Uniaxial compression tests were performed on different categories of weathering of three lithological units:Malanjkhand granite; Nagpur basalt; and Delhi quartzite, occurring in central and northern parts of India. The deformational behaviour is studied in terms of variation in tangent modulus (Et50) and initial modulus (Ei) due to weathering. The power relationship between uniaxial compressive strength (sc) and Et50 shows strong correspondence for weathering sequence of common rock types. This relationship has been established by regression analysis and
significant correlation parameter (coeYcient of determination, r2=0.87) for crystalline rocks. It is shown that there is a systematic decrease in stiVness ratio, that is, ratio of tangent modulus and uniaxial compressive strength with increased
weathering state. Comparison of Et50 and Ei values has shown that Et50 decreases more gradually than Ei, and reduction is more drastic for Ei values with an increased degree of weathering in all the three rock types. The mode of failure has been found to be influenced by weathering extent in rocks. A brief account is given of the intrinsic characteristics of fresh and weathered rocks and mineralogical changes produced by weathering investigated quantitatively. Correlation drawn between the petrographical and mechanical indices has shown that mechanical properties are apparently dependent on the intrinsic characteristics of weathered rocks.