## 3 Results
### Description of the Selected Soil Improvement Techniques
#### 3.2.4 Sustainable Stabilization (Chemical, Physical-Mechanical, and Biological Mechanism of Action)  
Sustainable and innovative stabilization techniques are employed to improve expansive soils due to the high costs and environmental concerns associated with traditional stabilizers such as cement and lime [13,14]. Cement production, in particular, contributes significantly to CO2 emissions and climate change issues [14,18]. Using processed waste is less expensive, more sustainable, and environmentally friendly, emitting significantly fewer greenhouse gases [14,18]. Various waste materials are employed, such as fly ash, bagasse ash, brick dust, GGBS, plastic waste, and fibers [6,14,18,21]. These additives improve the geotechnical properties of weak soils. They have been shown to significantly increase values such as the CBR and UCS [14,18,21]. The main applications of these techniques focus on subgrade stabilization to improve soil-bearing capacity [14,18]. They are crucial for road construction and are used in highway and airport engineering applications [14]. The objective is to improve the geotechnical properties of weak soils wherever possible. Faced with the limitations of traditional stabilizers such as cement and lime (high cost, CO2 emissions), industrial byproducts and waste offer sustainable alternatives. These are based on chemical mechanisms using ash from industrial or agro-industrial waste, physical-mechanical mechanisms using natural or synthetic fibers, granular waste particles, and biological mechanisms using plant extracts, organic waste, and microorganisms. Each of the mechanisms of action analyzed in the scientific literature is detailed, filtering those that employ sustainable and innovative techniques for improving expansive soils, which is the focus of this study, and subsequently identifying those that would be most viable to apply on the Ecuadorian coast. * Chemical mechanisms involve reactions between waste additives and the soil (see Table 1). The pozzolanic reaction is key: materials such as fly ash, granulated and GGBS [52], rice husk ash (RHA) [14; 18], and brick dust form cementitious compounds (C-S-H, CH gel) [14] when they react with water, binding particles [6]. Cation exchange occurs when cations (e.g., from marble dust [6] or eggshell [18; 21]) displace ions in the clays, reducing repulsion and causing flocculation and agglomeration [6]. That decreases plasticity and swelling potential [6], improving strength (UCS, CBR) [14; 18]. Other materials include silica fume [6; 18] and bagasse ash [6; 18; 21]. Other agro-industrial materials are also being explored. That includes a variety of residues such as palm oil ash (POFA), eggshell dust, burnt olive residue, press mud, rice straw ash, coconut fiber, and wood ash, among others [6; 21; 53]. Their mechanisms of action often combine chemical and physical-mechanical aspects. For example, silica-rich materials, such as RHA, bagasse ash, and POFA, can participate in pozzolanic reactions by reacting with activators and soil water, forming cementitious compounds such as C-S-H gels that bind soil particles and improve strength [6]. Calcium-containing waste, such as eggshell dust or burnt olive waste, can induce cation exchange and flocculation, reducing plasticity and swelling [6]. Using fibers like coconut fiber acts through physical reinforcement, providing tensile strength, controlling cracking, and increasing overall strength [6]. * Physical-Mechanical Mechanisms alter the physical structure of the soil. Reinforcement uses fibers (polypropylene, plastic [14] or recycled geotextiles [6; 18]) to improve tensile strength and control cracks. Mechanical interlocking occurs when granular waste particles (GGBS, fly ash) are physically interlocked [6; 14]. Dilution involves the addition of non-expansive materials. These methods improve strength (UCS, CBR) and reduce the expansion-contraction potential. * Biological Mechanisms.