| China has a special geographical landscape.There are three-fifths of the mountain areas in the country,especially in the western and southwestern regions.There are many mountains and the geological conditions are more complex,which leads to many technical problems in the field of tunnel engineering and has not yet formed a unified theory.In this context,the length and section size of tunnel excavation are developing towards the direction of extra-long and extra-large.The buried depth of the tunnel also increases due to topographic factors.The increase of the buried depth of the tunnel increases the initial ground stress of the excavation.Therefore,high ground stress will occur in tunnel construction.The problem of insufficient support caused by the increase of buried depth and the weakness of surrounding rock is very serious,which brings serious problems to the research of support technology,construction technology and long-term safety and stability of underground engineering.Based on this background,the following research work is carried out by means of indoor test,numerical test and field test.(1)Overview and comparison of steel tube concrete support systemBased on the full investigation and analysis of typical engineering cases of tunnel support structure instability,the failure modes and occurrence characteristics of tunnel support structure are summarized,and the inducing factors of engineering problems are analyzed.According to the main characteristics and mechanical mechanism,the classification research is carried out,and the failure mechanism of tunnel support structure is analyzed.The concrete-filled steel tube support system is proposed,and its concept,function,stress characteristics of each support structure,as well as the classification and characteristics of concrete-filled steel tube are introduced.Through the comparison of the economy and support effect of the bolt-shotcrete support and the initial support system of concrete-filled steel tube,the steel consumption of concrete-filled steel tube with 150 and wall thickness of 5mm is better than that of I20 b I-beam,saving 9.83 % of steel consumption per meter.The tensile strength of shotcrete is low.After crack propagation and interface slip,there is no obvious complete slip stage of I-steel arch,and the specimen has a certain later bearing strength.The bond-slip stage of concrete filled steel tube arch is more obvious,and the slip occurs faster.However,due to its high strength and later bearing capacity,the supporting structure has good ductility and deformation ability.(2)Experimental study on bearing mechanical properties of composite supporting structureThe influence of different arch spacing and steel fiber content on the bearing capacity of concrete filled steel tube composite support structure was studied.The results show that the experimental phenomena and crack development process of the specimen during loading are analyzed,and the loading failure process of the specimen is similar to that of the previous section.The ultimate load is linear with the arch spacing,and the ultimate load decreases with the increase of the arch spacing.The decrease of the arch spacing improves the bearing capacity of the plastic stage and the yield stage of the structure.The addition of steel fiber increases an upward trend after concrete cracking;compared with the 600 mm spacing arch specimens,the ultimate load of 800 mm and 1000 mm arches is reduced by 20.54 % and 32.62 % respectively.After adding 1.2 % steel fiber into shotcrete,the ultimate bearing capacity of composite supporting structure is increased by 5.7 %.The difference between the numerical test and the indoor test is small,and the error is within 4.49 %.(3)Study on bearing capacity characteristics of composite support structureA numerical model represented by three initial support structures was established.A series of comparative working conditions of different concrete spray layer strength,arch frame combination spacing and longitudinal connection bar diameter were set up,and the economic index = F / C was defined as the reference index to evaluate the cost performance of parameter design.The results show that the concrete strength has a significant effect on the composite support structure;the spacing of the arch combination has obvious influence on the stress distribution.The diameter of longitudinal connecting bar has little effect on the bearing capacity of supporting structure.Considering the economy and construction convenience,it is suggested that the strength of concrete spray layer can be C25;the arch spacing is 1.0 m;the diameter of the connecting rib is 28 mm.(4)Study on bearing capacity characteristics and deformation control of primary support of deep buried soft rock tunnelAiming at the bearing capacity characteristics and deformation control of the primary support of deep-buried soft rock tunnel,a tunnel excavation support model represented by three primary supports is established.Different concrete spray layer strength,different arch combination spacing,and different longitudinal connection bar diameters are set to 50 working conditions to study the bearing mechanical properties and control effect of the primary support.The results show that considering the economic factors and the utilization rate of strength,the strength of concrete spray layer should adopt C25 and C30;with the increase of arch combination spacing and longitudinal connection bar distribution spacing,the difference of vault settlement under different strength concrete spray layer conditions gradually decreases.With the increase of concrete spray layer strength,the difference of vault settlement between different arch combination spacing and the same working condition increases gradually,and the difference of vault settlement between different arch combination spacing decreases gradually.When the longitudinal connection bar diameter does not exceed 25 mm,its control effect on tunnel deformation is getting smaller and smaller;it is suggested that the spacing of arch combination should be adjusted between 0.8 ～ 1.2 m and the diameter of longitudinal connection bar should be adjusted between 25 ～ 28 mm under the condition of surrounding rock.(5)Design optimization and engineering suggestions for supporting system of tunnelThe same test section was selected to design two support schemes of steel arch and square steel pipe arch respectively.The ABAQUS was used to simulate the tunnel deformation and the stress distribution of the initial support members during the tunnel excavation process,and the effect of the square steel pipe concrete support scheme was studied.The following conclusions can be drawn : the displacement curves of the surrounding rock under the two support schemes show the same trend,and the concrete-filled steel tube support scheme can better control the deformation of the surrounding rock;the support scheme of concrete filled steel tube can better restrain the internal deformation of surrounding rock.The two support schemes have sufficient strength reserve and high safety factor.The stress variation law of the arch is basically the same,but the supporting effect of the concrete-filled steel tube supporting scheme is better. |
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