Rubber powder formed from discarded tire rubber is mixed with red clay to form a rubber-red clay mixture. The dynamic triaxial test was carried out on the mixtures under different conditions. The effects of rubber content, rubber particle size, moisture content of mixed soil, compactness, confining pressure, and vibration frequency on shear strain relation, dynamic shear modulus, and damping ratio of the mixture were investigated. The results show that under the same dynamic strain, the dynamic shear stress-strain curve of rubber mixed soil decreases with the increase in rubber particle content and moisture content and decrease in rubber particle size. On the other hand, it increases with the increase in compactness, confining pressure, and vibration frequency, and as the dynamic strain increases, the τd-γd curve becomes more nonlinear. In addition, with the increase in the rubber particle content, the dynamic shear modulus decreased while the damping ratio increased. When the content was 2%, the change was fastest. After continued addition, it gradually became stable, and when the decrease in rubber particle size also shows the same pattern, 2.00 mm rubber-red clay mixture shows better structure. The water content has great influence on dynamic shear modulus and damping ratio of rubber-red clay mixtures. With the increase in compactness, confining pressure, and vibration frequency, the interaction between mixed soil particles was enhanced, the dynamic shear modulus increased, and the damping ratio decreased.

Evaporation rainfall is one of the main causes of slope failure. Red clay belongs to special soil, and the failure mode of the slope is different from that of the general soil slope. In this paper, based on the theory of similarity simulation, a test model of red clay slope similar to the prototype is established in a certain proportion. Sensors are embedded in different positions of the slope to carry out the change rule of physical and mechanical indexes of the red clay slope under the condition of evaporation and rainfall. The results show that the water content of the slope increases during the rainfall period, the surface water content of the slope is obviously affected by the rainfall intensity, and the water content of the slope decreases in the drying period. Under the action of evaporation and rainfall, the internal temperature of slope is stable, and the surface temperature changes significantly. The pore water pressure of the top, shoulder and slope changes little with the times of evaporation and rainfall, and the pore water pressure at the foot of slope is obviously affected by evaporation and rainfall. Evaporation rainfall causes obvious horizontal displacement of red clay slope. The earth pressure increases in the rainfall period and decreases in the static and dry periods. With the increase of evaporation rainfall times, the earth pressure increases gradually.

Evaporation-rainfall is one of the main causes of red clay slope failure. In order to explore the evolution law and mechanism of red clay slope failure under evaporation-rainfall, this paper takes Guizhou red clay as the research object, establishes slope model in laboratory, and analyzes the failure characteristics, failure mode and failure mechanism of red clay slope under evaporation-rainfall. The test results show that the failure mode of red clay slope under evaporation-rainfall is: splash erosion and surface erosion stage → crack development stage → gully formation stage → gully development stage → local collapse stage → slope foot collapse stage. During the test, there is no obvious sliding surface on the slope. In the process of rainfall, the free water inside the slope converges to the foot of the slope under the action of gravity, which results in the maximum moisture content at the foot of the slope, and the strength of the soil decreases sharply. The local collapse occurs at the foot of the slope first. When several collapses are connected and the structure of the slope foot cannot support the weight of the upper shallow soil, the collapse occurs to the middle and upper part of the slope exhibition.

The stability of the red clay slope is poor, and the long-term exposure to the atmospheric environment is prone to instability and damage due to the dry-wet cycle. The simplified Bishop method to calculating the stability of the red clay slope is inconsistent with the actual situation. The failure characteristics and mechanism of the red clay slope are analyzed, and the existing problems of the red clay slope stability calculation are pointed out. The red clay slopes in Guizhou Province are dominated by shallow failures. Local collapse and collapse failures are very common. Overall destruction is relatively rare. A stability analysis method for red clay slopes based on composite sliding surfaces (fold-line - circular arc) is proposed. In the stability analysis of clay slope, the soil body is divided into three layers of upper, middle and lower fissure areas, and different strength indexes are selected in different areas. It is recommended to consider the effect of hydrostatic pressure generated by the water column in the fracture on the stability of the red clay slope.

The red clay is widely distributed in Guizhou province, which is characterized by high natural moisture content, difficult compaction and serious shrinkage and crackingin, and phosphogypsum is discharged for 5 million tons every year in Guizhou province. For the sake of effectively reducing the accumulation of phosphogypsum, mixtures were prepared with cement as a curing agent and mass ratios of cement to phosphogypsum of 1:1, 1:2 and 1:3 (low content phosphogypsum group) and phosphogypsum to red clay of 1:1, 1:2 and 1:3 (high content phosphogypsum group). Unconfined compressive strength, expansion, shrinkage and dynamic characteristic tests were conducted to analyze the behavior and mechanical properties of the mixtures. The modification mechanism of the mixtures by phosphogypsum was further explored by XRD (X-ray diffraction) and SEM (scanning electron microscopy). which provided a theoretical basis for the application of phosphogypsum in highway engineering and improved the engineering properties of red clay. The results show that the unconfined compressive strength of the phosphogypsum stabilized soil in the low content phosphogypsum group is greater than that in the high content phosphogypsum group. When cement:phosphogypsum = 1:2.2–1:3, the unconfined compressive strength of the mixture is maximum. When cement:phosphogypsum = 1:3, the maximum dynamic shear modulus of phosphogypsum stabilized soil is the largest. The absolute expansion rate and linear shrinkage rate of phosphogypsum stabilized soil in the low content phosphogypsum group are greater than those in the high content phosphogypsum group. When cement: phosphogypsum = 1:1–1:3, the absolute expansion rate is 6.5–12.3%, and the linear shrinkage rate is 1.3–2%. When red clay:Phosphogypsum = 1:1–1:3, the absolute expansion rate is 0.2–4%, and the linear shrinkage rate is 1–1.5%. The more phosphogypsum content, the smaller the expansion deformation and shrinkage deformation of the mixture. It is suggested that mass ratios of phosphogypsum to red clay is 1:1, and the cement content is 5%. Which can not only make full use of phosphogypsum solid waste, but also improve the engineering properties of red clay. Article Highlights The unconfined compressive strength of the phosphogypsum stabilized soil in the low content phosphogypsum group is greater than that in the high content phosphogypsum group. The absolute expansion rate and linear shrinkage rate of phosphogypsum stabilized soil in the low content phosphogypsum group are greater than those in the high content phosphogypsum group. The strength of phosphogypsum stabilized soil is derived from the comprehensive effect of cement and phosphogypsum. Due to the increment of the phosphogypsum and the decline of proportion of red clay, leading to the decrease in the content of hydrophilic minerals, the expansion deformation and shrinkage deformation characteristics are changed. Considering strength and deformation characteristics, it is recommended that mixtures with 5% cement content, and phosphogypsum to red clay of 1:1 as subgrade filler, which can not only make full use of phosphogypsum solid waste, but also improve the engineering properties of red clay.