Assessing the deformation response of double-track overlapped tunnels using numerical simulation and field monitoring

The unprecedented rate of metro construction has led to a highly complex network of metro lines.Tunnels are being overlapped to an ever-increasing degree.This paper investigates the deformation response of double-track overlapped tunnels in Tianjin,China using finite element analysis(FEA)and field monitoring,considering the attributes of different tunneling forms.With respect to the upper tunneling,the results of the FEA and field monitoring showed that the maximum vertical displacements of the ground surface during the tail passage were 2.06 mm,2.25 mm and 2.39 mm obtained by the FEA,field monitoring and Peck calculation,respectively;the heaves on the vertical displacement curve were observed at 8 m(1.25D,where D is the diameter of the tunnel)away from the center of the tunnel and the curve at both sides was asymmetrical.Furthermore,the crown and bottom produce approximately0.38 mm and 1.26 mm of contraction,respectively.The results of the FEA of the upper and lower sections demonstrated that the tunneling form has an obvious influence on the deformation response of the double-track overlapped tunnel.Compared with the upper tunneling,the lower tunneling exerted significantly less influence on the deformation response,which manifested as a smaller displacement of the strata and deformation of the existing tunnel.The results of this study on overlapped tunnels can provide a reference for similar projects in the future.

Dynamic interaction of twin vertically overlapping lined tunnels in an elastic half space subjected to incident plane waves

The scattering of plane harmonic P and SV waves by a pair of vertically overlapping lined tunnels buried in an elastic half space is solved using a semi-analytic indirect boundary integration equation method. Then the effect of the distance between the two tunnels, the stiffness and density of the lining material, and the incident frequency on the seismic response of the tunnels is investigated. Numerical results demonstrate that the dynamic interaction between the twin tunnels cannot be ignored and the lower tunnel has a significant shielding effect on the upper tunnel for high-frequency incident waves, resulting in great decrease of the dynamic hoop stress in the upper tunnel; for the low-frequency incident waves, in contrast, the lower tunnel can lead to amplification effect on the upper tunnel. It also reveals that the frequency-spectrum characteristics of dynamic stress of the lower tunnel are significantly different from those of the upper tunnel. In addition, for incident P waves in low-frequency region, the soft lining tunnels have significant amplification effect on the surface displacement amplitude, which is slightly larger than that of the corresponding single tunnel.

Performance of overlapped shield tunneling through an integrated physical model tests,numerical simulations and real-time field monitoring

In this work,deformations and internal forces of an existing tunnel subjected to a closely overlapped shield tunneling are monitored and analyzed using a series of physical model experiments and numerical simulations.Effects of different excavation sequences and speeds are explicitly considered in the analysis.The results of the physical model experiments show that the bottom-up tunneling procedure is better than the top-down tunneling procedure.The incurred deformations and internal forces of the existing tunnel increase with the excavation speed and the range of influence areas also increase accordingly.For construction process control,real-time monitoring of the power tunnel is used.The monitoring processes feature full automation,adjustable frequency,real-time monitor and dynamic feedback,which are used to guide the construction to achieve micro-disturbance control.In accordance with the situation of crossing construction,a numerical study on the performance of power tunnel is carried out.Construction control measures are given for the undercrossing construction,which helps to accomplish the desired result and meet protection requirements of the existing tunnel structure.Finally,monitoring data and numerical results are compared,and the displacement and joint fracture change models in the power tunnel subject to the overlapped shield tunnel construction are analyzed.

Investigation and measurement of old building response to the overlapped shield tunnel of multiple schemes in the sandy cobble stratum

Ground movements due to tunneling are becoming increasingly critical as buildings are located around construction sites.This study proposes a new combined reinforcement method using a foundation grouting oblique pipe roof.The former improves the bearing capacity of the subsoil,and the latter blocks the transmission of soil deformation,which weakens the influence of construction during overlapped tunnel under-crossing.Based on this new method,a case study of the shield tunneling response to an old building in Line 6 of China’s Chengdu Metro is presented.Additionally,three-dimensional numerical models without reinforcement,traditional foundation grouting reinforcement,and the new combined reinforcement schemes were compared.The numerical simulation performance was verified using a set of field instrumentation data,which demonstrated that the old building response to the overlapped tunnels was under control,and the maximum deformation,angular distortion,and principal tensile strain of the building were 5.25 mm,5.10
10–6 rad/m,and 0.0081%,respectively.Compared with the traditional reinforcement scheme,the deformation,angular distortion,and principal tensile strain in the combined reinforcement scheme were reduced by 54.78%,71.02%,and 70.22%,respectively.These results have important implications for the design and construction of shield tunnels and their response to old buildings.

Investigation and statistical modeling of InAs-based double gate tunnel FETs for RF performance enhancement

In this paper,RF performance analysis of In As-based double gate（DG）tunnel field effect transistors（TFETs）is investigated in both qualitative and quantitative fashion.This investigation is carried out by varying the geometrical and doping parameters of TFETs to extract various RF parameters,unity gain cut-off frequency（f_t）,maximum oscillation frequency（f_（max））,intrinsic gain and admittance（Y）parameters.An asymmetric gate oxide is introduced in the gate-drain overlap and compared with that of DG TFETs.Higher ON-current（ION）of about 0.2 mA and less leakage current（IOFF）of 29 f A is achieved for DG TFET with gate-drain overlap.Due to increase in transconductance（g_m）,higher ft and intrinsic gain is attained for DG TFET with gate-drain overlap.Higher f_（max） of 985 GHz is obtained for drain doping of 5×10^（17）cm^（-3） because of the reduced gate-drain capacitance（C_（gd））with DG TFET with gate-drain overlap.In terms of Y-parameters,gate oxide thickness variation offers better performance due to the reduced values of Cgd.A second order numerical polynomial model is generated for all the RF responses as a function of geometrical and doping parameters.The simulation results are compared with this numerical model where the predicted values match with the simulated values.