Investigation Of Variations Of Safety Factors Of Weakly Weathered Surrounding Rocks Of Tunnels During Excavation Using Back Analysis Of Displacements

Qingdao granite geology and hard rock provide good innate conditions for the construction of tunnels.However,the weathering and rupture of some areas of Line 8 are severe weathering and erosion of residual hills.It is more difficult in safety assessment,so we urgently need to find an effective evaluation index of surrounding rock safety in order to provide a safe and more reasonable method for underground engineering construction.According to the safety factor method,combined with the D-P yield criterion,in ABAQUS,the safety factor and displacement change of the tunnel section are simulated.The range where the safety factor is less than 1 is analyzed and the vulnerable area of the tunnel section is determined.The scope of application and the rationality of its application in the evaluation of the stability of surrounding rocks is verified.In Jiading Mountain and Anshan Road area,triaxial test and in-situ test are analyzed,and basic mechanical deformation behaviors of rock and soil are obtained.By analyzing the influence of basic mechanical deformation characteristics and engineering factors of weakly weathered granite in the study area of Qingdao Metro Line 8 on the safety factor of surrounding rock,the following conclusions were obtained:1.By reading the research data of a large number of domestic and foreign experts,it is found that the existing surrounding rock safety monitoring mechanism is relatively mature but some aspects of fracture mechanisms of surrounding rocks need to be studied.Using the geological survey,triaxial test research,on-site test of Jiading Mountain area and Anshan Road area,numerical simulation of mechanical deformation of soil,mechanical deformation characteristics of weakly weathered granite,engineering factors,etc,the safety of surrounding rock can be evaluated.In view of the inconvenience of laboratory tests and numerical simulations,the safety factor method is introduced.The research shows that the area of the tunnel surrounding rock where the safety factor is less than 1 is extremely small.2.Using a variety of theoretical and numerical methods,the internationally accepted method of analyzing the safety factor of surrounding rock,based on the strength reduction method,the practicability of the study on the safety factor of surrounding rock is obtained.The strength reduction method is used to calculate the strength reduction factor of the surrounding rock of the tunnel under the same two-dimensional condition,and then to judge the effectiveness of the section safety factor method.3.Using the D-P yield criterion and the golden section method to perform displacement inversion,the safety factor of the weakly weathered granite tunnel is obtained.By comparing the vertical displacement of the monitoring point with the actual displacement at that point,the range of elastic modulus and Poisson ratio is determined,and the safety factor at this point is obtained.The results show that the safety factors are affected by geological factors during the construction of weakly weathered granite.In addition,the variations of stresses and displacements in different positions are analyzed to determine the fragile area during rock excavation.4.Based on the survey data of Anshan Road,stress release rate,spaces between tunnels and tunnel depths affecting the safety factor of the surrounding rock of the tunnel are investigated.The direct relationship related to the quality of the tunnel supporting structure is to select the stress release rate of the surrounding rock.If an incorrect stress release rate is selected,the designed support structure will be over-protected.On the other hand,the designed support structure may not be strong enough to cause an accident.According to the stress release rate of 25,40,55,70,85,and 100,the variations of stresses and displacements under these conditions are simulated.Finally,the variations of safety factors under different stress release rates are summarized.5.Other factors will also affect the safety of tunnels.This article investigates the influence of different ground depths and the distance between the two tunnels on the safety factor of the roof of the two-hole tunnels.