Effect of Physical Properties on the Mechanical Behavior of Wyoming Siltstone Under Uniaxial and Triaxial Compression

ABSTRACT: Mechanical properties of siltstone such as compressive strength and Young's modulus are commonly used in the design of geotechnical structures. This paper highlights the effect of physical properties (i.e., porosity) and environmental conditions (i.e., water content and confining pressure) on the uniaxial and triaxial compressive strength, Young's modulus, and the strength parameters (i.e., cohesion and internal friction angle) of siltstone. A series of uniaxial and triaxial compression tests were conducted on siltstone formations from Wyoming. In addition, experimental data on siltstone from literature were compiled and integrated into this study. Prediction equations for the compressive strength and Young's modulus were established using regression analysis of the data. The results showed that UCS and Young's modulus decreased with the increase in water content and porosity. Also, triaxial compressive strength showed a positive relationship with the normalized confining pressure with respect to the porosity. Finally, the siltstone cohesion decreased with an increase in water content and porosity, while no relationship for the internal friction angle was observed. 1. INTRODUCTION Siltstones are fine-grained clastic sedimentary rocks with grain sizes lying between 1/256 to 1/16 mm (Travis 1970). Siltstones may be massive or laminated, composed of quartz and clay minerals, but unlike shales, they show little evidence or effect of bedding on its compressive strength. Siltstones are generally interbedded with shales or fine-grained sandstones and rarely form thick deposits like shale and sandstone. Independent studies of mechanical properties of siltstone like compressive strength and deformability are not commonly found. These properties are often deemed like other commonly occurring sedimentary rocks like sandstone. Although siltstones are not found in abundance, the effect of factors like temperature, confining pressure, mineral composition, porosity (ϕ), and water content (w) on mechanical properties should be studied to improve the design of our civil infrastructures. Empirical equations were developed for predicting the mechanical properties of specific siltstone from a specific location. Those equations cannot be generally applied on siltstones from other locations and regions. For example, only limited empirical equations were developed for predicting uniaxial compressive strength (UCS) of siltstone from Xinjing, China by Li et al. (2019) and siltstone from Australia by Yasar et al. (2010). In this paper, experiments and regression analysis of the mechanical properties of siltstone (i.e., uniaxial compressive strength, Young's modulus, peak triaxial compressive strength, and shear strength parameters) were conducted to better understand siltstone behavior under different environmental and loading conditions. The prediction of the proposed regression models is validated based on the residual standard error (RSE), Akaike Information Criterion (AIC), and Bayesian Information Criterion (BIC) values.