Influence of PGA of ground motion on seismic subsidence of soft soil
Vertical displacement of soil due to input El Centro ground motion with PGA of 0.15g was calculated at the bottom of the foundation for each monitoring point. Figure 6 shows seismic subsidence of ground surface derived from dynamic calculation on three-dimensional model of the soft soil site Fig. 6a suggests that the seismic subsidence propagates from the center, at which the most significant seismic displacement was observed, and gradually decreases until reaching ground surface at which significant plastic deformation and shear failure were observed. Figure 6b illustrates cross-sectional view of the foundation after seismic subsidence. Propagation of the seismic subsidence assumes a parabolic shape symmetrically distributed on the sides with low middle and high sides, and maximum depression angle of 3.44°. The largest vertical displacement is observed at bottom center section of the foundation. Impact distance of the seismic subsidence is elliptical, spanning from 5–25 m in X-direction and 6–24 m in Y-direction. Soil on X-direction and Y-direction of the foundation is compressed and uplifted by 15mm.
Vertical displacement of soil due to input El Centro ground motion with PGA of 0.30 g was calculated at the bottom of the foundation for each monitoring point. Figure 7 shows seismic subsidence of ground surface derived from dynamic calculation on three-dimensional model of the soft soil site. Figure 7a indicates that the seismic subsidence spreads from the center, at which the largest subsidence was observed, and gradually decreases until reaching ground surface at which significant plastic deformation and shear failure were recorded. Figure 7b illustrates cross-sectional view of the foundation after seismic subsidence. Propagation of the seismic subsidence adopts a parabolic shape symmetrically distributed on the sides with low middle and high sides, and maximum depression angle of 7.91°. Bottom middle section of the foundation demonstrates the largest vertical displacement. Impact range of the seismic subsidence is elliptical, covering 4.5–25.5 m in X-direction and 6.5–24.5 m in Y-direction. Soil on X-direction and Y-direction of the foundation is compressed and uplifted by 35mm.
Vertical displacement of soil due to El Centro ground motion with a PGA of 0.40g was calculated at the bottom of the foundation for each monitoring point. Figure 8 shows seismic subsidence of ground surface derived from dynamic calculation on three-dimensional model of soft soil site. Figure 8a implies that seismic subsidence propagates from the center, at which the most significant seismic subsidence was recorded, and gradually decreases until reaching ground surface at which significant plastic deformation and shear failure were detected. Figure 8b illustrates cross-sectional view of the foundation after seismic subsidence. Propagation of the seismic subsidence assumes a parabolic shape symmetrically distributed on the sides with low middle and high sides, and maximum depression angle of 8.0°. Bottom center section of the foundation exhibits the largest vertical displacement. Impact area of the seismic subsidence is elliptical, ranging from 4.5–26.0 m in X-direction and 6.0–25.0 m in Y-direction. Soil on X-direction and Y-direction of the foundation is compressed and uplifted by 46 mm.
Overall effects of PGA of ground motion on soft soil seismic subsidence are summarized in Fig. 9–11. As shown in Fig. 9, seismic subsidence generated by 0.15 g ground motion is approximately 104mm, which is 80.6% and 148.6% lower than the subsidence generated under the action of 0.3 g and 0.4 g ground motion (189 mm and 260 mm). This indicates that increase in PGA distinctly increases degree of seismic subsidence. Figure 10 illustrates development of seismic subsidence at different depth of soil. Lower seismic subsidence is observed at deeper soil layer. Nevertheless, 0.4 g ground motion produces notably higher seismic subsidence than 0.15 and 0.3 g ground motion regardless of the soil depth, indicating a non-linear stress-strain response of the soil.
PGA of ground motion influences uneven surface subsidence. As suggested in Fig. 11, the larger is the PGA, the more significant is the subsidence, the higher is the uplifted distance of soil on X-direction and Y-direction of the foundation, and the larger is the damage distance on horizontal surface. Impact of the seismic subsidence is larger in X-direction than Y-direction, as a consequence of the direction to which input ground motion is applied.
