Comparison of shear strength of sand backfills measured in small-scale and large-scale direct shear tests

Direct shear tests were conducted on 30 sand backfill materials having gravel contents ranging from 0% to 30% in a 64 mm square small-scale direct shear (SSDS) box and a 305 mm square large-scale direct shear (LSDS) box. The objectives were to compare the shearing behavior of a broad range of natural sand backfill materials tested in SSDS and LSDS and to determine if the same friction angle (phi') is obtained in SSDS and LSDS when the natural backfill material contains gravel. Triaxial compression (TC) tests were also conducted on four of the backfill materials for comparison with the SSDS and LSDS tests. Specimens tested in SSDS and TC included only material passing the No. 4 sieve (P4). Test specimens in LSDS included the P4 material as well as material retained on the No. 4 sieve (R4), to a maximum particle diameter of 25.4 mm. Friction angles corresponding to peak strength (phi') measured in SSDS and LSDS differed by no more than 48 for a given sand backfill, and in most cases were within 28. The friction angles also were unaffected by removal of the R4 material. Repeatability tests showed that statistically similar failure envelopes (p-value = 0.98) are obtained in SSDS and LSDS, and that highly repeatable friction angles (phi') are obtained using the SSDS (phi' +/- 0.25 degrees) and the LSDS (phi' +/- 0.45 degrees) methods. No statistically significant difference was found among the failure envelopes measured in SSDS, LSDS, and TC, suggesting that phi' for clean sand backfill with less than 30% gravel can be measured with similar accuracy using any of the three test methods.