Study On Yielding Support Theory Of Soft Rock Tunnel Based On Rheological Analysis

Deformation of surrounding rock of soft tunnel is a time-dependent process.There is no concept of time in the traditional theory of elasticity and elastoplasticity,which can not describe the change of deformation of surrounding rock with time.The time-dependent properties of rheology well describe and predict this process.It is a great practical significance to introduce rheological mechanics into soft rock analysis.The surrounding rock of soft tunnel often produces large deformation with time.In engineering,the support structures with high strength and stiffness are often used to restrain the large deformation caused by rheology,which result in the failure of the deformation energy of surrounding rock to be released well.The yielding support system is a kind of support structure which can provide enough support force to the surrounding rock so that the surrounding rock does not undergo large deformation and failure,and has certain flexibility to increase the displacement of the surrounding rock appropriately.The deformation energy of surrounding rock can be released appropriately by the way of yielding support.In this paper,appropriate viscoelastic and viscoelastic-plastic rheological constitutive models are selected to study the law of surrounding rock's displacement changing with time by the method of theoretical analysis.At the same time,the differences of surrounding rock displacement under different support modes such as yielding support are studied,and the applicability of theoretical deduction is verified by numerical analysis.The main contents and achievements of this paper are as follows:(1)Fractional Burgers viscoelastic rheological constitutive model is selected in the viscoelastic analysis of tunnel surrounding rock,and the analytical solution of displacement varying with time is deduced.By comparing with the solution of fractional Maxwell model and the Integer Burgers model,the results show that the displacement changes of the three models have the same trend,but they also have some differences.By observing the law of displacement variation,it is found that the increment of displacement is large in the short time after tunnel excavation.Therefore,it is necessary to support surrounding rock in time to restrain the large deformation of caused by rheology in practical engineering.(2)The fractional viscoelastic-plastic rheological constitutive model with Mohr-Coulomb criterion is selected in the viscoelastic-plastic analysis of tunnel surrounding rock.The stress release effect affected by the palm surface is considered in the calculation model,and the analytical solutions of stress,displacement and radius of viscoplastic zone of surrounding rock are derived.The applicability of the deduced results is verified by comparing with the solutions of the Nishihara model.(3)The surrounding rock is regarded as the Burgers model and the supporting structure as elastic model or Kelvin viscoelastic model.The analytical solutions of support force and displacement of surrounding rock with time under strong support or yielding support are derived.Taking the elastic support structure as an example,the difference of displacement variation of surrounding rock in two supporting modes is analyzed,the results show that the displacement produced by yielding support is larger than that of strong support,but the increment is not very large,which ensures that the support structure can provide sufficient support force and release the bearing capacity of surrounding rock appropriately.At the same time,the starting time and the amount of yielding support are discussed.(4)Two kinds of support modes are simulated by FLAC 3D software.According to the analysis of the displacement numerical solution,the results show that the error between the numerical solution and the theoretical solution is less than 2%,which proves the correctness of the theoretical derivation.