Seismic centrifuge modelling of earth dams

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1 Geomechanics and Geoengineering: An International Journal Vol. 5, No. 4, December 21, Seismic centrifuge modelling of earth dams Louis Ge a,yubao b, Chin-Kuan Ni c and Hon-Yim Ko b a Department of Civil, Architectural, and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, USA; b Department of Civil, Environmental, and Architectural Engineering, University of Colorado at Boulder, Boulder, CO, USA; c Department of Civil Engineering, National Taipei University of Technology, Taipei, Taiwan (Received 27 June 28; final version received 18 November 29) Three dynamic centrifuge models were tested to obtain data for safety evaluation of the Jen-Yi-Tan Dam in Taiwan subject to a strong earthquake. In these tests, recorded 1999 Chi-Chi earthquake ground motions were modified and used on the electro-hydraulic shaking table mounted on the 4 g-ton centrifuge at the University of Colorado at Boulder. All tests were conducted under centrifugal acceleration of 15 g, and the input acceleration was scaled accordingly in order to simulate the given earthquake. A rigid container and water as pore fluid were used in the tests. In both Models 2 and 3, no sign of soil liquefaction was observed in the tests although a noticeable amount of settlements were found from the earth dam cross-section profile after testing. Keywords: centrifuge; embankment; seismic; earthquake Introduction Geotechnical centrifuges provide capability of modeling prototype geo-structures such as slopes, embankments, retaining walls, piled foundations, and tunnels, etc., without losing mimicking the gravity-induced in-situ stress field. It is also a powerful tool to examine the failure mechanisms and calibrate numerical models. In dynamic centrifuge modeling of embankments and earth dams, Muraleetharan and Arulanandan (1991) performed tests on earth dams with three sand layers, a clay core, an upstream clay blanket and a downstream berm. A rigid container and water as pore fluid were used in their tests. The model was spun at 32 g and the applied base motion by a servohydraulic shaker corresponding to a.45 g,.9 Hz prototype earthquake with a 23-s duration. Peiris et al. (1998) carried out tests on gravel embankments on loose saturated sand foundations. An equivalent shear beam box and silicone oil were used as a pore fluid. The model was spun at 5 g when an earthquake motion was given by a stored-angular-momentum actuator at about 5 Hz for.3 s. Taboada-Urtuzuastegui et al. (22) examined seismic behaviors of a slope in liquefiable soil. Two tests were conducted at 6 and 12 g, respectively. Two corresponding substitute viscous pore fluids and a rigid container were used in the tests. The input motion from an electro-hydraulic shaker consisted of 2 cycles of a sinusoidal wave corresponding to a prototype frequency of 1 Hz and prototype peak acceleration up to.25 g. Adalier and Sharp (24) performed a series of four dynamic centrifuge tests on an Corresponding author. geyun@mst.edu embankment dam on liquefiable foundation at different relative densities. The tests were carried out at 125 g. During testing, a 3-cycle earthquake motion was given by an electro-hydraulic shaker. It corresponded to a prototype earthquake with a frequency of 1.5 Hz and a magnitude of.2 g. A rigid container and water as pore fluid were used. In this paper, three dynamic centrifuge models were tested to obtain data for safety evaluation of the Jen-Yi-Tan Dam in Taiwan subjected to a strong earthquake. In these tests, recorded horizontal ground motions during the Chi-Chi earthquake that took place on 21 September 1999, in Taiwan (M S = 7.2) were modified and used on the electro-hydraulic shaking table mounted in the 4 g-ton centrifuge at the University of Colorado at Boulder. All tests were conducted under centrifugal acceleration of 15 g, and the input acceleration was scaled accordingly in order to simulate the given earthquake. Description of the centrifuge models Model geometries Three models were constructed in this study, whose scale factors are 15. Model 1, as shown in Figure 1, was made to have a homogeneous section using the prototype core material (clay) of the Jen-Yi-Tan Dam. This model was founded on the impervious and rigid base of the aluminum model container. By avoiding any complications that might arise from dam foundation interaction, the effects of the simulated earthquake on the dam structure could be studied without being influenced ISSN print/issn online 21 Taylor & Francis DOI: 1.18/

