Humidity diffusion can stimulate soil deformation. However, evidence for triggering water-induced excessive floor deformation in underground structures remains elusive. This study investigated the issue by solving a case of a loess tunnel affected by floor heave. First, the study tracked the development of field damage during tunnel operation and examined the potential influence of base moistening on inverted arch uplift. Subsequently, a largescale field tunnel model test was conducted to analyze the moisture distribution, stress, and deformation development at the tunnel base during the humidification process. Finally, the mechanical properties of the base loess at the humidification stages were tested to assess the degradation caused by moistening. Results showed that the moisture distribution at the tunnel base changes from W- to U-shaped during long-term humidification. Moreover, after base humidification, the soil pressure at the arch foot initially decreases sharply and then increases, while the soil pressure at the inverted arch continues to increase. Furthermore, the lining at the arch foot shows an increase in compressive stress, while the inverted arch shows an increase in tensile stress. The differential settlement between the arch foot and inverted arch widens, transitioning to uplift deformation of the inverted arch and ultimately causing floor heave. Laboratory tests showed that the floor heave is primarily caused by the deterioration of the mechanical properties of the loess resulting from humidity diffusion in the tunnel foundation. A time-dependent floor heave model was established by combining the water content, shear strength, compressive strength, and compressibility of the tunnel-base loess, and its feasibility was verified. The model exhibited a sequential decrease in the influence of the internal friction angle, compressive strength, cohesion, and compression coefficient on the floor heave. The findings of this study are considerably important with respect to uncovering the mechanism of floor heave during the operation of loess tunnels and advancing the prediction of damage.

Floor heave deformation of railway tunnel is a common disease in the operation stage of high-speed railway in recent years. Due to its complex deformation mechanism, there is no specific and reliable evaluation method at present. In order to investigate the deformation rule and failure mechanism of the invert arch of railway tunnel when subjected to the expansion force of the bottom swelling surrounding rock, the tunnel floor heave loading test system is designed independently. The working conditions of the tunnel invert under different expansion forces are simulated. The test results show that (1) The circumferential stress on the outside of the arch foot and the inside of the invert arch is tensile stress, and the inside of the arch foot and the outside of the invert is compressive stress, and its maximum value is at the arch foot. The arch foot and the invert are more sensitive to the expansion of the basement, and it is easy to destruction. (2) After the invert bottom is bulging, the inner side of the invert and the outer side of the arch foot are strained. For concrete lining, the tensile strength is far less than the compressive strength, that is, these parts are prone to tensile damage, so they are the weak positions of the entire lining. It is recommended to properly reinforce at this position in practical engineering. (3) Under the action of basement expansion of the tunnel, the surrounding rock at both sides of the tunnel and the position of the arch is easy to fail and weaken the bearing arch effect, thus aggravating the deformation and failure of the lining.