7. Technical support

According to PLA [15], amorphous clays, high in allophane, are the main determinants of the very particular physical and mechanical properties of Andisols. They are responsible for the development of low-density bulk floors, high porosity, high water retention (high saturation, field capacity, and tension of 1.5 MPa), and high limits (upper plastic limit or liquid limit— LPS and liquid plastic lower limit—LPI) of plasticity. The retention of available water (field capacity humidity at 1.5 MPa) is also usually high and limit liquid or water flow in the form of water is near, in soils not altered to the point of saturation.

vegetation cover that counteracts the runoff associated with degadation phenomena gives origin to loss of soils and biodiversity and the alteration of the hydrological cycles of the basins or rivers of the region. This determines that areas of productive vocation, which are close to the micro-basins, that have lost their protective capacity of the ecosystems of strategic interest, are also affected due to the factors that undermine the stability of the soil, thus diminishing the potential to offer environmental services, of which the populated communities of the region are beneficiaries, limiting the production processes, and, therefore, their

Other determining factors of the drastic hydrological imbalances of the micro-basins of the region, which contribute significantly to the increase of flows, both surface water and infil-

• Increase in the change of land use from forests to paddocks. It has produced a drastic hydrological imbalance of micro-basins, significantly increasing the flows of surface and

• Excavation at the base of slopes and their over steepness, during the road construction

• Deficiencies in road rainwater management works (transverse, without debris to stable and/or well-protected sites, and without internal structures to dissipate energy). Specific fillings in some areas of the road corridor, with low technical specifications and coinciding

• Deposit of the materials resulting from the road cut, on the adjacent slopes, without any type of confinement. These "hillside fillings" coincide with the failed soils of some recent

• Specific problems of inadequate catchment, conduction, and delivery of surface water served in local homes (lack of channels and downspouts, deliveries of sewer networks to

The deforestation of the areas of interest for the protection of the micro-basins and the presence and increase of the areas in natural pastures in the study area are some of the causes of the decrease in water flows, which are associated with degradation of soils and aquatic and terrestrial flora and fauna and the alteration of the hydrological cycles of the basins, when climatic variables reach the most critical levels. The productive areas to intervene surrounding the micro-basins as ecosystems of strategic interest are also affected by climate change, which affects soils, reducing the supply of this environmental service to the

According to PLA [15], amorphous clays, high in allophane, are the main determinants of the very particular physical and mechanical properties of Andisols. They are responsible for the

trated, which are the cause of landslides and mass erosion phenomena, are as follows:

social and economic life.

94 Soil Moisture

infiltrated waters.

with sites of subsidence and settlements.

the hillside, soft areas without waterproofing, etc.).

processes.

landslides.

beneficiary communities.

7. Technical support

Although the gravimetric water retentions are usually very high (up to 2–3 times the mass of dry soil when saturated), they are not so much on a volumetric basis due to the low apparent densities, although they are still higher than in other soils. The high retention of moisture even at high voltages and the poor connection between pores means that in humid climates, even with good drainage, conditions of poor aeration at shallow depths that restrict root development remain in the Andisols. In any case, to achieve such high moisture retentions requires a degree of weathering of volcanic ash, with formation of halloysite and accumulation of organic matter, since with very recently formed ashes, generally with sandy loam to gravel, the volumetric capacity Water retention is usually very low [15].

With drying, up to 30–50% of the water-retention capacity and a large part of its plasticity are irreversibly lost. It has been pointed out that the change of the plasticity indexes with the drying of the soil is the main property that distinguishes the Andisols from other soils where crystalline clays predominate.

The drastic and irreversible changes of properties of the Andisols derived from changes in humidity have much to do with the erosion processes in these soils.

The greater the inclination of the soils, the instability increases soils and, with it, the greater the susceptibility to mass movements, the more rainwater is infiltrated and less lost by runoff (accumulation). Mass movements depend on the interaction of several factors, especially slope; lithology; soil type; intensity, duration, and continuity of rainfall; surface and internal drainage conditions; vegetation cover; and management.

By virtue of the above, it is technically demonstrated that in Andisols, where a limiting layer has been formed for internal drainage at shallow depths and a high rate of surface infiltration is maintained, increases in moisture content negatively affect the stability of the soil material facing landslides by:

