Analytical Theory Of Viscoelastic Deformation In Rock Tunnels Under The Influence Of Stress Relaxation And Pore Pressure

With the development of mining engineering as well as the water conservancy and hydropower engineering to the deep, long-term deformation and stability of tunnel under the condition of high ground stress and pore pressure has become increasingly prominent. On the one hand, the relaxation of original rock energy storage in high ground stress area which also been called the stress release effect is remarkable. On the other hand, pore water pressure degradation of surrounding rock is particularly prominent under the condition of high pore water pressure, which has unfavorable influence on the long-term stability of rock. The mechanical behavior of surrounding rock is assumed to obey the fractional Burgers creep model. Viscoelastic creep solution of deep-buried tunnel surrounding rock under the space effect of excavation face has been acquired by using elastic-viscoelastic correspondence principle. And then, based on Terzaghi effective stress principle, the equilibrium equation of the axisymmetric rock tunnel under the pore water pressure is established.Elastic stress field is calculated by using plane strain theory. The viscoelastic creep solution of the high pore water pressure degradation Kelvin rheological surrounding rock is acquired under the condition of keeping the volume strain rate being constant.On the basis of analyzing the solutions’ correctness and superiority, Regularity for the effect of the stress relaxation, pore water pressure, forming time and many related parameters on the creep behavior of axisymmetric tunnel. The results showed that:1. Using the creep model can fully reflect the viscoelastic creep features of surrounding rock with fewer creep elements which including creep attenuation and creep stable growth characteristic.2.Compared with the calculated results of many other creep models such as广义Kelvin Modle,BonaitinThomson Model(H/M)and so on,the fractional Burgers creep model can fully reflect the viscoelastic creep behavior of different lithologic rock including hard and soft rock by changing order.3. The deformation of surrounding rock is affected both by the space effect of4. Excavation surface and the time effect of hole formation. In the creep behavior which near and far away from the excavation surface of the surrounding rock, the space effect of the excavation surface and the time effect of hole formation are dominant factors. In addition, the Hoek equation, the Y.L.Lee equation and the M.Panet equation can fully express the stress relaxation responding which related to the space effect of excavation surface.5. When internal water pressure of the tunnel wall is relatively small, the degradation effect of pore water pressure is weak and the development of the tunnel wall displacement is small. As the pore water pressure increase, the stable stage of creep gradually develop to a unstable stage. Especially under high pore water pressure, the degradation and deformation capacity of surrounding rock has obviously increased. In addition, the transition time between the decay stage and metastable stage of surrounding rock creep is basically not affected by the internal water pressure of the tunnel wall.6. The lager the coefficient of viscosity, the faster the creep rate of rock decline,and the creep of surrounding rock will enter the stable stage soon. The coefficient of viscosity only has effect on the convergence process of tunnel wall, however has no effect on the final amount of convergence displacement. Under the degradation effect of pore water pressure, surrounding rock of rock tunnel creep is more sensitive to Poisson ratio.7. When fitting degradation effect of pore water pressure by using different function forms which is exponential and power function, the calculated results verify the applicability of these two fitting function. Particular attention should be giving to that due to the large difference between the creep parameters identification of different lithology rock, the calculated displacement and the duration of each creep stage are quite different in quantity, but it is consistent in the qualitative analysis.This study deepens the theoretical understanding of the stress relaxation and the aging deformation responding of the axial symmetrical rock tunnel under the degradation effect of pore water pressure, which providing theoretical support for the judgment of the long-term stability of surrounding rock. Truly hope to bring a slight improvement in method.