A case study of a deep-seated spoil lowwall instability controlled by a classical active- passive wedge mechanism at Anglo American Metallurgical Coal’s Dawson Mine is presented. Following a truck dump extension, widespread displacements were identified during conventional single forward pass de-coaling of Pit 6-8 strip E15. New through-spoil drilling, downhole geophysical logging, and XRD analysis identified a moisture-sensitive tuffaceous claystone unit containing a high proportion of medium-high swelling, mixed-layer illite-smectite clays 11-12 m below the lowwall floor. A novel soil mechanics approach was used to determine the mechanical properties of the tuffaceous claystone for which conventional rock mechanics tests could not be applied. Three-dimensional numerical modelling was then undertaken in FLAC3D to: a) validate the characterised mechanical properties; b) determine appropriate buttress slot widths for retreat mining of strip E16; and c) provide a validated base case for the predictive modelling and design of future strips. Operational controls for mining of strip E16 included: surface monitoring (radar, LiDAR); subsurface monitoring (TDR, VWPs); and an adaptive mine plan following the Observational Method. The TDR confirmed that the tuffaceous claystone unit at 11-12 m depth was acting as the sole basal horizon controlling the instability and mining of strip E16 was completed safely without coal sterilisation.