Cause Analysis And Treatment Of Lining Cracks Of Straight-middle-wall And Multi-arch Tunnel Without Middle Drift

The Kaidagu tunnel of Xianggelila-Lijiang Expressway in Yunnan Province is located in strongly and moderately weathered basalt stratum,and the design pattern of the line is multi-arch tunnel without middle guide～tunnel with small clear distance～separated tunnel,where the tunnel inlet nearly 400m section adapted the structure type of straight-middle-wall and multi-arch tunnel without middle drift.During construction,extensive lining cracking occurred in this section.Therefore,how to correctly analyze the causes of lining cracks and take effective remediation measures is the key challenge for the construction of the Kaidagu tunnel.In this paper,based on the lining cracks in the Kaidagu tunnel,the research methods such as literature investigation,field investigation and testing,numerical simulation and analysis are adopted.Firstly,the cracking situation of the on-site structure is investigated,and the location and development trend of the cracking are defined.Then,the three-dimensional simulation analysis model of Kaidagu tunnel is established,the stress characteristics of tunnel structure and the safety of lining structure under complex construction process are explored,and the causes of tunnel cracking are clarified.Secondly,the influence of typical structural design factors and different construction methods on the stability of surrounding rock,the stress of tunnel structure and the safety of lining structure are studied by means of three-dimensional simulation analysis.Finally,based on the three-dimensional simulation analysis method,the optimization scheme of tunnel design and construction is put forward,and the measures of structural treatment and repair of the cracked lining section are defined.The main contents and conclusions are as follows:（1）Detailed investigation and site monitoring were carried out for the cracked section of Kaidagu tunnel,the test results show that:the cracked lining area is mainly distributed in the area from the lower right arch shoulder to the lower right wall of the first cave,which should be the weak link of the design form of the straight middle wall without a guide;the cracked lining of the first cave is closely related to the construction of the later cave,and the time of the cracked lining is basically the time when the excavation and support of the upper step of the later cave is completed,but the construction of the back arch has not yet been carried out.（2）Based on FLAC^3D,the complex construction process of Kaidagu tunnel was simulated and the causes of lining cracking in Kaitaku tunnel were investigated.The results show that the structure is relatively safe in the excavation and support stage under the design form of tunnel without intermediate guide tunnel,while there is a significant change in the bearing pattern of the first tunnel under the influence of excavation of the later cavern divisions,and the internal force of the middle wall structure increases significantly and the lining safety factor decreases sharply,which finally leads to the structural collapse of the middle wall area due to excessive compressive stress.At the same time,the crack area and crack timing obtained from the numerical simulation match well with the actual situation on site,which verifies the correctness of the three-dimensional numerical simulation in this paper.（3）In the form of straight-middle-wall and multi-arch tunnel without middle drift,the stress pattern of the straight middle wall is complex,and there is a big difference in the load effect of the support structure of the backward cave in different parts of the straight middle wall,which makes the second lining of the right wall of the preceding cave have uneven stress distribution,significant increase in structural axial force and multiple reversal of bending moment,which is an important reason for the serious crack damage of the wall of the preceding cave in the relying project.（4）By means of three-dimensional numerical simulation,the typical factors of design and construction,such as the thickness of the right wall of the first cavern,the way and position of the initial support lap of the second cavern,the longitudinal spacing of the palm face,the cyclic feed and the construction method,are analyzed for their influence on the deformation of the surrounding rock and support structure,the distribution of the plastic zone of the surrounding rock,and the safety of the lining structure of the first cavern,which can provide a reference basis for the optimization of the design and construction of similar projects.（5）The comprehensive optimization measures including the downward shift of the primary lap position,the increase of the thickness of the middle wall,the optimization of the tunnel excavation method,and the simultaneous construction of the left and right holes were proposed and analyzed by numerical simulation,which verified that the safety of the preceding hole structure,especially the straight middle wall,was significantly improved after adopting the optimization measures,and the cracking of the lining structure could be effectively avoided.At the same time,based on the cracking situation and the results of cracking analysis,the structural cracking disposal measures are proposed,and the disposal process and remediation parameters are introduced systematically,which can provide reference for similar disease cases.There are 74 pictures,24 tables and 91 references in this paper.