LEAP-UCD-2017 V. 1.01 Model Specifications
This paper describes the specifications developed by and distributed to all of the centrifuge test facilities involved in LEAP-UCD-2017. The specified experiment consisted of a submerged medium dense clean sand with a 5-degree slope subjected to 1 Hz ramped sine wave base motion in a rigid container. This document describes the detailed geometry, sensor locations, methods of preparation, quality control, shaking motions, surface markers, and surface survey techniques.
KeywordsLiquefaction Centrifuge model test Specifications Round robin test
1.1.1 Differences Between This Paper and Pre-test Specifications
This paper documents the specifications for LEAP-UCD-2017 centrifuge model tests as they existed just prior to the testing for LEAP-UCD-2017. The primary goal of the specifications was to improve the specimen preparation, the testing procedures, and the accuracy of the data and in addition to provide data to quantify the uncertainties associated with the experiments. The specifications were first drafted for the LEAP-GWU-2015 project described by Kutter et al. (2017). Based upon experience in 2014–2015, the specifications were updated and improved for the LEAP-UCD-2017 exercise.
While some of the data in this paper (e.g., the maximum and minimum densities) were subsequently superseded, it was decided to maintain this document as it was prior to centrifuge testing as a record of the specifications. The remainder of this paper is therefore nearly a verbatim copy the specifications developed prior to the centrifuge testing.
1.1.2 Goals and Overview
The goals of this LEAP are to perform a sufficient number of experiments to characterize the median response and the uncertainty of the median response of a specific sloping deposit of sand to a specified ground motion. To put the uncertainty in context, it was considered critical to also determine the sensitivity of response to relative density, the sensitivity of the response to the ground motion intensity, and the sensitivity of the response to unspecified components of the ground motion that are superimposed on the specified ground motion.
The specific median soil deposit to be tested at each centrifuge facility is a 4-m-deep, 20-m-long deposit of Ottawa F-65 sand with a dry density of about 1650 kg/m3 and a ground slope of 5°. The specified median ground motion is a ramped sine wave input motion similar to the target motion for LEAP-GWU-2015. The primary response quantity of interest is the displacement and deformed shape of the soil deposit. Important secondary response quantities include time series data from acceleration, pore pressure, and displacement sensors.
1.2 Scaling Laws
The length scale factor, L∗, is defined as L∗ = Lmodel/Lprototype. According to the conventional centrifuge scaling laws, gravity will be scaled according to g∗ = 1/L∗. The gravity in the model gmodel = ω2Rref, where Rref is measured at 1/3 the depth of sand in the middle of the plan area of the soil deposit. Viscous pore fluid will be used in all the experiments and the viscosity should be scaled according to μ = μwater/L∗. Scaling laws are used in accordance with recommendations by Garnier et al. (2007).
1.3 Description of the Model Construction and Instrumentation
1.3.1 Soil Material: Ottawa F-65 Sand
Specific gravity and grain size characteristics of Ottawa F-65 sand used in LEAP-GWU-2017 (Kutter et al. 2017)
Maximum and minimum dry density for Ottawa F-65 sand (Kutter et al. 2017)
Min. density (kg/m3)
Max. density (kg/m3)
Cooper Labs (UCD)
ASTM D4254 and D4253
GeoTesting Express (RPI)
ASTM D4254 and D4253
Andrew Vasco (GWU)
ASTM D4254 and D4253
Andrew Vasco (GWU)
Lade et al. (1998)(using graduated cylinder)
Cerna Alvarez (UCD)
Lade et al. (1998) (using graduated cylinder)
Cerna Alvarez (UCD)
Modified ASTM D4254(a)
Parra Bastidas (UCD)
ASTM D4254, JIS A 1224
Wen-Yi Hung (NCU)
Yan-Guo Zhou (ZJU)
Average of tests
Stand. dev. of all methods
It appears from standard deviations given in Table 1.1 that the grain sizes of the tested sand were very consistent. (It should be noted that many sites used 0.25 and 0.125 mm sieve sizes without intermediate sieve sizes, so the values of D10–D60 were interpolated between the percentages retained on those two sieves; this interpolation is estimated to produce errors on the order of 0.01 mm for D30 and D50.)
