A coupled modelling approach for discontinuous subsidence at the Cadia East mine

Abstract

It is known that in addition to the production schedule, a number of orebody, geologic and topographic features can influence the nature and extent of surface subsidence related to a cave mine. Some of these factors include the shape and depth of the orebody/extraction level in plan, strength of the rock mass including the intact blocks, joints and geological features such as faults and dykes, ground surface profile, including historical surface mining and the presence of nearby underground excavations (Sainsbury 2012). Brady and Brown (2006) classify subsidence into two types, continuous and discontinuous. Continuous (or trough) subsidence refers to the formation of a smooth (gradual) change in profile that does not have step changes. Discontinuous subsidence involves large surface displacements (both tension and shear) and the formation of steps/breaks in the surface profile. Discontinuous subsidence is strongly affected by the presence of pre-existing features within the rock mass. As such, it cannot be accurately assessed without the inclusion of such features within a three-dimensional numerical model.A coupled continuous-discontinuous approach has been developed to predict caving and subsidence efficiently and accurately at the Cadia East mine over an eight-year production life (or 150 MT drawn). Numerically simulated results with the coupled approach are compared to in situ observations. It can be seen that in the case of sub-vertical structures within the near-surface rock mass, traditional strain criteria applied alongside continuum models are unable to capture the true shape or extent of cracking and that discontinuum models provide greater accuracy.