Insights on Threshold Fines Content

ABSTRACT: As increasing amounts of non-plastic silt are added to a sand, its classification transitions from being a sand to being a silty sand to being a sandy silt and eventually to being a silt. This transition leads to a change in the soil’s behavior from sand-like to silt-like with a corresponding increase in compressibility and decrease in both shear strength and resistance to liquefaction. Numerous studies have shown that this change in behavior occurs over a relatively narrow range of silt contents. This range is referred to by several names in the literature, including threshold fines content (TFC). The threshold fines content represents the silt content at which the soil begins to transform from a sand matrix, in which the silt particles are entirely contained in the voids between the sand grains, to a silt matrix that contains isolated sand grains. Below the threshold fines content, the soil behaves essentially as a sand; above the threshold fines content, the soil behaves essentially as a silt. While the concept and importance of the threshold fines content has been increasingly recognized over the last 20 years, several aspects of it have not been widely discussed in the literature. This paper will focus on four of these aspects: the existence of both an upper-bound and a lower-bound threshold fines content for a given soil, the range and distribution of threshold fines content upper-bound and lower-bound values for natural soils, the effect of relative density on the threshold fines content, and the behavior of soils with fines contents between the upper-bound and lower-bound threshold fines content.