Landslides associated with the colluvial soils overlying the Natal Group in the greater Durban region of Natal, South Africa

Abstract

In the last 25 years, many of the landslides that have occurred in the greater Durban region have been associated with the colluvial soils overlying the Natal Group, most of which occurred during the very heavy rains of September 1987. Subsequently, a very heavy rainfall event in February 1999 also gave rise to landslides. In fact, prior to 1987 these colluvial soils were considered relatively stable. A critical precipitation coefficient has been developed which included the cumulative precipitation up to a landslide event. In addition, an attempt has been made to establish a threshold value for triggering of landslides for the colluvial soils from a study of pluviometric data. The results indicate that when a rainfall event exceeds 12% of the mean annual rainfall, small-scale landslides are likely to occur. When a rainfall event is greater than 16% of the mean annual rainfall, a moderate number of landslides take place. Major landslides are associated with rainfall events with intensities in excess of 20% of the mean annual precipitation. An example of a landslide which occurred on the Natal Group due to construction operations is provided, as well as an account of those which took place during September 1987. In the latter case, most of the slides took the form of mudflows and were responsible for some of the worst damage which has occurred in the Durban region. The colluvial soils involved were relatively thin and therefore became quickly saturated by the heavy rainfall. In some places the situation was further aggravated by liquefaction of the soils.