Slope instability along the north-west coast in Malta

Abstract

Mass movement processes operating along the coastal zone, north of the Great Fault are examined. Slides fall under three main categories. Rotational slides and translational slides occur in the Upper Coralline Limestone whereas mudslides are found where Blue Clay outcrops. Two other processes are present: rockfall and soil creep. Rockfall can be considered as the most important mass movement process, whereas soil creep is the least significant identified at one locality. In this study particular attention is given to Blue Clay slopes. A coastal geomorphological survey was undertaken for the northern region. Two geomorphological maps were produced to determine the spatial distribution of coastal features, identify the main mass movement processes and establish a relationship between the geology and geomorphology. Three representative sites were selected based on the mapping exercise to conduct further research. Detailed geotechnical testing was performed on soil material collected from selected slopes at the three field sites. The physical and mechanical properties of Blue Clay were determined to assess the current state of stability of the slopes and the strength of the material. A series of stability analyses were performed on the selected clay slopes at each field site. The Simplified Bishop method was used to calculate Factor of Safety values and to establish the transition between stability and instability. Geomorphological and geotechnical investigations performed at previous stages of the research provided the necessary input data to be used in the stability analyses. Variation in the pore pressure ratio allowed the identification of the critical phreatic conditions at which the slopes fail.