Shear strength characteristics of sand-gravel mixtures in high gravel contents

Shear behavior of sand-gravel mixtures is an important issue in geotechnical engineering due to extensive use in earth dams, borrow materials and slope stability problems of natural or man made slopes. Experimental study of behavior of this type of material is usually difficult due to problems associated with use of large testing apparatus or large specimens. Several researchers have proposed methods for the test of these materials by removing larger fraction of them and testing finer matrix. In this research, a regular set of large direct shear tests with a 300*300*160 mm shear box performed to determine shear strength and dilation characteristics of sand-gravel mixtures in high gravel contents. Bad graded fine sand was used besides uniform gravel grains with maximum grain size of 12.5 mm as testing materials. The tests conducted on mixtures prepared in four gravel contents of 70, 80, 90 and 100 percents. Samples constructed in three relative densities of 35, 60 and 85% and tested by different surcharge pressures of 100, 200 and 300 kPa. Experiments conducted on completely dry samples in consolidated drained condition. Shear strength behavior of mixtures was investigated and it was concluded that both shear strength and friction angle increases with gravel content, relative density and surcharge pressure. However, dilation increases by increase in gravel content and relative density and reduces with increase in surcharge pressure. Results were compared with previous experiments on mixtures with less gravel contents and it was observed that in little gravel contents the behavior is controlled by the both sandy matrix and gravel grains. However, as gravel content increases from floating limit, the behavior is mainly controlled by gravelly part of mixture and sand fraction has little effects on its behavior.