This study proposes using a xanthan gum-biochar-coconut fiber composite

termed bio-matrix

to amend the topsoil of expansive soil fill slopes

addressing issues of cracking and poor vegetation growth. The response surface methodology (RSM) was employed to analyze the cracking and strength characteristics of expansive soil amended with bio-matrix at different ratios during dry-wet cycles. The analysis revealed the effects of individual factors and their interactions on the crack ratio and shear strength of expansive soil. Underlying mechanisms were further investigated using scanning electron microscope (SEM)

X-ray diffraction (XRD)

and tensile tests. Results indicated that all factors significantly influenced the crack ratio and shear strength of expansive soil. The primary interactions were observed between xanthan gum and coconut fiber

with negligible interaction among other factor pairs. The high viscosity and colloidal properties of hydrated xanthan gum enhanced the interfacial bonding between soil particles and fibers

intertwining with coconut fiber to form a stable three-dimensional network

thus improving soil tensile strength. Multi-objective satisfaction function optimization identified the optimal bio-matrix ratio: 0.60% xanthan gum

3.60% biochar

and 0.24% coconut fiber. Compared to unmodified expansive soil

the optimized bio-matrix reduced the crack ratio by 71.04%

increased shear strength by 93.73%

and enhanced vegetation coverage by 223.77%. These findings enhanced a theoretical foundation for the application of bio-matrix in ecological protection of expansive soil slopes.