Experimental investigation of mining induced fracturing

The safety of mining operations is significantly influenced by the in situ stress fields in which the mining excavations are created.  Many in situ stress measurements have been carried out for mining and civil engineering projects in Southern Africa over the past 30 to 40 years.  Some of these data have been published, but many are contained in mine, company and owner organisation records.  A collated and evaluated record of these data will provide a valuable input source for the planning of mining operations, allowing more valid stress calculations to be made.

This report describes such a record, which has been compiled into a computer database.  Data on stress measurements and observations have been obtained, evaluated, reanalysed where necessary, and graded.  The project described in this report is SIMRAC Project GAP 511b, "Evaluation and upgrading of records of stress measurement data in the mining industry".

A standard spreadsheet program has been used for capturing of the database.  The database is available on a stiffy disc in standard DBASE format.  In this format it can be accessed using any of the common database or spreadsheet packages such as ACCESS, QUATTRO PRO, EXCEL, LOTUS 123 etc, at least one of which should be available on every mine.

Various GIS and other graphical/map packages were investigated for possible use for visual display and interrogation of data.  Ultimately, the package ArcExplorer was chosen.  This is an easy to use Windows based package which provides all the functionality considered to be necessary for the project.  ArcExplorer has been developed by the Environmental Systems

Research Institute (ESRI), the organisation responsible for the development of the ARC/INFO and ArcView systems.  The main reason for the choice of ArcExplorer, however, was that it is freely available on the Internet, and can therefore be downloaded at no cost by any user who wishes to make active use of the database.  Instructions for downloading of the system from the Internet are given. ArcExplorer uses shape files containing the spatial information of each data point in the database, and a DBASE file containing the alphanumeric data of each data point.  Copies of these files are provided on the stiffy disc which accompanies the report.

Where possible, the data for each measurement entered into the database include:

Χ the location (coordinates and depth below surface);

Χ the mine or project;

Χ the rock type in which the measurement was made, and its deformation properties;

Χ the geology of the area;

Χ the in situ stress components in the north-south, east-west and vertical directions, and the corresponding shear stress components;

Χ the in situ principal stresses and their orientations relative to north;

Χ an estimate of the overburden stress;

Χ comments relevant to the measurement;

Χ gradings of the qualities of the individual measurements and the groups of measurements on a subjective/quantitative basis.

The purpose of the database is to allow users to extract the information that they require or may have an interest in.  It is therefore somewhat superfluous to present any results from the database.  However, in the report, some results are presented, which are of interest from the mining viewpoint and also serve to demonstrate the capabilities of the system.  The results available from the database indicate that the horizontal principal stresses in  Southern Africa commonly trend approximately north west - south east and north east - south west.  The major horizontal stress magnitudes are almost always equal to or greater than the vertical stress magnitudes.

Based on evaluation of the information it is considered that there is little to be gained by carrying out in situ stress measurements for the specific purpose of filling in "gaps" in the stress database.  Sufficiently reliable extrapolation of data, based on existing trends, can be achieved from the information in the database and from observations of dog earing behaviour in boreholes, shafts and raise bores.  It is recommended that data from future in situ stress measurement exercises, if available, should be added to the database from time to time, to maintain the database as up to date as possible. It is concluded that the availability of the database will contribute to the goal of safer mining conditions in South Africa.

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