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Although direct shear test has some limitations in test procedure and test results, still it is one the popular and prevalent test which is used in most laboratories for evaluation of shear strength parameters of soils. Most of direct shear test apparatus in laboratories are in the specimen dimensions ranging from 6*6 to 10*10 centimeters. There are very limited number of large direct shear apparatus in soil mechanics laboratories with specimen dimensions of 30*30 or larger. In order to perform direct shear tests on coarse graded materials, it is recommended to degrade material to finer gradation accepted by test standards relative to the box dimensions. It is clear that degradation by using exclusion or parallel method increases fine contents of soil samples in small shear boxes. When shear tests are performed on fine graded soil samples, the strength parameters become different from those of original coarse graded samples. Therefore it is necessary to investigate relation between these different parameters. The aim of this research is to compare the test results between shear strength parameters derived from both small and large direct shear tests. Three soil samples with 5,15 and 25 percent fine contents with three 80,90 and 100 percent relative densities were tested in large and small direct shear apparatus with box dimensions of 30*30*15 and 6*6*2.5 centimeters. Shear parameters were derived and compared carefully. The results indicated that degradation has great effect on the parameters. Also it was found out that degration by parallel method compared with exclusion method gives smaller parameters. It seams that the discrepancy is the result of fine content differences.             Comparing the fine contents in two samples prepared using both methods indicates that the fine content in samples produced by parallel method have greater amount.             Comparing resulting internal friction angles of original coarse graded material tested in large apparatus with the friction angles calculated from small degraded samples give larger values in the same relative densies. The increments are 2.5 degrees for degradation using exclusion method and 4.5 to 6 degrees using parallel method. Also the test result showed that the amount of increments increase in samples with higher amount of fine contents

Keywords: Direct shear, Specimen size, degradation, Shear stremght

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