The extended Mathews stability graph: Quantifying case history requirements and site-specific effects

and Resume Narrow vein underground stope design engineers in Australia have expressed significant interest in the requirements and methodology for developing site-specific stability charts. This is largely due to a concern that existing stope stability charts inadequately take into account factors influencing narrow vein stope stability, such as the effect of backfill abutments and the effect of in stope pillars. This paper presents conclusions from a comprehensive statistical examination of case history requirements for a site specific stability chart and critiques literature arguments for site-specific charts. A new statistical analysis technique and a large comprehensive stability database enabled the authors to quantify case history requirements and any site-specific effects the model exhibits. The analyses indicated that a reliable stable-failure boundary requires at least 150 case histories, of which a minimum of 10 percent should be unstable stope surfaces. Marginal site-specific effects were observed for the operating conditions captured within the database. The authors conclude that the apparent site-specific effects noted in previous literature are attributable to operating conditions inadequately represented in the database. Such operating conditions could induce erroneous stability predictions at any site, and are therefore not truly site-specific. Following from these conclusions, a strategy for taking into account operating conditions particularly relevant to narrow vein mining has been proposed. In particular, the development of a chart that accounts for the effect of backfill abutments on stope stability. Backfill abutment effects are particularly relevant to mechanised narrow vein mining methods such as modified Avoca. Collaboration from at least five sites is required to obtain access to relevant data and underground drives for geotechnical mapping. Preliminary estimations indicate that approximately three months fieldwork is required to meet case history requirements. CMS data in conjunction with 3-dimensional design software will be used to calculate equivalent linear overbreak/slough (ELOS). In contrast to stable, failure, major failure and caving categorical variables, ELOS is an objective and continuous stability variable and is therefore, more conducive to mathematical modelling. Introduction The Mathews Stability Graph method is an empirical stope design tool used extensively in Australia and Canada. Over the last few years a much larger database of case histories from a wide variety of geotechnical environments has been developed for the technique (see Trueman et al, 2000; and Mawdesley et al, 2001). Trueman and Mawdesley (paper awaiting permission to publish from sponsoring companies) have also extended the technique into the prediction of caveability. The current database comprises 486 case histories of stope surface stability of which 317 were stable, 154 had experienced failures or major failures and 15 continuously caved. This data was collected from 35 mines from Australia, Canada, Chile, the United Kingdom, the United States and Zimbabwe. A wide variety of geotechnical environments and stope surface geometries are included in this database, with Q' values ranging from 0.01 to 90, while hydraulic radii range from 1 to 55. Logistic regression was used to determine stability boundaries (Mawdesley et al, 2001). The original method of determining the brought to you by CORE View metadata, citation and similar papers at core.ac. uk provided by University of Queensland eSpace