The Effect of Confining and Pore Pressures on Fracture Toughness of Rocks

Abstract

Four-point bending tests were carried out in a pressure vessel in order to know the effect of both confining pressure and pore pressure on fracture toughness of Ogino tuff, Tohoku marble and Inada granite. In the first experiment dried specimens were sealed with vinyl film and copper foil to know the effect of confining pressure and in the second experiment water saturated specimens were supplied in pressurized water without sealing to know the effect of pore pressure which is equal to confining pressure. A fatigue crack of 2-3 mm in length was created from the tip of a notch prior to testing and the fracture toughness K_Q was evaluated by 5% offset method.
Main results obtained in this work are summarized as follows:
(1) The effect of confining pressure on fracture toughness is different between Ogino tuff and the crystalline rocks of Inada granite and Tohoku marble. The fracture toughness of the crystalline rocks significantly increases with confining pressure while that of Ogino tuff increases slightly.
(2) Fracture grows more straightly with confining pressure for all the rocks.
(3) The closure of pre-existing cracks in the directions parallel to the notch plane due to the components of confining pressure in those directions enhances the resistance against the fracture growth and also forces the crack to grow more straightly.
(4) Fracture toughness of water saturated specimen under atmospheric pressure decreases to about 90% of dried specimen for the crystalline rocks and to about 50% for Ogino tuff.
(5) Fracture toughness of the crystalline rocks can be described with the effective confining pressure where as that of Ogino tuff remarkably increases with pore pressure which is equal to confining pressure. This phenomenon in Ogino tuff seems to be due to the change of microstructure with pore and confining pressures since the main component of the rock is the weak minerals such as zeolite and montmorillonite.