Sandy slope erosion leads to coast degradation and exacerbates coastal zone instability and failure. As an eco-friendly engineering technology, microbial-induced calcium carbonate precipitation (MICP) can provide a protection method against sandy slope erosion. In this study, a series of flume tests were conducted to investigate the wave erosion resistance of the MICP-treated sandy slope. The penetration tests were conducted to measure the slope surface strength, and the calcium carbonate content was evaluated by the acid washing method. The scanning electron microscope (SEM) was employed to study the microstructures of MICP-treated sand particles. In addition, the influence of MICP treatment on the wave shape and the excess pore water pressure was also analyzed. Results show that after four MICP treatments, the erosion resistance of the slope is significantly promoted, and no apparent erosion occurs after wave actions. The penetration resistance is also improved after MICP treatments, and the maximum penetration resistance of untreated and four-time MICP-treated slopes are about 0.14 MPa and 2.04 MPa, respectively. The calcium carbonate content on the slope surface can reach 7%. SEM analyses indicate that the intergranular bridging calcium carbonate crystals promote the wave erosion resistance of the sandy slope.

The surface roughness, surface hardness, tensile properties and friction-wear properties were characterized, in comparison with those of the traditional turned cutting, electropulsing treatment (EP) and ultrasonic surface-rolling process (USRP) sample. The surface microstructure was obviously refined after USRP and EP-USRP, with a fine-grain depth of 60 μm and 100 μm, respectively. The surface roughness significantly decreases at first, and then gradually increases after surface-strengthening modification. The lowest roughness of 0.035 μm and 0.040 μm is obtained for the USRP and EP-USRP samples, respectively, which is about 12 times less than that of the turning surface roughness of 0.421 μm. The surface hardness increases from 280 HV to 360 HV after strengthening modification. The super tensile property of 30CrNi2Mo steel is obtained for the USRP, for which the yield strength, tensile strength, elongation and yield-to-strength ratio are 743 MPa, 961 MPa, 11% and 0.773, respectively. The friction coefficients in the turning state, USRP and EP-USRP are 0.37, 0.35 and 0.4, respectively. Ultrasonic composite-strengthening modification can increases the surface hardness, and obtains gradient microstructure on the material surface, which endows the material with better surface properties.