Influence of fibers on desiccation cracks in sodic soil

Abstract

Drying shrinkage of soil contaminated by alkaline solutions in ecologically fragile areas is a key issue that affects the strength, stability, and permeability of contaminated soil. Reinforcement with polypropylene fibers is a common physical treatment method. This paper presents experimental investigations of the shear strength, permeability, formation, and development of desiccation cracks in sodic soil with four different contents of polypropylene fibers: 0%, 2%, 4%, and 8%. The shear strength and permeability coefficient were tested by the SLB-1 triaxial shear permeability tester. Meanwhile, a series of crack images and water content evolution were obtained during free desiccation tests with self-designed monitoring equipment. The dynamic changes in the cracks are then quantitatively analyzed by using digital image processing and geographic information system (GIS) analysis. The experimental results show that shear strength increases and permeability coefficient decreases with the increase of fiber content. The evaporation process with different polypropylene fiber contents has three stages, namely, Stages I, II, and III that are in accordance with the slope of the evaporation curve. However, the polypropylene fiber content has little effect on the total duration of crack development. The crack development process has three stages. The crack ratio, fractal dimension, and average width decrease as the polypropylene fiber content increases. The fractal dimension gradually and linearly increases with time. The fractal dimensions of the cracks are reduced with increased polypropylene fiber content, which indicates that polypropylene fibers can inhibit the development of soil cracking. Because the elastic modulus of the fibers is very high, it is difficult for the fibers to be pulled out from the soil during cracking. Therefore, friction is produced at the interface between the fibers and soil particles, which can inevitably inhibit the relative sliding of the fibers and share the tensile stress caused by the structural shrinkage in the soil. Therefore, polypropylene fibers can dramatically improve the stability and integrity of soil, similar to a “bridge.” In conclusion, these research results provide an important explanation for dealing with alkali-contaminated foundations and soils.