Keywords

1 Introduction

This paper presents three centrifuge tests conducted at Ehime University (EU) in the LEAP-ASIA-2019 project. Two of them, models EU-1 and EU-2, were the same as those of tests in LEAP-UCD-2017, and one test, model EU-GS1, was newly conducted in the present project. Models were designed so that they simulated the same prototype at different scaling factors to verify the validity of the generalized scaling law (Iai et al., 2005). Model configuration, preparation techniques, and sand used are in accordance with “LEAP-UCD-2017 Version 1.01 Model Specifications” (Kutter et al., 2018) otherwise mentioned.

2 Centrifuge Model

2.1 Model Description

A rigid model container was used with internal dimensions of 50 cm long, 12 cm wide, and 23 cm deep. Three models were prepared at a target relative density of 65%, saturated with viscous fluid and tested at either 40 g or 20 g. Figure 7.1 shows a side view of the models. All the corresponding prototype dimensions are the same as those specified with an exception that the prototype width was approximately half of that specified.

Fig. 7.1
A side-view schematic diagram of a rigid container, with the slope of the sand and different instrument positions. It includes A V 1, 2, 3, 4, 6, 11, and 12, P 1, 2, 3, 4, 6, 9, and 10, C P T 1, 2, and 3, B E 1 and B E 2.

Schematic of models

Full size image

The prototype being simulated in this study is a fully submerged medium dense Ottawa sand slope with a slope angle of 5%. This prototype was modeled in two ways; one was a well-established centrifuge modeling technique where a model geometrically reduced by a factor of 40 was tested at 40 g centrifugal acceleration. The sand in the model was the same as the prototype but saturated with a fluid which was 40 times more viscous than water to closely simulate the prototype sand permeability. The other was the generalized scaling law. Iai et al. (2005) proposed a scaling law by combining the centrifuge scaling law with a scaling law for 1 g dynamic model tests. The scaling factors for the centrifuge test η = 20 and 1 g test μ = 2 were selected in this study to represent the same prototype. The model was prepared with the same sand but saturated with a fluid with different viscosity, subjected to 20 g centrifugal acceleration and shaken with similar base input motion with different acceleration amplitude and frequency. Scaling factors for tests in this study are summarized in Table 7.1.

Table 7.1 Summary of scaling factors
Full size table

2.2 Sand

Ottawa F-65 sand (Bastidas et al., 2017) was used in all the experiments, which was shipped out from UC Davis on March 2017. Sieve tests on the sand confirmed that grain size distributions of the sand used in EU were consistent with those shown in the specifications.

2.3 Placement of Sand

Dry sand stored in an air-conditioned room, where humidity was kept low, was pluviated into the container through a screen with an opening size of 1.0 mm, rather than 1.2 mm specified in the specification, because openings of the standard sieve in JIS (Japanese Industrial Standards) are slightly different from ASTM.

An arrangement of the screen masked off was used to achieve the target relative density. 12 mm slots spaced at 25 mm were used for preparing the sand bed with a relative density of 65%. After the pluviation of the sand, the surface of the model was leveled using a vacuum device, and the height of the model surface was measured to calculate dry density. The procedure of the measuring height was the same as that specified. The surface was sloped using the vacuum device to 5 degrees, and 18 surface markers were set (Fig. 7.2). The depth of the sand on the centerline was 10 cm for all models tested in this study.

Fig. 7.2
A photograph of the centrifuge model before saturation.

Model before saturation

Full size image

2.4 Saturation

The model container was moved into a vacuum chamber, and the air in the chamber was replaced by CO2. This was achieved by introducing vacuum pressure of −95 kPa and flooding CO2 gas. De-aired viscous fluid was dripped into the lower end of the model slope while keeping the vacuum of −95 kPa constant in the chamber and a fluid supply tank (Okamura & Inoue, 2012). The viscous fluid was a mixture of water and hydroxypropyl methylcellulose (type 60SH-50) termed Metolose from Shin-Etsu Chemical Company (Shin-Etsu Chemical Co., Ltd., 1997). This Metolose solution was prepared by dissolving 1.8% or 1.5% Metolose by weight in water, so as to achieve a viscosity of 40 or 33.6 times that of water (40 cSt or 33.6 cSt kinematic viscosity), respectively. The viscosity of the fluid in each model was measured at room temperature before and after the tests with a rotational viscometer and presented in Table 7.2.

