Study On The Mechanical Mechanism Of Yielding Supports In Squeezing Tunnels

Recently,more tunnels have been constructed in deep soft rock masses throughout the world.Normally,tunnels in such situations possess two characteristics;first,soft surrounding rock subjected to high ground stress is prone to time-dependent deformations and second,support structures failure often occurs when they are strictly restraining surrounding rock displacements.The current tunnel support design is mostly short-term based on the elasto-plastic theory,which is unable to satisfy the design requirements of corresponding tunnels.In deep soft rock tunnels,the time-dependent deformation triggered by rock rheology usually contributes most part of tunnel deformations.Hence,it is of great significance to carry out the long-term tunnel support design using rheology theory.When dealing with rock strong rheological deformations,it has been found that traditional stiff system is unable to restrain rock displacements without being damaged,and yielding supports based on the principle of “Energy absorption and deformation release” are gradually recognized as the most effective system.Taking into account the lack of yielding supports research,this paper,based on the theoretical analysis,numerical simulation and engineering application,investigates the influence of rock rheological behaviour on tunnel deformation and support performance.The main conclusions can be drawn as follows:(1)In order to make up the shortcoming of traditional rheological models that they are unable to describe the non-linear accelerated creep characteristics of rocks,an improved Burgers model,based on the fractional theory and damage mechanics,is established.The use of the improved rheological model can be divided into two stages based on whether the accumulated creep strain of rock reaches the threshold value of opening accelerated creep stage.This improved model can not only describe the comprehensive creep characteristics of rock,but also introduce few parameters,leading to a good engineering practicality.On the basis of this model,the accurate expression for time-dependent tunnel displacement is derived and then is well applied to deformation prediction of auxiliary tunnel of Jinping ?hydropower station.(2)Aiming at investigating the coupled effects of rock bolts and lining on surrounding rock reheological deformation,Hooke's spring is used to simulate the rock bolt behaviour and then by its parallel connection with rock rheological model a novel model for describing bolted rock time-dependent behaviour is established.Furthermore,the analytical solutions for tunnel displacement and lining pressure under the conditions of only lining support and rock bolt-lining coupled support are provided,respectively.Based on the above theoretical solutions,the analysis on the influence of rock bolt parameter on tunnel response is carried out.Compared with the case without rock bolt reinforcement,the application of rock bolts has a significant restraint effect on tunnel displacement.However,there is a corresponding threshold of rock bolt parameter.If it exceeds this value,the increase of rock bolt parameter will not result in the obvious reduction of tunnel displacement and lining pressure.(3)It is an important task to accurately evaluate the influence of excavation stoppage on tunnel stability.In order to study the influence of excavation stoppage on tunnel performance,the improved function expression of stress release coefficient is proposed.Then,the analytical solution for tunnel temporal and spatial deformations is derived,where excavation stoppage and the delay installation of lining are considered.Subsequently,the proposed theoretical analysis can be well validated by its application in Rongjiawan tunnel.Finally,a comprehensive parametric investigation is carried out,including duration and start time of excavation stoppage,tunnelling rate and installation time of lining.(4)A supporting scheme that a yielding layer is adopted and placed between the first lining and second lining is proposed,in order to address the problem of lining failure in deep soft rock tunnels.Then,exact visco-elastic solutions are deduced for the tunnel displacement and lining pressure under such support system.The reliability and effectiveness of analytical solutions are validated using numerical method.Parametric investigation shows yielding layer plays a good role in absorbing rock rheological deformations.For best supporting effect,the yielding layer should have sufficient thickness to prevent excessive pressure and displacement of the second lining,and its thickness should also be restricted to avoid excessive rock displacement.For tunnels excavated in a certain type of rock,adequate deformability of the yielding layer is required as well.(5)Because the yielding element experiences shortenings from elastic compression to plastic strain,the deformation process of circumferential yielding lining should be divided into three stages including elastic stage 1(both elastic shortenings of shotcrete lining and yielding elements),yielding stage 2(plastic strain of yielding elements)and elastic stage 3(elastic deformation of shotcrete lining).The calculation expression for support stiffness of circumferential yielding lining is determined for each stage.Based on the above calculation expressions for support stiffness,the effect of the time-dependent hardening property of shotcrete on the early safety of tunnel lining is analyzed.Furthermore,in order to determine the interaction mechanism of surrounding rock and circumferential yielding lining,a unified design approach for circumferential yielding lining is presented,where the lateral pressure coefficient is discussed.