SHEAR CHARACTERISTICS OF ROCKFILL-SAND MIXTURES UNDER HIGH STRESSES

Abstract

In several constructions eg. rockfill darns, 0'1_04- to 'arca granular piles etc. mixtures of rockfill and sand are used. Further, many a times the stresses in these rockfill—sand Tftw mixtures may be very high. Design of such structures necessitates an understanding of shear characteristics of these mixtures of coarse and fineg rained soil under high stresses. Conventional type of shear machines can not accomodate large size particles. Therefore shear machines which can accomodate large size samples are necessary. Large sample size direct shear machine designed and fabricated,by Ghanekar (1984) has been used in the present'investigation. The machine can accomodate samples (square in plan), 1000 sq.cm. in area and 25.0 cm. deep. The machine has the capacity of applying normal stress upto 20.0 Kg./cm.2 . Two materials namely rockfill (crushed quartzite rock particles) with patticle size ranging between 20 mm, to 50 and locally available river sand having particles ranging from 75i1 45.1μ have beeh used in the study. Sand is mixed with rockfill in five different percentages i.e. 0/ ,. 25/ 50/ , 75/ and 100/ by volume in dry condition to give samples for different test series . Tests have been performed at • two different relative densities of 60/ and 20/ . Tests 7eD have been carried out at five different normal stresses ranging from 1.0 to 20.0 Kg./cm.2 . Continuous record of shear stress and shear displacement are taken during each test which was continued upto failure. The variables considered in the study are - (0 applied normal stress (ii) percentage of sand in-a specimen and (iii) relative density. Based on the analysis of test data, it has been observed that in case of rockfill-sand mixtures under high applied normal stress, Mohr's envelop is max = curved and follows the law : T a a b where 'a and 'b' are constants. The values of these constants have boon obtained from various tests and factors influ-encing these constants are discussed. Comparision of the values of these constants has also been made with the data reported by various investigators. The analysis of test data reveals that shear strength of rockfill-sand mixtures is governed by the percentage, of finer vis-a-vis coarser material present in the sample, percentage strain_at failure remains practically constant, verticle compression iv ..JL' " • ' of sample depends on the stress level'lnd modulus of deformation increases with increase in applied normal-iiress, Fiiriher, spacing between two halves of the shear box is governed by the particle size of the material pre-sent in Larger percentage in the mixture and the particle breakage during the shear test is a function of normal stress being higher for rockf ill than for sand