Influence of water content on the strength of rock
Introduction
Hawkins and McConnell (1992) investigated the influence of the water content on the strength and deformability of 35 different British sandstones from 21 localities, ranging in age from Pre-Cambrian to Cretaceous. They published values for the measured uniaxial compressive strength, and for the tangent and secant deformation moduli, in the case of dry and fully saturated conditions. Vásárhelyi (2003) analyzed the published data and showed that there is a linear correlation between the dry and fully saturated uniaxial compressive strengths, σc0 and σcsat, respectively (see Fig. 1). The overall best-fit equation for the 35 investigated sandstones is:
Hawkins and McConnell (1992) carried out tests to determine the influence of the water content on the strength of 15 sandstones. They found that the relationship between water content and uniaxial compressive strength could be described by an exponential equation of the form:where σc(w) is the uniaxial compressive strength (MPa), w is the water content (%) and a, b and c are constants. It is obvious that the strength at zero water σc0 = a + c, the strength at full saturation σcsat = c. The parameter b is a dimensionless constant defining the rate of strength loss with increasing water content. The determined constants for each of the 15 different sandstones (published by Hawkins and McConnell, 1992) with the respective R-values are listed in Table 1.
Fig. 2 shows the best-fit lines plotted for the 15 different rock types for water content values up to 5%. It is apparent that the strength of the rock is very sensitive to the water content; an increase in water content of as little as 1% from the dry state can have a marked effect on strength. The parameter b characterizes this sensitivity, with larger values corresponding to more sensitive materials. Hence, the b parameter should be very important for rock engineering design, particularly in the context of abandoned mines where the groundwater will rebound (Li and Reddish, 2004).
Section snippets
Calculating the sensitivity of water content in absolute scale
The disadvantage of the analysis method of Hawkins and McConnell (1992) is that the saturated condition differs for each of the investigated sandstones, i.e. the absolute water content at full saturation can be very different. Further, the suggested fitting curve of Eq. (2) of Hawkins and McConnell changes if the relative water content goes to infinity.
For a better representation of the moisture dependence, we suggest a recalculation of the material constants b, with the water content expressed
Conclusion
A method for estimating the sensitivity of sandstone to its water content has been presented. From an analysis of the results of Hawkins and McConnell (1992), this sensitivity is found to be highly dependent on the effective porosity, and to be applicable to more than the strength reduction. An advantage of the presented method, is that less tests are necessary for calculating the influence of the water content on the rock properties. From measurements of the density and the uniaxial
Acknowledgements
The authors acknowledge the support of the Bolyai Scholarship, and the financial support of the Hungarian Research Foundation (OTKA No. D 048645, F 43291 and T48489).
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