DESIGN OF FILTER FOR GRADED CORE MATERIAL

Abstract

Attempt has been made to solve the problem of designing filter for dam core materials that are internally unstable as in number of cases the conventionally designed filters have not been able to check this instability . The experiments have been conducted for this purpose in a cylindrical container of internal diameter 60 cm and height of 140 cm . The layers of core material , fine filter and coarse filter having thickness of 20 cm each were placed one over the other, keeping the coarse filter layer at the bottom on an iron grid. A wide range of hydraulic gradient (3 to 50) was applied with the help of overhead tank and seepage/leakage and soil contents in it were measured . The first test (Test "A" ) has been conducted to test if an unstable material with conventionally designed filter is actually unstable and gives rise, to the formation of sinkhole. The second test (Test lign) has been per-formed in accordance with Sherard's recommendations with the provision of a fine to medium sand layer between the core and the fine filter layer. The third test (Test "C" ) has been conducted to find out if the guidelines given by the U .S.B.R . for design of filters for soils having maximum particle size more than 4.75 mm could be used for stabilizing the unstable core material with the provision of filters accordingly. Therefore, in the three tests the core material used is the same but the filtering layers are different . It has been observed from the test "A" that it is not possible to make the internally unstable core material stable with the provision of conventionally designed (Terzaghi filter criterion) filter. With the application of hydraulic gradient of about 50 a large quantity of leakage (iv) and the formation of sinkhole have been observed. In thetest "B" , it has been observed that the provision of the sand layer makes the unstable core material stable with respect to migration of particles and seepage. Therefore, the recommendation of Sherard are useful *Inthe test "C" also the core material was observed to be stable with the provision of filter layers designed with the, guidelines of U.S.B.R. Therefore, the problem of instability of core material could be solved by either provision of sand layer or designing filter according to U.S.B.R. guidelines. However, the second alternative seems better from the point of economy of construction.