Threshold fines content FCth is an important parameter to assess the grains contact state and the mechanical properties of sand-silt mixtures. Systematic laboratory tests are performed on 38-group test results for 14 types sand-silt mixtures. The FCth of the mixtures are evaluated by the traditional approaches in literature using different physical and mechanical properties. Furthermore

the factors affecting the FCth of the mixtures are discussed preliminarily. The results show that as the grains size difference of sand-slit mixtures increasing

the FCth determined by the static test results will be obviously higher than those by the dynamic test results. The Rahman’s empirical formula can not predict the mean threshold fines content well. The traditional approach using the maximum and minimum void ratio of clean sand or clean silt

can predict the of the mixtures with round-like grains observed in laboratory tests well. However

the of the mixtures with angular-like grains obtained by the traditional approach are obviously lower. The analysis indicates that the mean grain size ratio Rd is an important physical index for evaluating . It is seen that an increase in Rd causes a decrease in of the mixtures with Rd up to 10

thereafter causes an increase in the and remains constant ultimately. A new empirical formula is thereby established and validated based on Rd.