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Performance of piles with different configurations subjected to slope deformation induced by seismic liquefaction
Journal Pre-proof Performance of piles with different configurations subjected to slope deformation induced by seismic liquefaction Wuwei Mao, Bangan Liu, Rouzbeh Rasouli, Shogo Aoyama, Ikuo Towhata
PII:
S0013-7952(19)31357-2
DOI:
https://doi.org/10.1016/j.enggeo.2019.105355
Reference:
ENGEO 105355
To appear in: Received Date:
12 July 2019
Revised Date:
15 October 2019
Accepted Date:
19 October 2019
Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Performance of piles with different configurations subjected to slope deformation induced by seismic liquefaction Wuwei Maoa,b, Bangan Liu*c, Rouzbeh Rasoulid, Shogo Aoyamae, Ikuo Towhataf a
Department of Geotechnical Engineering, College of Civil Engineering, Tongji University,
Shanghai 200092, China b
Key Laboratory of Geotechnical and Underground Engineering, Ministry of Education, Tongji
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University, Shanghai 200092, China Email: [email protected] c*
China State Construction Engineering Corporation, Beijing 100000, China
Jacobs, Toronto, Ontario M2J 1R3, Canada
e
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Email:[email protected]
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Email: [email protected]
Kiso - Jiban Consultants Co., Ltd., Tokyo, Japan
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E-mail: [email protected]
Department of Civil Engineering, University of Tokyo, Tokyo, Japan
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E-mail: [email protected]
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Highlights
A series of shaking table tests were conducted for investigation of pile performance in a sloping ground subjected to seismic liquefaction.
Abundant information regarding the characteristics of excess water generation, pile bending moment and slope deformation were obtained.
Protection of both front row and rear row pile was suggested for effective reduction of pile 1
bending moment in engineering practice.
Abstract Liquefaction-induced lateral ground deformation has been a common observation during many past earthquakes. Piles are widely used in liquefaction-prone areas, and their performance under such circumstances has become an important issue for researchers and engineers. Previous studies on this
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issue have focused primarily on piles located behind quay walls or in level ground conditions, which have considerably different boundary conditions compared to those of an inclined sloping ground condition that is more common for waterfront structures. This study presents a series of shaking
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table tests modeling the performance of piles subjected to liquefaction-induced slope deformation.
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Different pile configurations, including single piles, piles aligned in one row parallel to the direction
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of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3×3 configuration, were tested. Several aspects regarding the behaviors of excess generation of pore
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water pressure, pile bending moment and lateral slope deformation under different pile configurations were revealed and discussed. The results obtained in this study can be used for
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validation of related numerical approaches, or as benchmarks that can further facilitate performancebased pile designs in liquefaction-prone areas.
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Keywords: Shaking table test; liquefaction; lateral flow; group pile; bending moment;
1 Introduction It is well known that when an earthquake occurs, strong ground motion not only directly damages facilities and structures but also causes serious geological disasters, such as liquefaction, collapse, 2
landslides and mudslides. Among them, seismic-induced ground liquefaction often causes serious damage to structures. Fully liquefied soil will completely lose its strength and result in a fluidized type of large deformation, such as lateral spreading and flow slides. Considerable large deformation induced by seismic liquefaction has been extensively reported during past earthquakes (Hamada et al. 1987, Yuan et al. 2004, Kanıbir et al. 2006, Huang and Jiang 2010, and Towhata et al. 2014), which can cut the service of infrastructures and lifeline systems. In cases of sloping ground, such
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damage could be intensified and lead to even more catastrophic consequences.
Observations from field earthquake surveys have shown that seismic liquefaction is one of the main causes of foundation damage in liquefaction-prone areas. Therefore, piles are widely used for
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buildings and structures to improve the bearing capacities of engineering sites. In the past few
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decades, liquefaction-induced large deformation and its impact on piles have been studied by many
al. 2018, Kheradi et al. 2019).
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researchers through laboratory and field tests (Dobry et al. 2003, Ashour and Helal 2017, Zhang et
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Fundamental issues associated with the performance of piles subjected to seismic liquefaction include pore water pressure generation, lateral ground deformation, bending moment and lateral
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forces exerted on piles. General findings from single-pile studies suggested that the deformation of the pile is much smaller than that of the ground in the cases of lateral spreading or flow sliding, and
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the bending moment in the piles is basically controlled by the lateral soil pressure (Horikoshi et al. 1998, Dobry et al. 2003, Su et al. 2016). Tokimatsu and Suzuki (2004) investigated the pore water pressure generation around the pile and emphasized the difference in local soil conditions regarding the compression and extension side of the pile. In practice, piles usually function in the form of a group, and accordingly, group pile behavior in 3
liquefiable ground has been a topic of great interest. In general, the group pile effect leads to the reduction in the total resistance relative to the superposition of the single pile capacity. Imamura et al. (2004) concluded from centrifuge experiments that when the pile spacing is more than 3 to 4 times the pile diameter, there are no interactions among the piles. Rollins et al. (2015a, b) performed lateral load tests on a full scale pile group with different spacings and found that the group pile effects decreased considerably as the pile spacing increased from 3.3 to 5.65 times the pile diameter.
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It should be noted that the number of piles used in the previous studies was relatively small (e.g.,
1×2, 1×3, 2×2, and 3×3) and with limited testing cases. Therefore, further investigation concerning the relevant issue is required.
