Research On Bearing Mechanism Of Arch Primary Support For Super Large Section Tunnel

In recent years,the scale of highway construction is increasing,resulting in the increasing number of super large section highway tunnels,such as one-way four lane and two-way eight lane.But because of the large span of four lane and two-way eight lane tunnel,low flat rate,complex construction process and many disturbance of surrounding rock,the failure of primary support strength and the loss of bearing capacity may lead to the instability of the palm surface and the tunnel collapse.The main reason for the above disaster is that the related design and construction codes are still based on the engineering analogy,and the applicability of the super large section tunnel is insufficient,and the research and understanding of the bearing characteristics of the supporting system are lack.Under this background,based on the successful experience of super large section tunnel construction,the following research work is carried out through the means of indoor test,numerical test and field test.(1)Study on full scale test of load-bearing mechanical properties of archBy improving the large-scale mechanical test system of underground engineering support structure,the full scale test of the mechanical properties of the arch is carried out,and the whole deformation form,local failure characteristics,bearing capacity,inplane(outer)instability form and distribution law of the stress(internal)force are analyzed,and the mechanical properties and the deformation and failure modes of the arch support are studied.The results show that the arch is the failure mode of the local strength damage resulting in the overall instability.The maximum load of the arch is 1242.43 kN,showing the unsymmetrical deformation of the arch roof,the arch waist and the twist of the arch foot.The numerical test and the indoor test arch bearing capacity difference is small,and the numerical difference rate is below 10%.Under the condition of pressure sharing,the arch roof gradually subsided with the load increasing,the load displacement showed linear increase,the displacement of arch shoulder was smaller,no obvious deformation,the load kept increasing,and the arch waist was gradually exprotruding,and the deformation and load were larger and faster.The radial displacement of the right arch foot is smaller and the load is slowly increasing until smooth.The lateral displacement of the left arch foot increases rapidly,the load increases first and then decreases,and the external instability occurs,which indicates that the strength failure of the part is lost and the bearing capacity is lost,thus result in instability.(2)Study on bearing characteristics of primary support arch for super large section tunnelIn view of the phenomenon that the single arch is easy to appear out of plane instability the indoor test,the combined structure model represented by three arches is established by numerical test.Considering the influence factors such as the spacing of the arch,the longitudinal connection distribution and the state of the spatial bias state,the bearing capacity of the combined structure of the arch is studied by different factors.Influence law.The results show that the arch spacing and longitudinal spacing have significant influence on the arch bearing capacity.The parallel action between the arch and the longitudinal connection makes the longitudinal stiffness and strength of the composite structure increase obviously,and the bearing capacity is magnified,which is 3.14～3,92 times the bearing capacity of the single arch.The difference of bearing capacity difference between different arch supports is 10 times,and the difference of bearing capacity between different longitudinal connections is less than 1 times.The influence of lateral bias ratio on the ultimate bearing capacity of the composite structure is related to the height span ratio of the tunnel.The longitudinal bias ratio has a significant decreasing trend in the bearing capacity of the arch structure,which is not conducive to the bearing capacity of the arch.(3)Study on bearing mechanics characteristics and deformation control of primary support for super large section tunnelIn view of the engineering background of the Magongci tunnel of,a tunnel excavation support model,represented by three primary support,is set up in view of the mechanical characteristics of the primary support of the super large section tunnel and the control of the tunnel deformation.50 sets of comparison conditions are set up to study the bearing characteristics and control effect of the primary support.The results show that the settlement and difference of vault decrease with the increase of concrete spray layer strength.Taking into account economic factors and intensity utilization ratio,C25 and C30 should be adopted for concrete spray layer strength.When the arch spacing is more than 1.2 m,the settlement difference of the vault is too large;when the vertical connection spacing is more than 1.5 m,the settlement difference of the vault is less.The concrete shotcrete layer takes on more axial forces,and the arch supports more bending moments.When the arch spacing is 0.4 m～0.8 m,the ratio between the axial force of the arch and the moment of the concrete shotcrete layer is too large.When the longitudinal connection spacing is 0.5～1.5 m,the ratio of axial force sharing ratio and concrete bending moment ratio of arch support is more reasonable.Therefore,the arch spacing is adjusted by 0.8-1.2m,and the vertical connection spacing is adjusted by 0.5～1.5m.(4)Study on field contrast test of support schemeIn order to compare the distribution spacing between the encrypted longitudinal connections and the effect of reducing the primary support of the arch spacing and the stability of the tunnel structure,two supporting schemes are designed,which are the distribution spacing of the longitudinal connection and the reduction of the distance between the arches,in the experimental section of the relatively fractured zone of the surrounding rock of the Magongci tunnel.Compared with the analysis of the tunnel deformation,the release law of the contact pressure of the surrounding rock and the primary support and the stress distribution law of primary support components,the support effect of the longitudinal distribution spacing support scheme is studied in comparison with the two schemes of tunnel excavation and support.The results show that the primary support of the two support schemes has the same effect on the control of the tunnel convergence and the settlement adjustment of the vault,and has the same restraining effect on the surrounding rock.The arch has enough strength reserve and high safety factor,all of which can be enough carrying capacity to ensure the stability of the tunnel structure in the short time.