Keywords: embankment, liquefaction, slope stability, finite element method, limit equilibrium method, seismic design.

The seismic safety evaluation procedure currently used in Japan first calculates the factor of safety using the -method, in which the excess pore water pressure is simply evaluated based on the simple liquefaction evaluation, or the seismic coefficient method assuming a circular slip surface. If the calculated factor of safety is smaller than the planned one, dynamic analysis has to be conducted to calculate the residual displacement under a strong earthquake. However, because the slip surface is most possibly non-circular for liquefiable embankments, and because the seismic characteristics are not considered for the -method or the seismic coefficient method, the residual displacement cannot be calculated with a satisfactory confidence. This paper proposes to use finite element approaches to evaluate the seismic safety of embankments under strong earthquakes; in detail, we use the finite element seepage analysis program VisualFEA+VGFlow [1] to calculate the pore water pressure before the earthquake. Then we use the fully coupled effective stress dynamic elastoplastic finite element program UWLC [2] to calculate the deformation and excess pore water pressure induced by a strong earthquake, and use the static elastoplastic finite element program GeoFEAS [3] with the shear strength reduction technique [4] to evaluate the safety of embankments after a strong earthquake considering the pore water pressure and excess pore water pressure calculated by finite element seepage analysis and fully coupled dynamic elastoplastic finite element approach. The factor of safety of embankments calculated using finite element approaches is compared with those of the conventional -method or the seismic coefficient method.

The proposed approaches are used to re-analyze the Lower and Upper San Fernando Dams. During the San Fernando earthquake of February 9, 1971 (magnitude 6.6), a major slide occurred in the upstream slope of the Lower San Fernando Dam; slide movements also caused a downstream movement of about 1.5m in the embankment of the Upper San Fernando Dam; however, a major slide did not occur [5]. For the Lower San Fernando Dam, the numerical results can reproduce the major slide with a large residual displacement and a factor of safety after the earthquake is 0.88 when the excess pore water pressure due to the liquefaction was considered. The numerical results show that the excess pore water pressure ratio in the upstream and downstream shell below the phreatic surface was quite high up to about 0.7. This indicates that the hydraulic fill materials in these regions may have either liquefied or been weakened severely. Additionally, the excess pore water pressure ratio in the upstream shell was still high even at 60s after the beginning of the earthquake.

For the Upper San Fernando Dam, the calculated lateral displacements were 2.5m at the downstream berm and 1.2m at the downstream toe; they were consistent with the observation of 2.2m at the downstream berm and 1.1m at the downstream toe. The excess pore water pressure ratio in the upstream shell of the Upper San Fernando Dam is about 0.6, and the excess pore water pressure ratio was still high 60s after the beginning of the earthquake. The excess pore water pressure ratio in the Upper San Fernando Dam is about 0.1 lower than that in the Lower San Fernando Dam. Because the Lower San Fernando Dam is 25m higher than the Upper San Fernando Dam, the shear stress before the earthquake in the Lower San Fernando Dam is larger than that in the Upper San Fernando Dam. This is the one of the main reason that a large slide occurred in the upstream of the Lower San Fernando Dam but only some residual displacements occurred in the Upper San Fernando Dam. The factor of safety for the downstream shell is larger than one even when the excess pore water pressure was considered.

1

Forum 8 Co., Ltd., "User's manual of VisualFEA+VGFlow", 2004 (in Japanese).

2

Forum 8 Co., Ltd., "User's manual of UWLC", 2004 (in Japanese).

3

Forum 8 Co., Ltd., "User's manual of GeoFEAS", 2005 (in Japanese).

4

D.V. Griffiths, P.A. Lane, "Slope stability analysis by finite elements", Geotechnique, 49(3), 387-403, 1999.

5

H.B. Seed, K.L. Lee, I.M. Idriss, F.I. Makdisi, "The slides in the San Fernando Dams during the earthquake of February 9, 1971", Journal of the Geotechnical Engineering Division, ASCE, 101(7), 651-688, 1975.

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