Research On Determination Method And Application Of Surrounding Rock Loose Circle Of Tunnel

During tunnel construction,the excavation disturbance on the rock mass causes the stress distribution of the surrounding rock so that the surrounding rock loose circle is produced.In the loose circle,the rock mass strength decreases,the crack increases,the stability is deteriorated,and the plastic slip is easy to occur.So,the hidden dangers of construction safety will be generated.It is of great importance to accurately determine the thickness of the surrounding rock loose circle to ensure tunnel construction safety and optimize supporting structure parameters.In this paper,through the combination of theoretical analysis,numerical simulation,laboratory test and on-site measurement,the method for determining the surrounding rock loose circle of tunnel is studied,the analysis and test are carried out based on the Shimen tunnel of Baohan Expressway.The main work of the thesis is as follows:(1)Based on the analysis of the surrounding rock stress of the Hoek-Brown strength criterion,the formula for calculating the radius of the loose circle is derived.The deficiencies of Mohr-Coulomb strength criterion and Hoek-Brown strength criterion in the elastic-plastic theoretical calculation of tunnel surrounding rock loose circle are analyzed.In the generalized Hoek-Brown failure criterion,the intermediate principal stress is introduced by the Lord parameter μσ to modify the Hoek-Brown strength.Based on the modified Hoek-Brown strength criterion considering the intermediate principal stress,the theoretical calculation formula for the radius of the surrounding rock loose circle are derived.The Hoek-Brown strength criterion and the modified Hoek-Brown strength criterion considering the intermediate principal stress were used to calculate the radius of the surrounding rock loose circle of the three-lane highway tunnel.Results show that considering the intermediate principal stress,the strength of the rock mass will increase,the rock mass damage will be more difficult,and the loose circle range will be smaller,and the influence of intermediate principal stress,support resistance and buried depth on the loose circle range will be obtained.(2)The determining theory of the tunnel based on the loose circle theory is proposed to take the surrounding rock tensile strain to reach the ultimate tensile strain of the rock mass as the initial time of constructing support based on the loose ciecle theory.Using Midas GTS numerical calculation software,the key points’ displacement,stress,tensile strain and loose circle thickness of surrounding rock under various stress release stages of Ⅲ,Ⅳ and Ⅴ surrounding rock are calculated respectively.we obtain that with the increase of stress release coefficient,the vault sinking value and the tensile strain is increasing continuously.And the compressive stress is decreasing at arch and increasing at side wall.According to the law of the occurrence and development of the surrounding rock loose circle,are determined.The initial support timing of the level Ⅲ,Ⅳ and V surrounding rock is best when the surrounding rock stress is released to 60%,40%,and 20% respectively.(3)The integrity coefficients of broken rock(Rv)and loose circle(Lv)are proposed to quantify the rock mass fracture state in surrounding rock loose circle.It is assumed that in the tunnel excavation,Rv is approximately equal to Lv.Then the criterion of surrounding rock loose circle based on integrity coefficient is proposed.Combined with laboratory tests and field tests,a method for testing the surrounding rock loose circle in acoustic wave method is developed.(4)Finally,in this paper,the engineering application of the determination method of the surrounding rock loose circle is studied with the Shimen Tunnel of Baohan Expressway as the engineering background,the thicknesses of the surrounding rock loose circle of level Ⅲ,Ⅳ and V are determined to be 0.50～0.85m,0.81～1.59m and 1.48～2.18m respectively.Based on this,the bolt length of the level Ⅲ surrounding rock is optimized from 3.0m to 2.0m,the bolt length of the level Ⅳ surrounding rock is optimized from 3.5m to 2.5m,and the bolt length of the level V surrounding rock is optimized from 4.0m to 3.0m.Initial support was applied to the surrounding rock at level Ⅲ,Ⅳ and V when the stress release was 60%,40%and 20%respectively.Through the monitoring and measurement of the construction optimization section,the optimized support parameters not only meet the requirements of safe construction,but also greatly save construction costs.