Research on the Mechanism of Composite Improvement of Loess Based on Quantitative Analysis of Microstructure and Mechanical Strength

Loess has poor engineering geological properties due to its loose and porous soil structure. To meet the engineering requirements of bearing capacity of foundations in loess areas, various stabilizers have been added to the loess during construction. Currently, cement, fly ash, fibers (natural and synthetic) and ionic curing agents have been widely used as stabilizers worldwide. In this study, the Composite Improvement Method (CIM) was used to improve the loess mechanical properties. A series of experimental tests with three factors and five-level orthogonal were conducted on loess samples with the inclusion of cement, curing agent and polypropylene fiber, to investigate the relative strength in terms of direct shear tests and unconfined compression tests. The test results were analyzed by range and variance analysis to quantify the significance of the improved materials, and the optimal improvement scheme was obtained. The results show that cement content has the greatest influence on shear strength, especially on unconfined compressive strength (UCS), followed by polypropylene fiber content and curing agent content. To reveal the internal mechanism of loess improvement by CIM, the quantitative microstructural information of the improved soil was extracted, and the microscopic geometric parameters such as apparent porosity and fractal dimension were calculated and analyzed as well. Based on the qualitative and quantitative analysis of microscopic images, it is observed that CIM increased the roundness of soil particles, decreased the total number of soil particles, and reduced the fractal dimension of particles and pores. Additionally, the composite improvement mechanism of CIM is further revealed. CIM can stimulate the activity of each other between the improved materials and work together to improve the engineering geological properties of the soil.