Correlations between chromaticity parameters and shear strength of granite residual soil with different free iron oxide content and moisture content

In hot-humid regions, granite residual soil's moisture and free iron oxide contents show marked spatiotemporal heterogeneity tied to seasons and weathering, creating an inherent connection between its visual color and mechanical properties. This study carried out indoor chromaticity parameter measurements and shear strength tests on residual granite soils in southeastern Guangxi with varying moisture and free iron oxide contents, acquiring CIE-L*a*b* color indices and shear strength parameters. Using path and regression analyses, it explored correlations between chromaticity parameters (L* [lightness], a* [red-green chroma], b* [yellow-blue chroma]) and shear strength indices (internal friction angle φ, cohesion c). Results showed that at constant moisture, rising free iron oxide content positively correlates with cohesion, a* and b*, but not with L*; φ has no significant links to any chromaticity parameter. At fixed free iron oxide content, cohesion has a nonlinear correlation with L*, a* and b* as moisture increases, while φ is significantly positively correlated with these chromaticity indices. The findings verify that chromaticity parameters can effectively and rapidly estimate granite residual soil's shear strength, providing valuable references for geotechnical evaluations.