A series of impact experiments is conducted on natural

soaked and saturated red sandstone samples. The effect of hydro-dynamic coupling on dynamic strength

deformation characteristics and energy mechanism of samples is analyzed by comparing with that of dry samples. Experimental results indicate that dynamic compressive strength and peak strain of samples are mainly affected by the water content and impact load

while the peak modulus is obviously influenced by the water content. The water softening effect leads to loosen expansion of granular structure

which further results in cementation weakening. In addition

the meso-mechanical effect of pore water might lead to low strength and high strain of samples at high strain rate. The strength of samples is strengthened and the plastic deformation is decreased by the effect of strain rate. A large amount of total absorbed energy U prior to the peak stress is transformed to the releasable strain energy Ue

but a small portion of U is transformed to the dissipation energy Ud. The strain energy of each part increases with the increase of the pulse intensity

however the variation with the water content is largely different. A modified brittleness index BIM (ratio of Ud to Ue) shows that both the water content and pulse intensity have a threshold resulting in the response of plastic deformation quite different. The effective impact energy index (Keff) shows that the strain rate effect significantly influence the impact tendency of dry and saturated samples. Water softening effect reduces the effect of strain rate

but the strain rate effect is enhanced when the saturation is reached.