Study On Construction Mechanics Behavior Of Asymmetric Small-Distance Section In Urban Bifurcation Tunnel

In the process of highway tunnel construction,sometimes due to the influence of linetype and nearby buildings,the uplink tunnel and downlink tunnel have to be set relatively close to each other.If the left and right tunnels are asymmetrical at the same time,it will become asymmetrical small-distance tunnel.Because the size and span of the left and right sections of the asymmetrical small-distance tunnel are not the same,the construction method and structural design are not the same for different sections,so the degree and law of the interplay between the left and right lines of the tunnel are also greatly different in different projects.However,at present,there are few researches on asymmetrical small-distance tunnel,and the relevant tunnel design codes are not explicitly involved.Therefore,it is very significant to study the construction mechanics behavior of asymmetrical small-distance tunnel.Based on the north extension of zengjiayan road project through the inner ring,this paper studies the construction mechanics behavior of the asymmetrical small-distance section of this road through the combination of data investigation,numerical simulation and model test.The main research contents and conclusions are as follows:(1)By means of numerical analysis,the strata movement,the displacement of supporting structure,the stress of supporting structure and the development range of surrounding rock plastic zone under different excavation sequences are compared and studied.The results show that it is more beneficial to control the above indexes to excavate the small-span connecting line tunnel first.(2)The mechanical property of the supporting structures of the left and right tunnels with different excavation methods are compared by similar model test.The results show that when left and right tunnels are excavated by step method and three-step method respectively,the force of the supporting structure is slightly unfavorable.But it can effectively control the displacement of surrounding rock,which is more reasonable than other construction methods.(3)On the basis of selecting the above construction method,the optimization analysis of construction method and support parameters is carried out through numerical simulation.The results show that: increasing the step dislocation distance and the height of the upper step are unfavorable to the control of the stability of the support structure and surrounding rock.It is more reasonable when the dislocation distance is 5m,and the height of the upper step should not be too large.Increasing the thickness of shotcrete and the spacing of steel arch can effectively improve the stability of support and surrounding rock.It is more reasonable when the thickness of shotcrete on the left and right lines is 22 cm and 24 cm,and the spacing of steel arch is 80 cm.(4)By means of numerical analysis,the influence law of the subsequent tunnel on the displacement of the supporting structure of the advance tunnel under different grade surrounding rock conditions is studied.The results show that: to a certain extent,the better the surrounding rock conditions,the greater the proximity influence.The proximity effect of the section with smaller clear distance is greater than that of the section with larger clear distance.The proximity influence of the supporting structure near the middle rocky column is slightly greater than that of the supporting structure not near the middle rocky column.