Numerical Simulation Of EPB Tunneling Process And Its Induced Ground Disturbances In 3D Complex Stratum

The Shield method is one of the most common techniques for developing urban underground space and is frequently employed in urban underground transportation tunnel construction.The shield tunneling process is essentially the dynamic coupling between the shield machine and the strata.The complex and varying strata properties make it difficult to predict the shield machine’s service behavior,easily resulting in engineering accidents like surface collapse,excavation surface destabilization,and shield machine attitude deviation.Shield excavation simulation can provide the stress state of the shield machine and the disturbance behavior of the strata during the shield excavation process.Thereby,the shield excavation simulation is an effective way of predicting shield construction risk and optimizing the excavation parameters of the shield machine.In most shield excavation simulations,the stratified strata are assumed to be parallel to each other.This assumption ignores the three-dimensional spatial variability of the actual strata properties,making it difficult to investigate the disturbance behavior of the actual strata.A shield excavation simulation approach incorporating three-dimensional geological modeling technology with geotechnical engineering numerical simulation technology is proposed to consider the three-dimensional spatial distribution of the excavating strata.The proposed method first establishes a shield tunneling model for 3D complex strata and then studies the disturbance behavior of the strata induced by the EPBS tunneling.The main results and conclusions of this research are summarized as follows:(1)Complex three-dimensional geological modeling is the basis for carrying out shield tunneling simulation in complex strata.For this reason,this paper uses C#programming language to build a three-dimensional geological model visualization platform based on the CsharpGL computer graphics library.In addition,the Kriging algorithm is introduced in this platform to interpolate geological borehole data and then establish a 3D complex geological model.Taking a section of Changsha Metro Line 4 as an example,the variation function model of 8 stratigraphic sub-interfaces was established using 34 borehole data of this section,and the spatial variability of each stratum was compared and analyzed,and it was found that the difference in elevation of stratum with variation function value less than 10 was small and the stratum was gentle,while the elevation of stratum with variation function value more than 30 was undulating.A geological modeling platform with independent intellectual property rights is developed,which can provide geometric models for carrying out 3D shield tunneling simulations of complex strata.(2)A conditional judgment algorithm based on a stratigraphic partitioning interface is proposed to realize rapid and high-precision meshing of 3D complex strata,and a numerical modeling pre-processing program of Flac3D is developed to improve the efficiency of complex strata modeling.Taking an interval of the Changsha Metro Line 4 project as an example,the numerical simulation of the earth pressure balance shield tunneling process was carried out based on Flac3D.The simulated values of shield excavation surface settlement matched well with the measured values,which verified the effectiveness of the complex stratigraphic grid division algorithm and the numerical model of shield excavation proposed in this paper.(3)The disturbance law of shield tunneling in typical upper soft and lower hard strata is studied.The ground stress along the depth direction and the displacement change law of the soil above the tunnel during shield tunneling were analyzed,and the ranges of the stable zone,soil arch zone,and loose zone above the tunnel were determined as 0～1.3D,1.3D～1.9D,and 1.9D～2.3D below the ground surface,respectively(D is the diameter of the tunnel).The changes in ground stress were studied for soft rock percentages of 25%,50%,75%,and 100%,respectively,and it was found that the height of the loosening zone increased from 0.25D to 0.5D when the soft rock percentage increased from 25%to 100%.(4)The stratigraphic disturbance law of the alternating interface between two strata from soft rock into hard rock and from hard rock into soft rock was studied,and the disturbance law of alternating soft and hard strata under different support forces was also discussed.The simulation results show that when the shield machine enters the hard rock section from soft rock 1,the change of surface settlement is less than 3 mm under each supported force,and when the shield machine enters the soft rock 2 strata from hard rock,the surface settlement of the soft rock stratum is reduced by 11.2 mm when the support force is 0.5 P1 compared with that of 0.05 P1(P1 is the initial horizontal ground stress in tunneling direction).In addition,the effect of shield tunneling soil arch in complex strata was investigated,and it was found that the shield tunneling from soft rock into hard rock did not show a combined arch pattern,but from hard rock into soft rock showed an asymmetric combined arch pattern.