Investigation of Water and Soil Migration and Mud Pumping of Subgrades under Traffic Load

Atmospheric precipitation leads to the increase of moisture in the subgrade. The moisture and soil migration in subgrade is a key scientific problem in evaluating the service performance of the subgrade and resolving or preventing mud pumping. Using a self-developed testing system as well as a numerical simulation model, a detailed study of the dynamic hydraulic characteristics, water migration mechanism, fine soil migration mechanism, and mud pumping properties of the subgrade was conducted. The results showed that water migrated into the upper layer of the subgrade under traffic load. Meanwhile, a pressure gradient for the pore water was generated in the subgrade. This kind of pressure gradient is beneficial for the moisture and fine soil migration. With rising groundwater level and increasing traffic load, the porosity of the subgrade soil differs at different depths. The fine soil migration is caused by water migration, which causes new migration channel for water in the subgrade. Then, a circulating system of moisture and fine soil is formed in the subgrade under a traffic load. After that, the upper layer subgrade soil is nearly saturated under the action of traffic load. Then, the nearly saturated soil liquefies instantaneously, becoming mud, under the action of traffic load. However, as the loading time progresses, the moisture and fine soil in the subgrade continue to migrate upward, resulting in the mud being pumped into the ballast and the gradual disappearance of liquefied soils at different depths.