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

Abstract

Loess has poor engineering geological properties due to its loose and porous soil structure. The mechanical properties and engineering geological properties of loess can be effectively improved by loess improvement, so as to meet the requirements of foundation bearing capacity for engineering construction in loess areas. The composite improvement method (CIM) is used to improve the loess mechanical properties. In this study, a series of orthogonal experimental tests were conducted on loess samples with the inclusion of cement, curing agent and mekralon, to investigate the relative strength in terms of direct shear tests and unconfined compression tests. The direct shear test results showed that the order of factors influencing shear strength is: cement content >curing agent content >mekralon content. Additionally, increasing the cement content could increase the shear strength of loess, while the improvement efficiency decreased significantly when the cement content exceeded 8%, and the peak shear strength was reached at the optimal mekralon content of 0.9%. The unconfined compression test results revealed that influence of cement content on unconfined compressive strength (UCS) is extremely significant. The order of factors influencing UCS is: cement content>>curing agent content > mekralon content, and the peak UCS was attained at the optimal cement content of 12%. Based on the qualitative and quantitative analysis of microscopic images, it is observed that the CIM increased the roundness of soil particles, decreased the total number of soil particles and the fractal dimension of particles and pores. It is found that due to reactions occured between cement and water, new substances generated could wrap, bind soil particles and crystals to form high-strength aggregates, and fill pores to make the soil denser. Additionally, mekralon is randomly arranged in the soil sample to form a three-dimensional fiber frame, which is intertwined with soil particles, preventing the tensile deformation of the soil sample and enhancing the strength of the soil.

Keywords: Improved loess, Composite improvement method, Mechanical strength, Microstructure, Improvement mechanism