7.4.4 Other differences with the Hardening Soil mo...
The shear hardening flow rule of both the Hardening Soil model and the Hardening Soil model with small⁃strain stiffness have the linear form：
The mobilized dilatancy angleψm in compression，however，is defined differently.The HS model assumes the following：
where φcv is the critical state friction angle，being a material constant independent of density；ψm is the mobilised friction angle：
For small mobilised friction angles and for negative values ofψm，as computed by Rowe's formula，ψm in the Hardening Soil model is taken zero.Bounding the lower value ofψm may sometimes yield too little plastic volumetric strains though.Therefore，the Hardening Soil model with small⁃strain stiffness adapts an approach by Li and Dafalias（2000）wheneverψm，as computed by Rowe's formula，is negative.In that case，the mobilised dilatancy in the Hardening Soil model with small⁃strain stiffness is calculated by the following equation：
where M is the stress ratio at failure andη＝max
Eq.7.18 is a simplified version of the void ratio dependent formulation by Li and Dafalias（2000）.
The mobilized dilatancy as a function ofψm for the HS small model is visualised in Figure 7.15.
Figure 7.15 Plot of mobilised dilatancy angle m and mobilized friction angle mm for HSS model（Brinkgreve，2017）