Stability of saprolitic slopes: nature and role of field scale heterogeneities

Heterogeneities in various forms and scales often control the mechanisms and locations of failure and deformation, and the factor of safety of saprolitic slopes. This paper presents a critical review of field scale heterogeneities and their roles in controlling the stability of saprolitic slopes. In particular corestones and relict joints are analysed, with emphasis on characterization and possible instability modes. Abnormal flow patterns, fast build-up and/or chaotic distribution of pore water pressure are the most common causative factors of landslides. As heterogeneities are often responsible for the occurrence of such localized abnormalities, realistic models incorporating effects of these features can help predict how and where abnormal flow/pressure patterns may develop. Potential pitfalls during ground investigation in landslide prone slopes are elucidated and effective investigation strategies to avoid these pitfalls are recommended. The uncertainties, for example, in distribution and volumetric percentage of corestones and in delineating zonal boundaries, require continuous upgrading of the engineering geological model during the construction stage of site investigations. Such uncertainties can be reduced in a cost-effective manner by recording drill penetration rates during installation of soil nails and horizontal drains. A better understanding of the interactions among the heterogeneities, the matrix and the engineering geological environment as a whole should enable the significance of discrete features in stability to be more consistently assessed, thereby providing a more rational basis for investigation and design practice in saprolitic profiles.