ABSTRACT

Irrigation is a practice used in water deficits areas to increase production. Nevertheless, inadequate irrigation practices may deteriorate chemical and especially physical fertility. All irrigation water, contain soluble salts, sodium, chlorine, and other elements. It is expected that problems associated with saline and sodic soils will increase in the future despite of the human use of good quality waters. Irrigation with poor quality water affects physical and chemical soil properties, reducing plant growth.  High ESP (Exchangeable sodium percentage) change pores location, increasing micropores with consequences in water and gases circulation as well as mechanical impedances derived from structure loses. 
Mechanical study of agricultural soils includes different problems due to its polifasic-insaturated-granulated-porous and generally cohesive nature. The agricultural soils mechanic behaviour understanding has been limited always, since classical soil mechanical techniques had been developed for saturated soil studios. Critical State mechanical provides a theory whose principal objective is to relate charges or tensions applied to a soil with changes in porosity. This theory is a strong tool because it makes possible the study of soils in p – q – v dimension, where p is Normal Tension, q is Shear Stress and v is Specific Volume. It says that soils under shear deformation, finally reach a Critical State in which they flow without new changes.
Tangencial shear machines are usually used in agricultural soils properties studios. Main advantages of this machine are that they can be used for shear as well as compression.  Furthermore share stress measurement, horizontal and vertical movements, makes it very useful to obtain mechanical soil parameters with Critical State Theory and Coulomb Law.
Main objective of this work was increase knowledge about damage caused by irrigation with sodic waters on agricultural soils mechanical properties. Hypotheses were: 1). Sodium falls risks of soil compression, 2). Sodium increase aggregates tensile strength in anyone size and 3). Soil chemical alteration causes displacements in Columb lines and Chritical State. 
Three reconstituted samples (Cardone, Ozzano y Cadriano) from three Italian localities were used. These soils were artificially sodified, simulating irrigation with highly sodic waters. Two treatments were obtain called A and B, whose main difference was the higher sodic contain in the exchange complex in Na+ with respect to B. Cadriano soil was evaluated using Brazilian test, whereas Cardone and Ozzano were put under share and compression. Furthermore, on Ozzano, tensile strength evaluations were made on individual aggregates. Results obtained on Cadriano soils don’t show differences between treatments with respect to Compaction tension nor Tensile strength. In Ozzano single aggregates bigger ones had less tensile strength then smaller ones and A treatment aggregates shows higher tensile strength B treatment. In compression an tensile strength treatment Specific volume was the most affected parameters by sodic, showing A treatment lower porosity and lower slopes in its compression lines. About Coulomb parameters, A treatments shows generally higher tensile strength and cohesion values, whereas angle of internal friction don’t change significantly with respect to B treatment. 
In field operations results obtained in this thesis can explain the higher energy requirement in tillage in high SAR soils (Sodium Adsorption Ratio). In Critical state theory, A treatment shows a sub-critical zone reduced with respect to B which would confer it a higher expansive tendency during share. Nevertheless this higher expansive tendency would only be in relative terms since its difficult they reach porosity values like a lesser sodified soil at the same Normal Tension. Sodic effects on soil porosity had such a magnitude that they wouldn’t allow to be assimilated by its higher expansive tendency. Its concludes that soil affected by high sodic irrigation water have stronger aggregates with higher tensile strength and lower compression vulnerability.  Also sodic affected soils shows an increase in its tensile strength, its cohesion values and, finally, an increase in super-critical region which confer a higher expansive relative capacity during share.