Research On Mechanical Properties Of Shield Tunnel Segments Strengthened By Ultra High Performance Concrete

During the operation of the shield tunnel,it is often disturbed by external construction,such as piled up above,excavation near the foundation pit,etc.Because the shield tunnel is an assembled structure with poor structural integrity,it is likely to cause excessive convergent deformation of the tunnel after external disturbance,open joints and cracks in the segment concrete,causing water leakage.To deal with these problems of shield tunnels,the method of strengthening the inside arc surface of the tunnel is generally adopted.Ultra-high performance concrete is a cement-based material with ultra-high compressive strength,ultra-high toughness,ultra-high durability,and ultra-low permeability.It has excellent coordination with ordinary concrete and is suitable for structural strengthening of reinforced concrete.In order to study the mechanical properties of UHPC-strengthened shield tunnel segments,this paper carried out a full-scale test of the UHPC-strengthened shield segment,and analyzed the flexural performance of UHPC-strengthened damaged segments under positive and negative bending moment loads.Based on the test of the UHPC-strengthened segment,a finite element model was established.According to the force mode of the segment at a specific buried depth,the force and deformation characteristics of the UHPC-strengthened segment were further studied,and the influence of axial force on the strengthening effect was explored.Finally,a refined model of the longitudinal joint of shield tunnel was established,the mechanical properties of the longitudinal joints of shield tunnel segments strengthened by UHPC were studied,and the failure mode and strengthening effect were analyzed.The main conclusions are as follows:(1)UHPC can significantly improve the bearing capacity,stiffness and ductility of the shield tunnel segment,and control the development of crack width.The greater the thickness of the UHPC layer and the thickness of the steel plate,the more obvious the strengthening effect.(2)Under the positive bending moment load,the segment and the UHPC layer did not peel off in the early stage of loading.The segment and the UHPC layer deformed in coordination,and the cross-sectional strain distribution as a whole conformed to the assumption of flat section.Peeling cracks appeared at the UHPC-RC interface at the later stage of loading.The segment and UHPC layer is no longer stressed as a whole.Under negative bending moment load,the interface did not peel off during the whole loading process,and the section strain distribution conformed to the assumption of flat section.(3)Under a sagging moment,the bearing capacity of the specimen increases with the increase of the thickness of the UHPC layer,and the growth rate gradually slows down.Under a hogging moment,the bearing capacity of the specimen increases with the increase of the thickness of the UHPC layer.Under a sagging or hogging moment,the initial stiffness of the specimen increases with the increase of the thickness of the UHPC layer.Under a sagging moment,the bearing capacity of the specimen increases obviously with the increase of the thickness of the steel plate;under a hogging moment,the bearing capacity of the specimen is less sensitive to the thickness of the steel plate,and slightly increases with the increase of the thickness of the steel plate.Under a sagging or hogging moment,the initial stiffness of the specimen increases slightly with the increase of the thickness of the steel plate.UHPC can still effectively improve the rigidity of the segment and control the deformation when the segment is under the stress state of the combination of bending and compression.Whether under positive or negative bending moments,the bearing capacities of initial and strengthened specimens with axial force are significantly higher than that of the specimens without axial force.The greater the axial force,the higher the bearing capacity.(4)The UHPC layer changes the stress and deformation mode of the longitudinal joints of the shield tunnel segments by sharing the stress.It delays the overall damage of the longitudinal joint causing by local damage and makes full use of the load carrying capacity of the segment,thereby improving the corner stiffness and load carrying capacity of the longitudinal joint.The greater the thickness of the UHPC layer,the greater the load carrying capacity and corner stiffness of the longitudinal joint of the segment.Compared with the positive bending moment,the increase rate of the corner stiffness of the specimen under the negative bending moment increases faster as the increase of the thickness of the UHPC layer.