Research On Construction Risk Control Of Hole Ingates With Super Small Clear Distance In Beijing Subway

Small clear distance subway tunnel.As a super first tier city,Beijing has more dense underground space.There are often more than three parallel small clear distance tunnels.When the multi parallel small clear distance tunnel is excavated from the cross passage,its overall excavation section is large and the number of construction disturbances is large,which is easy to lead to the instability of surrounding rock and the collapse of tunnel structure.Therefore,it is very important to ensure the safety of the construction of Matou gate of urban small clear distance group tunnels.In this paper,combined with the ultra-small clear distance group tunnel concealed excavation project of Tiantongyuan east station of Beijing metro line 17,the construction calculation model of ultra-small clear distance group tunnel horsehead gate is established,the excavation sequence,method and construction footage of ultra-small clear distance group tunnel horsehead gate are systematically analyzed,the optimization measures of ultra-small clear distance pre calculation,reinforcement and pre support are studied,and the optimization scheme of support mode is put forward for the dangerous area in the process of ultra-small clear distance group tunnel horsehead gate excavation,The main research results are as follows:(1)Based on the mechanical angle of tunnel excavation,the influence area of ultra-small clearance group tunnel horsehead gate excavation is determined,and the excavation sequence of small clearance group tunnel horsehead gate is studied.According to the classification of excavation cross-sectional area,it is obtained that when excavating small clearance group tunnel horsehead gate,the tunnel with large cross-section should be excavated first,and then the tunnel with small cross-section should be excavated;During the excavation of large section horse head gate,the double side wall pilot tunnel method shall be adopted for construction;When excavating small section horse head gate,the method of up and down steps can be used for construction;The numerical simulation calculation of different construction footage shows that when the construction footage is 1m,the excavation of horse head gate can give consideration to safety and efficiency.(2)Based on the optimal excavation method,the pre reinforcement and pre support scheme of horsehead gate of ultra-small clear distance group tunnel is studied.When the full section grouting pre reinforcement method is adopted,the optimum grouting reinforcement thickness is 1m;When the pipe shed pre support method is adopted,the pipe shed diameter of 159 mm can achieve better control effect;According to the pre reinforcement and pre support methods,four reinforcement schemes are proposed.Finally,the most suitable support scheme is determined as the support mode with the thickness of 1m in the grouting area and the pipe diameter of 159 mm.(3)The control optimization measures proposed in this paper are applied to the construction,and the construction process is monitored on site.By comparing the onsite monitoring with the numerical simulation,the trend of the monitoring results is basically consistent with the law of the simulation results,which shows the accuracy of the numerical simulation.At the same time,it shows that the above optimization control measures can ensure the safe construction of the Horsehead gate of ultra-small clearance group tunnel.