Mechanical behavior and damage constitutive model of biocement-geogrid reinforced sand under hydro-chemo-mechanical coupling

Enzyme-induced carbonate precipitation (EICP) has emerged as a promising technique for soil stabilization, notably enhancing the strength of sandy soils. However, EICP-treated sand exhibits low ductility and brittle failure behavior, limiting its application in geotechnical structures. To address this issue, this study proposes the incorporation of geogrid reinforcement to improve both the strength and ductility of EICP-treated sand. To evaluate the mechanical behavior of biocement–geogrid reinforced sand (BGRS), a series of laboratory tests, including unconfined compressive strength (UCS) tests and particle image velocimetry (PIV) tests, were conducted to investigate the influence of immersion duration in a weak-acid environment on both reinforced and unreinforced EICP-treated sand. Additionally, a digital image processing technique was employed to quantitatively analyze microstructural damage. A constitutive model under hydro-chemo-mechanical coupling was developed based on statistical damage mechanics theory. The results show that geogrid reinforcement significantly improves both the strength and ductility of EICP-treated sand. With increasing immersion time, the strength and elastic modulus of the BGRS decline progressively, accompanied by a shift in failure mode from shear failure to expansion failure. Microscopic analysis reveals that the degradation is caused by acid–base reactions and the effect of water weakening, which promote the development of internal voids and consequently reduce the cementation efficiency of EICP. Furthermore, the proposed damage constitutive model exhibits good agreement with the experimental data, effectively capturing the failure behavior of the BGRS under hydro-chemo-mechanical coupling. These findings provide useful insights for understanding and evaluating the mechanical behavior of biocement–geogrid composite systems under water weakening and weak-acid environments.