| By the end of 2018,rapid transit lines were operating in 35 cities in mainland China.The total operating mileage exceeded 5700 km,and most of that was in shielded tunnels.With the wide application and the increasing service life of shield tunnels,various tunnel defects are now emerging.Technology for strengthening shield tunnels is therefore of great significance.Steel plates and fiber-reinforced plastic are the two materials most commonly applied for strengthening subway tunnels.Steel plates consume a lot of steel and are therefore uneconomic.Fiber-reinforced plastic can only be applied to the roof of a tunnel.In this study strengthening using concrete-filled steel tubes was investigated by experiment and mathematical modeling:1.Strengthened segment joints were tested under sagging and hogging bending.The failure mode of the CFSTs strengthened segment joints was firstly discussed,and the strengthening effects of adopting the CFSTs and steel plates were compared.The impact of the CFSTs’ sectional height on the strengthening result was also investigated.It was found that:(1)the failure of CFSTs strengthened joints were mainly caused by the connection failure between CFSTs and segment;(2)the loading capacities and stiffness of segment joints were both obviously improved after strengthening with CFSTs or steel plates.Under sagging bending,the steel usage of the CFSTs is smaller than that of steel plates,but the strengthening effect of the CFSTs is better than that of steel plates,Under hogging bending,the steel usage of the CFSTs is smaller than that of steel plates.Under hogging bending,although the steel content of the CFSTs is less than that of steel plates,the strengthening effect of CFSTs and steel plates is roughly comparable;and(3)the variation of the sectional height of the CFSTs from 45 mm to 60 mm has a limited influence on the loading capacity and stiffness of the joints.2.High temperature treated segment joints were strengthened with CFSTs and tested under sagging bending.The influence of the joint gap on the temperature distribution was investigated,the residual mechanical properties of the fire damaged segment joint and the effectiveness of CFSTs for restoring fire damaged segment joins were revealed.It can be concluded that:(1)when the initial joint opening is smaller than 10 mm,the temperature atmost positions in the joint gap remains lower than in the neighboring concrete with over 60 min of exposure to fire.After 120 min,however,the temperature in the joint gap is higher than in the surrounding concrete;(2)the bearing capacity and flexural stiffness of the segment joints after a fire can be significantly improved by strengthening with CFSTs.They may even surpass those of untreated joints;(3)strengthening near a joint play a crucial role during initial loading,but its effectiveness gradually decreases,and strengthening far from the joint becomes more important as the loading increases.3.Steel plate-concrete specimens connected with anchors and adhesive(AN-AD specimens),anchors(AN specimens),and adhesive(AD specimens)were tested under shear loadings.Their shear properties were analyzed and compared with each other.Mechanical models for the AN-AD specimens were proposed.Test results show that:(1)before the peak load,the shear stiffness of the AN-AD specimens is basically the same as that of the AD specimens with the same connection length,and the displacement at failure point of the AN-AD specimens is approximately similar to that of the AN specimens with the same connection length;(2)under the fracture failure mode of the anchor,the utilization coefficient of the multi-anchor connections is about 0.92 regardless of the number of anchors;(3)when the connection length is the same,the ultimate bearing capacity of the AN-AD specimen is about 78% of the sum of those of the AN specimen and the AD specimen;(4)for the AN-AD specimens,the load is almost carried by the adhesive before the peak load,and then the load shared by the anchors increases gradually after the peak load,and the load carried by each anchor is generally similar;and(5)the modified C-T model has a good prediction accuracy in the ultimate bearing capacities of the AN-AD specimens and AD specimens;(6)the ultimate bearing capacities of the AN-AD specimens can be well predicted using the bonding-slip constitutive model.4.The calculation model of strengthened tunnel ring considering bond slip between the strengthening part and tunnel ring was established.The strengthening effect of concrete filled steel tubes(CFSTs)and that of steel plates for tunnel ring were compared.The influence of the interface connection performance,section height of CFSTs,wall thickness distribution of steel tube,strength of steel tube and core concrete,and subgrade reaction coefficient on the strengthening effect of CFSTs strengthening method were investigated,and then localstrengthening was discussed.It is found that:(1)when the steel consumption for CFSTs and steel plates were comparable,the strengthening effect of CFSTs was obviously superior to that of the steel plates under a high-level vertical soil pressure;(2)when the vertical soil pressure was at a high-level,the strengthening effect can be significantly improved by improving the section height of CFSTs or the interface connection performance between CFSTs and tunnel ring,however,the effect was not significant when the vertical soil pressure was at a low-level;(3)when the vertical soil pressure was at a high-level,the deformation of tunnel ring can be significantly reduced by increasing the subgrade reaction coefficient,but the influence of steel tube strength,concrete strength and wall thickness distribution of steel tube on the deformation of tunnel ring was relatively limited;(4)compared with that at the waist of tunnel,local strengthening at the roof of tunnel ring can achieved higher strengthening effect.When the vertical soil pressure was less than 650 kPa,local strengthening the tunnel at the top and waist simultaneously can achieve the similar strengthening effect as strengthening entire ring. |
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