As spoil dumps get higher, particularly in strip mining where most overburden is placed in-pit, consequences of slope failure become disproportionately greater. Current understanding of the shearing behaviour of spoil for stability design has involved a combination of laboratory-scale diagnostic testing and engineering judgment. This is a relatively empirical approach that provides a linear shear strength envelope for materials known to exhibit non-linear behaviour, particularly under high confining stresses. A shortcoming to the diagnostic testing is that oversize particles are usually scalped to accommodate the device capacity. The influence of prototype-size particles on the geomechanical behaviour of mine spoil is not truly captured. In response to concerns about overestimating the shear strength and stability of high spoil dumps, and current plans for coal mine dumps to exceed 400 m in height, there is a need to rationally define the stress-strain behaviour of more characteristic spoil masses under representative compressive and shearing loads. A Large Direct Shear Machine (LDSM) has been designed at The University of Newcastle to generate reliable stress-strain data on large samples of coal measures spoil (0.72 m x 0.72 m x 0.6 m) subjected to loads representative of very high dumps (~3.5 MPa). This paper reviews current methods for predicting shear strength parameters in the context of very high spoil dumps, and presents an overview of the design considerations of the DSM.