Cohron sheargraph data: interpretation using critical state soil mechanics

Abstract

Critical state soil mechanis was used to explain the differences in stress paths recorded in sheargraph tests in weak, strong and medium-strong soils. It was projected that the cohesion of both peak and ultimate strength lines would be near zero for weak and medium-strong soils, but the peak cohesion would be high for strong soil. The friction angle of the peak strength line was projected to be lower than that of the ultimate line for strong soils and less than or equal to that of the ultimate line for weak and medium-strong soils. The projections about the type of stress path and the relationship between cohesion and friction for peak and ultimate strength lines were tested by performing field sheargraph tests in a Vertisol. It was found that the fraction angle of the ultimate line was higher than that of the peak line, which suggests that in loose to medium dense soils it may be more appropriate to use the ultimate line for tine design procedures that employ the friction angle in calculations. The different forms of stress path recorded in field sheargraph tests were associated with different regimes of soil behaviour in laboratory shear, in which the height change during shear was measured. Tests with strong soil resulted in sheargraph tests that showed brittle stress paths with well defined shear stress peaks, and in laboratory shear tests that showed shear accompanied by expansion. Tests with weaker soil resulted in sheargraph stress paths without well defined peaks, and in laboratory shear accompanied by compression. Shear accompanied by expansion is desirable in tillage for seedbed preparation and therefore the sheargraph may be useful in identifying the right soil conditions for tillage.