Guanshan railway tunnel

located in Gansu Province

China

is a deep tunnel in hard and brittle diorite rock mass under high in situ stress. Meanwhile

this tunnel is also located in the Liupan Mountain

which is an active tectonic region of the compression uplift with a high horizontal movement rate and significantly changes of movement direction on the northeastern margin of Qinghai-Tibet block. On the contrary of the good rock mass classification predicted in investigation stage

the real situation after excavation shows a great change of rock mass parameters and structure

revealing not only the degradation of rock mass quality also the significant anisotropy of rock mass. The degradation and anisotropy of rock mass is described and analyzed based on the statistic analysis of the rock structural plane in field. Meanwhile

self-developed borehole camera is adopted to further observe the progressive deformation and failure of rock mass in an interval after excavation. As one of the hard and brittle plutonite

the uniaxial compressive strength (UCS) of diorite is a relatively high index on investigation stage

which is obviously larger than the in situ stress. However

the deformation and failure of diorite rock mass happened prevailingly. As the evidence obtained by borehole camera

the micro fracture or closed rock structural plane under high in situ stress will re-open and develop quickly after the excavation

which degraded the stability and quality of rock mass structure. To figure out the deformation and failure process

an angle rotation of uniaxial compressive test is designed. The uniaxial pressure is lower than the uniaxial compressive strength of diorite. This test has good agreement with the borehole camera test results

fully presents degradation of rock mass structure

induced by stress field change after excavation and proves the feasibility of the progressive deformation and failure process of hard and brittle diorite rock mass during the strongly stress field change after excavation. Besides anisotropy of rock mass

stress field change is always a significant consideration factor in deep excavation project under high in situ stress.