For aggregated soils the Mohr failure line can be separated into two straight lines, with different slopes and intercepts. In the range of low normal load, when the slope is very steep and the intercept is small, the failure line is defined by the friction and cohesion inter-aggregates, while for the higher load range the slope becomes smaller and intercept is larger, which defines the intra-aggregate friction and cohesion. Therefore, for aggregated soils the normal load range used in the direct shear test affects the final result. The present study was carried out with the objective of evaluating if the Mohr failure line of a Red Latosolic Nitisol can be subdivided in different segments, with different steepness and intercepts, and if this is related to the existence of soil aggregates. Initially, soil surface samples of a Typic Hapludox (Nitossolo Vermelho Distrófico latossólico, Brazilian Soil Classification System) were subjected to the direct shear test with seven normal loads (24.4, 48.9, 98.2, 196.4, 294.6, 392.8 and 491.8 kPa). Afterwards, the tests were carried out with aggregate samples of five diameter classes (< 0.25, 0.25-0.50, 0.50-1.00, 1.00-2.00 and 2.00-4.00 mm) with eight normal loads (24.4, 48.9, 73.5, 147.3, 294.6, 441.9, 589.2 and 736.6 kPa). The Mohr failure line could be divided into two straight line segments in all tests. Results indicate that the aggregation should be considered in the definition of the normal loads to be used in the direct shear test, as well as in the result analysis, once it affects the failure line. The studies here indicate that, under the given moisture conditions, the aggregates determine the shear strength for loads of up to 294.6 kPa for the evaluated soil.
soil structure; soil compaction; soil physics; soil mechanics
