Seismic Responses of Liquefiable Sandy Ground with Silt Layers

Transcription

1 Journal of Applied Science and Engineering, Vol. 16, No. 1, pp (2013) 9 Seismic Responses of Liquefiable Sandy Ground with Silt Layers H. T. Chen 1 *, B. C. Ridla 2, R. M. Simatupang 2 and C. J. Lee 1 1 Department of Civil Engineering, National Central University, Taoyuan, Taiwan 320, R.O.C. 2 Department of Civil Engineering, National Central University, Taiwan and University of Brawijaya, Indonesia Abstract This paper presents the numerical simulation results of liquefable sand-silt stratum with silt intralayers under strong earthquakes. The numerical simulation results showed that the existence of silt intralayers in a sandy soil stratum will reduce the ground settlement and the excessive pore water pressure above the silt layer will also become smaller than that in the regular sand stratum. However, the pore water pressure beneath the silt layer will become higher due to the impermeable character of silt layer. Although the existence of more silt layers decreases the ground settlement furthermore, the pore water pressure will have slower dissipation. Key Words: Liquefaction, Sandy Stratum, Silt Intralayers, Effective Stress Analysis 1. Introduction Liquefaction is a phenomenon that the structural and the geotechnical engineers concern most as it can result in serious damage to the ground and the building such as sand boiling, lateral spreading, excessive settlement, tilting and overturning of structures. For a long time, many liquefaction-related studies mainly treated the ground as sandy ground; however, in reality there may be layers of silt or clay embedded in the sandy ground. In some earthquakes the failure of ground did not occur during the earthquake but after the earthquake stopped. The investigations on such a phenomenon showed that it may be due to the existence of a silt layer in the sandy ground where a water film develops at the bottom of the silt layer with high pore water pressure [1]. This indicates that the sandy soil stratum with silt intralayers may become unstable even after the main shake, causing the sliding of slope. The purpose of this study is to investigate numerically the behavior of liquefable sand-silt stratum with many layers of silt under strong earthquakes. *Corresponding author. chenht@cc.ncu.edu.tw 2. Method of Analysis For the numerical simulation the three-dimensional nonlinear effective stress finite element method was adopted [2]. This method was developed on the basis of Biot theory for porous media. The nonlinear soil behavior was modeled using the Cap model with Mohr- Coulomb type failure line and the pore pressure model consistent with the Cap model was adopted [3]. The lateral boundaries can be modeled as either roller-type boundaries or absorbing boundaries, while the bottom bedrock is always fixed. This method adopts the U-W form of equation of motion [4] as follows: (1) where u is the displacement of soil particle and w is the displacement of water relative to soil particle. The vector {J} is made up of 1 s and 0 s to account for the de-

2 10 H. T. Chen et al. sired direction of input motion. is the input motion specified at the bedrock of soil stratum. 3. Verification and Validation In this study the validation and verification of numerical simulation was first conducted by using the results of centrifuge tests on three models [5]. Although the validation was made for all three models [6], here only the comparisons for two models are presented. Shown in Figure 1 are the two finite element models which were constructed in accordance with the models used in the centrifuge test. The model in Figure 1a denoted as Sand model corresponds to the sandy stratum which was divided into 11 layers with the top and the bottom layers having the thickness of 1.2 m and the remaining layers with thickness of 2.4 m for each layer. Figure 1b shows the model denoted as Sand-Silt 1 model where the silt layer of 1.6 m thick was placed at the depth of 5.6 m from the surface and the model was divided into 13 different layers. The input motions measured at the base of shaking table on the centrifuge platform was used as the input motion. Figures 2 and 3 show the comparison for the surface settlement and excessive pore water pressure development, respectively. It can be seen that the simulation results show the same trend as the experimental results and the agreement is acceptable. 4. Numerical Results and Discussions 4.1 Modal Description Shown in Figure 4 are the five models adopted in this study. Sand model consisted of sand only. For Silt 1 model a silt layer of 2 m thick was placed at the depth of 8 m from the surface. Two silt layers of 2 m thick for each were placed at the depth of 8 m and 20 m, respectively, from the surface for Silt 2 model. Silt 3 model was the Figure 1. Finite element models: (a) Sand model, (b) Sand-Silt 1 mode1. Figure 2. Comparison for surface settlement: (a) Sand model, (b) Sand-Silt 1 model.

3 Seismic Responses of Liquefiable Sandy Ground with Silt Layers 11 Figure 3. Comparison of excess pore water pressure development: (a) Sand model, (b) Sand-Silt 1 model. Figure 4. Finite element models: (a) Sand model, (b) Silt 1 model, (c) Silt 2 model, (d) Silt 3 model, (e) Silt 4 model. one where two silt layers of 2 m thick for each were placed at the depth of 8 m and 14 m from the surface, respectively. For Silt 4 model, three silt layers of 2 m thick for each were placed at the depth of 8 m, 14 m and 20 m from the surface, respectively. All the models had dimensions of 26 m 26 m 30 m (length width depth) and were divided into 15 layers with element size of 2 m 2m 2 m. Detailed properties of the models can be seen in the thesis by Simatupang [6]. A real earthquake motion recorded in 1999 ChiChi earthquake at Chiayi station (Chiayi input motion) was used for this 3D simulation study. Before the simulation, from the selected earthquake the maximum acceleration of all components was selected and normalized to 0.2 g; thereafter, the same scaling factor was applied to the motions of the other two directions. These three scaled component of motions were then used as the input motions for the simulation.

