- The effect of degree of saturation on the unconfined compressive strength of selected sandstones
- E. Barefield ; A. Shakoor
- Book Title / Journal: IAEG2006
- Year: 2006
- Rock Mechanics
- Keywords: Sandstone ; compressive strength ; saturation ; uniaxial tests ; density
- Unconfined compressive strength is one of the primary parameters by which sandstone rocks are evaluated for their usefulness as engineering materials. The unconfined compressive strength of sandstones is known to be controlled by such factors as loading rate, bedding orientation, presence of microfractures, and petrographic characteristics (grain size, grain shape, matrix-cement mineralogy, etc.). Research has also shown that unconfined compressive strength is significantly reduced upon saturation with water. The aim of this research was to characterize the relationship between unconfined compressive strength and varying degrees of saturation for sandstone rocks, and to explain this relationship using index properties and petrographic characteristics.
Eighteen NX-size cores were prepared from each of nine different sandstone formations that were sampled from Central Ohio through Central Pennsylvania, USA. 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%, 20%, 40%, 60%, 80%, and 100% saturation. The sandstones were also classified according to Okada's classification in order to further characterize them and provide a means of explaining the measured trends of compressive strength decrease with increasing degree of saturation for each sandstone formation.
Laboratory test results show that the sandstones tested have absorptions values ranging from 1.32% to 6.93%, dry density values of 126.7 pcf (2.03 Mg/3m) to 58.5 pcf (2.54 Mg/3m), porosities of 3.35% to 14.11%,
and dry compressive strength values ranging from 2426 psi (16.73 MPa) to 21700 psi (149.66 MPa). Data analysis indicates significant trends of unconfined compressive strength reduction with increasing degree of saturation. The trends are more consistent for high-strength sandstones than those of medium to low strength
which display less predictable behavior. Some sandstones show a majority of strength reduction having occurred by 20% saturation, with minimal to indiscernible strength reductions at higher saturation levels.
Strength reductions of up to 71.6% between dry and saturated states are observed. Based on these results, prediction equations are developed for sandstones that are grouped according to their absorption levels.