Influence of frequency characteristics of ground motion on soft soil seismic subsidence
Figure 12 shows seismic subsidence of ground surface derived from dynamic calculation on three-dimensional model of soft soil site. The seismic subsidence propagates from the center, at which the largest subsidence was observed, and gradually decreases until reaches ground surface at which significant plastic deformation and shear failure were indicated. Figure 12b illustrates X-direction and Y-direction cross-section of the foundation after the seismic subsidence. Propagation of the subsidence adopts overall parabolic shape symmetrically distributed on the sides with low middle and high sides, and maximum depression angle of 5.01°. The largest displacement was documented at bottom middle section of the foundation. Impact distance of the subsidence is elliptical, spanning from 4.5-25.5 m in X-direction and 6.0-25.0 m in Y-direction. Soil on X-direction and Y-direction of the foundation is compressed and uplifted by 24 mm.
Vertical displacement of soil due to input Taft ground motion with PGA of 0.30 g was calculated at the bottom of the foundation for each monitoring point. Figure 13 shows seismic subsidence of ground surface derived from dynamic calculation on three-dimensional model of soft soil site. Figure 13a suggest that the seismic subsidence spreads from the center, at which the largest subsidence was observed, and gradually decreases until reaches ground surface at which plastic deformation and shear failure were recorded. Figure 13b illustrates cross-sectional view of the foundation after seismic subsidence. The subsidence adopts an overall parabolic shape symmetrically distributed on the sides with low middle and high sides, and maximum depression angle of 7.66°. The largest vertical displacement was observed at bottom middle section of the foundation. Impact distance of the seismic subsidence is elliptical, ranging from 4.5–26.0 m in X-direction and 6.5–25.0 m in Y-direction. Soil on X-direction and Y-direction of the foundation is compressed and uplifted by 44mm.
Overall effects of frequency characteristics of ground motion on soft soil seismic subsidence are summarized in Fig. 14–16. Figure 14 shows that the three ground motions (EL Centro, Kobe, and Taft) produced vertical displacement curves with different growth characteristics. Kobe ground motion initiates soil subsidence at 7 s, which is consistent with onset time at which PGA of the Kobe ground motion is recorded. Figure 15 shows development of seismic subsidence at different depth of soil. Regardless of the soil depth, Taft ground motion demonstrates the most significant seismic subsidence followed by El Centro and Kobe ground motion.
Figure 16 indicates that the three ground motions generated different degrees of seismic subsidence, which suggests the influence of frequency characteristics of the ground motions on soft soil seismic subsidence. Natural frequency of the model site is 1.2 Hz. Similarly, amplitude of Fourier spectrum of Taft ground motion is located at 1–2 Hz, which is close to natural frequency of the site. This resonance significantly increases vertical displacement of soil. Despite exhibiting many low-frequency contents similar to the Taft ground motion, El Centro and Kobe ground motion demonstrate lower amplitude than the Taft ground motion. Therefore, Taft ground motion produced the most severe soft soil seismic subsidence among the investigated ground motions. Overall, these findings highlight the significance of low-frequency contents and amplitude of Fourier spectrum on soft soil seismic subsidence. Higher abundance of low-frequency contents and higher amplitude of Fourier spectrum promote a more severe seismic subsidence.
Despite this study contributes towards understanding on impact of PGA and frequency characteristics of ground motion on soft soil subsidence, these findings were derived from a simplified three-dimensional model. Hence, there may be mechanisms and/or factors related to actual soft soil subsidence during strong earthquakes, which are unaccounted by current model. On this account, a large-scale three-dimensional model which simulates actual interaction between foundation and soft soil is necessary to obtain more reliable conclusions from numerical analysis. Moreover, further research is encouraged to account for soft soil properties, upper load, and foundation-soil interaction for validating impact of ground motion characteristics on soft soil and deriving influencing factors of soft soil seismic subsidence.