Discussion on ‘A correlation between friction angle and particle shape metrics in Quaternary coarse alluvia’ by Cheshomi et al .

Abstract

The authors have investigated empirical correlations between descriptions of grain characteristics and friction angle of some coarse-grained soils. They have defined strength as the peak total stress ratio and described this by a Mohr–Coulomb criterion with values of cohesion in the range 48–165 kPa and friction angles in the range 33–54°. They have related the friction angles to grain characteristics. These values and correlations raise a number of important issues. Soil parameters fall into two classes. There are material parameters that depend only on the grains and there are state-dependent parameters that depend also on the current stress and voids ratio. (State-dependent parameters may be modified by structure described by fabric and bonding but material parameters depend only on the characteristics of the grains.) For example, the critical state friction angle ϕ′c is a material parameter. The parameters that describe peak strength are clearly state-dependent parameters and they depend on the current stress and voids ratio. Any empirical correlation between a state-dependent parameter such as peak friction angle and grain characteristics that does not explicitly include stress and voids ratio must have very limited application. It is clear from their figure 7a that the authors are considering strength at the peak total stress ratio, which occurred at a strain of about 2.5% in these tests. The peak strength may be described by a Mohr–Coulomb criterion with total stress parameters c p and ϕp or by a maximum stress ratio criterion ϕm (= tan−1τ/σ). For uncemented soil with zero pore pressure, the peak envelope must pass through the origin (the shearing resistance of uncemented dry soil is …