The weathering characteristics of Ankara andesites and their importance from the rock mechanics point of view are studied. First, a review of processes involved with the weathering of rocks is presented together with the engineering implications and available methods of describing and classifying the products for engineering purposes. The latest universal weathering grade rock mass classifying system developed by Dearman and his friends is used for the field studies of the research. A detailed investigation on the physico-mechanical properties of the weathered Ankara andesites is carried out in the laboratory and their relations with each other are studied. Mohr envelopes have been established for each weathering grade of different andesite types. The general trend of Mohr envelopes for weathered andesitic rock shows that Mohr envelope lines can be drawn to represent the each weathering grade. There is systematic decrease in the values of material constants with the degree of weathering.
Characterization of rock have been studied by different investigators. The rock mechanics approach is to deal with the physical and mechanical properties of rocks and to investigate the importance of these parameters for the design of structures. Weathering, defined as the process of alteration of rocks occuring under the direct influence of atmosphere and hydrosphere (Sounders and Fookes 1970) is an important process for the rocks. Since weathering is the main factor that weakens the rock, mechanical behaviour of the weathered rocks and their quantitative classification of the weathering grades are two important aspects for rock mechanics purposes. Many investigators tried to classify the rocks according to the purpose of the engineering projects they were involved in. Moye (1955) in Australia, and Ruxton and Berry (1957) in Hong Kong, Hamrol (1961), Iliev (1966) and Working Party Report (1972) have classified the weathered rocks. Most of the weathering classifications are based on Moye and Ruxton and Berry's classifications. Both, Ruxton and Berry and Moye's classifications are qualitative classifications rather than quantitative. Little (1967) quantified Moye's classification by giving grade numbers to different zones of weathering. Although Moye's classification system is a rock material classification, Ruxton and Berry's system is a rock mass classification. Hamrol (1961) found a relationship between the strength of the rocks and the quick absorption index for different weathering classes. Whereas Iliev (1967) used sonic velocity index to characterize different weathering grades. Dearman, Baynes and Irfan (1978) reviewing the weathering classifications put forward a universal weathering grade classification system to be used for engineering purposes and gave rock mass and rock material properties of weathered granites and gneises (Irfan 1977, Ibid 1978).
Previous work related to weathering are generally based on the granitic rocks. Little research has been done on andesites. Kossev (1970) in an attempt to correlate the physical and mechanica] properties of 14 different kinds of rocks including andesites, stressed upon the influence of composition of rocks on the engineering properties. Hoek and Brown (1980) analysed the test results from the Panguna andesites, as the host rock for the large copper deposits in.