4 12 H. T. Chen et al. 4.2 Discussions Figure 5 shows the time history of settlement on the surface for Sand, Silt 1, Silt 2, Silt 3, and Silt 4 models subjected to Chiayi input motion. The largest settlement occurs in the Sand model, which is around 0.87 m. The maximum settlements of Silt 1, Silt 2, Silt 3, and Silt 4 models are 0.55 m, 0.37 m, 0.34 m, and 0.22 m, respectively. Silt 1, Silt 2, and Silt 3 models have smaller settlement than the Sand model due to the existence of silt layer near the surface. Silt 4 model has the smallest settlement from all models due to the existence of three silt layers. Shown in Figure 6 are the excess pore water pressure ratios at different depths for Sand, Silt 1, Silt 2, Silt 3, and Silt 4 models subjected to Chiayi input motion. In this figure, it can be seen that the effect of silt layer in the sandy soil stratum is significant. At each depth, the behavior of EPWP was different. All five models liquefy at the depth of 1 m where Silt 1, Silt 2, Silt 3 and Silt 4 models have lower EPWP ratio and faster dissipation than Sand model. But at the depth of 5 m, only the Sand model liquefies, while the Silt 1, Silt 2, Silt 3 and Silt 4 models show almost the same development of EPWP without liquefaction. At the depths of 7 m and 9 m, which are inside the silt layer of Silt 1, Silt 2, Silt 3, and Silt 4 models, the development of EPWP is slower than that of Sand model before liquefaction and after the liquefaction occurs, the trend reverses. At this depth, there is a water film beneath a less permeable soil layers and it takes longer time to dissipate the EPWP. At the depths of 17 m, 21 m and 29 m, liquefaction does not occur for all five models; the development of EPWP for Sand model and Silt 1 model is almost the same, meaning that the EPWP is not affected by the existence the silt layer in Silt 1 model while a slight increase is observed for the Silt 2, Silt 3, and Silt 4 models at later time. Figure 7 depicts the initial effective stress and the EPWP profiles for all five models at several selected time. All five models show the similar behavior in the Figure 5. Time history of settlement for 5 models (Chiayi input motions). Figure 6. Time history of EPWP ratio at different depths for 5 models (Chiayi input motions).

5 Seismic Responses of Liquefiable Sandy Ground with Silt Layers 13 Figure 7. EPWP profile at different time for 5 models (Chiayi input motions). development of EPWP up to 5 seconds. At 15 seconds, for Silt 1 model, a jump in the EPWP occurs between the top and bottom of silt layer and for Silt 2, Silt 3, and Silt 4 models the jump occur between the top and bottom of each silt layer but there is no jump for Sand model. The variation of EPWP between the three silt layers of Silt 4 model is that the value of EPWP decreases to a value smaller than that of Sand, Silt 1, Silt 2, and Silt 3 models at the top of lower silt layer. The above phenomenon becomes less pronounced as the EPWP keeps increasing from 15 seconds to 40 seconds. At 40 seconds, EPWP in Silt 4 model is higher than that in Sand, Silt 1, Silt 2, and Silt 3 models. For the dissipation of EPWP, it starts from the bottom of soil stratum and proceeds upward. The Sand model shows fastest dissipation. Silt 1, Silt 2, Silt 3, and Silt 4 models show that the dissipation of EPWP is slow beneath the silt layer. As a result, Silt 4 model has the lowest dissipation rate, while the dissipation rate for Silt 1 model is the same as that of Sand model for the depth larger than 21 m and the dissipation rate for Silt 3 model is faster than that of Silt 2 for the depth larger than 19 m. After liquefaction (50 seconds to 80 seconds), the trend of EPWP profiles of Silt 2, Silt 3, and Silt 4 is similar to the post-liquefaction scheme predicted in [1]. 5. Conclusion The existence of silt intralayer in a sandy soil stratum will reduce the ground settlement. The excessive pore water pressure above the silt layer will also become

6 14 H. T. Chen et al. smaller than that in the regular sand stratum. However, the pore water pressure beneath the silt layer will become higher due to the impermeable character of silt layer. This can be dangerous especially when it is happened in the slope ground, because the water film will be produced during the motion and will remain even after the motion stops, leading to sliding or lateral movement of ground. Although the existence of more silt layers decreases the ground settlement furthermore, the pore water pressure will have slower dissipation. References [1] Kokusho, T. and Kojima, T., Water Film in Liquefied Sand and Its Effect on Lateral Spread, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 125, No. 10, pp (1999). [2] Jou, J. J., Study on Seismic Reponses Analysis of Pile Foundation Bridge, Dissertation, Doctor of Philosophy, Department of Civil Engineering, National Central University, Jhongli, Taiwan (2000). (in Chinese) [3] Pacheco, M. P., Altschaeffl, A. G. and Chameau, J. L., Pore Pressure Prediction in Finite Element Analysis, International Journal for Numerical Methods in Engineering, Vol. 13, pp (1989). [4] Zienkiewicz, O. C. and Shiomi, T., Dynamic Behavior of Saturated Porous Media; the Generalized Biot Formulation and Its Numerical Solution, International Journal for Numerical Methods in Engineering, Vol. 8, pp (1984). [5] Lee, C. J., Wei, Y. C., Lien, H. C. and Chen, H. T., Centrifuge Modeling on the Seismic Responses of Sandy Deposit with a Thin Silt Seam, 8 th International Conference on Urban Earthquake Engineering, Tokyo Institute of Technology, Tokyo, Japan (2011). [6] Simatupang, R., A Numerical Investigation on Stone Columns as a Countermeasure for Liquefaction of Sandy Soil Stratum with Interlayers of Silt, Master Thesis, Department of Civil Engineering, National Central University, Jhongli, Taiwan (2011). Manuscript Received: Nov. 12, 2012 Accepted: Jan. 20, 2013