Investigation of Bovills landslip, near Devonport, Tasmania

Bovills Slip occurs in weathered basalt colluvium at the base of a coastal scarp about 2 km east of Devonport on the north coast of Tasmania. The colluvium consists of red-brown fissured silty clay with rock fragments. Many landslips occur in colluvial soils on the coastal scarp and also in basalt-derived soils elsewhere. Thus a detailed investigation and stability analysis of Bovills Slip is relevant to the general slope failure problem in Tasmania. Pore water pressures measured with open standpipe piezometers show a correlation with rainfall, with peak pressures occurring during wet winter months. Effective shear strength parameters were determined by both multi-stage direct shear tests and consolidated undrained triaxial tests with pore pressure measurements. Different residual shearing mechanisms were recognised in the shear box tests. Significantly different values of residual strength were associated with these different mechanisms. The fully softened strength parameters appropriate for the analysis of first-time landslips were investigated by both triaxial and shear box tests. For the soil tested both the residual and fully softened effective friction angles showed a pattern of dependence on the plasticity. Surface movements have been monitored by repeated surveys, and subsurface movements have been monitored by regularly checking piezometer tubes for deformation. After heavy rain, in August 1981, the landslip moved by 20 to 30 mm. A two dimensional model of the August 1981 failure has been analysed by limit equilibrium methods. The factor of safety is most sensitive to variations in piezometric head and cohesion. Analysis has been used to assess the relative change in factor of safety (stability) caused by changes in the slope and by remedial measures. The stability was reduced when the slope was undercut by roadworks in 1973, and the first movements caused a decrease in shear strength of the soil. Downslope movements have produced shape changes which have tended to increase the factor of safety. Toe drainage, toe surcharge, 'and re-grading have already resulted in increased stability. Subsurface drainage, although effective, would be relatively expensive. Lime stabilisation and tree planting were also considered. In the long term well established trees may increase the factor of safety by as much as 50%. Possible future research on landslips in Tasmania is discussed in order to demonstrate how the results of this detailed investigation may be used as a starting point for regional studies.