Relationship between Unconfined Compressive Strength and Degree of Saturation for Selected Sandstones

Unconfined compressive strength of rocks is known to vary with changes in moisture content. However, previous studies report inconsistent results about this subject. The purpose of this study was to investigate the relationship between unconfined compressive strength and varying degrees of saturation for sandstone rocks and explain it in terms of index engineering properties and petrographic characteristics. Eighteen NX-size (2.125 inches or 5.4 cm in diameter) cores were prepared from each of the nine sandstones sampled from central Ohio through central Pennsylvania. Laboratory tests were conducted to determine absorption, dry density, specific gravity, and porosity values for each core. Cores were then tested for unconfined compressive strength at 0 percent, 20 percent, 40 percent, 60 percent, 80 percent, and 100 percent saturation. Statistical analyses were performed to determine the relationship between unconfined compressive strength and degree of saturation. The results indicate significant trends of unconfined compressive strength reduction with increasing degree of saturation and strength reductions of up to 71.6 percent between dry and saturated states. The trends are more consistent for high-strength sandstones than those for medium- to low-strength sandstones, which display less predictable behavior. Some sandstones show a majority of strength reduction occurring by 20 percent saturation, and minimal to indiscernible strength reductions at higher saturation levels. Based on these results, prediction equations were developed for sandstones, which were grouped into four classes on the basis of their absorption values. The input parameters required for the prediction equations include degree of saturation and one or more of the index engineering properties.