Effect of water on sandstone and limestone, fracture toughness, frictional parameters and brittle strength.

The presence of water causes a dramatic reduction of the strength of most rocks. Under compressive stress conditions, fracture mechanics models show the strength of a rock sample is in particular controlled by frictional parameters and the fracture toughness of the material. Previous studies suggested that these parameters could change significantly in the presence of water, but there is a paucity of data quantifying this. Here, we report fracture toughness, frictional and uniaxial compression tests performed on five sandstones and five limestones under dry and water-saturated conditions, that provide new insight into the mechanical influence of water on sedimentary rock strength. Our new data showed that on both sandstones and limestones, the presence of water causes a reduction of both the fracture toughness (from 0 to 50%) and the static friction coefficient (from 0 to 40%), suggesting that water weakening in these sedimentary rocks is mostly due to a reduction of these two parameters under the relatively high strain rate conditions investigated here. While for sandstone we found a reduction of the Uniaxial Compressive Stress between 0 to 35%, it was less variable in limestone, in most cases around 40%. The measured fracture toughness and frictional parameters were then introduced into two well-known micro-mechanical models (the pore-emanating cracks model and the wing crack model), which provide simple theoretical expressions for the Uniaxial Compressive Strength. We found that the predicted water-weakening based on our toughness and friction parameter measurements is in overall agreement with our strength measurements on dry and wet samples.