Understanding landscape evolution and the surface processes, both relict and active, that formed it is key to minimising geotechnical risk. The use of an engineering geological model (EGM) to the assessment of these hazards provides an essential framework for the derivation of appropriate mitigation strategy in that the “ground conditions at any site are a product of its total geological and geomorphological history which includes stratigraphy, the structure, the former and current geomorphological processes and the past and present climatic conditions” (Fookes et al., 2000).
The primary objective of the EGM for a landslide study is to understand the geological and geomorphological controls on instability and how that might impact the project. In particular, the EGM should identify the potential landslide types present as well as their likely magnitudes and frequency. The EGM is initially developed based on desk study data combined with knowledge and experience. Initial mapping is undertaken, typically based on remote sensing, and an initial landslide inventory developed. The initial mapping and inventory is then field verified, which also allows detailed engineering geomorphological mapping to be undertaken. The field mapping focuses on the validation of key features/areas identified from the initial assessment as potentially comprising a hazard to the proposed infrastructure layout, as well the evaluation of features not visible from remote sensing. The final mapping together with the revised landslide inventory provides the data for undertaken the hazard and risk assessment.
The approaches to estimating the probability that a landslide will occur can be divided into the frequentist approach and the degree of belief approach. The frequentist approach involves statistical analysis of events within the landslide inventory and, as such, can be seen to be “objective”. In comparison the degree of belief is viewed as being subjective and involves making judgments based on the available information and experience of the practitioner, with the focus on understanding the underlying causes.
The decision as to which approach is taken will broadly reflect the availability of information and the skills and experience of those undertaking the assessment. However, the reality is that most landslide assessments typically need to incorporate both approaches. Similarly, the decision as to whether a qualitative or quantitative assessment is undertaken will depend on the data, time and financial resources available.
Engineering geomophological map and subsequent interpretation of a landslide complex