The effect of lining density on the seismic response of ground penetrating shield tunnels: Shaking table testing and simplified semi-analytical solution

The Ground Penetrating Shield Tunnel (GPST) method optimizes construction in soft ground by eliminating deep vertical shafts, reducing construction time, and minimizing the environmental impact. The present study examines the effect of lining density on the seismic response of the tunnel. Large scale shaking table tests are conducted, comparing the response of two tunnel models with different lining densities. The segmental lining of the two tunnel models is constructed employing two different gypsum mixes, the first one with normal weight aggregates representing an ordinary lining, and the second one with lightweight aggregates representing a lightweight lining of 50% reduced density. Employing a detailed instrumentation system, the experimental results reveal the key differences in the seismic response of the two tunnel models, quantifying the effect of lining density. The derived insights are used to develop and validate a simplified semi-analytical model, which is subsequently employed for parametric analyses. The model is based on a beam resting on Winkler foundation approach, accounting for the variation of soil modulus with depth. The parametric analyses reveal a strong correlation between the dynamic response of the above-ground portion of the tunnel and lining density. The decrease of lining density leads to a decrease of the inertial response of the above-ground tunnel portion, and to a substantial decrease of its structural displacement. The lining density also has a significant effect on the frequency response of the tunnel, with its reduction leading to a shift of the tunnel's natural frequency away from the predominant frequency of the soil, further reducing the inertial response of the tunnel.