Study on Shear Strength Empirical Equation of Soil–Rock Mixture in the Qinba Mountains

Shallow geological disasters in the Qinba Mountains are frequently associated with soil–rock mixtures (S–RM) formed from the weathering and accumulation of weak metamorphic rocks. The complex structure of this material, combined with constrained in situ testing conditions, makes it challenging to accurately determine its shear strength parameters in engineering practice. To address this, a series of large‐scale direct shear tests were conducted to systematically investigate the influence of three key factors—water content, particle size distribution, and the contact area between the S–RM and bedrock—on its shear strength. The results indicate that the particle size of the natural S–RM is predominantly distributed within the 2–10 mm range, and shearing leads to a significant reduction in coarse particle content. The sensitivity of shear strength to the influencing factors, in descending order, is contact area > water content > particle size distribution. Based on the experimental data, quantitative relationships between each factor and the internal friction angle were established, respectively. Subsequently, an empirical equation for predicting shear strength was proposed by integrating these relationships, weighted by their normalized sensitivity coefficients, to account for the coupled effects of multiple factors. This model enables a straightforward and reliable estimation of the S–RM shear strength using parameters obtainable through conventional field investigations and basic laboratory physical property tests, providing a practical reference for stability evaluation of geotechnical masses in disaster prevention and control projects in mountainous regions.