Study On Stress And Deformation Characteristics Of Carbon Fiber Anchor Cable And Cast-in-place Pile Supporting Structure In Deep Foundation Pit Near Sea

With the continuous progress of infrastructure construction,the development and utilization of urban underground space presents a development trend of scale.Foundation pit engineering gradually faces the development trend of "deep,large,long,near",the stability of supporting structure and the security of surrounding environment of foundation pit become particularly important.Pile-anchor supporting structure is widely used in deep foundation pit supporting engineering because of its advantages of high bearing capacity,strong adaptability,active support and good deformation control.Carbon Fiber Reinforced Polymer(CFRP)has been gradually favored by researchers due to its advantages of light weight,corrosion resistance,high strength,good insulation and low stress loss.It has been applied in bridge reinforcement,component reinforcement,concrete reinforcement,foundation pit support,etc.Its excellent performance solves many engineering problems.Compared with the traditional steel stranded cable anchor cable,CFRP anchor cable(bar)has become an excellent substitute for metal anchor cable(bar)because of its good corrosion resistance and easy cutting characteristics,especially in the coastal area and the surrounding land control area is more strict to promote and use the value.Based on a deep foundation pit project in Yantai City,this paper carried out field tests on the stress distribution characteristics of the CFRP prestressed anchor cable.Through field measurement and finite element simulation,the stability of the deep foundation pit of the pile and anchor supporting system was analyzed.The main innovative work and research results are as follows:(1)Through the theoretical analysis of the support structure of prestressed anchor cable,the common calculation method and design theory of prestressed loss of anchor cable are grasped,and a set of mode of testing the prestressed loss of anchor cable is established.The realization process is to paste resistance strain gauge on the anchor cable,measure the strain value of the anchor cable during the service period,and deduce the law of prestressed loss of anchor cable through the stress-strain relationship.Combined with the existing research results,the load transfer mechanism and failure mode of CFRP bolt are revealed.The results show that the main reason affecting the anchoring failure mechanism of CFRP bolt is the coupling effect of bolt,grout and rock mass.(2)Through the field measurement of stress and deformation of three groups of CFRP prestressed anchor cables,the short-term and long-term stress loss laws of CFRP anchor cables are revealed,and the distribution characteristics of stress and strain in the anchoring section of CFRP anchor cables are defined.The results show that the stress-time curve of the cable is basically consistent,and the short-term stress loss rate of the cable is between 25.14% and33.54%,and the long-term stress loss rate is between 23.87% and 46.91%.The strain of CFRP cable decreases continuously from the free section to the anchoring section.The strain at each monitoring point of CFRP cable first increases,then decreases rapidly,then rises slightly,and finally tends to be stable.(3)Through the field measurement of the horizontal and vertical displacements at the monitoring points of the foundation pit,the deep horizontal displacements of the soil,and the settlement of the surface and adjacent buildings,the deformation characteristics of the deep foundation pit with pile-anchor supporting structure are defined.The results show that the relationship between foundation pit deformation and monitoring time changes regularly.The horizontal and vertical displacements of each monitoring point increase rapidly,and then the rate slows down and gradually becomes stable.At 0.5H(H is the excavation depth of foundation pit),the upper and lower ends of the foundation pit are more likely to be offset to the outside of the foundation pit,and the upper and lower ends of the foundation pit are more likely to be offset to the inside of the foundation pit.The settlement of ground surface and adjacent buildings in the early stage of foundation pit excavation accounts for about 10%～20% of the settlement during the whole excavation.(4)Through the numerical simulation analysis of CFRP supporting cable with different angles and lengths,the feasibility and superiority of CFRP supporting cable instead of traditional steel stranded cable are revealed,and the optimal supporting scheme of CFRP supporting cable and cast-in pile combined supporting structure is defined.The results show that the CFRP anchor cable has better supporting effect on soil,and the soil at the bottom of the pit is more stable.Within a certain range,the smaller the cable dip Angle and length,the more reasonable the soil stress distribution,the larger the cable dip Angle and length,the more easily the soil deformation can be controlled.