LEAP-UCD-2017 Centrifuge Test at IFSTTAR
In the framework of the LEAP 2017 exercise, two dynamic centrifuge tests on a gentle slope of saturated Ottawa-F64 have been performed at the IFSTTAR centrifuge. These tests were conducted in parallel with other tests performed in nine other centrifuge centers. The objectives were to compare the experimental results, e.g., effect of the experimental procedure or of test parameters on the results, and to provide a database for numerical modeling. In this framework, all the tests were performed on the same prototype geometry and the first base shaking was a 1 Hz ramped sine with an effective amplitude of 0.15 g at the prototype scale. Among some of the tests performed in the various centrifuge facilities, several parameters were modified, such as the density and the second input motion, in order to study their impact on the slope behavior when it is subjected to base shaking. This paper details the procedure followed at the IFSTTAR center for the buildup of dense sand and medium loose sand models and the deviations from the specifications provided by the leaders of the LEAP-UCD-2017 program. The main deviations are, for both tests, the absence of the measurement of the saturation degree and, for the first, the characterization of the soil properties with CPT test and bender element measurements during the test. Some results of the tests performed are briefly presented such as the acceleration, pore pressure buildup, and the deformation of the slope surface.
KeywordsLiquefaction Sand Gentle slope Centrifuge test Dynamic base shaking
Actual researches in numerical modeling on liquefaction phenomena, for instance, advanced numerical techniques based on multiscale approach in large deformation (Callari et al. 2010), highlight the need of experimental database for the calibration and the validation processes. In this framework one of the objectives of the LEAP-UCD-2017 research program is to provide high-quality laboratory and centrifuge test data. A total of ten centrifuge teams were involved in this experimental research work.
Following the model specifications (Kutter et al. 2019), each team has performed a series of dynamic tests on a gentle slope of saturated Ottawa F-65 sand. The objectives of the specifications were to minimize the discrepancies between the experimental procedures followed in each centrifuge team in order to evaluate the quality of liquefaction centrifuge tests and the effects of procedure deviations on the obtained results through cross testing. In addition to this repeatability step, additional tests with different densities and with different second and eventually third base shaking were performed.
Two tests were performed at the IFSTTAR center with the same base shakings but with different densities. In this paper, the procedure and some of the results obtained on the dense sand model, S02, and on the medium loose sand model, S03, are presented.
15.2 As Built Model
15.2.1 Soil Material and Placement of the Sand by Pluviation
The soil selected for the LEAP-UCD-2017 centrifuge experiments is Ottawa F-65, a clean sand with less than 0.5% of fines. Control quality checks on the grain characteristics (granulometry and maximum and minimum densities of the sand delivered) were performed following the model specifications established for this project. The maximum and minimum densities were measured based on the modified Lade et al. (1998) method and the modified ASTM D4254 method, respectively. In addition, the French NF P94–059 procedure was used for both. The average values for the maximum densities are 1737 and 1775 kg/m3 for the NFP and the modified Lade et al. method, respectively. The average values for the minimum densities are 1509 and 1472 kg/m3 for the NFP and the modified ASTM methods, respectively. From the LEAP-UCD-2017 Version 1.0 model specifications, the average values of all the tests that have been performed with different methods are 1475 and 1756 kg/m3 for the minimum and the maximum densities, respectively. Due to the dispersion that has been observed, the values obtained by IFSTTAR are in the range of the previous ones (data are available in Kutter et al. 2019).
The Ottawa sand was dry pluviated through a sieve attached to the bottom of the hopper of the IFSTTAR dry pluviation system (Ternet 1999). The initial target densities for S02 and S03 tests at the IFSTTAR center were 1703 and 1651 kg/m3, respectively. Due to the lack of data concerning medium dense sand, the target density for the S03 test has been increased to 1599 kg/m3. To obtain these request densities the specifications concerning the opening size of the sieve were not followed (e.g., for S02 a sieve with an opening of 1.2 mm and for the flow restriction the recommended design was three slots of 1.2 mm width with a center to center spacing of 26 mm).
First, due the standard sieves in France, the selected sieve for the calibration has an opening of 1.25 mm. In the case of the dense sand, following the recommended design for the slots, a first air pluviation test was performed but the sand flow stopped rapidly. As suggested in the specification document, the clogging effect could be due to the humidity of the room that was measured at 65%. Consequently, the geometry of the open part was modified. The width of the slots was increased up to 6 mm that corresponds to three-mesh width of the sieve and the center to center distance between the opening slots was 22 mm. Furthermore, due to the container width (i.e., internal width 200 mm) the slot opening was 240 mm long. Finally, a drop height of 50 cm and a horizontal velocity of 48 mm/s of the hopper were selected to obtain a density of 1696 ± 4.22 kg/m3 (three measurements).
