Study On Construction Control Technology Of Meiziling Small Clear Distance Shallow Buried Tunnel

With the rapid economic development in the Yangtze River Delta,the increasing traffic flow has brought great impetus to the improvement and expansion of expressway construction.At the same time,in order to optimize the highway line,shorten the highway mileage and improve the transportation efficiency,under the guidance of protecting the ecological environment and adhering to the concept of green and sustainable development,the construction demand of large section shallow buried tunnels in the eastern region,especially in some hilly areas,has shown an increasing momentum of development in recent years.In terms of terrain,the elevation of Hilly and mountainous areas is generally low and the terrain fluctuation is small.The form of small clear distance shallow buried tunnel is often used to improve the linear design index of the route.However,the exposed surrounding rock in the Yangtze River Delta is generally weak and broken due to strong weathering.At the same time,the rainfall in this area is concentrated and large.If the tunnel excavation is in the period of rainfall concentration,it will pose a great threat to the safe construction of the tunnel;In addition,the multi lane tunnel itself has the characteristics of large section,large span and low flattening rate,which is bound to aggravate the mutual influence of excavation between double track tunnels,which is also quite unfavorable to the stability of surrounding rock.At present,the common research of small clear distance tunnel is mostly seen in the two lane working condition.There is no exact systematic technical and theoretical support for the construction control of large section tunnels with three lanes and above,especially the small clear distance tunnel in the eastern hilly area.Therefore,through the on-site monitoring and analysis of meiziling tunnel and the use of three-dimensional numerical modeling technology,it is necessary to systematically analyze and study the key technology of construction control of shallow buried small clear distance tunnel in the hilly area of the Yangtze River Delta.The main research contents and research results are as follows:(1)Based on the monitoring data of surrounding rock settlement of meiziling tunnel and surrounding rock settlement of meiziling tunnel,it is concluded that the surrounding rock settlement of meiziling tunnel is poor during construction,and the influence of meiziling tunnel on the stability of surrounding rock settlement is poor;The geological conditions changed during the tunnel exit,and the settlement of the arch crown of the right tunnel increased by 44.4% and that of the left tunnel increased by 59.7%;In the double wall heading method,the excavation of the pilot tunnel has little effect on the settlement of the arch crown;The distance between the left and right tunnel faces will affect each other within 60 m.The law reflected by the monitoring data is verified by numerical calculation,and the rationality of the numerical calculation model and its parameters is verified by comparative analysis.(2)A three-dimensional numerical calculation model is established to analyze and evaluate the excavation scheme of meiziling tunnel.The sensitivity of a series of key construction control parameters,such as the geometric parameters of the tunnel steps,the reasonable left and right clear distance of the tunnel,the reasonable front and rear face distance of the tunnel,and the safe buried depth of the tunnel,is analyzed.By summarizing the sensitivity of each construction control parameter,it can be obtained that the excavation combination of double wall heading method has the best effect on controlling the deformation of surrounding rock;The longer the bench length,the smaller the deformation of tunnel surrounding rock and the faster the convergence rate.The upper bench length should be controlled within 3m ～ 5m;The height of the upper step of the double side wall heading method and the single side wall heading method has a great influence on the horizontal convergence displacement.When the height of the upper step of the step method is ≥ 4m,the lining is more stable It is stable,but it is not conducive to controlling the displacement of the tunnel face.(3)Considering the typical surrounding rock conditions in the eastern region,when analyzing the excavation stability under multiple working conditions,it is considered that in the process of reducing the clear distance between the left and right tunnels,the increase rate of internal force of different excavation methods is bilateral wall heading method(38.8%)< unilateral wall heading method(132.4%)< bench method(396.5%);The safety distance in front of and behind the tunnel of class V surrounding rock is 2B,and the safety distance in front of and behind the tunnel of class IV surrounding rock is 1.5b;The safe burial depth in grade V surrounding rock of the tunnel is 32 m,and the safe burial depth in grade IV surrounding rock is 17m;In case of class V broken surrounding rock encountered during the tunnel exit,the excavation method needs to be changed to double wall heading method at least 10 m in advance.The reinforcement range of advance small conduit grouting reinforcement measures is 120 °,the settlement is reduced by 11.9% compared with the unreinforced state,and the axial force of lining is reduced by 70%;The rainfall intensity is 200 mm/d,the rainfall lasts for 3 days,and the water inflow is 8 times of the normal water inflow;The water inrush velocity of the lower bench method is the largest,and the water inrush velocity of the double wall heading method is the smallest;The double wall heading method has the best effect on controlling the deformation of tunnel surrounding rock under rainfall infiltration.