LEAP-UCD-2017 Centrifuge Test at University of California, Davis
Three centrifuge experiments were performed at the University of California, Davis, for LEAP-UCD-2017. LEAP is a collaborative effort to assess repeatability of centrifuge test results and to provide data for the validation of numerical models used to predict the effects of liquefaction. The model configuration used the same geometry as the LEAP-GWU-2015 exercise: a submerged slope of Ottawa F-65 sand inclined at 5 degrees in a rigid container. This paper focuses on presenting results from the two destructive ground motions from each of the three centrifuge models. The effect of each destructive ground motion is evaluated by accelerometer recordings, pore pressure response, and lateral deformation of the soil surface. New techniques were implemented for measuring liquefaction-induced lateral deformations using five GoPro cameras and GEO-PIV software. The methods for measuring the achieved density of the as-built model are also discussed.
KeywordsLiquefaction Centrifuge model test Lateral spreading LEAP-UCD-2018 LEAP Round robin test
The current phase of LEAP, LEAP-UCD-2017, involved centrifuge experiments conducted at nine different research facilities, including the University of California, Davis (UCD). The experiment, similar to LEAP-GWU-2015 (Kutter et al. 2017), consisted of a submerged clean sand sloped at 5 degrees, subjected to a 1 Hz ramped sine wave ground motion inputted at the base of a rigid model container. Three experiments were performed on the 1 m radius Schaevitz centrifuge at the Center for Geotechnical Modeling. The 1 m centrifuge performs shaking in the circumferential direction of the centrifuge. Detailed specifications by Kutter et al. (2019a) were provided to facilitate replicability among the different centrifuge facilities. Discussed herein are the specifications that pertained specifically to the UCD experiments and deviations from the specifications, both intended and unintended. The implementation of a new technique for measuring slope deformations during strong shaking is discussed and the process for measuring the achieved dry density of a constructed model and saturation protocol are detailed. Kutter et al. (2019b) provide a detailed comparison of all centrifuge experiments from the nine participating facilities, including the results presented herein.
13.2 UC Davis Test Specific Information
13.2.1 Description of the Model and Instrumentation
A flat soil surface in a centrifuge represents a hill in prototype. To model a flat surface in prototype, the model surface should be curved with the same radius as the centrifuge. A 5-degree slope from the normal of the radial g-field would have a varying radius along the slope of the model surface, which is described theoretically by a log-spiral. Carey et al. 2017 showed that rotating a circular arc by 5 degrees is a reasonable approximation for the log-spiral surface.
The number of sensors placed in the model was limited by the capacity of the data acquisition system; therefore, only the required pore pressure transducers (P1, P2, P3, P4, P9, and P10) and accelerometers (AH1, AH2, AH3, AH4, AH11, AH12, AV1, AV2) were included.
13.2.3 Scaling Laws
The scaling laws for LEAP-UCD-2017 are provided by Kutter et al. (2019a). The length scale factor L∗ is defined as L∗ = LMODEL/LPROTOTYPE = (0.457 m)/(20 m) = 1/43.75. Using the conventional centrifuge scaling law for gravity, g∗ = 1/L∗ = 43.75. An angular velocity of 194 RPM was determined to produce g∗ = 43.75 at the effective radius of 1.033 m (the radius to 1/3 of the depth of soil).
13.3 Test Results
UCD performed three experiments with a target dry density of 1651 kg/m3. The measured dry densities of the three models ranged from 1648 to 1665 kg/m3. Densities were calculated by the measurement of mass and volume. A more detailed explanation of the method used to measure the volume of each model is described later in this paper. Each specimen used the standard pluviator, which consisted of a No. 16 sieve with three slots (Kutter et al. 2019a) to place the soil. To provide the best likelihood to obtain the target density, the drop height from the standard pluviator to the model surface was adjusted before each experiment, based on the previous model’s measured density. The calculated achieved density does, however, seem to be affected by unaccounted for factors such as humidity, temperature, personal habits and uncertain measurements of mass and volume.
13.3.1 Achieved Ground Motions
Ground motion PGAs for the three UCD experiments
Destructive motion 1
Destructive motion 2
13.3.2 Accelerometer Records During Destructive Motions
Destructive motion 1: The surface accelerometers recorded an almost identical motion as the base input motion. Both motions appear to remain in phase with one another, implying the soil moved nearly as a rigid mass, with small shear strains, and no evidence of liquefaction.
