## 2 Study Area  
In Spain, expansive soils cover approximately 30% of the territory and primarily belong to the montmorillonite group [34]. Additionally, 67% of the country is located in climates where significant moisture changes can occur in the soil, with dry periods ranging from two to eight months [35]. The study area is located in the southeastern region of Spain, specifically in the provinces of Murcia and Almeria. Figure 1 presents the potential swelling risk map of Spain [35] along with the locations of the two primary sample sources used in this study. The samples were obtained in the context of different engineering projects, including road infrastructures, slope stability works, building structures, and foundation reinforcements for existing structures. As such, they are disseminated. In this way, the study areas were defined based on whether the samples belong to geotechnical units exhibiting similar swelling behaviours, according to the potential ranges established in Figure 1. As can be observed, the studied areas fall into the category of expansive clays with a moderate to high swelling potential (expected \(\rm p_{s}\) between 125-300 kPa) and high to very high potential (expected \(\rm p_{s}\) upper than 300 kPa). They consist of clayey-marly soils with medium-to-high plasticity and contain clays from the montmorillonite group, belonging to Neogene materials. According to the authors' experience, they can achieve relatively high \(\rm p_{s}\) values (above 400 kPa). In the study area, there are numerous cities, as well as associated structures, roads, railway lines, and slopes. If adequate measures are not taken, all of these may experience pathologies  
Figure 1: Risk map of expansive clays in Spain, indicating the location of the study area. Taken from [PERSON] et al. [35]. due to cyclic swelling and shrinkage. Specifically, buildings can suffer differential settlement, triggering diagonal or stair-step cracks in walls and foundation displacement, particularly in lightweight structures with shallow footings. Slopes develop vertical or subvertical desiccation cracks during droughts, followed by superficial landslides upon wetting due to a reduction in shear strength. Roadways develop longitudinal cracks parallel to the direction of traffic due to differential heave between the centreline and shoulders. Concurrently, edge uplift occurs along rigid boundaries (e.g., medians, curbs) caused by moisture-induced swelling pressure gradients. In concrete pavements, differential heave or settlement causes edges or corners of the concrete slabs to lift or sink, creating uneven surfaces. This process induces cracks at slab edges or joints, as well as mid-slab longitudinal cracking, along with minor associated transverse cracks. Figure 2 shows damage in recently constructed structures on expansive clays, where the authors were involved in structural rehabilitation and underpinning work. In Figure 2a, typical cracks associated with soil expansion and shrinkage phenomena are observed, in this case at a 45\({}^{\circ}\) angle, with a crack monitor to track movement. Figure 2b presents a similar case, but with more severe damage due to the combined effect of highly expansive clays and construction defects. In Figure 2c,d, it can be seen how swelling affects the lighter parts of the structure such as rigid concrete floor slabs and garage slabs in direct contact with expansive clays. Figure 2: Damage associated with expansive clays in the study area, based on the authors’ experience. (**a**) A 45\({}^{\circ}\) crack with an installed crack monitor; (**b**) Severe damage due to the synergy between expansive clay and construction defects; (**c**) Differential heave in floor slab; (**d**) Longitudinal cracks in garage slabs. This is, therefore, a very common problem that needs to be anticipated, with \(\rm p_{s}\) being the most important parameter for its assessment. Likewise, when implementing the necessary reinforcement measures (e.g., underpinning), \(\rm p_{s}\) remains the key soil-related factor for their design.