Shear modulus should be the same despite saturation, but it is observed that the saturated shear modulus is often lower than the dry one, and this is described as shear weakening effect. In many areas, e.g. reservoir engineering, problems such as permeability changes can occur through shear weakening. "Buntsandstein", Grey Berea Sandstone, limestone, dolomite and granite samples were taken to determine mass, porosity, permeability, compressional and shear wave velocity over time. Porosity and permeability values did not change. Vp and vs were determined in the laboratory with an ultrasonic device. The samples were measured dry and saturated (1000ppm NaCl), hence shear and bulk modulus were calculated. Vp and mass over time is presented, increasing mass does not increase vp consistently. Shear and bulk modulus over time is figured, thereby it can be seen that the often cited shear weakening effect occur on the limestone and granite samples. Shear weakening was expected for the limestone samples due to the complicated pore structures, but it was not predictable for granite samples. Probably an explanation could be that granite contains a mineral that is able to adsorb and/or release water which influences the shear modulus, thin sections may provide information.


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