In the case of the medium loose sand, the minimum density that can be obtained with the pluviation system was 1624 ± 3.8 kg/m3. It has been obtained with two slots of 2.9 cm width that correspond to 15 meshes and a center to center distance of 4.8 cm.
During the calibration procedure of the pluviation, the measurement of the density of the sand was performed using cylindrical density boxes (approximate volume 390 mm3). Once the pluviation was made the top surface of the density box was leveled using a specific tool to avoid unintentional densification of the sand.
15.2.2 Rigid Container Configuration and Sensor Layout
Due to the shaker configuration, this rigid container was fixed through 12 screws into an ESB. In addition, the shearing movement at the extremity of the ESB container was blocked by four rods (one at each corner, Fig. 15.2). Before the first test, a series of drives was performed in order to obtain the time histories of the commands to the servo valves of the shaker (i.e., drives) that correspond to the request base shakings. This preliminary test that was performed on the two-container assembly highlighted the presence of non-negligible high frequencies not far from the predominant one (i.e., 50 Hz at the model scale). Consequently, a sufficient amount of sand was put in place between both containers to remove these high frequencies that should be due to resonance phenomena of the embarked system (Fig. 15.2).
The relative rigidity of cables of the pore pressure transducers (Druck PDCR81) induces some problems for their positioning during the sand pluviation. In order to facilitate their positioning and limit the length of cables in the sand mass, cable glands were inserted on one of the lateral side of the container (Fig. 15.2). This configuration enables the placement of the pore pressure sensors inside the container with the request length of cable before the starting of the pluviation. Sensors were taped up on the lateral side of the container and put in place once the request level of sand was pluviated.
The required accelerometers and pore pressure sensors (AH1 to AH4, AH12, AH11, AV1, AV2, P1 to P4, and P9 and P10) were all put in place during the pluviation. In order to facilitate the positioning of the sensors, the pluviation has also been calibrated to drop off a layer of approximately 1 cm thick each back and forth movement of the hopper, which is the model distance between each level of sensors of the central array (AH1 to AH4 and P1 to P4). In addition, two other accelerometers and pore pressure sensors (S02, P5 and P7; S03, P6 and P8) were installed to complete the instrumentation.
The location in the coordinate system defined in Fig. 15.3 was measured during placement and during excavation after the test. The difference between as-built and “after test” coordinates of the sensors embedded in soil could be partly due to errors, such as inaccurate measurement that was performed using a steel rule, inadvertent tugging on wires after placement or during excavation, and displacement of the sensors relative to the liquefiable soil. These location data are analyzed in the Sect. 15.4.2. of this paper and displacement vectors are provided.
A pair of bender element has also been installed. The pair of bender elements is the same as the fixed one presented by Brandenberg et al. (2006). However due to problem of amplification, the measurement of the shear wave velocity has not been performed in the model S02. In addition, as requested in the specifications no CPT test was performed during this test. These two main deviations from the model specifications do not enable the characterization of the soil properties for the S02 models. In the case of the S03 test performed on medium loose sand, both CPT tests and bender element measurements have been performed.
The shear wave velocity measurements were made just before and after each base shaking. For each soil state, 6 series of 36 measurements were made (2 series of measurement for the 3 selected input frequencies 5, 7.5, and 10 kHz). The use of several frequencies and the staking of data increase the reliability of the determined values. The CPT device used was developed and built at the University of California, Davis, and had an external diameter of 6 mm at the model scale. CPT measurements were made to characterize the initial state and the soil and the state after each base shaking. Due to the experimental configuration, the CPT was removed from the centrifuge before each shaking.
In the case of the IFSTTAR 1D shaker the direction of the solicitation is parallel to the axis of the centrifuge (Chazelas et al. 2008). In order to keep constant the radius between the surface of the soil in a transverse cross section and the center of rotation of the centrifuge, the surface should have a curved shape in the direction perpendicular to the base shaking. The distance between the axis of rotation of the centrifuge and the center of the soil surface is 5.063 m. Considering that the inner dimension of the container’s width is 0.2 m, the difference in height between the midpoint and the corresponding point at the lateral side should be 1 mm. As this value is in the range of precision of the leveling of the surface, the soil surface was not curved in the Y direction.
