Internal erosion, especially suffosion, can detach fine particles and enlarge the void spaces within the soil matrix. This can lead to mass loss, soil strength deterioration, severe deformation, instability, and, eventually, geo-structure failures. In recent decades, significant research has been conducted on soil behaviours subjected to internal erosion, including laboratory tests and DEM simulations. These tests have revealed various aspects of soil behaviours during and after erosion, such as the decline in soil strength or the transition from dilative to contractive behaviour. Overall, these findings provide valuable insights into the complex mechanical responses of soil subjected to internal erosion. On the other hand, constitutive modelling for internal erosion has had limited development, with only a few models addressing post-erosion soil behaviours and neglecting the erosion process itself. To address this gap, this study established a unified constitutive model based on observations acquired from DEM simulations of drained triaxial erosion tests. The newly established model incorporates the Clay and Sand Model (CASM) as its foundation with a new hardening law induced by mechanical loading and erosion. A simple sub-loading surface concept is also incorporated to provide a smooth transition from contraction to dilation responses. The modified CASM model for erosion, namely CASME, can predict post-erosion behaviours such as peak strength reduction, transitions from dilation to contraction, and soil collapses observed in DEM simulations. More importantly, the CASME model successfully captures soil behaviours throughout the entire erosion process across various pre-shearing stress ratios or levels of mass loss, encompassing erosion-induced sample deformations and the movement of CSL as the reference void ratio evolves. Applications of the CASME model to boundary value problems using the newly developed five-phase SPH framework recently developed by the authors will also be demonstrated in this presentation.