A revision to the minimum required specimen width to maximum particle size ratio in ASTM D3080/D3080M for direct shear testing of coarse granular materials

ASTM D3080/D3080M, Standard Test Method for Direct Shear Test of Soils under Consolidated Drained Conditions, is widely used in geotechnical engineering to perform direct shear test (DST). It specifies a minimum specimen width to maximum particle size ratio (W/dmax) of 10. Although this is commonly applied to coarse granular materials, recent findings by the authors indicate that a ratio of 60 is necessary to eliminate specimen width effect and ensure reliable friction angle measurements. A W/dmax ratio of 60 is easily achieved for soils with dmax ≤ 1 mm, but it poses a significant challenge for coarse granular materials such as gravel, rockfill, and waste rock, where dmax may range from 5 mm to several tens of centimeters. To address this issue, an empirical correction method is proposed to adjust friction angles obtained from DSTs with W/dmax ratios between 10 and 60. The method is derived from published and accountable data. New DSTs and repose angle pile tests were conducted on waste rock samples in their loosest state. Validation of the proposed correction is supported by strong agreement between the corrected friction angles (W/dmax < 60), those directly measured on large specimens (W/dmax ≥ 60), and the repose angles from pile tests. The proposed solution offers a practical means to improve DST result reliability when large specimen sizes are not feasible. However, as an empirical approach, further refinement is needed through additional testing on a wider range of materials and conditions, particularly with W/dmax between 10 and 60. Additional research is also recommended to determine the appropriate gap between shear box halves and to validate ASTM’s suggested specimen thickness (T) or T/dmax ratio. Lastly, it is noted that the friction angle of a cohesionless material in its loosest state corresponds to the static repose angle, which exceeds the dynamic repose angle.

Mots clés

• coarse granular material
• friction angle
• static repose angle
• direct shear test
• specimen size effect
• minimum required specimen width
• ASTM D3080/D3080M
• reliability