Correlation of mineralogical and textural characteristics with engineering properties of selected granitic rocks from Turkey
Introduction
The properties of rock are influenced by the mineral composition, texture (grain size and shape), fabric (arrangement of minerals and voids) and the weathering state (İrfan, 1996). Granitic rocks include a wide range of rock types varying in their mineralogy, petrographic characteristics and engineering properties. Many investigators have developed a number of petrographic techniques to document and quantify the mineralogical and textural characteristics of granitic rocks using optical microscopy (Mendes et al., 1966; Willard and McWilliams, 1969; Hallbauer et al., 1978; İrfan and Dearman, 1978; Onodera and Asoka Kumara, 1980). Most of these techniques concentrated on quantifying the degree of weathering. Large variations in mechanical properties have been attributed to variations in the petrographic characteristics of granitic rocks. Petrographic characteristics known to affect mechanical properties include grain size, shape of grains, degree of interlocking, type of contacts and mineralogical composition. For typical fresh igneous rock, the mineralogy and texture combine to give high strength and excellent elastic deformation characteristics (Johnson and De Graff, 1988). Several investigators have studied the effect of grain size on the mechanical properties of rock. In general, the strength of rocks is greater for finer grained rocks (Brace, 1961). Onodera and Asoka Kumara (1980)reported that the strength decreased significantly as the grain size increased in igneous rocks. They determined a linear relationship between the grain size and strength, that is, as the grain size of the granite decreased, the strength increased. Relationships between mineralogical composition and mechanical properties of various granitic rocks have been previously investigated. However, these correlations were only based on quartz content. Merriam et al. (1970)found a definite relationship between the quartz content and the tensile strength of granitic rocks they investigated. İrfan and Dearman (1978) developed a quantitative method of assessing the grade of weathering of granite in terms of its microscopic petrography. They proposed a micropetrographic index as a percentage ratio of sound constituents to unsound constituents. They found good correlation between the micropetrographic index and the geomechanical properties of granitic rocks. Mendes et al. (1966)proposed that quantitative micropetrographic data could be used to formulate rock quality indices, which could be closely correlated with mechanical characteristics. They made a modal analysis of the mineralogical composition of the granite samples, together with an analysis of their texture and microstructure, and demonstrated that petrographic characteristics had a good correlation with the mechanical properties. A number of petrofabric techniques were developed by Willard and McWilliams (1969)in an attempt to gain a better understanding of the mechanical behaviour of rocks in relation to their micro-structure. They reported that microfractures, grain boundaries, mineral cleavages and twinning planes influence the ultimate strength of a rock and may act as surfaces of weakness, which control the direction in which failure occurs.
Although the relationships between the mechanical properties and petrographical characteristics of granitic rocks have been studied by different investigators (Merriam et al., 1970; Onodera and Asoka Kumara, 1980), the effect of mineralogical and textural characteristics on the engineering properties of this type of rock is not well known. The purpose of this study is to quantify the relationships between the petrographic characteristics, physical properties and mechanical properties of selected granitic rocks from Turkey, which have been widely used in buildings or as ornamental stone. The results of this and other similar studies could be used to predict the mechanical properties of intact granitic rocks based on petrographic characteristics and physical properties.
Rock samples were collected from different parts of Turkey (Fig. 1 and Table 1). The specimens were first subjected to thin section examination. The samples were cut and peels taken to give a petrographic description. Specimens were then tested for physical and mechanical properties. Before testing, all specimens were cut and trimmed to a length-to-diameter ratio of 2:1 for the tests and were oven dried. The engineering properties analysed included specific gravity, dry and saturated unit weight, water absorption, effective and total porosity, P-wave velocity, Schmidt hardness, point load strength index, uniaxial compressive strength, tensile strength and modulus of elasticity. Finally, petrographical characteristics and engineering properties of the granite specimens taken from different quarries were correlated by statistical analyses.
Section snippets
Petrographic descriptions
The mineralogical and textural characteristics of the samples were studied by optical microscopy. The thin sections were examined under a petrographic microscope for mean grain size, grain shape, type of grain contacts and modal composition. The location and descriptions of each rock type are given in Table 1. All rock tested were unweathered or slightly weathered according to Anon (1995), and exhibited an hypidiomorphic-granular texture. Quartz, plagioclase and K-feldspar were the main
Physical properties
The physical properties of the granitic rocks were determined by a variety of laboratory tests. The specimens were prepared and tested generally in accordance with the procedures given in ISRM (1981). The rock blocks were cored to give cylindrical test specimens. Physical properties investigated included specific gravity, dry and saturated unit weight, water absorption, effective and total porosity and P-wave velocity.
The unit weight and water absorption were determined using the saturation and
Mechanical properties
The mechanical properties of the granitic rocks were determined by a variety of laboratory tests in accordance with the procedures given in ISRM (1981). Schmidt hardness was determined on the block samples. Mechanical properties determined on the core samples included the point load strength index, the uniaxial compressive strength, the tensile strength and the elasticity modulus.
A Type L-9 Schmidt Hammer (ELE International, UK) was used in the laboratory on joint-bounded blocks to estimate the
Statistical analysis of test data
The results were statistically analysed to determine the range, mean, standard deviation and variance for each property investigated. Selected petrographical, physical and mechanical properties of the granitic rocks were plotted against each other in order to predict and estimate one property from another. The correlation coefficients and best fit curves were calculated by the `least squares curves fit' method using a computer. Some of the important results are illustrated and discussed. The
Correlations between the petrographical characteristics and the engineering properties
In order to determine the influence of the petrographical characteristics on the engineering properties, the ratio of quartz to feldspar (plagioclase and potash feldspar) and grain size were correlated with the engineering properties using regression analyses.
Porosity identifies the relative proportion of solids and voids; the unit weight adds information about the mineralogical or grain constituents (Goodman, 1989). The quartz content has an influence on dry unit weight and total porosity.
Correlations between the physical and the mechanical properties
There is a linear relationship between the total porosity and dry unit weight. The dry unit weight decreases as the porosity increases. There is also a similar relationship between the total porosity and P-wave velocity [Fig. 8(a) and (b)]. Theoretically, the velocity with which stress waves are transmitted through rock depends exclusively upon their elastic properties and their density (Goodman, 1989).
There are statistically significant correlations between uniaxial compressive strength and
Conclusion
Interrelationships between the petrographical characteristics and the engineering properties of the 19 different granitic rock samples were determined by simple regression analysis. The conclusions of the study are as follows:
- 1.
The mean grain (mineral) size has impact on the strength of the rock. The strength increases as the mean grain size decreases.
- 2.
Mineralogical composition is one of the main properties controlling the rock strength. Concerning the mineral composition, the variation in the
Acknowledgements
This work was supported by the Research Fund of the University of Istanbul, Project No. 1060/031297. The authors would like to express their thanks to Dr Yildirim Güngör for his help in petrographical studies, and to the reviewers for their comments and suggestions.
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