In late 2016, however, a different batch of sand delivered from US Silica to UC Davis had quite different grain sizes; D50 was approximately 0.28 mm and some variability was noticed from bag to bag. The 2016 batch of material was considered to be non-satisfactory for LEAP. All of the sand shipped from Davis in early 2017 to LEAP participants was therefore taken from the original batch of sand shipped to UC Davis in 2014. Unfortunately, the inconsistency in the later delivery has raised concerns about the quality control of the sand in the future. Therefore, it is important for each LEAP site to perform quality control checks on the grain characteristics of the Ottawa F-65 sand. A sufficient number of bags to prepare an entire model should be mixed together first, and then the grain size analysis and maximum and minimum tests should be conducted on this mixture. If the grain size is significantly different from that in Table 1.1, researchers should scalp the material so that D50 and D10 conform to be within one standard deviation of the data reported in Table 1.1. In addition, at least one maximum and minimum density test, as described below, should be performed on the mixed material used for each centrifuge model.
Grain size parameters of Ottawa F-65 sand published by US Silica
To facilitate frequent quality control, a simple and consistent method of measuring maximum and minimum densities is required. Researchers are requested to measure maximum and minimum densities at least once for each model test. The methods described below should be used (modified from ASTM procedures). In addition, researchers may decide to measure the index densities using common standard procedures used in their country. A new section of the data reporting template for reporting the results of the grain size and maximum/minimum densities will be developed soon.
Independent measurement of any of the soil properties should be reported using a spreadsheet with format consistent with the formats of existing soil properties in the LEAP Soil Properties and Element Test Database (https://nees.org/resources/13689/).
Modified ASTM D4254 Method C for Minimum Dry Density
A glass graduated cylinder of 1000 ml will be used for measurement of the minimum density. The humidity of the “dry” sand source should be measured by burying a humidity sensor into the sand until the reading stabilizes. The humidity of the room should also be measured. About 500 g of dry sand should be carefully weighed and placed inside a 1000 ml graduated cylinder. The top of the graduated cylinder should be covered with a sheet of latex and held by hand to seal the top of the cylinder. The sample should then be turned upside down and steadily rotated upright within about 60 s. The volume of the loose sample can then read from the graduated cylinder. Data to be recorded include date, researcher, mass of sand, volume of loose sand, humidity of the laboratory, humidity of the sand source, temperature of the room, and the minimum dry density. The calibration of the volume marks on the side of the graduated cylinder should be checked by weighing it with a known volume of water and assuming the density of water is 998 kg/m3.
Modified Lade et al. (1998) Method for Maximum Density
Place approximately 500 g of soil in approximately 50 g increments into a plastic 1000 ml graduated cylinder. After placing each 50 g increment, the side of the graduated cylinder should be firmly tapped eight times (two times each on the north, south, east, and west sides of the cylinder) with the plastic handle of a screwdriver. The mass of the screwdriver should be approximately 140 g and total length of about 250 mm. To consistently apply the firm taps, the operator should hold the screwdriver by the metal part, and the plastic handle should be about 250–300 mm away from the cylinder between taps to produce consistency. After the last 50 g increment of soil is placed and tapped, each of four sides of the graduated cylinder should be lightly tapped six times (24 total) up and down the sample. To level the top surface for purposes of accurate reading, five or ten very light taps are made while the cylinder is tilted. The volume of the sand may be read from the graduated cylinder and the maximum density calculated. Data to be recorded include date, researcher, mass of sand, volume of loose sand, humidity of the laboratory, humidity of the sand source, temperature of the room, and the maximum density. A video of the recommended procedure for checking maximum and minimum density is posted in the General Report for the data archive for LEAP-UCD-2017 ( https://doi.org/10.17603/DS2N10S).
