In order to solve a series of highway diseases caused by poor water stability, insufficient strength, large deformation and obvious expansion and contraction in Guizhou Red Clay Subgrade Project. This paper takes the undisturbed red clay of a highway subgrade in Guizhou as the test material, improves the red clay by using the curing agent, and carries out the indoor test by adding different content of curing agent into the natural red clay The hydraulic properties and mechanical strength characteristics of the modified red clay were studied. Combined with the construction and detection of the field test section, the scientific evaluation of the red clay composite material was carried out. The results show that: with the increase of curing agent, the liquid plastic limited water content moves to different directions, the soil resistance to water damage is enhanced, and the expansion and contraction characteristics are effectively controlled. Moreover, the unconfined compressive strength, bearing capacity and resilient modulus of the modified red clay are significantly improved, which can be compensated the engineering defects of red clay can improve the quality of highway engineering and extend the service life. At the same time, through the test section for field test results analysis, the strength index of red clay composite cured soil to meet the engineering design standards.

In order to reveal the pore evolution law and shrinkage mechanism of Guiyang red clay in the process of water loss at room temperature, shrinkage test, mercury intrusion porosimetry test and X-ray diffraction test were carried out on the red clay. Crystal layer osmotic pressure theory and capillary theory were adopted to explain its microscopic mechanism. The study results show that: the water content promotes the shrinkage of the red clay, and the shrinkage was anisotropic. The shrinkage goes through three main stages: First, the normal shrinking stage, when the volume of the pores, no matter macro-or meso-, between the aggregates decreases and more and more small pores and micropores appear with the gradual reduction of water content; second, the residual shrinkage stage, when a large number of small pores become micropores within the aggregates; third, the zero shrinkage stage, when all pores remain constant within and between the aggregates. The red clay is high in montmorillonite, and the osmotic pressure difference between the inside and outside of its crystal layer causes the migration of water molecules into the pore, leading to the narrow of the distance between the crystal layers, which macroscopically manifests as soil shrinkage. Through Kelvin capillary theory, we established the relationship between the saturation and the suction of Guiyang red clay at each shrinkage stage, and explained the internal mechanism of controlling of the matrix suction to the shrinkage process.