In addition to the sensors, surface markers were placed in a grid pattern at the surface of the soil to measure the displacement during soil liquefaction. The diameter of the surface markers used was two times smaller than the recommended design (improved design with an external diameter of 13 mm at the model scale). The location of the markers in the X and Y directions was performed with a steel rule with an estimated precision of 1 mm and the Z location was performed with a laser sensor. The use of a laser sensor to measure the soil surface required a partial immersion of sensor (at least 10 mm) and consequently the water level was between 45 and 43 mm above the top of the slope (Fig. 15.3). The precision of the Z position is smaller than 0.5 mm as requested in the specifications. The surface markers have been put in place before the saturation process and their location has been measured at 1 g before the first spin up of the centrifuge and after each base shaking (Motion#1 and Motion#2) once the centrifuge was spun down.
15.2.3 Viscosity of Pore Fluid
The viscous fluid used for the test is a mixture of tap water, HMPC (Culminal MHPC50), and biocide that is added in order to avoid decrease of the viscosity with time. A series of viscosity measurements was performed on several mixtures with different amounts of HPMC (from 28 to 23.5 g per liter).
15.2.4 Saturation Process
The saturation process was performed under vacuum at 1 g. Prior to the de-air fluid flow into the soil model, the rigid box was closed using an airtight lid. The container, the fluid tank, and the various connections were de-aired following the recommendations of Kutter (2013). Two cycles of vacuum/CO2 were performed. Each cycle consists of, first, the application of a vacuum up to an absolute pressure of 100 to 94 mbar followed by flooding of CO2 up to the atmospheric pressure. By this way, it is supposed that 99% of the air resting inside the container has been replaced by CO2. After these two cycles, an absolute pressure of 94 mbar was applied and maintained during the fluid flow into the container. The fluid was dripped onto the surface near the slope tip (a sponge was put in place above the soil surface to prevent perturbations). The drip rate was not measured but once a pool was formed at the slop tip, the fluid flow was adjusted by hand to maintain the pool. The time request for saturation was around 10 h which corresponds roughly to a drip flow of 6.5 ml/min. The degree of saturation was not checked with the method proposed by Okamura and Inoue (2012) at the end of the saturation process.
One nonconformity took place during the saturation process in the case of the S02 test. Some air leakage was observed on the lateral side where the cable gland for pore pressure sensors P1 to P4 was located. However, the few air bubbles that appear concerned only a small localized zone of about 2–3 cm2 at the surface.
15.3 Achieved Ground Motions
As previously mentioned the tests were performed at 50 g. The angular velocity (93.8 RPM) of the centrifuge was calculated to produce the 50 g centrifuge acceleration at a radius of 5.09 m (the radius to 1/3 depth, i.e., 26.7 mm at the model scale, below the soil surface at the central array of accelerometers).
15.3.1 Horizontal Component
The sampling frequency selected for the tests was 10,240 Hz except in the case of Motion#1 of the S03 test that was only 1024 Hz. The analysis of the frequency content of the base shaking for all the base shakings highlights three high-frequency components that are non-negligible especially in the case of Motion#1: 7 Hz, 9 Hz, and 16.6 Hz components. It should be noticed that due to the sampling frequency used for Motion#1 of the S03 test, only the 7 Hz and the 9 Hz components were recorded. The 16.6 Hz component corresponds to the dither associated to the servo valve of the hydraulic shaker and cannot be removed. However the velocity and the displacement induced by this high frequency are negligible due to the 16.6 Hz frequency value compared to the 1 Hz component.
Characteristics of the measured base motions for S02 and S03 tests
PGA 1 Hz component (g)
% Due to high frequencya
PGV 1 Hz component (m/s)
% Due to high frequenciesa
Frequency of the first harmonic
15.3.2 Vertical Component
From the frequency analysis of Motion#1 of test S02, a non-negligible part of the vertical acceleration is due to components with a frequency equal of higher than 7 Hz (up to around 50 Hz). As a consequence, as previously mentioned, due to an error in the selection of the sampling frequency (1024 Hz) for Motion#1 of test S03, the acceleration data are only relevant to frequency components up to about 10 Hz at the prototype scale and consequently the maximum vertical acceleration represents only 5% of the maximum horizontal acceleration against 71% in the case of Motion#1 of test S02 (the percentage 71% is based on the value of only one accelerometer).
For Motion#2 the maximum vertical acceleration represents 66.5 and 74%, respectively, of the maximum horizontal acceleration. Even if the level of vertical acceleration is non-negligible compared to that of the horizontal acceleration applied at the base of the container, due to the frequency content of the vertical acceleration, the vertical displacements are largely lower than the horizontal displacement. For Motion#1 of tests S02 and S03 the maximum dynamic vertical displacements are 4% and 3.4%, respectively, of the maximum dynamic horizontal displacement. In the case of Motion#2 the values are 8 and 3%, respectively.
The time representation of the dynamic vertical displacement for Motion#2 of S03 test illustrates an opposition phase between AV1 and AV2 that characterized a rotation of the container. In the case of Motion#1 both vertical accelerometers are in phase opposition but the displacement amplitude varies differently with time. In both cases the rotation is limited (it is estimated around 3.10−4° and 8.10−3° for Motion#1 and Motion#2, respectively).
