## 1 Introduction  
Expansive soils are materials characterized by their remarkable capacity for volume change (swelling and shrinking processes), mainly related to variations in moisture content [1; 2; 3; 4; 5; 6; 7]. These materials represent a significant global geotechnical challenge worldwide due to their behavior, which can cause severe damage and costly interventions in infrastructure, such as building foundations, roads, and pavement bases [2; 3; 4; 6; 8; 9]. Soil stabilization may solve these problems, becoming a crucial technique to improve soil behavior and its engineering properties and mitigating volume change issues [2; 7; 10; 11]. Damage caused by expansive soils is significant worldwide, and it can have a greater impact than other natural disasters [12; 13]. Annual damage costs are estimated in billions of dollars in several countries, e.g., over $15.0 billion in the US and over $3.7 billion in the UK [12]. However, construction in areas with expansive soils is often complex and challenging to avoid [6]. Expansive soils contain high proportions of clay mineral such as montmorillonite, illite, and vermiculite [12; 13]. The structure of those clay minerals is found in sheets and layers [14], which allows them to exhibit an important volume change when moisture varies [12]. These minerals possess a net electronegative ionic arrangement and an extraordinary capacity to absorb water [13]. Water enters between the clay sheets, separating them and causing swelling [14], ultimately affecting the soil structure and reducing its resilient capacity. Swelling depends on the particles' total internal and external area [6; 14]. Stabilization seeks to improve the soil geotechnical properties, such as the soil's bearing capacity, while controlling or reducing its swelling potential. Thermal stabilization uses heat treatment to alter expansive soil's mineralogical properties, inducing transformative changes in clay minerals [15]. Chemical stabilization methods with additives such as lime and cement have been traditionally used [6; 7; 9; 14; 16; 17; 18; 19]. However, the high cost and growing environmental concerns related to their production and use have driven the search for more sustainable alternatives [6; 7; 9; 14; 18; 19]. That has led to a growing interest in innovative techniques that use industrial and agricultural waste materials and other non-traditional additives [4; 6; 7; 8; 9; 10; 14; 18; 20; 21; 22; 23; 24; 25], seeking more economical and ecological solutions. Materials such as fly ash [7; 8; 9; 20; 22; 26], agricultural waste (rice husk ash, sugarcane bagasse) [10; 11; 20; 21; 27], expanded polystyrene (EPS), sand and jute fibers [1], recycled plastics [20; 28], nanomaterials [29; 30], and rubber particles obtained from end-of-life tires [31] are being investigated for their potential. Although significant improvements in properties such as uniaxial compressive strength (UCS), California bearing ratio (CBR), and swelling reduction were achieved [1; 8; 9; 10; 11; 18; 26; 28; 32], the effectiveness of stabilizers may vary depending on the specific composition of the expansive soil. Another challenge is the lack of clear standards for applying stabilization techniques [2; 7]. Furthermore, long-term durability under various environmental conditions and potential environmental risks (leaching) requires further research [6; 7; 9; 22; 32; 33]. The main objective of this paper is conducting a review on the innovative and sustainable techniques available for improving expansive soils, focusing on those techniques with potential applications in Ecuadorian coastal soils. Figure 1 shows Ecuador's general classification of soils, based on their predominant composition. Blue areas correspond to areas where clayey soils predominate, while red areas represent other soil types. According to regional reports and previous geotechnical studies, the dashed black line delimits an approximate region within which clayey soils exhibit expansive characteristics.