Intergranular state variables and stress-strain behaviour of silty sands

Relative contributions by the coarser and finer grains in a silty sand to its stress-strain response are affected by the intergranular matrix structure. The nature of this contribution is illustrated using an intergranular matrix phase diagram in terms of void ratio (e), fines content (FC), and intergranular and interfine void ratios (e(s) and e(f)). New integranular state parameters (ψ(s), ψ(f)) and (e(s), e(f)) are introduced as state variables to characterize silty sands; e(s) and e(f) dictate the steady-state characteristics of silty sand at low and high fines contents, respectively; ψ(s) and ψ(f) reflect the plastic compressibility characteristics at low and high fines contents, respectively. Using these state variables, the anticipated stress-strain-strength behaviour of silty sand in comparison to that of the host sand is presented. Similar stress-strain behaviour is expected at the same e(s) and initial confining stress σ'(c), with a few exceptions. At a constant void ratio e, e(s) increases while e(f) decreases with addition of fines; a silty sand passes through different states. First, at low fines content, e(s) (< e(max,HS), the maximum void ratio of the host sand, case 1), and high e(f), the stress-strain behaviour is primarily governed by intergranular friction between the coarser grains. The steady-state line (SSL) is primarily dependent on e(s) and is fairly independent of σ'(c); When compared at the same e(s) and at the same σ'(c) (or at the same ψ or ψ(s)), the silty sand and the host sand show similar (not identical) stress-strain behaviour; with addition of fines, as e(s) increases, the collapse potential increases and the stress-strain response becomes weaker. Second, with further addition of fines, when e(s) approaches or exceeds e(max,HS), the SSL is influenced by e(s) and σ'(c). When e(s) is near e(max,HS) (case 2), the stress-strain curves for silty sands are similar at the same e(s) and the same σ'(c) (or at the same ψ or ψ(s)), but different from and stronger than that of the host sand. At very loose states (e(s) > e(max,HS); case 3), the stress-strain curves, normalized with respect to σ'(c), are similar at the same e(s) (or at the same ψ or ψ(s)). Third, with further addition of fines beyond a threshold value, e(f) becomes sufficiently low (case 4); the fines impart a significant influence, while the role of intergranular (coarser-grain) friction diminishes; the silty sand is expected to behave similarly (not identical) to the host fines at the same e(f) and ψ(f); at this stage, with further addition of fines, the collapse potential decreases.