Experiment on the frost resistance of Modified Phospho Gypsum: A case used to Improve Baozhong Railway Subgrade loess

Abstract

It has been widely recognized that loess has a low stability and permeability, and it is susceptible to a sudden decrease in total volume or collapse upon wetting. When the railway subgrade was under the dynamic trainload, the loess subgrade was prone to instability and liquefaction. Bao Zhong railway is a key railway for Guyuan city, in Ningxia province of China, which is an important city of the Belt and Road. Due to the influence of large-area flood irrigation on the farmland, the subgrade had a degree of settlement. The settlement had not been alleviated after three treatments, which seriously affected the safety of the train. For this reason, cement, Modified Phospho Gypsum (MPG) and MPG-cement were used to reinforce the subgrade loess, and the unconfined compressive strength test, permeability test and freeze-thaw cycle test were carried out. Then the compressive strength, impermeability and frost resistance of the three were analyzed and compared. The results indicate that the compressive strength of the MPG-cement modified loess is higher than that of the cement modified loess, but lower than that of the MPG modified loess. However, the coefficient of permeability for the MPG-cement modified loess has an opposite result, and the MPG-cement modified loess specimens have the best frost resistance. In addition, the mechanism of MPG-cement modified loess is discussed. It is found that cement and MPG have two hydration reactions with water in loess. Ettringite, the hydration reaction product, which not only fills the pores, restricts the movement of the soil particles, but also acts as a connecting soil particle in the soil particles. Therefore, the strength of the modified loess continues to increase, and the physical properties of the modified loess are improved.