A good number of projects have been constructed and several others are under construction/investigation in Eastern India. Experience in working in these projects dictates that the weak features of these sites are of various types and mostly hidden under thick overburden cover. The tropical climatic condition and high rain fall are responsbile for the formation of thick overburden and vegetation cover. In the high altitudes of the Eastern India including the Himalayan terrain, there are the extensive action of ice and water which have formed thick glacial, fluvioglacial and hill wash deposits. In general loose surficial deposit and weathering of rock to various extent are seen in most of the river valleyproject sites of Eastern India. For example, the Umling Dam Site (Fig. 1) of Meghalaya consists of thick riverine material and also thick surficial deposit followed by a mantle of weathered rock of considerable thickness. Sub-surface exploration mainly by drilling followed by water pumping tests is therefore carried out in almost all river valley projects to evaluate the rock condition at depth. Weak features like faulting, shattering, shearing and jointing are very prolific in the rock in many of the projects of Eastern India. Presence of soft clay seams, kaolinization of rock and karstic condition are some of the other weak features recorded from some project sites. This paper deals with the importance of water pumping test to reveal such weak features in hard rock with secondary permeability. Analyses of these weak features in hard rock indicate that the weak zones are highly permeable. It is, therefore, of importance that permeability characteristics of the weak zones be determined in relation to the intensity of fractures to identify and delineate the exact position of weak zones at depth.
Through a drill hole, a perforated tube fitted with two rubber packers at the two ends is lowered down to the depth where test is to be conducted. Water from a tank is pumped to the test section under varying pressures. A pressure gauge and a water meter record the applied pressure and water loss respectively.
The most important consideration for this test is the use of right type of packer which can be tightly fixed with the rock wall of the drill hole without any chance of leakage through the contact planes of the packer and the rock. In recording the pressure it is to be ensured that the indicator moves equal distance to and froe with respect to the 'mark' noted as the applied pressure.
The quantum of water that permeates through the rock fractures in a test zone per minute should be the same in the consecutive minutes provided there is uninterrupted flow. In practice test is conducted for 5 to 10 minutes and average value of per minute flow is taken so that in case of clay filling in joints, this will be flushed off in process of testing.