A series of laboratory tests were conducted to investigate the properties of fiber-reinforced underwater flowable solidified soil (UFSS) as a novel material for scour protection in marine structures. The tests included flowability, underwater anti-dispersion, unconfined compressive strength (UCS), and anti-scour resistance. Results showed that adding fibers reduced UFSS's flowability and significantly enhanced its underwater anti-dispersion, exhibiting a similar trend with increasing fiber content. Increasing fiber length initially decreased and then increased flowability, with the opposite trend for anti-dispersion. The least favorable fiber lengths for flowability were 6 mm for PVA fiber and 9 mm for both basalt and glass fibers, whereas these lengths were optimal for antidispersion. Fibers improved both UCS and anti-scour resistance of UFSS, with both properties first increasing and then decreasing as fiber content and length increased. Excessive fiber content or length reduced both properties. In this study, the optimal fiber content for improving UCS was 0.3% for PVA and 0.2% for basalt and glass fibers, with an optimal length of 6 mm for all three. An empirical exponential relationship between UCS, critical scour resistance velocity, and critical scour shear stress at typical times (t = 3 h, 5 h) was established for rapid prediction of UFSS's anti-scour resistance.

To enhance the resistance to local scour around offshore wind turbine monopiles, 15 mixtures were designed based on Response Surface Methodology (RSM). Cement content, sodium silicate content, and rubber powder content were selected as independent variables. After determining their flowability, the compressive strength and shear strength were measured after curing in pure water and artificial seawater for 3 days, 7 days, 14 days, and 28 days. Experimental results indicate significant improvement in the mechanical properties of the modified soil, including increased Unconfined Compressive Strength (UCS), internal friction angle, and cohesion. The optimal mix ratio is identified as CSR40-10-15, consisting of 40% cement, 10% sodium silicate, and 15% rubber powder. The strength variation mechanism is elucidated from both macroscopic and microscopic perspectives. Finally, numerical simulations using Computational Fluid Dynamics (CFD) software validate the scour resistance performance based on the optimal mix ratio of flowable solidified soil, offering a new approach for local scour protection around offshore wind turbine monopile.