Abstract：Utilizing the 20 MN servo-controlled triaxial rock testing machine with high temperature and high pressure developed by the authors and using optical observation instrument of borehole deformation made by theory of optics，the intensive study of deformation laws and instability critical conditions of large-size granite with f 40 mm borehole samples of f 200 mm×400 mm within 600 ℃ and triaxial hydrostatic stress in depth of 6 000 m is carried out. The research results are as follows：(1) The deformation of borehole in granite bored with the increase of temperature and stress performance is obvious at different stages with high temperature and high pressure. Within hydrostatic stress of 4 000 m in depth and temperature at 400 ℃，drilling performance is obvious at the moment of viscoelastic stage. Although borehole diameter reduces but still in stable condition，and damage does not occur. The hydrostatic stress of 4 000–5 000 m in depth and the temperature at 400 ℃–500 ℃，the deformation of borehole is at viscoelastic-plastic stage and the surrounding rock near drilling shows a destructive trend，and the aperture begins to increase. Under 5 000 m hydrostatic stress in depth and temperature more than 500 ℃ when the cracked rock-like particles block in plastic zone gradually fall down from the hole wall，the borehole is in damage. (2) When the surrounding rock of granite in more than creep threshold of stress and creep threshold of temperature，which is over hydrostatics stress of 5 000 m in depth and temperature than 500 ℃，the borehole is in destruction and the collapse of a borehole phenomenon occurs. At the same time，the particles of surrounding rock in granite are falling down；and the hole diameter is increasing. (3) At different temperatures and hydrostatic stresses in depths，the critical hydrostatic stress of rheodestruction of surrounding rock is 5 000–6 000 m (that is 125–150 MPa)；and the critical temperature of rheodestruction is 500 ℃–600 ℃. The failure modes are of compression fracture，transpression or both of them. (4) The critical condition of borehole instability in granite under high temperature and stress is hydrostatic stress of 4 000–5 000 m in depth and temperature at 400 ℃–500 ℃. These research can provide a scientific basis and theoretical guidance for hot dry rock(HDR) development to utilize the drilling in the maintenance of stability and Chinese continental scientific drilling(CCSD) in the aspect of ultra-deep hole and deep hole in the construction of drilling encountered in the process of stability.
