Dynamic Mechanical Properties of Dry and Water-Saturated Siltstones Under Sub-Zero Temperatures

The effects of low temperature and strain rate on the dynamic mechanical properties of dry and saturated siltstones under sub-zero temperatures were investigated and presented in this paper. The siltstone specimens were first frozen to different sub-zero temperatures (from - 10 to - 50 degrees C), and then were tested at those temperatures using a split Hopkinson pressure bar (SHPB) system. The results indicated that compared to the dry specimens, the saturated specimens exhibit a much shorter compaction phase in the dynamic stress-strain curve due to the presence of the pore water or ice at sub-zero temperatures. The dynamic elastic modulus (E-d) and dynamic uniaxial compressive strength (UCSd) monotonically increase with the increase in the strain rate for both the dry and saturated specimens, and the dry specimens are more sensitive to the strain rate effect with respect to the UCSd. Furthermore, for both the dry and saturated specimens, the E-d and UCSd first increase with the decrease in the temperature from 18 to - 30 degrees C, and then decrease with a further drop in the temperature from - 30 to - 50 degrees C. Manifold reasons are responsible for this phenomenon, including the shrinkage of mineral grains, enhancement of the ice strength and interaction of the water/ice mixture with rock as the temperature drops. Using the NMR technique, the mechanisms of the mixed water/ice weakening and strengthening effects on the dynamic mechanical properties of siltstones at sub-zero temperatures were discussed.