The influences of heating and uniaxial loading on granite subjected to liquid nitrogen cooling

Understanding and controlling rock breaking is critical in many geological projects, such as mining, tunneling and other underground engineering projects. Tunnel boring machine (TBM) is widely used in such geological projects owing to its security and better working environment. However, it is difficult in tunneling hard rock which would reduce the life of cutters. In this paper, an auxiliary method, heating followed by sharply cooling, is used to reduce the strength of hard rock before mechanical drilling. To reveal the softening effect of such thermal loading on hard rock, this work studied the influences of heating, liquid nitrogen (LN2) cooling and uniaxial loading on granite which is representative hard rock. First, we investigated the properties of granite after heating from 25 degrees C (room temperature) to 1000 degrees C, which was followed by LN2 cooling treatments. Second, an ultra-low temperature resistant uniaxial loading chamber was designed and constructed, which was then used to investigate the simultaneous influence of both uniaxial confining stress and LN2 cooling on the mechanical properties of granite. The features of this chamber include: (1) the maximum uniaxial load applied by this chamber is able to reach 20.5 MPa; (2) the uniaxial load is able to be applied stably when the working temperature is about - 200 degrees C. The experimental results indicate that, overall, the volume and porosity of the LN2-treated granite increase as the heating temperature rises, meanwhile the bulk density, P-wave velocity, uniaxial compression strength (UCS) and elastic modulus decrease. Except for P-velocity which shows a continuous decreasing trend, all the changes of above parameters can be divided into two stages with the increasing of heating temperatures: from room temperature to 500 degrees C, they change very slowly, while all of them change sharply and linearly when the granite is heated to above 500 degrees C followed by LN2 cooling immediately. The optical microscopy observations indicated that the micro-structures of samples are almost same when the heating temperature is below 500 degrees C; the micro-cracks increase significantly when the heating temperature is above 600 degrees C. The strength of LN2-treated samples with 600 degrees C decrease about 30% compared with that samples with room temperature, which indicates that the feasible heating temperature for auxiliary rock breaking is above 600 degrees C. Additionally, uniaxial loading restricts the evolution of thermal cracking in samples during LN2 cooling, which results in the increase of the UCS and elastic modulus.