###### Abstract  
Coarse-grained soil (CGS) is a vital geotechnical material extensively used in engineering projects. Scale effects pose challenges to accurately assessing its shear strength, and the influences of scaling methods and moisture content remain underexplored. This study investigates the effects of maximum particle size (\(d_{\text{max}}\)), scaling techniques, and moisture content on the shear strength of CGS through large-scale direct shear tests and in situ direct shear tests. CGS samples from the Jilangtan platform in Qinghai Province, originally with a \(d_{\text{max}}\) of 60 mm, were scaled to 30 mm using the Equivalent Substitution Method (ESM) and Similar Gradation Method (SGM). Tests were conducted at moisture contents of 0.91% (air-dried), 2.45% (natural), and 7.66% (saturated). Results show that ESM consistently yields higher shear strength than SGM across all moisture levels. Scale effects are minimal under air-dried conditions but become pronounced at higher moisture contents. While all large-scale laboratory test results deviate somewhat from in situ findings, shear strength parameters derived from SGM align more closely with field observations.  
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Numerous studies have investigated these challenges. [PERSON] et al. (2024) [20] compared laboratory triaxial tests with in situ direct shear tests, finding that the Scaling Method accurately predicts the critical shear strength of mine waste rock under high normal stress. However, under medium-to-low stress, current \(d_{\text{max}}\) specifications may systematically overestimate shear strength. [PERSON] (2021) [21] proposed a modified particle size distribution method for small-scale direct shear tests, demonstrating reduced maximum shear strength, peak dilation, and friction angle. This suggests that small-scale equipment can effectively measure the CGS friction angle. [PERSON] et al. (2022) [22] conducted 24 direct shear tests and found that shear strength increases logarithmically with \(d_{\text{max}}\) but decreases as gradation uniformity declines. [PERSON] et al. (2022) [14] reviewed scaling techniques for rockfill shear strength, observing that all methods have limitations and cannot fully replicate in situ test results. Collectively, these findings underscore scale effects as a persistent challenge in determining CGS shear strength.  
Although prior studies elucidate how scale effects and scaling methods affect CGS shear strength, accurately determining the shear strength parameters of in situ materials via laboratory direct shear tests remains challenging. This study addresses this gap by integrating large-scale laboratory direct shear tests with in situ tests to investigate the effects of \(d_{\text{max}}\), scaling methods, and moisture content. Using undisturbed specimens with natural \(d_{\text{max}}\) and scaled specimens obtained from two common scaling techniques, we evaluate these samples in a large-scale direct shear apparatus and compare the results with in situ data. This approach elucidates how \(d_{\text{max}}\), scaling, and moisture content influence CGS shear properties, providing insights to improve scaling techniques and engineering design accuracy.