Destructive motion 2: The second motion, M2, was the strongest of all the destructive motions. Initially, the central array sensors are in-phase with the input motion. As liquefaction occurs, beginning at the shallowest sensors first, dilation spikes, characterized as sharp, sudden accelerations, occur. The magnitudes of the spikes decrease with depth since the severity of liquefaction is decreasing with depth.
Destructive motion 1: The input base motion had high-frequency components superimposed on the 1 Hz signal, especially from 12–16 seconds. It is believed that this behavior may have been caused by insufficient pressure in the supply accumulator for the hydraulic shaker. When the achieved velocity of the base motion (found by integrating the achieved base motion) is compared with the specified velocity in Fig. 13.4, good agreement is observed. This indicates the superimposed high-frequency accelerations had minimal effect on the base input motion velocity.
Destructive motion 2: The input motion contained a larger (0.18 g) 1 Hz component than M1 (0.15 g), but M2 had a smaller high-frequency component (0.083 g) compared to M1 (0.122 g) (see Table 13.1). The reduction of the high-frequency components was partly from increasing the hydraulic pressure to the accumulator. The dilation spikes for M2 have larger magnitudes than the spikes in M1, indicating more severe liquefaction in M2 than in M1. This also indicates that the severity of liquefaction dilation is more sensitive to the low-frequency components of the input motion.
Destructive motions 1 and 2: The input motions for M1 and M2 are nearly repeated in terms of magnitude, frequency content, and duration. Similar dilation spikes occur in both motions, but differences in the magnitudes can be observed. The spikes for AH4 are larger during M1 compared to M2, but the AH3 spikes are larger during M2. This could indicate localized zones of loose and dense material.
13.3.3 Excess Pore Pressures
Destructive motion 1: Similar to observations made of the acceleration response, the pore pressure response during M1 does not indicate that liquefaction occurred. The excess pore pressure ratios for P1 through P4 were 34%, 45%, 62%, and 90%. Once the magnitude of the base motion started to decrease at approximately 12 s, excess pore pressures immediately began to dissipate, indicating that at the depths of the PPTs the pore pressure never reached a constant state equal to their respective vertical effective stresses.
Destructive motion 2: The generation of excess pore pressure starts slow but as the magnitude of shaking increases each transducer reaches its initial vertical effective stress. Deliquefaction shockwaves, manifested as a sudden drop in porewater pressure due to the sudden dilation of a liquefied soil, are observed after the magnitude of the excess pore pressure is equal to the initial vertical effective stress (Kutter and Wilson 1999). Once shaking intensity decreases, pore pressures at P1, the deepest sensor, begin to dissipate first. Dissipation is followed then by the shallower sensors.
Destructive motions 1 and 2: The effect of the high-frequency component of the base motion is not easily seen in the pore pressure response for M1 and M2, indicating that the overall effect to the model performance is minimal. The stronger 1 Hz component in M2 is noticeable with the quicker accumulation of excess pore pressures for P1 and P2, compared with M1.
Destructive motions 1 and 2: Both M1 and M2 led to the fastest generation of excess pore pressure of all the tests conducted at UCD. At roughly 8 s the excess pore pressure ratio for P4 is already close to 100%; the other sensors indicated liquefaction later. Following shaking, the excess pore pressures were not maintained for the same duration when compared with the other experiments. The dissipation time for P1 and P2 after M2 was noticeably faster than after M1. This is possibly due to densification during M1.
13.3.4 Cone Penetration Tests
UCD3 shows an approximate 5% increase in qc at 2 m depth between M1 and M2. UCD2 shows very small (less than 2%) decrease in qc at 2 m depth between M1 and M2. qc increases by approximate 5% at 2 m depth after UCD1 M1, but an approximate 5% decrease in qc at 2 m depth after M2. As previously discussed, UCD1 M1 did not liquefy the model; therefore, the variation in the cone profiles may be attributed to a combination of effects of spatial variability, lateral stresses, and soil density.
13.3.5 Surface Marker Surveys
13.4 Nonconformities with Specifications
Several nondestructive motions were performed prior to and following each destructive motion as discussed earlier. For UCD1, additional destructive motions were applied after M2 for the purposes of calibrating shaker command motions for subsequent model tests; these motions are not analyzed in the present paper.
13.5 Advancements in Centrifuge Testing
Differences in surface marker displacement measurement between GEO-PIV and manual pixel counting
Surface marker 1
Surface marker 2
Surface marker 3
Surface marker 4
Surface marker 5
Surface marker 6
13.6 Method of Measuring Density
Achieved density of intermediate lifts and final model density
Depth of lift (mm)
Std Dev. (kg/m3)
Depth of lift (mm)
Std dev. (kg/m3)
Depth of lift (mm)
Std dev. (kg/m3)
Curved surface (final density)
Curved surface (final density)
Curved surface (final density)
The load cell used to measure the mass of the model was accurate to about 0.1 kg. The LEAP-UCD-2017 test template contains supplemental documentation of density calculation for each lift during construction of each model.
