The correct choice of foundation type is a fundamental aspect of civil engineering, especially in the design and construction of structures on terrains with specific geotechnical characteristics. Some soils can behave particularly negatively for the structures built on them, which influences the type of foundation to be used in each specific case. In the context of solar installations, whether solar trackers or fixed structures, the most commonly used solution is foundation via driven piles in the ground. These profiles are designed to transmit lateral, tensile, and compressive loads from the structure to the underlying ground. However, under certain conditions, it is necessary to consider additional actions that the terrain may exert on the structure.
A well-known case of problematic terrain is expansive clays. These clays have a significant capacity for expansion and contraction in response to variations in moisture content, which can generate upward pressures that are detrimental to foundations, potentially even lifting them. To mitigate the risks associated with problematic soils, it is essential to conduct an adequate soil characterization. Before carrying out in-situ studies, it is advisable to collect information from specialized literature that includes aspects such as topography, geology, geotechnical maps, and climatic data. Many countries have maps that compile this critical information; for example, the risk map for expansive clays in Spain, developed by the Geological and Mining Institute of Spain, can be a valuable tool for identifying potentially problematic areas.
Once the bibliographic information has been gathered, a geotechnical campaign should be performed in the project area if signs of expansive soils are detected. During this investigation, it is crucial to identify characteristics indicating the presence of expansive clays, such as soil colors in gray, greenish, or bluish tones, the presence of wide and deep polygonal cracks in dry conditions, and highly moldable wet soil. The precise identification of these characteristics allows for their classification into four groups based on presumed expansiveness (Grade I for negligible or low expansiveness to Grade IV for very high expansiveness). This classification is based on reference parameters such as fine content, liquid limit, and plasticity index, although direct testing is recommended for precise results.
With the soil characterization complete, foundation design can proceed. In the case of driven piles, the resistance of the soil to the extraction efforts generated by the expansiveness of the clays is due to the shaft resistance, which must counteract the extraction forces. To ensure the accuracy of design calculations, all parameters must be clearly defined in the geotechnical report. It is essential to correctly establish the depth of the active layer to avoid over-dimensioning the structure. The problem of expansiveness is intrinsically related to changes in soil moisture. During drought periods, the soil loses moisture due to evaporation, leading to contraction and cracking. When rain returns, water can saturate the soil, causing an increase in volume in the active layer. Additionally, soil retraction poses significant risks, such as differential settlement of the structure and loss of resistance in the surface layer of the ground.
To mitigate these risks, various strategies can be implemented: controlling soil moisture through an adequate drainage system can prevent soil oversaturation; controlling compaction can reduce soil density; and, in some cases, replacing the soil with non-susceptible material in the active layer or chemically stabilizing soil properties may be necessary. However, except for the drainage system, which is usually implemented in photovoltaic projects, these solutions can increase both execution complexity and project costs. For this reason, the main strategy is to counteract the effects of expansiveness through adequate depth of the driven piles.
In conclusion, the proper management of risks associated with expansive clays is fundamental to constructing safe and durable solar structures. Understanding soil behavior and applying effective solutions ensures the integrity and functionality of photovoltaic plants, ensuring that these installations meet the required safety and performance standards today.