Critical state behaviour of an agricultural soil

Soils bear natural and artificial structures and sustain agricultural and biological activities. Understanding soil behaviour is the key to appropriating the benefits derivable from this material. Engineers are usually interested in the mechanical aspect of soil behaviour. This involves the study of volume change and shear response to applied stresses. In the past, soil shear and volume change behaviour were studied independently. But many practical problems involve shearing accompanied by volume change, therefore, it is desirable to consider this interaction in modelling soil behaviour. The critical state theory (Roscoe et al. 1958) was developed to combine both shear and volume change behaviour of soils. Early soil mechanics theories including the critical state theory, were developed for saturated soils. Because unsaturated soils are encountered in many situations, there is the need to adapt earlier theories or to formulate new ones for unsaturated soils. The major differences in the physical behaviour of saturated and unsaturated soils are due to an internal stress called matric suction formed at the air-water interface in the soil pores. Theories developed for unsaturated soil mechanics are based on the inclusion of the effect of matric suction on soil behaviour. In this study, the applicability of two major frameworks (i.e., critical state theory and unsaturated soil mechanics) to the study of agricultural soil behaviour was investigated. Soil behaviour was considered within a modified critical state framework that permits the application of critical state theories to an unsaturated soil. An experimental program involving tests in a modified state-of-the-art triaxial testing system was undertaken. Critical state soil parameters were obtained and state boundaries identified. The role of matric suction and soil structure on state parameters are presented. Changes in matric suction caused a shift (translation) in state boundaries, while changes in structure caused a rotation of the state boundaries. Results also showed that the assumption that matric suction is a constant variable depends on the stress level in the soil. Under low stress regimes, this assumption seems justifiable, however this is questionable under high stress regimes. This study also showed the importance of consideration of suction in the analysis of agricultural soil behaviour