## 1 Introduction  
Expansive soil is rich in strong hydrophilic minerals such as montmorillonite and illite. It is generally considered a problem soil with a high expansion potential, with expansion contractility, multi-fracture, and over-consolidation [1]. The internal factors affecting soil expansion and contraction include mineral composition and microstructure. Meanwhile, the external factors include water and climatic conditions. Expansive soil is known as \"catastrophic soil\" because of the cyclic process of shrinkage and expansion: it characteristically expands when it absorbs water and contracts when it loses water [2]. This generally damages the superstructure, particularly the slight structure, e.g., cracking and destruction of foundations [3], roadbeds [4], dams [5], and channels [6]. According to statistics, the losses caused by expansive soil worldwide amount to billions of dollars. Within this, 10 million square meters of houses were destroyed due to expansive soil in China, and the economic losses exceeded USD 1 billion [7]. Therefore, it is necessary to improve expansive soil foundations to reduce the adverse effects of expansive soil on project services. The currently proposed methods to improve the engineering properties of expansive soil mainly involve chemical, physical, and biological aspects. Chemical methods improve the soil conditions through the use of additives, such as fly ash, cement, and lime [8; 9; 10; 11; 12]; physical methods improve soil conditions via geosynthetics, soil substitution, and moisture control [13; 14; 15; 16; 17]; and biological methods improve the soil conditions through the catalytic action of biological enzymes [18; 19; 20; 21; 22]. The use of soil replacement methods may considerably increase construction costs. Although the use of additives such as cement effectively suppresses the expansion and contraction of expansive soil, the carbon dioxide emittedduring the production of cement is uncontrollable. This has an adverse impact on the environment and a low long-term advantage. Therefore, it is necessary to study energy-saving and environmentally friendly methods to improve expansive soil. The resource utilization of solid waste is an important part of China's green, low-carbon, and circular economic system. Additionally, it is a strategic measure to achieve \"carbon peak and carbon neutrality\". In February 2022, to promote the standardized utilization of renewable resources, eight departments, including the Ministry of Industry and Information Technology, issued the \"Implementation Plan on Accelerating the Comprehensive Utilization of Industrial Resources\". It promotes the comprehensive utilization of renewable resources such as scrap steel, waste non-ferrous metals, waste tires, waste paper, and waste mobile phones. Research has shown that solid waste recycling and reuse are key to achieving eco-friendly, eco-efficient, and sustainable infrastructure development on a global scale. The growing number of scrap tires, one of the largest sources of solid waste, has generated severe disposal challenges. It is estimated that a billion tires are discarded globally annually. This number is projected to increase to 1.2 billion by 2030 [23]. In the past ten years, rubber generation and recycling volumes in China have increased, as shown in Figure 1. In 2022, the amount of waste tires generated was 13.5 million tons, and the recycling volume was 6.5 million tons. This corresponded to a recycling rate of waste tires of 48.15%. The rate is relatively low, and the recycling capacity is relatively insufficient. This implies that there are relatively abundant waste tires for recycling and advantageous reuse. With the increasing awareness of environmental protection and resource conservation, the reuse of waste resources has become a new trend in scholarly research. Studies have shown that adding rubber fibers to soil can mitigate swelling potential, improve ductility, and increase shear strength. [PERSON] et al.