To address scour hazards surrounding offshore foundations, a new method employing novel alkali-activated cementitious grout (AACG) has been proposed for improvement of seabed soil. Ground granulated blastfurnace slag (GGBFS) was replaced by fly ash (FA), steel slag (SS) or FA + SS to prepare precursors, the replacement amounts were 10 %, 20 %, 30 % and 40 %. Fresh-state and mechanical properties, minerals and microstructures were investigated. A novel scour simulation test device was developed to simulate engineering conditions of scour and remediation. Flow-soil coupled scour resistance tests were conducted, shear tests and SEM measurements of solidified soil were carried out. The results showed that the optimal ratio of GGBFS:FA:SS was 6:2:2 for AACG. The optimized AACG has better fluidity and lower brittleness, and its 28 d unconfined compressive strength (UCS) achieves 13.5 MPa. For AACG solidified soil, the maximum scour depth was reduced by 33.3 % and the maximum sediment transport amount was decreased by 53.2 %, which were compared to those of cement - sodium silicate (C-S) double slurry. Moreover, the increase degrees of internal friction angle, cohesion and critical shear stress were 700 %, 7.9 % and 786 %, respectively. The scour resistance of AACG solidified soil was superior. The inherent relationship between UCS and critical shear stress was discussed. UCS can be used to rapidly assess the scour resistance of consolidated soil. This study introduced an eco-friendly AACG as an innovative stabilizer for soil reinforcement around offshore structural foundations, offering significant application and environmental values for scour control.

A new scour countermeasure using solidified slurry for offshore foundation has been proposed recently. Fluidized solidified slurry is pumped to seabed area around foundation for scour protection or pumped into the developed scour holes for scour repair as the fluidized material solidifies gradually. In the pumping operation and solidification, the engineering behaviors of solidified slurry require to be considered synthetically for the reliable application in scour repair and protection of ocean engineering such as the pumpability related flow value, flow diffusion behavior related rheological property, anti-scour performance related retention rate in solidification and bearing capacity related strength property after solidification. In this study, a series of laboratory tests are conducted to investigate the effects of mix proportion (initial water content and binder content) on the flow value, rheological properties, density, retention rate of solidified slurry and unconfined compressive strength (UCS). The results reveal that the flow value increases with the water content and decreases with the binder amount. All the solidified slurry exhibits Bingham plastic behavior when the shear rate is larger than 5 s-1. The Bingham model has been employed to fit the rheology test results, and empirical formulas for obtaining the density, yield stress and viscosity are established, providing scientific support for the numerical assessment of flow and diffusion of solidified slurry. Retention rate of solidified slurry decreases with the water flow velocity and flow value, which means the pumpability of solidified slurry is contrary to anti-scour performance. The unconfined compressive strength after solidification reduces as the water content increases and binder content decreases. A design and application procedure of solidified soil for scour repair and protection is also proposed for engineering reference.