A precise understanding of strength–dilatancy response in marine coral sand containing fines is crucial for ensuring the stability of offshore infrastructure. This study systematically investigates the impact of fines on friction angle and maximum dilatancy angle, as well as their relationship, through meticulously controlled geotechnical tests, complemented by in-depth discussion and interpretation. The findings reveal that as fines content rises, the friction angles at both peak ( \({\varphi }_{\text{ps}}\) ) and critical states ( \({\varphi }_{\text{cs}}\) ), as well as the excess friction angle ( \({\varphi }_{\text{ex}}\) ) and maximum dilatancy angle ( \({\psi }_{\text{max}}\) ), show a notable decline. Conversely, a higher relative density leads to an increase in \({\varphi }_{\text{ps}}\) , \({\varphi }_{\text{cs}}\) , \({\varphi }_{\text{ex}}\) , and \({\psi }_{\text{max}}\) . Additionally, higher stress levels lead to a reduction in \({\varphi }_{\text{ps}}\) and \({\psi }_{\text{max}}\) , while their impact on \({\varphi }_{\text{cs}}\) and \({\varphi }_{\text{ex}}\) remains inconclusive, possibly due to minimal particle breakage under elevated stress conditions. Furthermore, this study defines the upper and lower bounds of variation in \({\varphi }_{\text{cs}}\) and \({\varphi }_{\text{ex}}\) relative to \({\varphi }_{\text{ps}}\) across different stress levels and density states. The transition region where the mixture shifts from sand-controlled to fines-dominated behavior was examined. A significant observation is the relationship between \({\varphi }_{\text{ex}}\) and \({\psi }_{\text{max}}\) , showing that Bolton’s strength–dilatancy theory, initially developed for clean sand, still applies to marine coral sand provided the fines content stays below a specific limit. This insight highlights the necessity of accounting for fines content when applying established dilatancy model to marine coral sand.
