# Experimental Studies of Scale Effect on the Shear Strength of Coarse-Grained Soil
###### Abstract  
Shear strength is an essential index for the evaluation of soil stability. Test results of the shear strength of scaled coarse-grained soil (CGS for short) are usually not able to accurately reflect the actual properties and behaviors of in situ CGS due to the scale effect. Therefore, this study focuses on the influence of the scale effect on the shear strength of scaled CGS, which has an important theoretical significance and application for the strength estimation of CGS in high earth-rock dam engineering. According to previous studies, the main cause of the scale effect for scaled CGS is the variation of the gradation structure as well as the maximum particle size (\(d_{\text{max}}\)), in which the gradation structure as a characteristic parameter can be expressed by the gradation area (S). A total of 24 groups of test soil samples with different gradations were designed by changing the maximum particle size \(d_{\text{max}}\) and gradation area \(S\). Direct shear tests were conducted in this study to quantitatively explore the effect of the gradation structure and the maximum particle size on the shear strength of CGS. Test results suggest that the shear strength indexes (i.e., the cohesion and internal friction angle) of CGS present an increasing trend with the improvement of the maximum particle size \(d_{\text{max}}\), and thus a logarithmic function relationship among \(c\), \(\varphi\), and \(d_{\text{max}}\) is presented. Both cohesion (\(c\)) and internal friction angle (\(\varphi\)) are negatively related to the gradation area (\(S\)) in most cases. As a result, an empirical relationship between \(c\), \(\varphi\), and \(S\) is established based on the test results. Furthermore, a new prediction model of shear strength of CGS considering the scale effect is proposed, and the accuracy of this model is verified through the test results provided by relevant literature. Finally, the applicability of this model to different types of CGS is discussed. c 2022 2021 2022 2021 2022 2021 2022 2022 2021  
an in-depth study of the scale effect on the shear strength of scaled CGS is very important to understand the mechanical behaviors of CGS. This study on the scale effect aims to (1) understand the difference of mechanical properties between scaled and in situ CGS and (2) accurately evaluate the mechanical properties of the prototype CGS [4]. Unfortunately, it is difficult to carry out reliable and accurate full-scale field tests using the existing test technology. At present, research on the scale effect on the mechanical behavior of CGS is mainly investigated through numerical simulations as well as laboratory experiments [3]. However, previous studies' results regarding the mechanical properties of CGS caused by the scale-induced effect usually deduce significant differences and even contrary conclusions [5; 6]. [PERSON] [7] conducted a triaxial shear test on two groups of CGS with similar gradation and different maximum particle sizes. It was found that the larger the particle size, the lower the shear strength with the same dry density. [PERSON] et al. [8] used a similar gradation method and the same dry density sampling standard to carry out triaxial shear tests on CGS with different sizes. The results showed that the maximum particle size decreased as the maximum particle size increased. The shear test results were consistent with the findings of [PERSON] et al. [9]. This is because the difference among the soil sample preparation standards significantly affected the test results. Test results suggest that when soil samples of CGS have the same dry density, as the maximum particle size (i.e., \(d_{\text{max}}\)) increases, the shear strength of CGS also increases. Nevertheless, there is still not a clear understanding of the scale-induced effect on shear strength when the sample preparation is conducted according to a standard with the same relative density [3]. For instance, [PERSON] et al. [10], [PERSON] et al. [11], and [PERSON] et al.