2 248 L. Ge et al #3 #4 # #7 # # Clay (CL) 76.2 # accelerometers Pore pressure transducers # # # #6 Clay (CL) # # (All dimensions are in mm) Figure 1. Placement of instrumentation devices in Model 1. by a compliant foundation. Model 2 in Figure 2 was analogous to Model 1 except that a thick layer of silt foundation replaced the rigid base. The body of the model dam was again constructed as a homogeneous section using the prototype core material. Model 3 depicted in Figure 3 differed from Model 2 by constructing its section to mimic the actual zoned section in the prototype. In addition, sand replaced silt as the foundation material in Model 3. Fidelity towards the prototype foundation was achieved by duplicating as closely as possible the density and moisture conditions. Model containers Two containers were designed and used. A container having inside dimensions of 122 mm long, 35 mm wide and 229 mm high was used for Model 1. The second container with inside dimensions of 122 mm long, 35 mm wide and 432 mm high was used for Model 2 and Model 3. Both containers were made of 663 grade aluminium (12.7 mm thick). Appropriate reinforcements were used to minimize the deflections, and all joints were sealed with silicon sealant to make the containers watertight. Soil properties Table 1 lists the properties of the soils used in the three models. Comparing the soil properties of the models with those of the prototype, fidelity towards the prototype was achieved by duplicating as closely as possible the density and moisture conditions of the various soils. Shaking table and input motion The electro-hydraulic shaking system consists of a servo-valve combination, a hydraulic power supply system, a linear actuator, a linear variable differential transformer (LVDT), linear bearings and bearing mounts. The detailed information about the shaking table can be found in Ketcham (1989) and Ketcham et al. (1991). The shaking table was mounted on the centrifuge platform in such a way that the model container was shaken along its longitudinal axis, thus minimizing the adverse effects of the variation of the g-level along the width of the container. Since the shaking table is a displacement-controlled system, the desired acceleration history is numerically integrated to obtain the displacement history. According to the scale factor 15 used in this project, the frequencies of the model earthquake need to be 15 times as high as that of the prototype earthquake and the shaking duration time needs to be shortened to 1/15 of the prototype earthquake. However, due to the limited capacity of the shaking table, which can generate motions with frequency up to 4 Hz in-flight, the higher frequencies in the prototype ground motions were filtered out so that the corresponding model earthquake will not contain frequencies higher than 4 Hz in-flight. Figure 4 shows the ground motions originally recorded from the Chi-Chi earthquake and its filtered data along with its corresponding Fourier transform spectra, where frequencies higher than 2.5 Hz were filtered out. Scaling law conflict According to the centrifuge scaling relations, dynamic phenomena take place N times faster and consolidation processes N 2

3 Geomechanics and Geoengineering: An International Journal accelerometers 55. # #3 # # #6 # Clay (CL) 76.2 # Silt (ML) 221. Pore pressure transducers # Clay (CL) # #4 # # Silt (ML) # (All dimensions are in mm) Figure 2. Placement of instrumentation devices in Model 2. times faster in the model than in the prototype. The inconsistency has implications for centrifuge modelling when a saturated soil structure is loaded dynamically and excess pore water pressure generation and consolidation occur simultaneously. This can be solved by employing a substitute pore fluid N times more viscous than water to slow down the consolidation event (Ellis et al. 1998, Dewoolkar et al. 1999). However, the substitute pore fluid was not used in the model tests because the centrifuge was testing at a high g-level (15 g), and the substitute pore fluid would be too viscous to handle. Consequently, this led to a faster excess pore water pressure dissipation process without using the substitute pore fluid. Model preparation Two sets of wooden moulds were made to accommodate the scale models, which all yielded the same shape as the Jen- Yi-Tan Dam. One set was for Model 1 and Model 2 while an additional set was made for Model 3 because of its zoned section. By stacking wooden blocks of the moulds, the outer shape of the model dam was formed eventually. The soils were the natural materials obtained from the field in Taiwan and shipped to the University of Colorado at Boulder. The soils were premixed at the prescribed moisture contents and were placed in the model container to be compacted in layers by dynamic compaction for Model 1 and by 3-kips Baldwin compression machine for Model 2 and Model 3. The weight of each layer was calculated to produce the prescribed dry density. Transducers were embedded in pre-selected locations during the model preparation. Upon completion of the model construction, the wooden moulds were removed. Model 1 The model dam was prepared layer by layer from bottom to top in a container of size mm. A set of wooden moulds was manufactured for the sample preparation. A dynamic compaction method was used for the dam preparation. For each layer, a pre-calculated amount of soil was placed into the container as well as its corresponding wooden mould. The hammer used in the modified Proctor compaction

