. These approaches, in which engineering and geological judgment is of paramount importance, are still found to be useful. In particular, comprehensive Type I assessment data can be very useful for regional or Type II analyses.
There is an increasing recognition of the need for more systematic and quantitative approaches which makes use of historical and observational data to the fullest. Type II analyses explore spatial and temporal relationships along the whole area considering a number of influencing factors as well as the triggering agent such as rainfall. This has enabled the preparation of relevant hazard maps for the study area. Type III analyses are detailed site-specific analyses which include conventional slope stability assessment as well as the recurrence intervals and hence the annual probabilities of landsliding. These are the dominant aspects of hazard. In this thesis, these relatively rigorous approaches are extended further to quantitatively assess the consequences to human life and the related risk. The assessments are tailored not only to all available geotechnical and rainfall data but also the information concerning the railway operations and the infrastructure. Consideration of economic loss and other environmental consequences on a quantitative basis was outside the scope of this thesis. Even these quantitative approaches requires a considerable component of engineering judgment.
As part of this thesis a comprehensive Type I method along with an original set of field sheets utilising a weighting approach has been developed. It has been named the University of Wollongong (UOW) method. In this approach risk is assessed by means of hazard and consequence matrix and a substantial component of consequence assessment framework is incorporated, an aspect in which other similar methods used in New South Wales (Australia) were found to be lacking. The UOW method and several other Type I methods were tested and compared. This process allowed strengths and weaknesses to be identified for each method.
It is widely recognised that predictions concerning slope stability are often difficult make. Hence, deterministic approaches must be complemented by the use of probability concepts. Therefore, one aspect of the methods proposed in this thesis is the estimation of annual failure probabilities and/or recurrence intervals of landsliding. This can be particularly useful for Type II and Type III approaches, provided relevant data are available. The main triggering agent in the study area is rainfall. Therefore, available historical data on rainfall and rainstorms has been very useful in carrying out Type II and Type III analyses when combined with observational data on subsurface water levels and on slope movements.
Type II analyses have been found useful to identify the most susceptible geological units. GIS-based hazard maps of the Unanderra to Moss Vale Railway Line have also been produced on the basis of such an assessment. Type III analyses was carried out for three Case Studies which consist of 4 sites. Risk of human casualty for a passenger either on a tourist train or on a freight train was determined on a quantitative basis for Type II and Type III analyses. In most cases risk was found to be above the tolerable or acceptable levels as proposed by some previous researchers. These three Types of hazard and risk assessment complement each other. An integrated approach using these three types of methods can be a very powerful tool for managing risk.
A key feature for quantitative hazard and risk analyses (Type II and Type III analyses) the estimation of the annual probability of landslide recurrence, based on the recurrence interval. In this thesis the concept of critical rainfall triggering landslide event has been proposed. Each landslide site may have a unique landslide triggering rainfall. A quantitative estimation of the conditions for landslide recurrence was made for each of the three landslide sites taking into consideration the historical rainfall data over several decades as well as the detailed observational data for each site.