STUDY ON ROCK CHARACTERISTICS UNDER THE EFFECT OF CHEMICALLY ACTIVE AND SILT CHARGED WATER

Abstract

Acid rain has much more serious consequences for the environment than mere previously known' according to recent research carried out at Frankfurt University in Germany. In addition to the vast expanses of trees which are dying, birds atter -re cent 66til sricut are also feeling its impact. A few years agov the upper reaches of Kashmir Valley also experienced monsoon rains having chemically active rain water, Therefore, the rockmass exposed to atmosphere may not remain unaffected due to water having chemical impurties. Rock mass is exposed to rain water, sea water, and river water(generally fed from snowy mountains). Water acts as an important agent in weathering of rocks. The condition may become critical when normal water become chemically contaminated. Such conditions may exist in rocks adjacent•to underground scheme prepared for waste disposal from industries, storage for explosives, oils, hard water and various toxic chemicals due to their leakage, evaporation etc. Some times effluent dicharged from various industries such as fertilizers,- pesticides, tannery etc. are toxic in nature. When such effluent come in contact with normal water, it alters the quality and may have a negative effect on strength characteristics of adjacent rocks. The rivers in Himalyan region carry lot of sediment in the form of boulders, sand, silt etc. flowing with water at high velocity and adjacent rocks are in continuous attack of the high velocity silt charged water. This causes the abrasion of rocks in diversion tunnels as well as various components/structures of hydro electric projects such as spillway profile, turbine blades etc. It is with this view that the present work has been taken-up to study the effect of chemically active water on the strength characteristics of rock material. Besides, the effect of silt laden high velocity water jet has also been studied separaterly for the abrasive resistance of different rocks. In order to predict probable effect of chemically active water on rock characteristics, 84 nos core specimens of 38/54 mm diameter with length to _diameter ratio of nearly 2 from different type of rocks namely sand stones, lime stone, and silt stone were prepared and tested under different conaitions of tests Viz. acidic, alkaline, and normal water. Besides this some physical/mechanical properties of the rocks were also determined, which essentially includes porosity, water absorption, specific gravity and point load strength index on rock lumps. The analysis of test results indicate that in general, the unconfined uniaxial compressive strength reduces significantly with soaking period in acidic and normal water, whereas reduction in the strength in alkaline water was observed only during initial stage upto :3 days and thereafter the strength increases with soaking period The results also indicate that the tensile strength reduces more rapidly as compared to compressive strength under different conditions of tests. In order to have an idea about the abrasion resistance of rocks under the impingement of sediment laden water flowing at high velocity, specimens of three different type of rocks namely sand stone, lime stone and quartzite -were tested in a high velocity water jet appratus with sand feeding arrangement for 13/26 hrs under similar conditions. The results of abrasion study indicate that average loss in mass per hour is maximum in sand stone (i.e. 2.54 gm/hr) and minimum in quartzite (i.e. 0.27 gm/hr). However in case of lime stone it is slightly higher than quartzite (i.e. 0.35 gm/hr). This is because of the fact that the quartzite and lime stone having very high compressive strength offer more resistance to abrasion as compared to sand stone of low strength.