Abstract: The content of coarse grained soils and their grain sizes have an important influence on the stressstrain characteristics of fined grained soil mixtures. This paper presents the valuation influence of coarse grain content on the geotechnical properties of fine grained soil mixtures throughout large scale insitu direct shear tests. After each test, the grain size distribution of each serious of sample were laboratory tested. The results indicate that: The peak shear strength value is proportional to the content of coarse grained materials. When the grain size range D60 is limited within 0.02 to 0.05 mm, both the shear strength and internal friction angle increase with the increase of grain size; when the grain size D60 is greater than 0.05mm, with the increase of grain size, both the rate of internal friction angle increase and the apparent cohesion reduction rate increase.

Key words: fine grained backfill soils, coarse grained group, stressstrain characteristics, shear strength, insitu large scale directd hear test