Research On Cracking Mechanism And Prevention Technology Of Tunnel Support Structure In Extremely Weak Surrounding Rock

With the extension of highway construction to the western China and mountainous areas,a large number of mountain tunnels need to be built.When the tunnel passes through the soft rock stratum or fault fracture zone,security emergencies such as the large deformation of surrounding rock,the instability and cracking of supporting structure cause serious risks to tunnel efficient construction and safe operation.In the present study,the cracking of tunnel support structure in extremely weak surrounding rock of four tunnels in Pinglian Expressway in Jiangxi Province has been analyzed.Field investigation,monitoring measurement,theoretical analysis,numerical simulation,indoor test and field test have been used to study the cracking characteristic mechanism of support structure,the grade evaluation of crack safety status and prevention technology of tunnel crack systematically.The main research contents and results are as follows:(1)The cracks of tunnel supporting structure have been classified based on factors such as the cause,position,strike,state,force and crack characteristics.According to crack classification,the contents and methods of crack detection after tunnel cracking are introduced.Based on the field investigation and monitoring measurement of tunnel cracking,the crack characteristic parameters such as length,width,depth,distribution range,strike and location have been analyzed statistically.The results show that the cracks of the tunnel supporting structure are mainly developed in the section with Vgrade surrounding rock and the section near the cave entrance.They are mainly circumferential cracks and distributed on the side walls.(2)The causes of cracking of tunnel support structures have been comprehensively analyzed from the aspects of surrounding rock characteristics,geological structure,support design,construction quality and temperature variation.The results show that the main causes of tunnel structure cracking are loose and weak surrounding rock,frequent cavity collapse in tunnel site,more lining construction defects and severe tunnel terrain bias.In addition,the variation of ambient temperature influences on the fracture development near the cave entrance.According to the above analysis,The deformation and damage characteristics of tunnel structures under different types of back cavities,lining defects and terrain bias have been studied by ABAQUS numerical simulation.(3)The analytic hierarchy method and fuzzy mathematics comprehensive evaluation have been utilized for establishing a fracture safety status classification model of extremely soft surrounding rock tunnel support structure.In this model,the geometric size,distributed development and stratigraphic conditions of cracks have been selected as the first level indicators,and the crack length,crack width,fracture depth,fracture expansion,fracture location,fracture direction and support type were selected as the basic indicators.The model has been used to evaluate the safety status of tunnel cracks in the study area.The results show that there are 6 light cracks,17 medium cracks,2 heavy cracks and 6 severe cracks.(4)The crack prevention technology of tunnel supporting structure of extremely weak surrounding rock have been proposed systematically.Based on the principle of yielding pressure support,different types and spacings of embedded structures are designed for testing of deformable steel arch connections.The results show that the compression performance and stability of the triangular steel version embedded in the7 cm spacing are the best.It has been applied to the supporting structure of Lianhua Tunnel.The results of arch frame stress monitoring show that the compression member absorbs part of the surrounding rock deformation,which reduces the arch stress and initial support intrusion and prevents the cracking of the supporting structure.(5)Research and classification of tunnel crack repairment and reinforcement treatment technologies have carried out.According to the safety status assessment results,the lining crack treatment scheme with different crack levels has been proposed,and the treatment effect has been evaluated by monitoring the typical cracks after treatment.For the inverted arch cracks,a combination of grouting reinforcement and buried pipe grouting repairment has been applied.Field grouting tests have been carried out to reveal the stress variation of the inverted arch and optimize the hole distance,hole depth and water-cement ratio.Dynamic and static loading tests have been designed to verify the effect of invert arch cracking treatment.