Landslides are a common feature of the basalt terrain or north eastern New South Wales. Failures typically occur through residual soil layers, at strength between the peak soil substance strength, and typical residual strength values. Shear strength on soil fissures approach residual strength. Failure therefore seems to occur through a combination of turbulent or rolling shear of the soil substance and sliding shear on fissure surfaces. Fissure properties such as surface condition continuity, spacing and orientation need to be observed when investigating sites in the area, or in similar geological terrains to allow consideration of the likely influence on overall soil mass strength.
North eastern New South Wales is a region of rapid growth. The geology of the region consists of an accumulated sequence of interlayered basalt flows, overlying Tertiary/sediments, sedimentary rocks, minor volcanics, and a metamorphic basement. Mass movements are common on the slopes surrounding the remnants of the basaltic shield resulting in urban development, and major transport and communication links being constructed on potentially unstable slopes. Two predominant soil groups have formed from the basaltic rocks, the pedalferic krasnozem soils, and the pedovocalic brown soils. The krasnozems are locally referred to as ‘red’ soils, and are generally found to the east of Lismore. These are strongly structured clay soils with little horizon differentiation. The brown soils occur predominantly to the west of Lismore and are characterised by chocolately brown colour, and a clayey, cohesive texture and often heavily fissured structure. The boundary between the eastern and western soil types is sharp, and correlates well with rainfall patterns. The purpose of this study was to evaluate data from twelve case study sites, with the aim of identifying mechanisms of strength loss in the soils, and slope processes which work to promote strength loss and ultimately initiate failure.