Engineering properties of weak rocks have been determined as per Indian (ISI) and International (IS) standards on cubes of 10 cm X 10 cm X 10 cm and NX size drill cores having L:D ratio of 1:1 and 2:1. Tests on 2:1 size samples needed correction to bring at par with lSI, and these values also provide fairly accurate data for application with values of 10–15 % more than I.S. Hence, the values given in the text as per lSI may have to be reduced by 10–15 % while correlating with I.S. The tests include specific gravity, porosity, crushing strength (dry & wet), absorption, modulus of elasticity, impact and abrasion values, permeability etc. Central India covers an area within and around the states of Madhya Pradesh and Maharashtra.

The various tests have been conducted on over 100 samples and the data have been correlated with field characteristics for assessing their suitability as construction material for the various civil engineering structuees like masonry, concrete, earth and rockfill dams, bridges and national/state highways, embankments and construction aggregates in the light of certain specifications given by Indian Standards Institution. Specifications as per Indian standards are given below:

(Table in full paper)

The pre-Cambrian and Archaean suite of rocks have suffered intense folding and fracturing compared to the overlying layered sedimentary and volcanic rocks. These have controlled weathering and deterioration in the strength of rocks to varying degrees. The dominant physical characters rendering weakness in rocks are erodible and friable nature, fissile planes and pulverisation and intense weathering especially due t o alteration of rock mass. On the basis of these characters, the weak rocks were earlier classified into four sub-units (Vaidyanath and Ghosht,1980). Subsequently, Ghosh (1980) carried out studies on the weathering of granites and basalts and presented an engineering grade classification of these rocks for various types of civil engineering structures-dams, buildings, bridges and use as construction material. In the present context, the physical and engineering properties of weak rocks have been correlated and further classified on the basis of compressive strength limiting to 500 kg/cm2, porosity of more than 8% and low elastic properties.

Regardless of timescale, there is remarkable contrast in their physical Character, though some of the engineering properties do not show much variance.

Weathered granite-gneiss

The fine to coarse grained weathered granitic rock and associated material occupy the lower pediment zones and have quartz and felspar as predominat constituents. Four unit zone-highly stained and disintergated rock, partially structured rock with crumbly patches, structureless rock with granite boulder and granitic soil, clay and s and have been recognised from bottom to top of weathered profile(Ghosh,80).

Weathering is pronounced in the felspar rich varieties. The felspar alters into clay mineral or is sericitised The weathered granitic rock, whether partially/completely decomposed or moderately disintegrated, provides heterogeneous matrix with large core stones of unaltered rock mass set in the decomposed matrix.

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