| Since the fifties of the last century,a total of 14 major ground fractures and several sub-fractures have been discovered in Xi’an urban areas,which have caused varying degrees of damage to various kinds of engineering buildings and transportation facilities.To Xi’an City brought not small economic losses.With the expansion of urban construction scale,Xi’an plans to develop urban rapid rail transit.Currently under construction of Metro Line 4 from north to south across the 14 ground fractures.Metro as a linear engineering,in view of the actual conditions of the site,the metro tunnel part of the driving section can not avoid ground fractures,so it is necessary to cross the ground fractures in the engineering;Therefore,it is clear that the genetic mechanism and activity law of ground fractures along Metro Line 4,as well as the influence mechanism on tunnel structure when the lines cross the ground fractures at large and small angles.It is of great significance to solve the engineering disaster problem of metro line 4crossing ground fractures.In this paper,based on the research background of the Xi’an Metro Line 4 tunnel acrossing the ground fractures,combined with the previous data,the distribution and activity of the ground fractures along the Line 4 are introduced firstly.Among them,the lines and ground fractures f3,f8,f9 intersect at a small angle,and the other ground fractures intersect with the lines at large angles.These ground fractures will affect the construction and normal operation of tunnel project of Metro Line 4.Secondly,this paper analyzes the genesis mechanism of ground fractures along Metro Line4.Based on the analysis and arrangement of the previous data,it is concluded that tectonic movement is the main internal factor of the occurrence of the ground fractures in Xi’an.Combined with the unique landform of Xi’an,the fractures shows its tectonic properties.Over-exploitation of underground confined water is an external dominant factor in the formation of ground fractures in Xi’an.The movement of fault layers leads to structural differences on both sides of the strata,and the influence of long-term over-mining of underground confined water makes the strata prone to differential settlement.Over-exploitation of underground confined water is the direct cause of the abnormal activity of the ground fractures.The MIDAS/GTS NX finite element program is used to establish the numerical model of the metro tunnel structure crossing the ground fractures under the small angle(25~o)and large angle(75~o)respectively,and the interaction between the tunnel structure and the ground fractures.The model is simulated and analyzed in the aspects of deformation,damage characteristics,influence range and stratum displacement.It is pointed out that when the metro crosses the ground fractures at a small angle(25~o),the deformation of the tunnel structure is mainly shear,plus the torsional deformation,the deformation range is mainly within the range of 2.8 times the diameter of the tunnel near it,and the range of the lower wall is slightly larger than that of the upper one.When the metro crosses the ground fractures at a large angle(75~o),the deformation of the tunnel structure is mainly tension-shear,with local torsion and bending deformation.The deformation range is mainly within the range of 0.35 times the diameter of the tunnel near the tunnel,and the range of the lower wall is slightly larger than that of the upper one.Finally,according to the contents of the previous chapters,it can be concluded that when the ground fractures activity reaches a certain value,the structure of the metro tunnel will produce rupture and destruction.In order to ensure that the metro tunnel structure can successfully cross the ground fractures,this paper puts forward the prevention and control measures from aspects of structure,waterproofing,monitoring and early warning. |