Table 7.2 Model properties and test conditions
Full size table

On completion of the saturation process, the vacuum in the chamber was released, and the model was rested in the atmospheric pressure for a few hours. A small change in the pressure of approximately 10 kPa was applied to the chamber at a constant rate approximately 5 kPa/min, and water level was measured with a LED displacement transducer with the resolution of 10 μm. The degree of saturation of the models was measured with the method developed by Okamura & Inoue (2012) and summarized in Table 7.2. The degree of saturation was in a range between 99.3% and 99.5%. In the centrifuge, hydrostatic pressure of the model was enhanced, and most of the remaining air and CO2 bubbles in soil pore are considered to have dissolved (Kutter, 2013), which increased further the degree of saturation.

2.5 Test Procedure

Three tests were conducted in this study. Models EU-1 and EU-2 were tests at 40 g, while the model EU-GS1 was tested at 20 g and applied the generalized scaling law to further scale up to the prototype. Shaking tests and CPT were carried out at 40 g or 20 g as schematically illustrated in Fig. 7.3. The centrifuge was spun up and down to mount and unmount the CPT device. Locations of the surface markers were measured with a ruler, and the relative density of the model was estimated from the average settlement.

Fig. 7.3
A scatterplot of acceleration versus time. It has 3 sets of trapezoidal trends for E U 1 and 2 and E U G S 1 at accelerations of 20 and 40. 2 plots of C P T 1 are at the peaks of the first set, followed by plots of destructive shaking in the second, and C P T 2 in the third.

Centrifuge test sequence

Full size image

Cone penetration tests were performed before and after the destructive ground motion with a CPT device designed and fabricated at UCD. Two penetration tests were conducted at a time at intervals of 5 cm (12.5 times the cone diameter). The rate of cone penetration was 0.6 mm/s in model scale, which was slower than that specified.

Two destructive shaking tests, Motion #1, were conducted with the tapered sine wave of a maximum prototype acceleration amplitude of 0.15 g. In the second destructive shaking tests, the same motion as the first one was imparted again to evaluate the evolution of the behavior of the model due to the previous shaking event.

All the data was recorded at a sampling rate of 2000 per second during shaking and after shaking for approximately 20 s until the generated pore pressure completely dissipated. In the subsequent section in this paper, all the test results are in prototype scale otherwise mentioned. Note that data were not applied any numerical filtering.

3 Results

3.1 Input Acceleration

Input base accelerations measured at the base of the container are shown in Fig. 7.4. In order to duplicate the specified tapered sine wave by the mechanical shaker, rotation rate of camshaft changed with time accordingly. The shaking initiated at the time t = 0, and the rotation rate increased linearly with time until t = 10 s and decreased thereafter. Therefore, the acceleration frequency was very close to 1.0 Hz for t = 9–13 s and between 0.7 and 0.9 Hz for t = 5–9 s and t = 13–15 s.

Fig. 7.4
A line graph of the input base acceleration versus time. It plots two lines for E U 2 and E U G S 1. The tapered sine wave changes with time according to the mechanical shaker and rotation rate of the camshaft. Both trends overlap.

Input accelerations of Motion #1 for EU-1 and EU-GS1

Full size image

3.2 CPT

CPT was conducted at two locations indicated as “CPT1” in Fig. 7.1 before shaking. Vertical profiles of cone tip resistances before shaking are indicated in Fig. 7.5. The tip resistances for EU-GS1 at two locations are almost the same and very similar to that of EU-1, confirming the model uniformity and reproducibility.

Fig. 7.5
A line graph of the depth versus cone tip resistance. It plots 4 declining trends for E U 2, E U 2, E U G S 1, and E U G S 1 from the origin.

