| Xigeda stratum is a special weak stratum in southwest China.It is easy to mud when water content reaches 25%,and its mechanical properties are significantly reduced.Therefore,this paper relies on the Chengdu-Kunming Railway Reconstruction Project,and based on domestic and foreign research data for analysis.Through theoretical analysis,numerical simulation and other research methods to study the bolt effect and steel frame selection of Xigeda stratum tunnel.Research shows:(1)The force transfer mechanism of bolt is solved by the expression formula of radial displacement of surrounding rock and the formula of shear stress distribution of bolt in Xigeda stratum tunnel.The calculation results show that the bolt at the arch position bears less pressure and the sidewall position bears more pressure.Further analysis shows that the deformation mode of different parts of surrounding rock is the key factor affecting the pressure state of bolt.Because of the slow attenuation of displacement and settlement and the small displacement gradient at the arch of tunnel,the pressure of anchor bolt is small.The displacement gradient of the sidewall is large and its attenuation is fast,so the pressure of the bolt is large.(2)The strength reduction method is used to quantitatively evaluate the overall stability of deep and shallow Xigeda tunnel under different support forms.Taking the safety factor as the evaluation index,the research results show that the increase rate of the safety factor of the system bolt under shallow burial is 4%.Under the condition of deep burial,the lifting rate of safety factor of system bolt is 3.1%.It shows that the effect of the system bolt on the overall stability of the tunnel is low.(3)Based on the convergence-restraint principle,the initial displacement of the support is determined by using the function relationship between load release coefficient and time,and combining with the actual operation time of the site process.The safety factor defined by maximum support force is used as evaluation criterion to study the safety of initial support of Xigeda formation tunnel.The results show that when the rate of water content is less than 20%,the supporting capacity of the two kinds of steel frames can meet the safety requirements.When the rate of water content increases to 26.6%,the support strength of the grille steel frame does not meet the needs.When the rate of water content increases to 30%,the supporting pressure provided by the combined support system of I20 b steel and shotcrete designed at this stage can not meet the safety requirements.In order to ensure the construction safety and the overall stability of the tunnel,it is necessary to strengthen the support.(4)Using FLAC3 D software to simulate the tunnel construction process.The calculation results show that the support capability of section steel-spray mixing is stronger than that of grille-spray mixing.There is a big difference between them in the restraint capacity of horizontal displacement around the tunnel.However,there is little difference in the restraint effect of the settlement at the arch position and in the overall pressure of support. |
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