## 4 Discussion  
The findings presented in this review confirm that Ecuador's coastal expansive soils pose a serious challenge for civil engineering, especially for constructing road infrastructure and buildings on highly plastic soils [42; 129; 130; 131]. This statement is consistent with previous studies conducted in other tropical and subtropical regions, such as India, Nigeria, and the southern United States, where the cyclic swelling and shrinkage behavior of these soils and their adverse structural effects have been documented [16; 27; 58; 83; 121; 132]. The collection of general information and selection of documents demonstrated the paucity of literature on the topic addressed in this study in Ecuador, with only seven documents from the country and four from neighboring countries with expansive soils. That contrasts with the numerous international studies that study 86 materials as stabilizers for expansive soils. Of the sustainable soil improvement techniques consulted, those with a chemical mechanism (55 publications) are the most researched, followed by those with a physical-mechanical mechanism (31 publications) and biological mechanisms (11 publications), which opens up the possibility of research into the last two mechanisms of action. In the bibliometric analysis of keyword co-occurrence in the fifth search string, two thematic research streams were observed: the first focused on traditional materials and proven methods, and the second oriented toward sustainable and innovative alternatives, a growing trend over the last decade. El Nino and La Nina phenomena significantly influence soil behavior in Ecuador's coastal region, particularly in provinces such as Manabi, Guayas, Esmeraldas, and El Oro. [129] El Nino events, especially those of 1982-1983 and 1997-1998, caused extreme rainfall that triggered widespread landslides and severe damage to road infrastructure, such as on the Rocafuerte-Tosagua highway in Manabi [129; 130]. The prolonged phases of humidity associated with El Nino saturate expansive soils, reduce their shear strength, and increase the likelihood of landslides, directly affecting homes, agricultural areas, and access roads [44; 45; 46]. Furthermore, La Nina events are often associated with prolonged drought conditions, influencing soil behavior in this region [44]. The alternating cycles of humidity and drought caused by El Nino and La Nina directly affect the volumetric stability of expansive clays. During wet periods, soils absorb water and expand; during dry phases, they lose moisture and contract, generating cracks, differential settlements, and loss of bearing capacity [44; 130]. This behavior is especially critical in soils with high plasticity and low initial moisture content, which can absorb up to 35% of water, aggravating structural damage [45; 130]. Furthermore, fluctuations in the water table intensify these effects, particularly in the active upper soil layer, which is highly susceptible to seasonal cycles of expansion and contraction. The description of soil improvement techniques included conventional techniques such as chemical, physical-mechanical, and thermal, as well as sustainable techniques with their chemical, physical-chemical, and biological mechanisms of action. Chemical stabilization is an effective technique for mitigating the problems associated with expansive soils by altering their properties through key chemical reactions. The proper selection and dosage of stabilizers are essential to achieve optimal improvements in soil plasticity, compaction, strength, and volumetric change behavior while also considering sustainability aspects. Physical-mechanical techniques address the behavior of expansive soils by modifying their physical structure (compaction, reinforcement) to limit water ingress and restrict volumetric changes or by diverting loads through problematic layers to more stable strata (deep foundations).