Investigating statistical relationships among clay mineralogy, index engineering properties, and shear strength parameters of mudrocks

• Mudrocks are common, diverse, and problematic in analysis & engineering.
• 49 samples were collected from 11 states to characterize mudrock strength.
• Standard index properties proved variable, asymmetrical, and non-Gaussian.
• They explained only 50% of variation in cohesion & 36% in friction angle.
• Slake durability, % expanding clay, & absorption are the most predictive.
• Additional research and conservative engineering practices remain necessary.

Clay mineralogy and standard index engineering properties measured for a large and diverse set of mudrocks show high variability and clearly relate to shear strength parameters. From the best-correlated variable downward, cohesion is related to slake durability index, specific gravity, percentage of expandable clay minerals, and liquid limit, whereas friction angle is related to percentage of expandable clay minerals, absorption, percent of clay < 2 μm, and liquid limit. Using all parameters (with transformations to maximize normality) explains 50% of the total variation in cohesion and 36% of friction angle. The variability and unpredictability of shear strength parameters result from the heterogeneity of mudrocks, and the low R2 values indicate that additional factors must be sought in order to explain and predict strength parameters at levels useful for engineering purposes.