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The direct shear test is a fundamental method in geotechnical engineering that provides crucial soil shear strength parameters, including cohesion (c) and the angle of internal friction (ϕ). These parameters play a pivotal role in structural design, slope stability assessment, and soil stability evaluation. However, achieving a uniform normal stress distribution within the shear box remains a challenging task, which can result in inaccuracies in test results. This study investigates the impact of shear box shape, specifically comparing circular and square configurations, on the outcomes of the direct shear test. The findings reveal that the choice of lower or upper box movement has a minimal effect on test results. Moreover, circular boxes demonstrate superior normal stress distribution, leading to reduced variations in comparison to square boxes. Wall friction effects lead to lower shear capacity measurements, with circular boxes yielding higher shear levels when contrasted with square boxes. Additionally, the soil along the sides and corners of the specimen experiences diminished shear stress due to reduced normal stress. This research contributes significantly to our comprehension of how shear box shape influences the determination of shear strength parameters in direct shear tests, ultimately enhancing the reliability of geotechnical engineering assessments.
Dastgerdi et al. (Fri,) studied this question.