Cone tip resistance before shaking for EU-2 and EU-GS1

Full size image

3.3 Excess Pore Pressure Response

Figure 7.6 shows excess pore pressure ratios (EPPRs) measured on the centerline of the models EU-1 and EU-GS1. EPPRs of EU-1 reached unity except for p1, indicating that the soils in EU-1 liquefied from the surface to the depth close to the bottom. While for EU-GS1, EPPR increased in a quite similarly manner at the beginning of shaking event, however, the maximum value of EPPRs was slightly lower than those in EU-1 and short to the liquefaction condition (EPPR = 1) followed by initiation of pore pressure dissipation during shaking.

Fig. 7.6
Five line graphs of excess pore pressure ratio and acceleration versus time plot E U G S 1 and E U 1 lines, each for the input motion A H 11, P 1, P 2, P 3, and P 4. The fluctuating trends in P 1 to P 4 stabilize and rise, while the fluctuations in A H 11 are high between 5 and 20 seconds.

Time histories of excess pore pressure ratio

Full size image

Figure 7.7 compares long-term excess pore pressure (EPP) responses for EU-1 and EU-GS1. Duration for liquefaction condition lasted after shaking and the time for pore pressure dissipation were much longer for EU-1 than EU-GS1. A possible reason for the difference in pore pressure responses is permeability of the soil. The sand in EU-GS1 was saturated with the viscous fluid of 33.6 cSt, and the corresponding permeability was 1.7 (=33.6/20) times lower than the prototype sand. Therefore, the permeability of the sands does not explain the difference in EPPR response between EU-1 and EU-GS1. The other reason may be the difference in stress level. The self-weigh stress in model EU-GS1 is half of that in EU-1 and the sand behaved more dilative.

Fig. 7.7
Five line graphs of excess pore pressure ratio and acceleration versus time plot E U G S 1 and E U 1 lines, each for the input motion A H 11, P 1, P 2, P 3, and P 4. The fluctuating trends in P 1 to P 4 stabilize, rise, and fall, while the fluctuations in A H 11 are high between 5 and 20 seconds.

Long-term comparison in excess pore pressures

Full size image

3.4 Deformation of the Model

Figure 7.8a shows locations of surface markers in model scale before and after the shaking. For EU-2, the upmost and the downmost markers near the side walls of the rigid container stayed almost in the same location due to the side wall confinement. Except for those locations, the surface subsided in the upstream side and heaved in the downstream side. Horizontal displacement was in the range from 0 to 3 mm, while for EU-GS1, both vertical and horizontal displacements of the surface markers were half or less as compared with EU-2. Figure 7.8b indicates prototype horizontal displacement. Horizontal displacement for the EU-2 was on the order of half of EU-2. This may be due to the fact that the extent of liquefaction of sand in EU-GS1 was limited.

Fig. 7.8
2 line graphs of height from the bottom and horizontal prototype displacement versus horizontal distance. A plots 2 decreasing lines for E U 2 and E U G S 1 each before and after shaking. B plots a fluctuating trend for E U 2 and the E U G S 1 line rise and stabilize. A schematic has the marker locations.

Vertical and horizontal displacement of surface markers

Full size image

4 Conclusion

This paper describes three centrifuge tests conducted at Ehime University for LEAP-2017 and LEAP-ASIA-2019. The tested models were fully saturated uniform sand slope with a relative density of 65%. All the models were subjected to the same simulated prototype base motion consisted with ramped sine wave.

Although the soil profiles including the relative density and cone tip resistance were quite similar for all the models, excess pore pressure was slightly lower and dissipated swiftly after shaking for model EU-GS1 (conducted at 20 g) as compared with the other two tests, EU-1 and EU-2 (performed at 40 g). Deformation of the sand for EU-GS1 was also smaller than EU-1 and EU-2. These differences may be due to the different pressure level, which resulted in more dilative behavior for EU-GS1 conducted at lower centrifugal acceleration of 20 g.