It is of great importance to study the behavior of loess moisture infiltration based on the loess permeability velocity to prevent geological disasters by loess collapsibility and get effective solutions for the problems of practical engineering. In this study, a custom-made permeation device was used to measure the permeation velocity of collapsible loess before and after water immersion and consolidation. The infiltration characteristics of the undisturbed collapsible loess under different consolidation pressures were discussed, and the influence of the pore characteristics on the permeability velocity was analyzed by the mercury injection test. Meanwhile, the correlations of various types of pores on the permeability velocity were studied by the gray relational entropy method to further reveal the water infiltration mechanism in loess. The results showed that the soil is compressed after consolidation and immersion tests, and the pore volume and pore diameter decreases. However, permeability velocity increases rather than decreases, and the self-weight collapsible state has the most significant permeability velocity. To the collapsible loess, the permeability velocity decreases with the increase of soil pore area. Additionally, small pores show the strongest correlation with permeability velocity. The conclusions of this study have a positive role in guiding foundation treatments and slope stability in collapsible loess areas.

It is practical to carry out sponge cities to manage rain and floods in collapsible loess areas where water resources and water disasters are prominent. The infiltration laws of the partial anti-seepage bioretention in collapsible loess fields are helpful to ensure the effectiveness and safety of sponge city, which were learned from the field test and numerical model. The seepage field and displacement field of loess sites with different collapsibility grades were compared during rainwater infiltration of the bioretention with the numerical model; the suitability and optimization suggestions for foundation treatment of this structure in various sites were proposed. It is found that the infiltration characteristics can be divided into three stages, and the infiltration range of bioretention increases with increasing infiltration time under the same site type, and the higher the collapsibility level of the site is, the more significant the rise in infiltration range. The settlement of adjacent roads in class II and III collapsible fields is far greater than that in class I and is greater than the settlement standard. The facilities’ bottom part foundation can be replaced to ensure the functionality of the facilities and the safety of the surrounding roads in the actual project.

Surcharge loading compromises the original stress state of soil strata, generating detrimental additional stresses on the tunnel that critically threaten operational safety. Given this, the tunnel is equated to the Timoshenko beam resting on the three-parameter Kerr foundation model for this study. The theoretical models of the existing shield tunnel structural response under surcharge, considering and ignoring the impact of lateral soil, are proposed and solved using finite difference method. On this basis, the parameter sensitivity analysis is conducted. Finally, the prediction formula for the vertical deformation of the existing tunnel under surcharge is developed. The results show that the calculation results obtained by proposed theoretical models match well with the observation results from the finite-element method, existing analytical method, and field-measured data. In particular, the theoretical model considering the impact of lateral soil has higher calculation accuracy. Meanwhile, the parameter analysis shows that an increase in the area and amount of the surcharge causes increasing the tunnel longitudinal displacement. Conversely, increasing the elastic modulus of soil, the tunnel buried depth and the distance between the tunnel axis and the loading center decreases the tunnel longitudinal displacement. The proposed prediction formula has sufficient engineering accuracy and can serve as a valuable tool for quickly calculating the existing tunnel longitudinal deformation under surcharge.

An embankment is needed to satisfy the requirements for the longitudinal slope of railway lines, and ground reinforcement is also generally required in loess regions. The present study attempted to understand the effects of different ground reinforcement measures on the dynamic characteristics of a track–embankment–ground system. To this end, the critical speeds and the distributions of dynamic stress and environmental vibration were analyzed using a 2.5D finite element method. Three typical ground reinforcements, including dynamic compaction ground (DCG), soil–cement compacted pile composite ground (SCG) and CFG pile composite ground (CFGG), were used. The results indicate that the train speed (critical speed I) at which the maximum vertical displacement of the track occurs is universally higher than that (critical speed II) at which the wave propagation phenomenon occurs. The lower boundary limit of the peak region in the dispersion relationship can be selected as the reference value of critical speed II. Moreover, the values of critical speed I obtained using the DCG, SCG and CFGG models were around 92, 105 and 127 m/s, respectively. For critical speed II, the values were 75, 80 and 115 m/s. Once the train speed exceeded critical speed II, the vibration was confined to the embankment in the CFGG model, as evidenced by the isolation of the wave propagation from the embankment to the ground as well as the increasing dynamic stress in the embankment. After reinforcement, the dynamic stress, dynamic influence depth (DID), critical speed and resonant frequency increased. Additionally, the DID stayed around the 3–6 m range at all speeds.

The effective and reasonable construction of the low impact development (LID) facilities in loess area depend on the functionality of typical LID facilities and the safety of surrounding structures in areas. A full-scale field test on rainwater concentrated infiltration of bioretentions in a collapsible loess site was conducted in this study. The water content and deformation law of the site were analyzed, and the water movement law of the rainwater-concentrated infiltration at bioretention facilities in the loess site was determined. The site settlements were calculated as per the wetting deformation curve and infiltration depths were calculated on an improved infiltration depth model tailored to the loess area. The rainwater infiltration rules of different bioretention structural forms are different in the collapsible loess field. The diffusion rate of the retaining wall type in loess decreases over time, while that on a sloping type does not. Within the same infiltration time, the retaining wall has a stronger influence on the site than the sloping type. When the water is concentrated in the site, its influence on the subgrade settlement is small (generally less than 1.5 mm) enough to satisfy the relevant engineering requirements. A modified Green-Ampt model based on assumed loess saturated unsaturated stratification can be used to predict the infiltration depth of facility water at the site. The adverse effects of water infiltration related to stagnant bioretentions can be mitigated by adjusting the initial water content and saturated water content at the loess site.