Slope failures in opencast coal mines in the Northern Waikato Coal Region commonly occur in the vicinity of major faults where they define pit limits. In-pit engineering geological mapping at the Kopuku Sector of the Maramarua Coalfield demonstrates the existence of defects both parallel and conjugate to the faults as well as bedding plane shear zones. A bi-wedge mechanism is considered appropriate to analyse the stability of slopes where failure is controlled by these defects. An important part of this mechanism is the requirement for up-dip movement along the bedding plane shear zones. The residual strength of bedding plane shears and the peak strengths of simulated defects were measured in the laboratory, and further analyses with the bi-wedge failure mechanism were used the study the influence of the angle of dip of bedding plane shear zones, overall slope angle and pore water pressure on stability.