1.3.2 Placement of the Sand by Pluviation
If the screen or the geometry of the open parts of the screen is significantly modified, calibration curve (density as a function of drop height), mass flow rate during deposition, as well as the dimensions of the container used to perform the calibration will be reported in the data report.
The elevation of the screen above the container during pluviation should be continually adjusted to maintain a constant vertical spacing between the sand surface and the screen; the drop height should not change by more than 5% during deposition. The sieve should be steadily and continuously moved (by robot or by hand) to avoid local mounds with side slopes that affect the deposition. Some tapping of the sieve is necessary if/when the sieves become clogged.
Calibration curves for the above three screen masking patterns are presented in Fig. 1.3. If researchers notice a significant difference in the calibration curve, they should report their observed calibration curve (density vs drop height).
It is suspected that humidity of the sand and electrostatic forces developed during repeated handling of the sand could affect the results. To investigate these issues, a sideview photograph of the sand flowing should be included in the sample preparation report. A humidity sensor should be embedded into the sand after deposition to measure the temperature and humidity of the sand after placement. The humidity of the room should also be measured. Suitable humidity sensors, for example, are available in the USA for approximately $75 (http://www.testequipmentdepot.com/extech/pdf/rh300.pdf).
1.3.3 Measurement of Density of the Sand
To measure the as-deposited density of the sand in the model container, an accurate method of measuring the container dimensions and the volume of the sand in the container is required. Container dimensions may be measured accurately using a rigid steel ruler and then the volume of the container checked by filling the container with water, covering it with a flat plate, eliminating air bubbles, and measuring the mass of water that it holds. Confirm that the volumes computed from dimensions and water mass are consistent; check and report their repeatability.
The mass of the sand should also be measured to better than 0.5%. The accuracy and the repeatability of the measuring method should be verified.
1.3.4 Geometry of the Model
1.3.5 Saturation of the Model
To facilitate dissolution of gas bubbles that are trapped in the sand, the model container will be repeatedly evacuated and flooded with CO2 to replace 98% of the air in the chamber by pure CO2 prior to saturation with de-aired viscous fluid (Kutter 2013). If 90% vacuum is applied, then the process must be repeated at least twice to remove more than 98% of the air. If 50% vacuum is applied, then the vacuum and CO2 flooding should be repeated at least five times. After 98% replacement of air by CO2, de-aired viscous pore fluid should be dripped into the low end of the model slope followed by infiltration with de-aired water while under vacuum as shown in Fig. 1.6. The de-aired viscous fluid may be prepared by letting it sit under a vacuum of at least 80 kPa (absolute pressure is 21 kPa or less). The vacuum should be continuously applied to the water supply reservoir while it is being introduced to the specimen. If this is not possible, steps should be taken to prevent gas from re-dissolving in the fluid prior to infiltration. Flow of viscous fluid to the model from the reservoir may be driven by gravity feed or peristaltic pump.
Documentation that the method of saturation is successful either by the measurement of p-wave velocity or by measuring the raising and lowering of the water level due to applying vacuum to the entire container (as described by Okamura and Inoue 2012) is required.
1.4 Instrumentation of the Model
1.4.1 Required Instrumentation
The required instrumentation is very similar to that for LEAP-GWU-2015 as indicated in Fig. 1.5a, b. AH1–AH4, AH11, AH12, AV1, AV2, P1–4, P9, and P10 are required sensors. AH6, AH9, P6, and P8 are highly recommended. AH5, AH7, AH8, AH10, P5, and P7 are recommended sensors. Bender elements are also recommended be used to monitor the evolution of the shear wave velocity during the centrifuge testing. To the extent possible, cables from the sensors should run in the transverse direction toward the side walls of the model container to minimize the reinforcing effect of the sensor cables. The routing of cables should avoid regions were CPT tests could intersect the cables.
1.4.2 Displacement Measurements
As described by Kutter et al. (2017), it is possible to determine dynamic displacements by integration of accelerometer records. Acceleration records should therefore be reported with as little analog and digital signal processing as possible; if filtering is necessary, the characteristics of the filter must be indicated. The corner frequencies of the data acquisition system should be reported, especially for the accelerometer sensors.
