Determination of the Shear Strength of Rockfill from Small‐Scale Laboratory Shear Tests: A Critical Review

Abstract

Determining the shear strength of rockfill is a key task for the design and stability analysis of rockfill structures. When direct shear tests are performed, the well-established ASTM standard requires that specimen width and thickness must be at least 10 and 6 times the maximum particle size (dmax), respectively. When the value of dmax is very large, performing such tests in laboratory with field rockfill becomes difficult or impossible. Four scaling-down techniques were proposed in the past to obtain a modeled sample excluding oversize particles: scalping, parallel, replacement, and quadratic. It remains unclear which of the four scaling-down techniques yields reliable shear strength of field rockfill. In this paper, an extensive review is presented on existing experimental results to analyze the capacity of each scaling-down technique to determine the field rockfill shear strength. The analyses show that previous researches followed an inappropriate methodology to validate or invalidate a scaling-down technique through a direct comparison between the shear strengths of modeled and field samples. None of the four scaling-down techniques was shown to be able or unable to predict the field rockfill shear strength by extrapolation. The analyses further show that the minimum ratios of specimen size to dmax dictated by well-established standards are largely used but are too small to eliminate the specimen size effect. In most cases, this practice results in shear strength overestimation. The validity or invalidity of scaling-down techniques based on experimental results obtained by using the minimum ratios is uncertain. Recommendations are given for future studies.

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

“…However, when the specimens are too small, the measured shear strength can be significantly different from that of the tested material in field conditions. Thus, the tested specimens must be large enough to eliminate any specimen size effect (SSE) [2][3][4][5][6][7][8][9][10][11]. The minimum required specimen volume to avoid any SSE is called the representative element volume [12][13][14].…”

“…To overcome this problem, a common practice is to prepare test samples by excluding the oversized particles [10,[27][28][29][30][31][32][33][34][35][36][37][38][39][40]. The method is called the scalping scaling down technique.…”

“…Scalping technique is probably the simplest and earliest method to obtain laboratory samples having an admissible d max from field materials [10,29,33,37,[40][41][42][43][44][45][46][47]. By applying this technique, the particle size distribution curve (PSDC) of the obtained samples can differ from that of the field material due to the reduction in coarse particles.…”

“…The fourth scaling down method, called the quadratic grain-size technique, produces a PSDC by following an equation that has nothing to do with the PSDC of field materials [10]. The physical meaning of the proposed modification is unclear, and it will not be discussed further in this study.…”

“…Until now, the parallel scaling down technique has been the most popular and the most widely used [10,15,17,[30][31][32]38,39,[57][58][59][60][61][62][63][64][65]. This is mainly because the parallel scaled down samples are considered to be the most faithful to the field material, due to the similarity between the PSDC of scaled down samples and that of the field material [10,32,39,40,[50][51][52][58][59][60][61]65,66]. This is, however, a not valid justification.…”