4 25 L. Ge et al # #4 # # #6 # Silt (ML) Clay (CL) 76.2 # accelerometers Sand (SM) 221. Pore pressure transducers # Clay (CL) Silt (ML) # #4 # # Sand (SM) 221. # (All dimensions are in mm) Figure 3. Placement of instrumentation devices in Model 3. Table 1. Properties of model soils. SM ML CL Specific gravity (G s ) Dry density ρ d (g/cm 3 ) Saturated water content w (%) Unsaturated water content w (%) N/A 11.4 N/A method was used to dynamically compact the soil until the correct thickness was obtained. After finishing the compaction for a layer, trenches and holes were excavated to accommodate instrumental devices if necessary. Then the surface was roughened to eliminate and prevent the interface effects. The same procedure was repeated for subsequent layer until the dam sample was completed. Figure 1 shows the placement of accelerometers (ACC) and pore pressure transducers (PPT) in Model 1. Model 2 Due to the weaker silt (ML) dam foundation in Model 2, an inverted compaction method was adopted. Instead of using the modified Proctor hammer, a Baldwin compression machine with a capacity of 3 kips was used to compact the soils. Also, a second aluminium container was made to accommodate the dam model with foundation. The container has the size of mm. In the inverted compaction procedure, the container was first fitted with a lid and turned upside down. The top layer of the embankment model was compacted first, using the appropriate wooden moulds previously employed in Model 1 preparation to help guide the finished slopes both upstream and downstream. Compaction was effected by applying a static force from the Baldwin machine over a thick aluminium plate placed over the soil surface until the proper thickness of the soil layer

5 Geomechanics and Geoengineering: An International Journal 251 G level Original Fourier Amplitude Original Frequency (Hz) G level Filtered Fourier Amplitude Filtered Frequency (Hz) Figure 4. Originally recorded and filtered Chi-Chi earthquake ground motions: time history and Fourier transform spectra. had been achieved. The aluminium plate was then removed and the soil surface was roughened to receive the soil for the second top most layer in the embankment section, while the wooden moulds for the second layer were also placed over those for the first layer. This process was repeated until the entire embankment section had been prepared. In between layers, where appropriate, placement of accelerometers and pore pressure transducers was carried out as previously described for Model 1. The leads of the accelerometers were hidden in wooden moulds and would be retrieved later when the moulds were removed after the process had been completed. The leads of pore pressure transducers were taken directly through port holes on the side of the container. Figure 2 shows the placement of accelerometers (ACC) and pore pressure transducers (PPT) in Model 2. The silt foundation was then compacted in six layers to reach the required thickness. The bottom was properly sealed with caulking. The container was taken off the Baldwin machine so that it could be turned right side up. The top lid was then removed, followed by the wooden moulds, while care was exercised to retrieve the accelerometer leads as they became exposed. From this point on, the preparation of the test proceeded in the same manner as in Model 1. Model 3 In Model 3, there was a clay core zone in the embankment so that an additional set of inner wooden moulds were needed. The same inverted compaction procedure as for Model 2 was employed for Model 3, with an added step used to prepare the upstream and downstream silt shells. In each layer, the clay core was compacted first, using both the outer and inner moulds. Then, the inner moulds were removed, allowing for silt materials to be placed in the spaces vacated by them. The silt was then compacted to the appropriate thickness. This procedure was repeated for each layer until the entire embankment had been compacted. The sand foundation was then compacted in 6 layers and the model preparation was followed in the same manner as in Model 2. Figure 3 shows the placement of accelerometers (ACC) and pore pressure transducers (PPT) in Model 3. Test results and discussions Due to the limited space, only selected test results are presented and discussed in this paper. All the test results presented here are given in model scale. Model 1 Figure 5 shows the soil acceleration time histories during shaking for selected accelerometers. ACC 4, located at the dam crest, shows some amplification of input motion. It can also be found in Figure 6, where the corresponding Fourier spectra are presented. ACC 5 and ACC 7, both placed at the same elevation (76.2 mm from the bottom of the model container), have slight difference in soil acceleration time history. ACC 7 shows more intense vibration than ACC 5, which can also be found in their Fourier spectra. One of the explanations might be due to the coupling between pore fluid pressure and soil deformation. In addition, ACC 7 is located on the upstream side of the dam, which was under fully saturated condition during the shaking, while the phreatic

6 252 L. Ge et al. 2 Input Motion ACC 4 g level ACC ACC Figure 5. Soil acceleration time history during shaking, Model 1. 5 Input Motion ACC 4 Fourier Amplitude ACC ACC Figure 6. Fourier spectra of soil acceleration during shaking, Model 1. Frequency (Hz)

