Study On Prevention Measurs For Lining Defects Of Large Cross-Section Rectangle Tunnel Constructed By Pipe Jacking

The rectangular pipe-jacking tunnel has several advantages,such as little influence on ground transportation,high section utilization rate and strong adaptability of shallow covering soil,which has been widely used in the fields of pedestrian passages,subway station entrances and exits.In order to satisfy the needs of faster transit system and higher traffic flow,the structure of pipe-jacking tunnel presents the trend of a large span and large cross-section development.Therefore,it is significantly necessary to carry out research on large-section rectangular pipe-jacking tunnel to promote and improve the application of this technology.In this paper,the safety and health status of the lining structure of the rectangular pipe-jacking tunnel during operational period were investigated.The lining structure diseases were divided into six categories based on the survey data,and the causes of disease formation were further analyzed.Based on a large-section rectangular pipe-jacking tunnel project under the existing municipal road,a three-dimensional numerical model of lining structure was established using FLAC3 D.Moreover,the influence of uneven foundation settlement on the lining structure and the corresponding prevention and control measures have been emphatically studied.The main research results are as follows:(1)When there is lateral softness in the lower soil layer of the rectangular pipe-jacking tunnel,the lateral distribution of the soft soil layer,the difference of the soft soil layer and the vertical distribution thickness have significant influences on the surface settlement caused by the pipe jacking construction,the deformation of the tunnel pipe joint,and the stress distribution of the tunnel lining structure.If other conditions were the same,when there was 20 m silty clay in the lower soil layer on the right side of the tunnel center line,the maximum surface settlement increased by about 37.7% compared with the uniform soil layer,and the maximum tensile stress of the pipe joint increased about 77.7%;when the soft soil layer changed from silty clay to silt soil,the maximum value of land subsidence increased nearly 1 times under 50 m distribution range of the soft soil layer,the maximum tensile stress increased 132.3%,and the concrete pipe joints showed were inclined and deflecting phenomenon.(2)The increase in the span and depth of the rectangular pipe-jacking tunnel was extremely unfavorable for the lining of the pipe joint.For example,when the span was increased from 6~7m commonly used in pedestrian passages and subway entrances and exits to about 10 m in two-lane municipal road tunnels,the span was increased by 3m,however the maximum tensile stress of the pipe joints was increased by nearly 3 times;the buried depth of pipe jacking tunnels was 8.0m and 12.0m,the maximum tensile stress of the pipe joint was 3.68 times and 1.44 times at the buried depth of 4.0m and 8.0m,respectively.The large-span,deep-covering soil caused a sharp increase in the tensile stress of the tunnel pipe lining structure,which was the root cause of large-section rectangular pipe-jacking tunnel structural disease occurrence during operational period.(3)Grouting reinforcement and cement-soil mixing piles in the lower soil layer of the tunnel are two effective methods to prevent and control the lateral uneven settlement of the pipe-jacking tunnel.Using grouting reinforcement,the maximum settlement of the surface could be reduced by 34.3%,and the maximum tensile stress of the pipe joint was reduced by 59%;cement soil mixing pile could reduce the maximum settlement of the surface by 53.4%,and the maximum tensile stress of the pipe joint was reduced by 53%.These two reinforcement methods can tremendously reduce the maximum surface settlement and the influence range caused by the weak and uneven soil layer in the lower part of the tunnel,and the maximum tensile stress of the pipe joint could be significantly reduced below the design allowable value,which effectively prevented occurrence of structural diseases of the pipe-jacking tunnel.