Experimental Study On Creep Performance Of Air-filled Expansion-controlled Anchor Rods

Inflatable control anchor is a new type of friction anchor,which is mainly used in the support system of foundation pit project.Compared with the traditional grouting anchor,it has the advantages of no grouting,producing anchorage force immediately after inflation,adjustable inflation pressure to control the anchorage force to meet the actual project requirements,and complete recovery by anchor pressure release after the project.With its own advantages,the inflatable expansion control anchor can produce good results in the support system and has a broad application prospect.Most of the pit projects take months to complete,and the anchor rods are subject to creep in the long term,resulting in a gradual decrease in anchorage force and a failure to ensure the safety of the support system.Thus,in order to ensure the safe and stable operation of the foundation pit project during construction,the ultimate pullout force of anchor rods can no longer meet the engineering requirements,and the creep performance of anchor rods should be included in the study.In this paper,creep tests were designed for three sets of inflatable expansion control anchors at different inflation pressures of 0.1MPa,0.2MPa and 0.3MPa,and the creep curves were used to draw graded loading creep curves and isochronous curves to investigate the long-term pullout resistance of the three sets of inflatable expansion control anchors.The main research work and results are as follows:(1)The analysis of the creep curve patterns of anchors shows that the three sets of inflatable expansion control anchors with different inflation pressures do not show significant creep phenomenon at low load levels.When the load is less than 40% of the ultimate pullout force,no creep occurs and the anchor rod has excellent anchorage performance.(2)The creep curve of graded loading was plotted as an isochronous curve by using the "isochronous curve method",and the long-term pullout resistance and the ratio to the respective ultimate pullout resistance of the inflatable expansion control anchor rods were obtained by analyzing the curve for three sets of different inflation pressure states.The long-term pullout force of the inflatable control anchor at 0.1MPa,0.2MPa and 0.3MPa were 6k N,16 k N and 22 k N,and the ratio of the respective ultimate pullout force was 66%,69% and 70%,respectively.(3)The long-term pullout resistance of inflatable control anchor and grouted anchor were compared with the control test of grouted anchor,and it was found that the long-term pullout resistance of inflatable control anchor at 0.2MPa was closer to that of grouted anchor,but the long-term pullout resistance of grouted anchor was smaller than that of inflatable control anchor at 0.3MPa,indicating that the long-term pullout resistance of inflatable control anchor was lower than that of inflatable control anchor at0.3MPa.However,the long-term pullout force of the grouted anchor is smaller than the long-term pullout force of the inflatable control anchor with 0.3MPa,indicating that the inflatable control anchor with 0.3MPa can provide a greater long-term working safety reserve in actual engineering applications.(4)The Burgers model was selected as the creep instantiation model of the inflatable expansion control anchor,and the polynomial coefficients were obtained by polynomial fitting of the creep curve of the anchor with graded loading,and the mechanical parameters,such as,were solved by the joint cubic equation system with the creep instantiation model.The analysis of the mechanical parameters revealed that the inflatable expansion control anchors produced microscopic damage during the creep process.Finally,the Burgers model was selected as the creep intrinsic model for the inflatable expansion control anchor to verify the correctness.