Experimental Study on the Minimum Required Specimen Width to Maximum Particle Size Ratio in Direct Shear Tests

Conducting laboratory direct shear tests on granular materials is a common practice in geotechnical engineering. This is usually done by following the ASTM D3080/D3080M-11 (hereafter named ASTM), which stipulates a minimum required value of 10 for specimen width (W) to the maximum particle size (dmax) ratio. Recently, a literature review performed by the authors showed that the minimum required W/dmax ratio given in the ASTM is not large enough to eliminate the specimen size effect (SSE). The minimum required W/dmax ratio of ASTM needs to be revised. In this study, a critical analysis is first made on existing data in order to identify the minimum required W/dmax ratio. The analysis shows that more experimental data obtained on specimens having W/dmax ratios between 10 and 50 are particularly necessary. To complete this need, a series of direct shear tests were performed on specimens having different dmax by using three shear boxes of different dimensions. The results show once again that the minimum required W/dmax ratio of 10, defined in the ASTM, is not large enough to eliminate the SSE. Further analysis on these and existing experimental results indicates that the minimum required W/dmax ratio to remove the SSE of friction angles is about 60. These results along with the limitations of this study are discussed.