Sand-specific interpretation of state via a modified relative dilatancy index

This paper presents a new, simple strength–dilatancy framework for interpreting the state parameter ( ξ 0 ) in sands using cone penetration test (CPT) data. The approach employs the relative density ( D r ) as a state variable, alongside the mean effective stress ( p′), instead of the traditionally used void ratio ( e). Existing industry-acknowledged CPT– ξ 0 correlations are generally found to be challenged by a stress level bias. This paper argues that stress dependency is often better captured in widely used CPT– D r correlations than in CPT– ξ 0 correlations, suggesting that inferring ξ 0 through correlations with D r may improve in situ interpretations. To facilitate this, a modified parameter ( I Rx ) is introduced, adapted from Bolton’s (1986) relative dilatancy index ( I R ) to ensure compatibility with the widely accepted linear approximation of the critical state line (CSL) in e–log 10 p′ space. The I Rx -based approach is considered applicable for stress levels below approximately 0.5–2.0 MPa, depending on sand type, due to the bilinear nature of the CSL in e–log 10 p′ space. Based on recent laboratory studies, this paper also investigates sand-specific dependencies in the parameters of Bolton’s I R , which share similarities with the parameters of I Rx . This way, I Rx is presented as a parameter useful for stress ranges covering many engineering applications, offering a promising pathway for incorporating sand-specific dependencies into strength–dilatancy relationships and CPT-based state parameter interpretation.