Degree Name

Doctor of Philosophy


Department of Civil and Mining Engineering


The open stope mining method is the most common underground extractive technique used in Australian metalliferous mines. The crown pillar as it stands in the vertical plane between two open stopes is an integral part of the global stability of an underground metal mine. The stability of crown pillars are significantly affected by the mechanical and physical properties of the rock mass, structural weaknesses, the initial state of the horizontal stress and the geometry of the crown pillar.

To date no integrated design methodology is available in the public domain and generally crown pillar design relies on past experience and rule of thumb. In this thesis the objective was to develop a method which will aid engineers in designing the optimum crown pillar at any mine no matter what the local conditions. T o achieve this objective various methods, including empirical, numerical and theoretical methods of crown pillar design were investigated. This part of the study was used as a guide-line for modification of the available techniques and development of a complete design method for crown pillars.

In chapter 3 the voussoir beam and tributary area theories were modified. Using the modified versions a combined empirical and theoretical method of crown pillar design was developed. This method allows the engineer to determine an initial value for pillar span and thickness for the given conditions, and also to get a first estimate of the stress level in the pillar. Also in this chapter is a review of the work on stope design of Mathews et al (1981) and Potvin et al (1989). Finally a preliminary design methodology is presented.

Chapter 4 has two basic sections dealing with the techniques used for gathering and reducing field and laboratory data essential for design. The first section deals with the techniques, such as scanline surveying, used in the Held to determine the structure of a rock mass. The second describes the various tests which were conducted to determine the mechanical properties of the rock mass.

Crown pillar stability assessment based on data collected from case studies of a copper mine (CSA Mine, Cobar) and a lead-zinc mine (NBHC Mine), was carried out in chapters five and six. CSA Mine, NSW, Australia was chosen as the first site for evaluation of the stability of crown pillars. Cobar is a copper mine where open stoping operations are carried out in several parallel orcbodies which dip between 75" to 85° and have an average thickness of 12 m. The NBHC Mine, Broken Hill, NSW, Australia was the second mine chosen for a crown pillar case study. In this mine the orebody has been formed from several massive and thick discrete lodes or lenses. The geometries of the stopes and crown pillars are much more complicated than at CSA Mine and the stress distribution due to mining activities is also complex. The crown pillar span varies from one area to another; this variation being influenced by the grade of ore and the geometry of the orebody

The results from the joint surveys and rock tests were used as data in order to determine the applicability of the various design methods and back analyse stable and failed pillars in these mines. Part of the back analysis included the use of UDEC (a distinct element program) to simulate the action of the crown pillars under various stress regimes and various mining sequences. To gain a better understanding of the failure mechanism and modelling capabilities of UDEC, a series of parametric studies were also carried out. Finally, after comparing the different methods, conclusions are drawn and a methodology for crown pillar design is suggested and recommendations for future work are given.