13.7 Pore Fluid Viscosity and Saturation
13.7.1 Pore Fluid Viscosity
Methylcellulose was used for each of the UCD experiments to scale the viscosity of the pore fluid in accordance with the scaling law, μ = μwater/L∗. Prior to centrifuge testing, several batches of viscous solution were mixed to determine the correct proportions of Dow F50 Food Grade methylcellulose powder and water by mass to achieve the desired viscosity of μ = 43.75. The ratio of mass of methylcellulose to mass of water was 2.2%.
Warm roughly ¼ of the required deionized water to 90 °C. Add methylcellulose with a mass that is equal to 8.8% of mass of the deionized water.
Mix solution for 45 min.
Dilute the mixture with the same mass of water from step 1 at room temperature. Mix for additional 10 min. Following mixing, this stock solution should be at roughly double the required concentration.
Using an approximately 200 g sample of the stock solution, 208 g of room temperature deionized water was added and mixed. Viscosity of the solution was checked using a Cannon instrument size 2 Ubbelohde viscometer.
Step 5 was repeated if necessary, adjusting the amount of deionized water added, to determine the ratio of deionized water and stock solution to produce the desired viscosity.
Lastly, the entire batch of methylcellulose was mixed with the ratio of water and stock found in Step 6.
13.7.2 Model Saturation
Each of the UCD models followed the same procedure for saturation. Initially, the dry model and container were placed in a vacuum chamber. 97 kPa of vacuum was applied, then the vacuum was shut off, and the chamber was flooded with CO2 up to until the vacuum was reduced to about 2 kPa. Then, the CO2 flow was shut off and the 97 kPa vacuum was reapplied. This cycle was repeated two times and following the second cycle the vacuum was reapplied to 97 kPa. Following the third evacuation, the residual concentration of nitrogen and oxygen gas in the chamber should be ((101.4–97)/101.4)3 times its initial concentration in atmospheric air, and the partial pressure of CO2 gas would be about 4% of an atmosphere. The saturation chamber and model were inclined so the toe of the model (left-hand side of Fig. 13.1) was the lowest point in the container. The methylcellulose solution was de-aired and dripped on a sponge on the top surface of the sand to maintain a pool of de-aired methylcellulose solution in the lowest edge of the container. As the wetting front progressed toward the top of the model, the size of the pool was allowed to grow. The top corner of the slope was the last location to saturate, allowing for any residual gas and air to escape from the model. Nine-seven kPa vacuum was maintained throughout the infiltration of methylcellulose. Once the model was completely saturated and the surface submerged, the vacuum was slowly released.
This paper described the three experiments performed on the 1 m centrifuge at the University of California, Davis, as part of the LEAP-UCD-2017 exercise. The model performance and results for the two destructive motions were presented, and the nonconformities from the specifications were also discussed. The process to mix methylcellulose pore fluid was presented and the protocol to saturate and measure the degree of saturation of the model was also explained.
Motion 1 in UCD1 did not cause liquefaction, which is evident by the low excess pore pressures, and the nearly rigid acceleration response of the model. The second experiment had larger than expected high-frequency component superimposed on the 1 Hz specified input motion. The achieved velocity closely matched the specified velocity for both motions of the second test. The third experiment showed the best repetition of input motions and model performance, and can be used to determine if numerical simulations correctly model the evolution of soil properties from repeated shaking events. Other minor deviations from the specifications were discussed, such as alternate placement of surface markers, and the use of minor nondestructive ground motions before the destructive motions.
A new technique was presented that uses GoPro cameras mounted on a wave suppressing window acting like a glass-bottomed boat to record images of the deforming surface. GEO-PIV was used to process the images and create displacement time series. Results from the procedure were shown to be reliable when compared against hand measurements, sensor data, and the manual processing of images.
A protocol for measuring the achieved density of a specimen was described. The final density was calculated by measuring the depth of soil along three longitudinal transects and fitting a curve to those depths in AutoCAD. This method to find density, while still relying on point measurements, proved to be helpful for accurately determining the final density and the associated uncertainty of the density measurements.
These experiments were supported by NSF CMMI Grant Number 1635307. The authors would also like to thank the staff at the Center for Geotechnical Modeling (CGM) for their assistance and technical insight throughout the series of tests.
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