| With the rapid development of Chinese economy,current civil-infrastructures have been short to meet people’s communication and travel requirements.In order to enable citizens to enjoy more convenient transportation,Chinese government has put strength in developing highspeed railway,intensifing highway density and urban subway and other basic transportation facilities.Phyllite is one of the representative geological conditions with which railways or expressways’ construction has to deal.Based on the Xiangga Mountain Tunnel of Lalin Railway,this thesis revels a tunnl support technical method of double-layer preliminary support,which aims to deal with the problem of large deformation of phyllite tunnel or other tunnels that have suffered from large deformation problem during the constrcution period.Compared with singlelayer preliminary support,double-layer preliminary support has more flexibility in design and more influencing parameters.The existing research results are mostly focused on engineering practice,and the related theoretical research results are lacking.Therefore,through theoretical analysis and numerical simulation,this paper systematically studies the support effect of phyllite large deformation tunnel with double-deck preliminary support under different excavation methods,different preliminary support thickness ratio and different construction time.The main research achievements are as follows:(1)The reasons for the large deformation of surrounding rock of Xiangga Mountain Tunnel are analyzed and the main causes of large deformation are due to the mechanical weakness of phyllite,also high in-situ stress which generated by tunnel depth,and the water-seepage in tunnel face make further weakening of phyllite property.(2)In the view of the shortcomings of the original design scheme and the supplementary scheme,the technical scheme of double-deck preliminary support is put forward,and the working characteristics of double-deck preliminary support are analyzed.Based on the elasticplastic hypothesis,three working modes of double-deck preliminary support are put forward: 1)single supporting mode: the first stage of preliminary support working independently,the second stage of preliminary support not being implemented;2)common supporting mode: twodeck preliminary support has been implemented,and joint deformation bears the surrounding rock load;3)elastic-plastic working mode: one layer of support in two-deck preliminary support achieves the ultimate bearing capacity.The loading force enters the plastic state,while the other supporting layer has not reached the state of ultimate bearing capacity.However,when the preliminary support of both layers reaches the ultimate bearing capacity,if it can not balance with the surrounding rock pressure,the double-layer support system will be destroyed.(3)Analysis and theoretical deduction are made on the stress distribution of the crosssection of the double-layer preliminary support.It is concluded that the preliminary support load of the first layer consists of two parts,namely,the surrounding rock load borne by the first layer when the preliminary support works alone,and the "additional load" generated by the two layers when the preliminary support works together,while the preliminary support load of the second layer comes from the "additional load".Through theoretical deduction,it is concluded that the load distribution of cross-section has the greatest correlation with the following parameters:(1)the load of surrounding rock;(2)the size of preliminary support cross-section of each layer;and(3)the elastic modulus of preliminary support material of each layer.If the stress difference between the two preliminary supports is too large,the shear failure of the bonding layer will occur.Therefore,the bonding layer between the two preliminary supports should be made of materials with early strength and shear resistance.(4)Through numerical simulation,the relationship between flexural stiffness ratio and supporting effect of double-deck preliminary support under three excavation methods,i.e.full section method,upper and lower step method and three step method,is analyzed.It is considered that the excavation method of upper and lower steps can control the deformation of surrounding rock and ensure the convenience of construction.The stress equilibrium distribution between two layers of support can be better achieved by adopting the preliminary support thickness ratio of 20/20.(5)By means of numerical simulation,the influence of different time combinations of double-deck preliminary support on supporting effect is analyzed when the upper and lower steps method is adopted and the thickness ratio of 20/20 preliminary support is selected.The calculation shows that the time of preliminary support construction in the first layer has the greatest influence on the deformation of surrounding rock.The later the first layer is applied,the greater the deformation of surrounding rock.There is an uneven difference in the profile deformation of the same tunnel section,and the most obvious difference is the arch waist.The time of the first support of the second layer has the greatest impact on the load distribution of the double-layer support.The later the second support is applied,the more the load sharing of the first support is,the less the load of the whole support is.The comprehensive research results show that it is appropriate to select the upper and lower step method for the construction of large deformation section of Xiangga Mountain Tunnel on Lalin Railway,adopt double-layer preliminary support,select 20/20 preliminary support thickness ratio,implement the first layer preliminary support closely following the face of the palm,and delay the second layer preliminary support by 4-8 times the length of circular footage,so as to control the deformation of surrounding rock and ensure that the support structure has a certain degree of safety remain. |
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