In the process of expanding ballasted railway capacity, there is a significant increase in train axle load and speed, which leads to significant mud pumping disease under multi-stage/multi-frequency train load-wetting coupling, and its mechanism is still unclear. Mud pumping model tests from ballasted track subgrades under multi-stage/ multi-frequency train load-wetting (MSC-W test/MFC-W test) coupling were conducted. The test results show that in the unsaturated state, the accumulated deformation of MSC-W test is more significant than that of MFC-W test, and the compactness of the subgrade filler is greater without significant particle migration. Under saturated or near saturated conditions, the MSC-W and MFC-W tests produces significant mud pumping by the driving force of dynamic pore water pressure. The amounts of mud pumping, fine particle layer displacement and void contaminant index (VCI) of the MFC-W test are significantly higher than those of the MSC-W test.

Existing ballasted track subgrades are prone to complex particle migration problems due to intermittent train load-rainfall wetting coupling, which causes mud pumping in severe cases. In this work, a model test on a ballast layer overlying a fine particle layer was conducted under intermittent load-wetting coupling conditions. The experimental results indicate that the coupling effect of intermittent loading and wetting has a significant effect on the increase in the volumetric water content and pore water pressure. The changes in the accumulated deformation, resilient modulus, damping ratio, and particle migration phenomenon mainly occur in the first three loading stages (LS1-LS3 stages), and the changes are most significant in the second loading stage (LS2 stage) because of the high saturation and low density of the soils. During the subsequent loading stages, the changes in the accumulated deformation, resilient modulus, damping ratio, and particle migration phenomenon are not obvious because of the high density of the soils. A low level of resilience occurs during intermittent periods (IS4-IS7). At the end of the test, the ballast fouling index (FI) was 16.4%, reaching a moderate fouling level. Timely replacement and rectification should be conducted for sections that produce mud pumping and ballast fouling.

Microbially Induced Calcium Carbonate Precipitation (MICP) provides an environmentally friendly solution for reinforcing large diameter monopiles for offshore wind turbines (OWTs). This study presents an investigation into the lateral responses of monopiles with precast microbial reinforcement using a low-pH one-phase method. Both static and cyclic loading tests were carried out. The results of static loading tests show that the failure mode of the bio-reinforced monopile was an overall overturn failure. The lateral bearing capacity was increased by 50% and the bending moment was reduced by about 25% with the bio-reinforcement. Further investigation was conducted on the secant stiffness, damping ratio, and accumulated deformation of the bio-reinforced monopile under various cyclic loadings. With the bio-reinforcement, the accumulated deformation under one-way cyclic loading can be reduced by 30%-60%. The influences of cyclic loading parameters and loading sequence on pile stiffness were clarified. The growth ratio of pile stiffness due to bio-reinforcement under one-way loading was between 1.65 and 2.82, and decreased with increasing load amplitude. The ratio was smaller under two-way loading. Three competing factors on pile stiffness were identified: cyclic compaction of the unreinforced sandy soil, weakening of the bio-reinforced soil and soil subsidence around the bio-reinforced soil.

The cumulative deformation properties of subgrade soil under cyclic traffic loads are critical for optimizing pavement structure design and ensuring long-term highway structural performance. This study aims to investigate the coupling effect of freeze-thaw cycles and cyclic loads on the cumulative deformation behaviors and meso-structure of coarse-grained saline soil (CGSS) subgrade filling in high-cold areas. Dynamic triaxial tests and computed tomography (CT) scanning were conducted to analyze the CGSS under different working conditions. The research focused on the dynamic deformation development and damage evolution under varying freeze-thaw cycles and load amplitudes. The research results show that the cumulative deformation behavior of CGSS under cyclic loading is relatively sensitive to the freeze-thaw process. The cumulative dynamic strain increases as the freeze-thaw cycles, with a critical freeze-thaw cycle number of five. The stable cumulative dynamic strain curve exhibits clear three-stage characteristics when plotted in semi-log coordination, with critical loading cycles at 20 and 1,000. After 10-100 loading cycles, the cumulative strain curve quickly shows failure. The CGSS's low density and pore regions greatly increase after a freeze-thaw cycle. The rise in dynamic stress amplitude notably affects the bonding between soil particles and crystalline salts. The coupling effect of the freeze-thaw cycle and dynamic activity exacerbates the deterioration of soil structure, resulting in variations in CT values within the scanning layer in the final state.