EFFECT OF MOISTURE CONTENT ON STRENGTH OF ROCKS

Abstract

The strength of intact rock is an important parameter in evaluating its stability under the loads imposed by the construction of civil engineering structures. The measured laboratory strength is influenced by a number of factors which could be categorised as (i) mineralogical factor e.g. mineral composition,-- cementing material„ structure and fabric,' (ii)( mechanical factors e.g. sampling technique, mode of sample: --- )reparation, shape and size of the specimen;' type of testing machine used, rate of loading etc, (iii) environmental factors e.g. moisture content, nature of pore-fluid, temperature etc. By keeping the Mineralogical & mechanical factors same;, the strength - - 1,(te behaviour for a given rock type is essentially governed by environmental factors. Among these factors, the effect of moisture content is significant. Most rock(17-Os their strength on coming in contact with moisture. It is well known fact that most of underground rocks contain moisture varying from less than 1 percent for some evaporites to over 35 percent for highly porous rocks. In most of underground excavations rocks are nearly saturated. The presence of moisture invariably reduces the rock strength.. In case of foundation works for heavy structures e.g. dams, power houses etc., compressive strength of foundation rock is an important parameter to be used in foundation design. Also tension failure of rock is one of the most commonly observed phenomenon during underground excavation operation and during working of floors and roofs in mines and tunnels. It is thus imperative that the influence of varying moisture on compressive strength and tensile strength characteristics of rocks should be studied. It is with this view that the present work has been taken up to study the effect of moisture content on the four types of rocks e.g. Delhi quartzite, Vindhyan sandstone, Shivalik sandstone-I and Shivalik sandstone-II. The influence of moisture content on uniaxial compressive strength, tensile strength and point load strength index has been evaluated. Attempt has also been made to establish relationship between different strength characteristics. The moisture content is found to have varying influence on strength characteristics of these rocks. Study indicates that there is no significant influence of moisture content on strength. characteristics of Delhi quartzite. For sandstones, reduction in compressive and tensile strength has been observed. By comparative study, it has been observed that low strength sandstone suffers more reduction in strength than do high strength sandstone. Also, effect of moisture was found more on tensile strength of rocks as compared to compressive strength.