5. Erodability

In Colombia, the natural slopes in soils of volcanic origin reach heights between 10 and 20 m with slopes greater than 60 [8, 13, 19]. Despite this, the slopes are susceptible to instability, erosion, and cracking depending on the climatic conditions and vegetation cover. In the Colombian Coffee Region, landslides detonated by intense rainfall or locally intense earthquakes are often reported. These landslides can have a high potential for destruction in densely populated areas in mountainous reliefs of great length and high slope.

The soils of the region are characterized by steep slopes of 30 (67%) to 35 (78%), extensive slope lengths; the shape of the concave slope is favorable to the accumulation of surface and sub-surface waters. In addition to the detonating agent, the occurrence of a landslide is determined by previous conditions related to deficient plant cover, or the misuse or management of the soil, the poor disposition of agricultural production systems, the indiscriminate felling of forests for planting of pastures and livestock production and their precarious management and essentially physical causes inherent or intrinsic to these soils.

The coffee axis is located in a tropical zone that presents great climatic changes due to altitude changes and has a bimodal climatic regime given by two humid periods and two dry periods. The zone receives an annual precipitation varying between 1500 and 2250 mm. Surface landslides (depth < 1.5 m) are usually activated during periods of heavy rains, April to May and October to November, in which the accumulated rainfall during 1 or 2 days exceeds 70 mm [13, 25].

The superficial soils predominant in the area have deficiencies in the properties of resistance to the cut, since they are recently formed volcanic ash, unconsolidated, and sandy (Ruiz and Cerro Bravo volcanic complex in the Department of Caldas). These materials generally have low plasticity and cohesion due to their loose grain condition with sandy textural appreciations. The cohesion is drastically reduced (or even disappears) when the soil becomes saturated (reduction in the suction capacity), during the occurrence of intense rainfall, for example (the suction is lost and the natural cements dissolve).

The landslides have a flat and irregularly shaped surface defined by the contact between the layer of soils derived from volcanic ash and the layer that underlies it, composed of materials of vulcano-detrital origin, that are moderately or slightly weathered and/or evolved and they often come in slices. Slides of greater depth (depth: 3–10 m) are produced with detonating precipitation less than 50 mm, when the previous accumulated precipitation exceeds 200 mm [13, 25]. Dramatic differences in the permeability of these strata layers or horizons of these soils lead to the formation of a hung phreatic level that reduces effective efforts and increases instability or susceptibility to erosion.