There is clearly a fundamental difficulty associated with measuring residual or permanent displacements from contact sensors founded in submerged liquefied, laterally spreading ground. A concerted effort is required to obtain more accurate displacement measurements than has been achieved in the past.
Careful Before and After Photographs of the Model and Surface Markers
Prior to spinning the centrifuge, after stopping after the first destructive motion, and after all of the destructive shaking events, photographs must be taken. The photographs should use a camera looking vertically down at the center of the specimen at the same distance and in the same lighting conditions. The location of the camera should be carefully placed above the top center of the model, and good lighting should be used to enable these reference photos to be used to determine the lateral deformation at various stages of the testing. These reference photographs should be taken using the same camera, with the same lens with the same amount of water on top of the specimen and with similar lighting. Make sure that the surface markers are clearly visible in the photographs.
Lateral Displacements from Cameras Mounted on the Centrifuge
Residual Settlements from Pore Pressure Sensors
The effect of electric noise on pore pressure sensor records can be dealt with by taking many data points over a period of time longer than the period of the noise. For example, if the noise is 50 or 60 Hz or higher, we could take the Residual Pore Pressure Average (RPPA) reading of data recorded at 2000 data points per second for a full second. This RPPA recording must be repeated before spinning the centrifuge, just before each shaking event, after complete dissipation of pore pressure from each shaking event, and after stopping the centrifuge.
As we are trying to resolve very small residual pore pressures to accurately measure residual settlements, it is crucial to ensure complete pore pressure dissipation. A good way to do this is to visually inspect the pore pressure record, find the time t99 required for pore pressures to drop to within 1% of the initial absolute pore pressure, and then wait ten times longer than t99 before recording the residual pore pressures. This might take about 10 min total between shaking events.
Sensors P9 and P10 accurately located in the bottom corner of the model containers should read the same RPPA increase during spinup from 1 g to the test acceleration, and ideally, they should return to the same RPPA after shaking and the same 1-g RPPA after stopping the centrifuge. Reasons for the difference between P9 and P10 include friction in the bucket hinge, the container base not being normal to the resultant g-vector, leakage or evaporation from the model container, or drifting of the centrifuge acceleration. Thus, the data from P9 to P10 sensors are crucial to allow us to compensate for small changes in g-level and water table elevation. Report RPM to an accuracy better than 0.5% before and after shaking. All of the RPPA values for all of the pore pressure sensors should be reported on one worksheet of the template.
Many facilities zeroed out the pore pressure sensors prior to each shaking event in LEAP-GWU-2015. This should not be done because it will defeat our effort to determine sensor settlements from residual pore pressures. If it is necessary to offset the electronic zero of the pore pressure sensors (e.g., for some electronic instrumentation limitation), RPPA values should be recorded before and after each electronic offset.
Direct Measurements of Sensor and Surface Marker Locations
The X, Y, and Z coordinates of the 15 surface markers shall be surveyed. Record the surface survey data in the spread sheet template showing the X, Y, and Z coordinates of the top center of each marker. The required accuracy of the measurement is 1 mm for horizontal displacements and 0.5 mm for vertical settlements. The markers may be located using photogrammetric techniques, a laser scanner, or a depth gauge with a Vernier caliper that measures the locations relative to a rigid guide frame such as that pictured below. Suggested surface markers to facilitate analysis of lateral movements are specified in section (circular disks described above).
Colored Sand Layers, Noodles, and Sensor Locations During Dissection
Horizontal colored sand layers will be placed at the elevations of the central array of pore pressure sensors. The elevation of the sand relative to the model container reference coordinate system and relative to the pore pressure sensors should be measured before and after the tests.
Vertical markers will consist of spaghetti noodles placed vertically in the dry sand. The noodles should be placed in the sand after about half of the slope has been placed. An array of noodles along the window and along the longitudinal centerline should be placed before the test. The length of the noodles should be adjusted so they stick out about 1 or 2 mm above the ground surface before saturation. The deformed shape of the noodles should be exposed by excavation and photographed at the end of the shaking events.
