Materials 'strength is a critical factor together with geological structure, groundwater and mining strategy in determining slope stability in surface coal mining. A wide variety of instability mechanisms are operative in British surface coal mines which involve failure within intact rock, discontinuities and weak zones. The nature of the Carboniferous Coal Measures entails that mine slopes are discontinuous rock masses comprising a wide range of sedimentary rocks. This paper aims to characterise the strength properties of the British Coal Measures encountered in surface coal mines and highlight the nature of the discontinuities and weak zones, which are of particular significance in determining slope stability.
Table 1 reports the physical properties (porosity, n, dry density, pd, and saturated density, ps) determined in accordance with ISRM standards (1970) for Coal Measures rock types from 35 British surface coal mines. These properties show a wide range, governed mainly by grain size and mineralogy. The overall relationship between dry density (tonne/m3) and porosity (%) for Coal Measures was found to be
n = 61.7 − 20.3 pd
The unconfined mechanical properties of Coal Measures rock types are summarised in table 1, as derived from over 2000 laboratory tests. The uniaxial compressive strength (σc), indirect tensile strength (σt, Brazilian method) and elastic modulus (EY were derived where possible from both dry and saturated specimens, (ISRM standards 1972). 'The general non-linearity of stress-strain curves determined the measurement of elastic modulus at 50% of ultimate strength. The σc data can be used to classify the Coal Measures for engineering purposes by a system such as that proposed by Deere and Miller (1966). This indicates that most of the Coal Measures encountered in British surface coal mines may be regarded as low to very low strength rocks. The data also highlights the reduction in σc resulting from saturation, generally 10 to 50% but as high as 90% in the argillaceous rocks. This form of classification however does not account for the effects of weathering and anisotropy, in addition to moisture (Hassani 1980).
The shear strength and internal angle of friction for Coal Measures rock types were evaluated by over 900 individual triaxial tests. The relationship of normal stress (σ1) versus lateral stress (σ3) in each test was analysed by linear regression. The slope of such fitted lines (tan β or K) was generally in the range of 2 to 5. It was found that significant differences occurred between the tested σc value of each rock type and the theoretical value of σc, (C), obtained by such analysis. This originated from the curvilinear nature of the triaxial σl versus σ3 plots, with C generally being in excess of σc The linear regression analysis was intended to simplify further calculation of shear strength (T) and internal angle of friction (Ф) by employing the method proposed by Obert and Duvall (1966)- Corresponding values of C and σc were substituted within equation (2) to calculate the equivalent shear strength values
