Experimental Study on the Reliability of Scaling Down Techniques Used in Direct Shear Tests to Determine the Shear Strength of Rockfill and Waste Rocks

Abstract

The determination of shear strength parameters for coarse granular materials such as rockfill and waste rocks is challenging due to their oversized particles and the minimum required ratio of 10 between the specimen width (W) and the maximum particle size (dmax) of tested samples for direct shear tests. To overcome this problem, a common practice is to prepare test samples by excluding the oversized particles. This method is called the scalping scaling down technique. Making further modifications on scalped samples to achieve a specific particle size distribution curve (PSDC) leads to other scaling down techniques. Until now, the parallel scaling down technique has been the most popular and most commonly applied, generally because it produces a PSDC parallel and similar to that of field material. Recently, a critical literature review performed by the authors revealed that the methodology used by previous researchers to validate or invalidate the scaling down techniques in estimating the shear strength of field materials is inappropriate. The validity of scaling down techniques remains unknown. In addition, the minimum required W/dmax ratio of 10, stipulated in ASTM D3080/D3080M-11 for direct shear tests, is not large enough to eliminate the specimen size effect (SSE). The authors’ recent experimental study showed that a minimum W/dmax ratio of 60 is necessary to avoid any SSE in direct shear tests. In this study, a series of direct shear tests were performed on samples with different dmax values, prepared by applying scalping and parallel scaling down techniques. All tested specimens had a W/dmax ratio equal to or larger than 60. The test results of the scaled down samples with dmax values smaller than those of field samples were used to establish a predictive equation between the effective internal friction angle (hereafter named “friction angle”) and dmax, which was then used to predict the friction angles of the field samples. Comparisons between the measured and predicted friction angles of field samples demonstrated that the equations based on scalping scaling down technique correctly predicted the friction angles of field samples, whereas the equations based on parallel scaling down technique failed to correctly predict the friction angles of field samples. The scalping down technique has been validated, whereas the parallel scaling down technique has been invalidated by the experimental results presented in this study.

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

“…Using a standard direct shear test box (6 cm for most cases) with field materials and respecting the minimum required specimen sizes to d max ratio are impossible. Scaling down techniques are thus employed by removing oversized particles during sample preparation to make laboratory tests possible [16,[58][59][60][61][62][63][64][65][66][67]. For most cases, the d max of the resulting samples can still be too large compared to any available standard or nonstandard direct shear test apparatus.…”

“…For most cases, the d max of the resulting samples can still be too large compared to any available standard or nonstandard direct shear test apparatus. The minimum required specimen size to d max ratio of 10 [42,68] has thus been universally used in direct shear tests [11,16,62,[69][70][71][72][73][74] even though the validity of this minimum required value of 10 has never been verified. In other words, it remains unclear if the SSE of direct shear tests is eliminated with specimens having a width to d max ratio of 10.…”

“…The direct shear test is a very old but still regularly used method to determine the shear strength of geomaterials [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. For a given project, one usually needs to take samples from the project site.…”

Shear strength of angular granular materials with size and shape polydispersity

“…Although numerous research has been reported about the effect of these techniques on the shear strength of granular materials, the results widely vary up to the point of being contradictory. While some studies have indicated that shear strength decreases with grain size [Marachi et al, 1972, Ovalle et al, 2014, Varadarajan et al, 2003, Xiao et al, 2014, Zeller and Wullimann, 1957, others have shown the opposite behavior [Al-Hussaini, 1983, Cao et al, 2020, Deiminiat and Li, 2022, Linero et al, 2007. The sources of this apparent contradiction are presumably found in several sample and testing characteristics, and inherent material properties.…”

Particle size distribution (PSD) estimation using unmanned aerial vehicle (UAV) photogrammetry for rockfill shear strength characterization

“…It follows, by analogy, that friction angles of field samples, when evaluated using the scalping scaling-down technique, can be accurately predicted. In contrast, predictions based on the parallel scaling-down technique may be deemed unreliable for active mines or real-case scenarios [36].…”