Settlement Gage Sensors
Tactile Pressure Sensors
Tactile pressure sensors may be attached to the container boundaries to measure the transient and residual soil pressure at the soil-container interface. Such sensors were used by RPI for LEAP-GWU-2015. The RPI group used tactile pressure sensors manufactured by Tekscan, Inc. (Cambridge, MA). The sensors need conditioning, equilibration, and calibration. Since the sensors are not waterproof, they were laminated, prepared for the specific soil-container interfaces, and then installed in the model container prior to the model construction (Figs. 3.3.1 and 3.3.2). The preparation procedure is described in detail in El Ganainy et al. (2014) and Kokkali et al. (2018).
1.5 Cone Penetration Testing
A new cone has been designed with a top load cell and no custom strain gauges at the cone tip as indicated in Fig. 1.11. The drawings are in a separate file in the shared box.com folder. Estimated cost for fabrication of this cone at UCD is approximately $1800 which includes a $500 load cell.
Thus if the test is done at 1/20 scale and μ∗ = 20, the velocity of penetration would be 5 mm/s, model scale. The data reported includes time, force, and actuator displacement in model units and qc vs depth below ground surface in prototype units. The tip resistance should be reported at depth intervals of 1 cm or less, prototype scale.
1.6 Shear Wave Velocity
Shear wave velocity measurements should be made before and after every destructive shake. About half of the test sites are expected to retrieve shear wave velocity data in some LEAP-2017 experiments.
1.7 Ground Motions
1.7.1 Destructive Ground Motions
For the first destructive motion, we will target a ramped sine wave very similar to that used for LEAP-GWU-2015. In that exercise we learned that many facilities shakers introduce high-frequency noise superimposed on the smooth ramped sine wave motion as shown, for example, in Figs. 1.14 and 1.15. Figure 1.16 shows that the velocity time series is much less affected by the high frequency that is so apparent in the acceleration time series of Fig. 1.14. Studies performed since LEAP-GWU-2015 indicate that the higher-frequency components have some but relatively less effect on the behavior of the model. To account for the reduced effect of high frequency, we have a working hypothesis that the effective PGA is
For the second destructive motion, pending additional discussion, discretion is given to the researchers at the test site. If, for example, the target for Destructive Motion #1 was missed in the first destructive motion, then Destructive Motion #2 would attempt to more accurately hit or to bracket the target. Another option for Destructive Motion #2 would be to attempt to impose the target motion for Destructive Motion #1 in a subsequent test for purposes of calibrating the shaking equipment. This would be a “practice” to enable more accurate performance in the subsequent test, and would also allow comparison of performance with different shaking histories. Other possibilities for Destructive Motion #2 are to try to mimic more realistic ground motions, to intentionally introduce noise in the input motion to explore the validity of Eq. 1.2. Some researchers may also precisely repeat the first destructive motion to more rigorously evaluate the evolution of the behavior of the model due to the previous shaking event. Others may decide to vary the centrifuge acceleration, thereby increasing the prototype depth of the model, prior to the second destructive shaking event.
The experiment should be terminated before deformations are catastrophic because this will allow meaningful interpretation of post-test sensor location measurements, photographs of colored sand, surface marker locations, and spaghetti noodle markers.
Duplicate an achieved input motions of another LEAP site.
Investigate effects of higher predominant frequency input motion.
Investigate stress level effect by imposing higher/lower centrifuge g-level (different prototype layer thickness).
Compare response in tapered sine to response in realistic earthquakes.
