Landslides Associated with the Pietermaritzburg Formation in the Greater Durban Area, South Africa: Some Case Histories

Abstract

The Pietermaritzburg Formation consists of shales which exhibit variations in mineralogy which give rise to its variability in fissility. Fissile beds alternate with hard, dense non-fissile beds. The fissile shales often tend to be more weather resistant than the non-fissile beds. Weathering along bedding planes also gives rise to thin clay layers along which sliding has taken place. The residual and colluvial clays on top of the shales also have been involved in sliding. The latter slides can be either translational or rotational. The former usually occur along the soil-bedrock contact and the latter where the soils are more deeply developed. One of the most notable landslides on the Pietermaritzburg Formation occurred at Mayat Place in Durban. This rotational slide occurred in weathered shale with subsequent development of retrogressive slides. The slide gave way to a mudflow towards the toe of the slope. Several houses were destroyed by the landslide and others had to be demolished. The slope was regraded and an effective drainage system was installed. Every so often slides occur along the railway line north of Durban where it passes through the shales, weathered shales and residual soils associated with the Pietermaritzburg Formation. These slides frequently are rotational but at times a rotational slide may merge into a one of translational character. The slide surfaces often occur between the residual soil and weathered shale beneath.