Landslides on Gault: geomorphological identification and qualitative risk assessment

Abstract

Abstract A geomorphological assessment was undertaken to determine the risk of landslide damage to 38 km of the M25–M26 motorway underlain by the Gault Formation (a stiff clay to weak mudstone). Conventional methods for geomorphological mapping using stereoscopic aerial photography were found to be unhelpful, owing to the low topographic expression of landslips in the study area. In contrast, Light Detection and Ranging (LiDAR)-generated topographic contour maps proved invaluable, giving the landscape a pseudo-relief and allowing identification of the subtle landslide features. Relict (inactive) landslides were found to lack a definable backscarp and were mainly discernible by their indistinct toe lobes. Two types of active landslides were identified within the study area: slow creep failures barely distinguishable from relict slides, and rapid slides. A specific list of descriptors for landslides on Gault is presented based on the results of the geomorphological investigation. Four factors were found to be of particular importance in governing the potential for landsliding: the presence of relict slips; slopes of 7° or over; an abundant source of water; and close proximity to the top of the Gault where extremely high-plasticity Upper Gault crops out. The results of this investigation indicated that the section of motorway most affected by historical landsliding was also most at risk from future landslides. The similarity in morphology between active creep-type landslides and relict slides has implications for landslide management on Gault.