Numerical study on the use of soft material walls to enhance seismic performance of an existing tunnel

Abstract

Seismic risk is one of the biggest challenges for tunnel safety, and several mitigation techniques have been proposed to enhance the seismic performance of existing tunnels. This paper aims to investigate the effectiveness of an innovative approach for reducing the seismic risk of existing tunnels by using soft material walls (SMW) symmetrically installed in the surrounding soils. The investigation is performed with a two-dimensional numerical model and the effectiveness of SMW in mitigating the seismic-induced lining forces is quantitatively evaluated by reduction ratio. The influences of nonlinear properties of soil, SMW and tunnel lining on the isolation effectiveness are also discussed. The parametric studies show that the computed reduction ratio is strongly affected by the modulus ratio between the SMW and the soil, the wall geometric parameter, and the flexibility ratio. It is more effective for the thick and soft isolation walls that are inserted near a stiff tunnel in the soft soil. The tunnel seismic response can be reduced by up to 50% for the scenarios investigated. Notably, the parametric study identifies an optimum normalized depth of SMW and recommends a relation between the maximum isolation effect and the flexibility ratio. Finally, simple charts are suggested in this work for estimating the isolation effect in specific conditions of the soil and the tunnel. Along these lines, the results of this work may be used in the seismic retrofitting of an existing tunnel, aiding the preliminary design of the isolation walls.

Keywords

Tunnels / Earthquakes / Seismic analysis / Isolation walls / Soft material

Declaration of competing interest

Daniel Dias is an editorial board member for Underground Space and was not involved in the editorial review or the decision to publish this article. All authors declare that there are no competing interests.

Acknowledgement

The work was financially supported by the Chunhui Program of the Natural Science Foundation of Hebei Province (Grant No. E2022201021) and the Research Fund for Talented Scholars of Hebei University (Grant No. 521100221063).

Appendix.

An academic case is used in this study, and no physical models are close enough to verify the developed two-dimensional numerical model. The numerical model is verified against the analytical results of Luco and De Barros (1994), who used an indirect boundary integral method to obtain the dynamic displacement on the ground surface. The tunnel lining is removed during the verification (unlined cavity) and the wavelength (λ) of the Rick wavelet is 16.67 m, which corresponds to the dimensionless frequency η = 0.5 and depth ratio h/r = 1.5 (r is the tunnel radius). The wave propagates vertically from the bottom of the numerical model. It can be seen from Fig. A1 that the calculation results are consistent with the results of Luco and De Barros (1994), indicating that the developed numerical model can well simulate the seismic response of geotechnical media. In Fig. A1, U_x, U_y are the total displacements of the surface points in the x and y directions, respectively; U_s is the displacement of the incident motion on the ground surface.

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