1.7.2 Nondestructive Ground Motions
The small amplitude 1 Hz motions used as nondestructive motions in the 2015 LEAP were found to be not very useful for characterizing the stiffness of the soils. It is possible that a high-frequency motion (with wavelengths comparable to the model thickness will be more useful). If Vs = 200 m/s and the wavelength is λ = 4 H (first mode resonance), the required frequency would be 12.5 Hz in prototype scale for the 4-m-deep deposit. It may be possible for the existing shakers to produce a very small vibration at this frequency Hz; if so, this could be useful for characterizing the soil between shakes. Caution must be exercised to avoid damaging or changing the state of stress of the model by shaking it too intensely. Banded white noise in the frequency range 6–20 Hz (or higher) prototype scale could be useful. RPI may use a Ricker wavelet to produce a short pulse containing these frequencies (RPI now in trials with practice test).
1.7.3 Assessment of Tapered Sine Wave (TSW) Ground Motions
1.8 Data Reporting Anticipated Plan/Concept
A specification for centrifuge test data will be detailed in a separate document; this will include templates for data submission and methods of uploading and sharing data.
1.8.1 New Leap Database
Results from LEAP-GWU-2015 were archived in a NEEShub Database. As part of this project, the database may be migrated to the new tools developed by the US NHERI DesignSafe Cyber Infrastructure Center. Experimenters should expect to fill out an excel workbook template for each model tested. The excel workbook data template will be similar to that used for LEAP-GWU-2015. In the meantime, we will share information using shared folders on box.com. Participating researchers should contact Bruce Kutter and Trevor Carey if they do not yet have access the LEAP-UCD-2017 Box.com files.
1.8.2 Dynamic Shaking Sensor Data
The dynamic sensor data must be recorded and reported at greater than 50 Hz prototype scale. Ideally, the data acquisition rate should be 200 samples per second (prototype time).
Acceleration data during shaking events should be reported in two formats: (1) as absolute acceleration without any baseline correction and (2) absolute acceleration with baseline correction. Baseline correction parameters must be reported. A spreadsheet template will be provided.
1.8.3 Pore Pressure Long-Term Time Series Data
Residual pore pressure changes from before and after the earthquakes will be used as explained above to determine the residual settlement of the pore pressure transducers during the earthquakes. A separate tab in the spreadsheet workbook will be used to report the sensor data (at approximately one sample per second) obtained during the entire spin. Alternatively, Residual Pore Pressure Averages (RPPAs) will be recorded and reported at several stages of the experiment as described above.
1.8.4 Summary of Other Anticipated Report Requirements to Be Detailed in a Separate Document
Sensor locations and marker locations.
Description of the saturation process and documentation.
Density calculations, photograph, and/or drawing of tools used to measure mass and volume of model.
Quality control checks (grain size analysis and emax, emin).
Description of any deviations from the specifications.
Dimensions, mass, and part numbers of sensors.
Description of how the model surface was curved.
Photographic record of the top view of the surface markers at every stage of the test, and table of surface marker locations at various stages of the test.
Photographic record of dissection and post-test sensor location measurement, and table showing locations of sensors, colored sand layers, and noodles before and after the testing.
Results of inspection of the model and description of any special features of the deformed model surface, e.g., presence of sand boils, cracks at the boundaries, non-symmetric deformation, etc.
Signal processing (analog filters, baseline correction, other corrections) to sensor data. If custom software is used, provide the software if possible.
The experimental work on LEAP-UCD-2017 was supported by different funds depending mainly on the location of the work. The work by the US PIs (Manzari, Kutter, and Zeghal) is funded by National Science Foundation grants: CMMI 1635524, CMMI 1635307, and CMMI 1635040. The work at Ehime U. was supported by JSPS KAKENHI Grant Number 17H00846. The work at Kyoto U. was supported by JSPS KAKENHI Grant Numbers 26282103, 5420502, and 17H00846. The work at Kansai U. was supported by JSPS KAKENHI Grant Number 17H00846. The work at Zhejiang University was supported by the National Natural Science Foundation of China, Nos. 51578501 and 51778573; Zhejiang Provincial Natural Science Foundation of China, LR15E080001; and National Basic Research Program of China (973 Project), 2014CB047005. The work at KAIST was part of a project titled “Development of performance-based seismic design,” funded by the Ministry of Oceans and Fisheries, Korea. The work at NCU was supported by MOST: 106-2628-E-008-004-MY3.
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