Construction Mechanical Behavior And Construction Technology Of Shied Machine Barrier-free Launching And Receiving

Shield construction is divided into three stages: launching,tunneling and receiving.Shield machine launching and receiving are the two parts that frequently cause accident among them,including soil body collapsing,groundwater inflow and sand inflow,pipeline fracture and so on.Much attention must be paid to the security of them.According to safety situation of frequent accidents during conventional shield machine launching and receiving,the author adopted theoretical analysis,numerical analysis and field test methods to investigate the stability of soil at entrance,construction mechanical behavior,and construction technology on the basis of the existing research results.And the achievements can be drawn as follows:1.By using the method of theoretical analysis,a new stability model for soil at entrance during shield machine barrier-free launching and receiving has been developed.Besides,a mathematical expression has been proposed to determine the stability of soil at entrance.The results are as follows:(1)The GFRP reinforced concrete structure at entrance should meet the requirements of the bending and shearing bearing capacity of water-earth pressure simultaneously.(2)It is considered that there is no sliding failure phenomenon of soil at entrance of shield machine barrier-free launching and receiving.Furthermore,it is suggested that shaft consolidation can be canceled.2.By means of the Flac3 D numerical analysis,studying for macro construction mechanical behavior of shield machine barrier-free launching and receiving with the research objects of GFRP reinforced concrete retaining piles,excavated surface soils at entrance,and ground surface settlement.The results are as follows:(1)The increase amplitude of horizontal displacement reached 125.71% when the pile is cut to half during barrier-free launching.At this time,the total thrust should be controlled to prevent the excessive ground surface heave caused by the passive earth pressure behind the pile body.(2)The horizontal displacement and maximum principal stress of pile body are within the allowable value range,and the soil behind the retaining pile body is stable and there is a slight heave on the ground surface during shield machine barrier-free launching.(3)The shield machine enters the finite soil from the entrance at 1 time tunnel diameter plus 2~3 rings segment during barrier-free receiving.After that,the chamber pressure with finite earth pressure coordinate changes,so as to satisfy simultaneously deformation requirement both retaining piles and ground surface.3.Studying the micro cutting process and mechanism of pile body by means of PFC3 D numerical analysis.The results are as follows:(1)The GFRP reinforced concrete cracking is a process of crack formation,crack propagation and crack coalescence during cutting pile body by shield machine.The variation of pile stress performs as follows: firstly,it increases gradually,then decreases,and finally tends to be stable.And the change rule of cutting force is similar to that of stress of pile body.(2)In the process of cutting piles,it is necessary to avoid the occurrence of integer collapse accident of the retaining pile body to foundation pit side.(3)The cutting mechanism of GFRP reinforced concrete pile is shear compression failure during shield machine barrier-free launching.Moreover,in the case of shield machine barrier-free receiving,the cutting mechanism is a "two stage model" ——shear compression failure then shear failure.4.Depending upon the in-situ experiment on the NO.16 subway line in Beijing,the field application of the construction technology was carried out.The results are as follows:(1)Barrier-free launching is divided into three stages: cutter head attachment retaining structure,cutting retaining structure,establishing chamber pressure and achieving stable earth pressure balance.Barrier-free receiving is also divided into three stages: entering finite soil,cutter head approaching and attachment retaining structure,and cutting retaining structure.(2)It is beneficial to the rapid establishment of the chamber pressure and earth pressure balance of shield machine barrier-free launching.(3)The shield machine barrier-free receiving can effectively improve engineering safety and efficiency.In addition,the penetration should be controlled at about 6mm during cutting pile body.