7 Geomechanics and Geoengineering: An International Journal PPT Excess Pore Pressure (kpa) PPT PPT PPT Figure 7. Excess pore water pressure during shaking, Model 1. surface most likely did not reach ACC 5 located on the downstream side. Figure 7 shows the excess pore pressure generation on selected pore pressure transducers (PPTs) during the shaking. Both PPT 1 and PPT 3 on the downstream side underwent slight change in pore pressure. PPT 4 and PPT 5 on the upstream side had about 17 and 8 kpa excess pore pressure, individually. By comparing the hydrostatic water pressure at those PPT locations before shaking, it is found that the pore pressure ratio r u was about.5 for PPTs 4 and 5, and about.3 for PPTs 1 and 3. Model 2 The acceleration time history of ACCs 2, 5, and 7, located on the crest, downstream side, and upstream side of the dam, respectively, is shown in Figure 8. They look quite similar, compared to the input motion. The Fourier spectra of the ground acceleration are shown in Figure 9, which can be seen that the Fourier amplitude of ACCs 2, 5, and 7 were almost identical and greater than that of input motion. Hence, it may conclude that the clay embankment sitting on silt foundation moved as a rigid body motion during the shaking. The excess pore pressure of PPTs 1, 3, 5, and 6 is shown in Figure 1. PPTs 1 and 3 were in the clay embankment while PPTs 5 and 6 were in the silt foundation. PPT 1 just had a small amount of excess pore pressure during the shaking due to its shallow embedment; however, its pore pressure ratio was about.5, which was the highest among these four PPTs. PPT 3 had a lower pore pressure ratio about.3 and both PPT 5 and 6 reached about.5. Model 3 ACC 2 in Model 3, placed on the clay crest of the dam, shows some amplification effects, as seen in Figure 11. ACC 5 located in silt shell embankment underwent slightly more vibration than ACC 7. The Fourier spectra given in Figure 12 show the Fourier amplitudes for ACCs 2, 5, 7 were greater than input motion. PPT 1, located in silt shell embankment, and PPT 3, placed in clay core, show little excess pore pressure generation during the shaking; however, PPTs 5 and 6, both located in sand foundation, were found having greater excess pore pressure. The little change of excess pore pressure in PPT 1 might be due to the fact that the seepage process took place quickly right after the testing of the centrifuge so that the water level in the reservoir dropped lower than the elevation of PPT 1. Conclusions Three centrifuge model tests were conducted to examine seismic behaviors of earth dams subject to an earthquake. A rigid container and water as a pore fluid were used in the tests.

8 254 L. Ge et al. 2 Input Motion ACC 2 g level ACC ACC Figure 8. Ground acceleration time history during shaking, Model 2. 5 Input Motion ACC 2 Fourier Amplitude ACC ACC Figure 9. Fourier spectra of ground acceleration during shaking, Model 2. Frequency (Hz)

9 Geomechanics and Geoengineering: An International Journal PPT Excess Pore Pressure (kpa) PPT PPT PPT Figure 1. Excess pore water pressure during shaking, Model 2. 2 Input Motion ACC 2 g level ACC ACC Figure 11. Ground acceleration time history during shaking, Model 3.

10 256 L. Ge et al. 5 Input Motion ACC 2 Fourier Amplitude ACC ACC Frequency (Hz) Figure 12. Fourier spectra of ground acceleration during shaking, Model 3. 1 PPT Excess Pore Pressure (kpa) PPT PPT PPT Figure 13. Excess pore water pressure during shaking, Model 3.

11 Geomechanics and Geoengineering: An International Journal 257 A modified 1999 Chi-Chi earthquake ground motion was generated by an in-flight electro-hydraulic shaker table. In both Models 2 and 3, no sign of soil liquefaction was observed in the tests although a noticeable amount of settlements were found from the earth dam cross-section profile after testing. Acknowledgements The financial support from Taiwan Water Supply Corporation is acknowledged. References Adalier, K. and Sharp, M.K., 24. Embankment dam on liquefiable foundation dynamic behaviour and densification remediation. Journal of Geotechnical and Geoenvironmental Engineering, 13 (11), Ellis, E.A., Soga, K., Bransby, M.F. and Sato, M., Effect of pore fluid viscosity on the cyclic behaviour of sands. Centrifuge, 98, Dewoolkar, M.M., Ko, H.-Y., Stadler, A.T. and Astaneh, S.M., A substitute pore fluid for seismic centrifuge modelling. Geotechnical Testing Journal, 22 (3), Ketcham, S.A., Development of an Earthquake motion simulator for centrifuge testing and the dynamic response of a model sand embankment. PhD Thesis, University of Colorado at Boulder. Ketcham, S.A., Ko, H.-Y. and Sture, S., Performance of an earthquake motion simulator for a small geotechnical centrifuge. Centrifuge, 91, Muraleetharan, K.K. and Arulanandan, K., Dynamic behaviour of earth dams containing stratified soils. Centrifuge, 91, Peiris, L.M.N., Madabhushi, S.P.G. and Schofield, A.N., Dynamic behaviour of gravel embankments on loose saturated sand foundations. Centrifuge, 98, Taboada-Urtuzuastegui, V.M., Martinez-Ramirez, G. and Abdoun, T., 22. Centrifuge modelling of seismic behaviour of a slope in liquefiable soil. Soil Dynamics and Earthquake Engineering, 22 (9 12),

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