Research On Damage Evolvement And Progressive Failure Rule For The Tunnel Surrounding Rockmass During Excavation

Currently, the theory concerned with the tunnel surrounding rockmass does not catch up with the demand of tunnel construction in China. In the geotechnical engineering world, increasingly extensive attentions have been paid to the issue—how to correctly understand the failure mechanism of surrounding rockmass for safe excavation and support optimization. Under the external loading condition, cracks gradually expand and transfix, finally lead to failure because there are many microscopic and macroscopic deficiencies (damages) included in the surrouding rockmass. In theory, the rockmass stability during excavation and the support system design must base on the research on damage evolvement and failure rule for the tunnel surrounding rockmass.With this issue, the main jobs of this paper are briefly summarized as follows:1.Based on continuous damage mechanics(CDM), using elastic wave velocity to characterize crack expanding, the author established the relationship formulas between the quality index Q,RMR,BQ and the initial damage of surrouding rockmass. And the range of initial damage for every level was also calculated.2.Based on the progressive failure property of rockmass, according to Hock's physical experiment and a mass of studies on tunnel stability, the failure process which usually elvoves through four situations, as well as the tunnel destruction patterns were concluded. Under the unloading condition, the failure of the surrounding rockmass is the accumulation of the damage along with the time.3.With the damage parameter defined by elastic wave velocity, the damage constitutive relation for brittle rockmass was established. Taking the stress variation in account, the damage evolvement equations under tensile and press-shear situation, were respectively listed.4.Taking Longgang tunnel under construction in Jilin province as an example, this paper described the damage evolvement of the surrouding rockmass of different excavation methods, through the analysis on ultrasonic wave test data.5.Considering the operating mode of full excavation without any support, the author selected the typical surrounding rockmassⅡ,Ⅲ,Ⅳand the different rockmass situations with different initial damages (0.48,0.55,0.62) in rockmassⅢ, then made an elastic-plastic numerical simulation.The aim was to investigate the displacement, stress and plastic strain distributions after excavation when the surrounding rockmass situation unceasingly declines (the damage gradually increases).6.Finally, the existing calculation methods of surrounding rockmass pressure were summed up and analyzed in this paper. The disadvantages of these methods were also pointed out and discussed.