INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS Int. J. Numer. Anal. Meth. Geomech., 2003; 27:495–511 (DOI: 10.1002/nag.283) The key-group method A. R. Yarahmadi Bafghi y and T. Verdel n,z Laego, Ecole des Mines, Parc Saurupt, 54042 Nancy Cedex, France SUMMARY This paper proposes an extension to the key-block method, called ‘key-group method’, that considers not only individual key blocks but also groups of collapsable blocks into an iterative and progressive analysis of the stability of discontinuous rock slopes. The basics of the key-block method are recalled herein and then used to prove how key groups can be identified. We reveal that a key group must contain at least one basic key block, yet this condition is not entirely sufficient. The second block candidate for grouping must be another key block or a block whose movement-preventing faces are common to one or more single key blocks. We also show that the proposed method yields more realistic results than the basic key-block method and a comparison with results obtained using a distinct element analysis demonstrates the ability of this new method. Copyright # 2003 John Wiley & Sons, Ltd. KEY WORDS: key-block method; key-group method; joint pyramids; vectorial technique; slope stability 1. INTRODUCTION Over the past 30 years, the key-block method has been developed and successfully used to analyse the stability of fractured rock masses, relative to either rock slopes or underground excavations. This success has mainly been due to the simplicity (and resolution speed) of the method compared to far more complex continuous or discontinuous analyses carried out with the finite-element, finite-difference or distinct-element method. Implementation of the key-block analysis has been manifested into two main forms: the vectorial technique by Warburton [1] and the graphical technique developed by Goodman and Shi [2]. A key block is primarily a block around an excavation, which if not reinforced can become unstable and lead to the progressive instability of other blocks. It is defined by four major conditions: * active (showing contact with the excavation), * finite (limited by discontinuities and the excavation), * geometrically mobile, and * key for other block movement. Received 7 June 2002 Revised 30 January 2003 Copyright # 2003 John Wiley & Sons, Ltd. y E-mail: ali-reza.yarahmadi-bafghi@mines.inpl-nancy.fr z E-mail: thierry.verdel@mines.inpl-nancy.fr n Correspondence to: T. Verdel, Laego, Ecole des Mines, Parc Saurupt, 54042 Nancy Cedex, France