SHEAR CHARACTERISTICS OF GRAVEL-SILT MIXTURES UNDER HIGH STRESSES

Abstract

Natural stream beds usually has- coarse ranging from boulders to sand and fine-Matr1X.Irom fine sand to clay. The proper under—standing of thear.character4stic of formations with mixtures of coarse and fine-gral,led soils is important for the safe and economic design of foundations, problems of stability of slopes etc.`- utilizing such formations. Due to small size of: the sample the .conventional type of direct shear machifes.can not accomodate samples with par- certain size, Thus it is essential to utilize shear testing machine with:CaPability of accomo-dating large size Samples. Also the'MaChinespreferably should have the capability to test sample's under high stresses which is of relevance in case of large daMp,and other simi- lar structures. A large size direct shear maaine designed and fabri-cated by Ghanekar (1984) has been used in the investigation. The machine has the capability to accomodate sample of 1000 sq. cm. in area and with maximum normal stress capacity of 20 Kg/cm.2 The performance of the machine has been demo- nstrated by Ghanekar (1984) through tests on gravel—sand mixtures under high magnitudeomormal stress. Two materials namely gravel comprising of particle size 50 mm to 6.3 mm and silt comprising of particle size 75 p 2 p has been 'used in the present study. The finer material i.e. silt was mixed with coarse material i.e. gravel in different percentages ranging from 0 to 100 by volume in dry state. Tests were conducted on various mix-tures using values Of normal stresses varying from 3.53 to 19.48 Kg/cm.2. Test's Were conducted on'satUrated samples also. Thus the Variable percentage of finer material in a sample, void ratio, normal stress and, effect of saturat-ion of sample were the variables considered in the study. Based on the analysis of.test data it has been concl-uded that (i) for gravel-silt mixtures under high normal stress Mohr's envelope is curved and follows the law : max = a o- b where 'a' and 'b' are constants. The const-ants 'a' and 'b' have been evaluated and also a. relation-ship established between the two. (ii) the percentage ti of finer matrix present in the sample plays a significant role in the shear strength of the mixture. (iii) horizon-L,11 displacement at failure for samples. varies in a wide range ( 6 to 20 mm) whereas in saturated samples the range is much narrower. ( 6 tol4 mm ) (iv) at a constant void ratio and at a particular normal stress the maximum shear stress (- max ) and angle of internal friction (CI)) decrease continuously with the increase in the finer fraction with highest value at 100 percent (v) at a particular normal stress with constant percentage of finer fraction the magnitude of maximum shear stress and angle of internal friction decrease with increase in void ratio (vi) In case of dry mixes at a constant percentage of finer fraction, the modulus of deformation increase linearly with increase in applied normal stress.