In the following all the data are given at the prototype scale otherwise mentioned.
15.4.1 Pore Pressure and Acceleration Responses
In this example, considering the initial position of the pore pressure sensors measured during the buildup of the model S03 and the level of the water table above the soil surface, the maximum pore pressure buildup, considering that there were no waves, was around 29.4, 47.1, 5.8, and 13.2 kPa for the P10, P0, P8, and P6 pore pressure sensors, respectively. These values are lower than the maximum pore pressure buildup measured during Motion#2 of the S03 test (i.e., 56.6, 63.6, 13.6, and 19 kPa for the P10, P9, P8, and P6 sensors, respectively). In addition, the variation of the pore pressure measured by sensors P9 and P10 is phase opposite such as for the sensors P8 and P6. Even if further analysis is requested, the amplitude of the pore pressure and the phase opposition suggest that one part of the pore pressure fluctuations recorded by these four sensors is due to the waves. This analysis suggests that a wave reduction system should be built for future tests to avoid non-negligible effect of waves near the extremities of a rigid container.
In the case of the central array of pore pressure sensors, the shape of the pore pressure buildup such as the maximum value suggests that the effect of the wave is limited in the middle of the container.
In the case of Motion#2 the ratio of the excess of pore pressure over the initial vertical effective stress reached 1 for all the reliable recorded pore pressure (Fig. 15.10). Compared to the case of the P4 sensor for Motion#1, sharper peaks of the pore pressure appeared for both levels of measurement. The variation of the pore pressure reached −4.9 and −30.9 kPa for the P3 (z = 2.2 m) and the P4 (z = 1.05 m) sensors, respectively. Taking into account the initial vertical effective stress at the location of the pore pressure sensor P4 (i.e., 10.9 kPa), a negative vertical effective stress is reached on very short periods at shallow depth. Due to the main objective of this descriptive paper, a deeper analysis of this observed phenomena will be provided in another paper.
The peaks that appeared in the pore pressure recordings (Fig. 15.11) are in accordance with the sharp peaks of horizontal acceleration that appear for all the accelerations measured except for the AH1 that is the acceleration at 3.5 m depth. It should be supposed that the phenomena of dilatancy occurred up to 2.5–3.5 m depth.
In the case of test S03 (medium loose sand, ρd = 1624 kg/m3) the pore pressure ratio for Motion#1 reached 88%, 100%, and 100% for P2, P3, and P4, respectively. The pressure buildup is more rapid than for the dense sand. Furthermore, the pressure decreases more slowly than for the dense sand.
Contrary to the dense sand, acceleration spikes appear at all the levels whatever the base shaking applied. However, the amplitude of the spike is generally larger and in the two directions (positive and negative) when they appear in dense sand. Furthermore, in the case of the medium loose sand subjected to large base shaking (S03 Motion#2), the number of spikes decreases with the decreasing of the depth.
15.4.2 Surface Maker Response
Based on the surface markers, in the case of the S02 test, the settlement does not exceed 10 cm in the upper part and -2 cm near the toe of the slope. In addition, the maximum average horizontal displacement is about 10 cm and was reached in the middle of the slope. For the medium loose sand, the displacements are largely higher with a maximum settlement of 47 cm in the upper part of the slope and −17.5 cm near the toe of the slope. The maximum horizontal displacement reaches 97 cm and is also located in the middle of the slope.
This paper summarized the buildup and some results of the two centrifuge tests performed at IFSTTAR in the framework of the LEAP-UCD-2017 series of tests.
Two centrifuge tests were performed by IFSTTAR; the tests were done on a dense and a medium loose Ottawa-F65 sand. The main nonconformities of the experiment buildup were the absence of the measurement of saturation degree for both tests and the measurement of the soil characteristics through CPT tests and bender element measurements.
Despite all care taken during the determination of the drives, non-negligible high-frequency components were recorded for Motion#1. Motion#2 with higher amplitude was less perturbed by high-frequency components.
For the two motions the recording of pore pressure sensors highlighted sharp pore pressure peak that illustrated dilatancy phenomena. The phenomena were more pronounced in dense sand and with increasing the PGAeff. The dense Ottawa sand and in a more limited way the medium loose sand should have reached under the test conditions the phase transformation line.
The density has a large effect on the observed sensors and marker displacement; in the case of the dense sand the displacement is limited contrary to the case of the medium loose sand.
This experiment has been made in the framework of the LEAP-UCD-2017 series of experiments. The authors would like to thank all the participating centrifuge teams that share their knowledge.
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