Mine portal design: considerations, methods and practices

Abstract

Mine access portals are routinely developed to provide access to underground mines; typically, they are either developed within an existing open pit or in a specially developed box cut. Portals are critical mine access excavations requiring rigorous design and construction practices. In some cases, the portal will serve as a single means of access and egress for a significant period and this requires that design acceptance criteria be commensurate with the acceptable risk being adopted. Failure of a portal can result in both safety and economic consequences. In the authors' view, the rigour around design effort, acceptance criteria and construction quality control are not always at the required level.Mine access portals can be developed in a range of rock mass conditions including soils, weak weathered rocks such saprolites, to competent good quality rock masses. Portal design requires an understanding and consideration of the geotechnical conditions, likely failure mechanisms, hazards and risk, as well as the intended period that it will serve as the main access to the mine. A range of design methods (empirical, analytical, numerical etc.) can then be used to assess likely loading conditions and the required ground support. Portal designs also need to consider the stability of the slope into which it is developed; in a box cut it is possible to engineer a stable slope whilst in an open pit it is necessary to deal with an already cut slope. This paper discusses typical hazards and risk associated with mine access portals as well as a range of design methods. A framework for portal design is presented including construction aspects and monitoring. Examples for several portals developed in a range of different rock mass conditions using different design methods and support methods are presented.