Study On Shield Tunnel Performance Under Various Assembly Schemes In Limestone Strata

According to statistics,carbonate rock covers one-third of China's national territorial area.With such large a scale of limestone areas,the engineering problems of urban rail transit construction in limestone areas have become increasingly prominent under the background of comprehensive and rapid urbanization.As specifications and research basis for the location of the key block is still not well developed,and for the existences of complicated karst geological conditions,it is difficult to pre-design a complete set of assembly schemes for the entire process of shield tunneling in limestone strata.Therefore,it is of great significance to study the performances of tunnel lining under various assembly schemes in limestone strata,for assembly adjustment during the construction process.In view of this,this paper carried out the following research work:With regard to the performance study of different assembly schemes in the non-karst development area of limestone strata,this paper develops various assembly schemes and then uses the general finite element software Midas GTS NX to establish three-dimensional finite element models for simulation analysis.Results show that each scheme has a certain degree of stress concentration around the joints.Compared with the stagger-jointed schemes,the stress concentration in straight-jointed schemes is more severe.For the dislocation of segments,each of the stagger-jointed schemes performs alike,but the performances of straight-jointed schemes are various from different locations of key block.However,the T-shaped joints near bottom or two sides of the tunnel are prone to a large dislocation under the condition of stagger-jointed schemes,where attention is needed in the waterproofing project.For the study of the performance of different assembly schemes in the karst development area of limestone strata,this paper firstly analyzes the initial geostress of the karst conditions that may occur under or on the side of the tunnel,and then test schemes for the karst area are made based on the results of the initial geostress analysis.Results of all the tests show that the increase of the size of the cave and the decrease of its distance increase the influence of the initial stress on the surrounding rock of the tunnel,and the effects of the two on the deformation,displacement and joint condition of the tunnel lining are similar.The development of the cave leads its upper rock mass to be unloaded,while stress is concentrated on both sides of the cave.When the cave is located below the tunnel,the more the initial stress of the tunnel's surrounding rock is affected by the cave,the smaller is the overall deformation of the tunnel.While it goes the opposite when the cave is located on the side of the tunnel.The development of the karst cave affects the stress difference of the tunnel's surrounding rock,which also affects the overall displacement of the tunnel.As the differences of assembly of lining segments affect the stiffness distribution over every part of the lining structure and the surrounding rock,specific performances under various assembly schemes and karst conditions in terms of deformation,displacement and joints are also compared in detail.