『Abstract
　In the recent past, several weathering indices have been proposed to characterize the extent of weathering and weatherability depending upon the nature and requirement of the study. The weathering index provides a quantitative measure of the extent of weathering of rock; hence it can provide input to the prediction models to assess the strength and deformational properties of rocks and classifications of weathered rock material. In the present study some of the important weathering indices, broadly categorized as chemical, (micro)petrographical and engineering weathering indices, are reviewed and studied experimentally for three common rocks of India, namely granite of Malanjkhans, basalt of Nagpur and quartzite of Delhi, along with results of other rocks reported by other researchers. The study reveals that none of the existing chemical weathering indices is valid for genetically different common rock types and useful for engineering purposes. However, loss on ignition (LOI) may provide an approximate estimation of altered minerals (clays and hydroxides) in tested rocks. It has also shown good correlation with petrographic indices and engineering index properties. Among the petrographic indices, crack density (ρcr) and unsound constituent (an input to the micropetrographic index) indices provide good correlation with engineering index properties. To quantify the extent of weathering in terms of strength degradation due to weathering in rock, an index is suggested - strength ratio (Rs) - which is the percentage of uniaxial compressive strength (σc) of weathered rock with respect to σc of fresh rock. Its significance is shown statistically through the relationships with other indices for several rock types including sedimentary and metamorphic rocks.

Keywords: Weathering indices; Uniaxial compressive strength; Engineering index properties; Chemical weathering index; deformational modulus; Crystalline rocks』

Introduction
Previous work on weathering indices
　Chemical weathering indices
　Mineralogical and petrographical indices
　Engineering indices
Rock material
　Malanjkhand granite
　Nagpur basalt
　Delhi quartzite
　Recognition of weathered rock material and sampling
Laboratory investigation
　X-ray diffraction method
　Transmitted light microscopy
　Geochemical study: X-ray fluoroscopy
　Geotechnical study
　　Specimen preparation
　Physical index properties
　Strength index tests
　　Schmidt hammer test
　　Unconfined compressive strength test
Mineralogical and textural changes
　Delhi quartzite
　Malanjkhand granite
　Nagpur basalt
Geochemical variations
Index properties
Results and discussion
Chemical weathering indices
Petrographical indices
　Microfracture index
　Crack density
　Micropetrographic index
Engineering indices
　Strength index properties as weathering indices
　Quick absorption index
　Coefficient of weathering
　Slake durability
　Proposed new index
Conclusions
References