Research On Protection Measures For The Impact Of Deep Excavation On Existing Tunnels In Loess Areas

Urban underground rail transport has experienced significant development in recent years due to its ability to greatly enhance travel efficiency.Because of its high economic value,many infrastructure projects have been built near it to improve their own economic value.However,excavation of deep excavations naturally poses a challenge in terms of the adverse effects on nearby tunnels.Therefore,designing reasonable protection measures to reduce the impact of deep excavation on tunnels is of great significance.(1)Based on the impact of deep excavation on tunnel deformation,orthogonal experiments were designed to study commonly used protection measures including isolation piles,soil reinforcement,anti-extraction piles and footings,and pile loading.The protective effects of these design parameters on tunnel deformation were analyzed,and their importance was evaluated.It was found that for tunnels on the outside of the pit,the most influential design parameter for isolation piles is burial depth,followed by horizontal distance from the tunnel,pile top burial depth,and diameter.For soil reinforcement design,the width,depth,and strength of the soil reinforcement are the most influential parameters in descending order.For tunnels beneath the pit bottom,full-floor soil reinforcement is the best form of reinforcement.If only the upper part of the soil can be reinforced,high-strength reinforcement soil is preferred,while low-strength reinforcement soil is more cost-effective if the soil can be reinforced to the bottom of the tunnel.The diameter of the anti-extraction pile and the length,length,and thickness of the bottom plate are the most influential parameters for anti-extraction pile and footing design.Although larger pile loading provides better protection for the tunnel,appropriate pile loading should be selected based on actual engineering conditions.(2)Using the Xi’an Northwest Business Center project,which spans over an existing subway double-line tunnel,the deformation of the retaining structure,the uplift of the pit bottom,and the deformation of the existing tunnel at the pit bottom during the unloading process of the Multi-stage excavation were analyzed using the finite element software to establish a numerical model.It was found that excavation of the deep excavation causes the retaining structure to move and deform inward as a whole in the horizontal direction,resulting in uplift of the pit bottom soil,mainly concentrated in the center of the deep excavation.With the increase of unloading amount,the vertical displacement of both tunnels increases first and then decreases along the longitudinal direction of the tunnel,with maximum vertical displacement occurring at the middle position of the pit bottom,and the curve approximates a normal distribution trend.The vertical displacement of the tunnel is greatly affected by the excavation depth of zone 2 above the tunnel,and the vertical displacement of the tunnel changes rapidly within the range of the pit bottom,gradually decreasing beyond the range and tending toward zero.Additionally,the left tunnel is closer to the center of the deep excavation and is more affected by unloading in zones 1 and 3,resulting in larger maximum vertical displacement compared to the right tunnel.(3)Based on the importance of the design parameters for various protective measures,protective designs were carried out for the subway tunnel in this project.The effects of soil reinforcement,applied load,anti-pull pile,and bottom plate as protective measures to control tunnel deformation were analyzed separately.It was found that although soil reinforcement can control tunnel deformation,soil reinforcement above the tunnel top is greatly affected by the rebound caused by unloading from the deep foundation.Considering the safety and economy of the tunnel,it is generally necessary to grout the soil down to the bottom of the tunnel.Soil reinforcement is better at controlling the deformation of shield tunnels than shallow-buried or cut-and-cover tunnels.Anti-pull piles and bottom plates can effectively control the vertical deformation of existing tunnels.Increasing the diameter and length of anti-pull piles can achieve better results in controlling tunnel deformation if permitted by the engineering conditions.However,applied loads are limited by the materials used and the site conditions and are not as effective in controlling the vertical deformation of existing tunnels.Finally,the combined effect of multiple protective measures on controlling tunnel deformation was studied.It was found that the combination of two protective measures is more effective than a single protective measure in controlling the vertical deformation of the tunnel.Under three protective measures,the vertical displacement of the tunnel was the smallest,and the protection effect was the best.