Though not clear, this may be related to the earlier-stated difficulties with the tensile test. ) generally does not include tensile testing in his guidelines for the calculation of mi (and sci), even though tensile strength is an important element of the failure criterion. Other writers have not always been consistent with Hoek’s approach in their application of the Mogi line as an upper limit to testing data. CALCULATION OF mi VALUES The Hoek-Brown failure criterion is intended for use in the brittle range of rock behaviour, and this controls the range of minor principal stress ( s 3´) values over which triaxial testing to determine mi is carried out. Alternatively, in the absence of laboratory tests, a guideline table can be used to obtain estimates of mi (sci is similarly determined, or descriptive tables used in the absence of testing data).Ģ. Hoek recommends that wherever possible the value of the mi constant should be determined by statistical analysis of the results of a set of triaxial tests on carefully prepared core samples. However, for general criterion use they considered it preferable to treat m simply as an empirical curve-fitting parameter because of the difficulty in adopting the uniaxial tensile strength as a fundamental rock property. INTRODUCTION In the case of intact rock material where s = 1.0, Hoek & Brown noted that m ˜ s ci /? s t?where s t is the uniaxial tensile strength of the intact rock. In the absence of such testing, the use of R as a means of estimating mi is considered to be a preferable alternative to relying on guideline charts.ġ. The paper concludes that the most accurate method of assessing mi values remains as statistical analysis of data from a full set of triaxial tests plus tensile tests. The databases, together with the results of further uniaxial, triaxial and tensile laboratory testing, have been used to assess the relationship between the ratio of uniaxial to tensile strength (R) and mi. Our analysis of existing databases on rock material properties (such as Sheorey, Douglas and Rocscience) demonstrates that there is a very poor correlation between lithology and mi values. In the absence of laboratory testing, guideline charts with mi values based on rock type have been provided by Hoek & Brown and others. The intact rock constant, mi, is one of the three basic input parameters required for the widely used Hoek-Brown failure criterion.
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