4. Soil-water relationship

In a general way, it can be said that the structure, the state of efforts, and the flow of water in any type of soil change when it is exposed to the intense cycles of drying and wetting, typical of the climatic conditions of the tropics. These changes affect the physical properties and mechanical behavior of the soil, which can lead to geotechnical problems (e.g., erosion, slope instability, etc.).

Soils derived from volcanic ash in Colombia are located in regions where a bimodal rainfall regime occurs during April to May and October to November and very dry periods occur between these stages. During periods of low precipitation and high temperature, high water evaporation occurs between the pores of the soil, causing its drying.

The evaporation produces contraction and increase of the suction forces in fine soils (silts and clays), for the states of complete saturation or partial saturation, respectively. The desiccation evolves occasionally toward the formation of cracks. These cracks can be understood as a consequence of the stresses produced by desiccation. Cracks in the surface of the soil make up areas susceptible to problems of erosion and instability, often observed on slopes with little plant cover, continuously exposed to drying processes. On the other hand, during humid periods, characterized by permanent and intense rains, the infiltrated water reduces the capillary effects and causes volumetric changes that can lead to swelling or collapse of the soil structure [13].