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Apart from the configuration of the piles, it should be noted that many studies have focused on the
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behavior of piles behind quay walls. Ashford et al. (2006) reported that the rear-row piles (near the
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quay wall) carried a larger load (bending moment) than those of the front-row piles. Such behavior was reproduced by other researchers when testing group pile behavior behind quay walls (Motamed
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and Towhata 2009, 2010). Several studies on group piles with a level ground model, however, obtained completely different results that the front-row piles are subjected to the largest load
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(McVay et al. 1998, Imamura et al. 2004, Rollins et al. 2015a, b). During a level ground model test, the lateral spreading or flow sliding of the ground is usually modeled by lateral loading of the piles.
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Such implementations emphasize that the boundary conditions of the piles are considerably different from those of piles installed behind quay walls, as would be the corresponding deformation and stress properties. For piles located in an inclined sloping ground, the problem becomes different again. Motamed et al. (2010) showed that both front-row piles and the trailing-row piles carried larger loads than those of the inner-row piles. Haeri et al. (2012) reported a shadow effect to explain 4
the lower soil pressure exerted on downslope piles in an inclined sloping ground test. They aimed to reveal the neighboring pile effects on lateral soil pressure, but the number of piles was limited (maximum of 3 piles aligned in one row). Takahashi et al. (2016) compared the efficacy of ground improvement with regular and irregular pile arrangements. To date, there has been a lack of experimental study on the response of piles installed in inclined sloping ground. In particular, the seismic performances of piles with different configurations under such situations have scarcely been
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investigated. There is an ongoing demand to clarify these issues in the practice of engineering design. The main objective of this study is to examine the response of piles located in an inclined slope
subjected to liquefaction-induced slope deformation. A series of shaking table tests were performed
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by considering piles of different configurations, and a sloping ground was made to model the
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waterfront structure that is a common situation as observed in past earthquake investigations.
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Aspects including pore water pressure generation, pile bending moment, and lateral slope deformation are revealed, and several issues regarding group pile behavior are discussed.
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2 Experimental details
2.1 Shaking table device and scaling laws
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The tests presented in this study were performed using the 1-G shaking table at the geotechnical laboratory of the University of Tokyo. The shaking table has dimensions of 3m×2m, biaxial shaking
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direction, a maximum loading capacity of 7 tons, a frequency range of 0~50 Hz, a maximum displacement of ±200 mm and a maximum acceleration of 1000 Gal. The stress level in a 1-G shaking table condition is much lower than that in reality. Therefore, the scaling laws should be considered when the results are intended to be interpreted on prototype conditions. Table 1 lists the general scaling laws for different parameters as suggested by Iai et al. 5
(2005) with a scaling factor of 20. Note that the measured experimental values are presented in the subsequent descriptions. Hence, prototype quantities can be estimated based on the scaling laws as presented in Table 1. Meanwhile, other laws of scaling can also be considered for the prediction of real foundations. 2.2 General arrangements of the experiment Fig. 1 shows the general layout of the experimental setup. The model ground was prepared in a soil
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container with 2.65 m in length, 0.6 m in height and 0.4 m in width. Shock absorbers were attached on two sides of the soil container to reduce wave reflection due to the rigid boundaries. Acrylic tubes were used as the model piles and were fixed on the plate with preset screw holes so that
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different configurations can be realized by connecting the piles to different positions (Fig. 2a, b).
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The pile end was closed to avoid the intrusion of water or sands, and the strain gauges were pasted
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on the surface of the pile to measure the bending moment. The basic properties of the model piles are an outer diameter of 32 mm, an inner diameter of 27 mm, and an EI (the flexural rigidity) of
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68.54 N·m2.
Pore water pressure (PWP) sensors were used to measure the excess pore water pressure generation
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and dissipation during the tests. All the PWP sensors were attached to the piles at designed locations and secured with tape as shown in Fig. 2c. The PWP sensors were set in pairs at each level with
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respect to the upstream and downstream of the pile shaft. It is important to saturate the PWP sensor and remove any air bubbles before it is installed to ensure measurement accuracy. Accelerometers were used to measure the input motion at the shaking table and on the soil container. Inclinometers were installed at two different locations (the upstream and downstream sides of the piles) to measure the displacement of the soil at different heights. 6
2.3 Materials Toyoura sand was used for model ground preparation. The physical and mechanical properties of Toyoura sand have been extensively tested in past studies (Koseki et al. 2005, Yang and Sze 2011). The main properties of Toyoura sand are as follows: the specific gravity is 2.65, the mean particle diameter is 0.21 mm, the maximum void ratio is 0.97 and the minimum void ratio is 0.62. The model ground was constructed using the water sedimentation method. That is, a certain depth
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of deaerated water was first filled in the soil container (10 cm above the ground surface in this study); then, the sand was poured into the water slowly layer by layer; the water level was gradually raised
in this process to maintain constant submerge depth. By repeating the above procedure, the whole
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sample was finally completed. The relative density of the model ground prepared with this method
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was measured to be approximately 30%. In 1-G shaking table tests, a reduced ground density is
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usually used to compensate for lower stress conditions (Towhata 2008). The relative density of the experiment should be added by 20% to give the equivalent prototype relative density, i.e.,
2.4 Input motion
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approximately 50% of the relative density in the prototype was modeled in this study.
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A sine wave with a frequency of 10 Hz and a duration of 25 s was used as the input motion for all the tests performed in this study. A rising and decaying period was designed for the first 7 s and the
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last 7 s respectively, as shown in Fig.3. The frequency of the input wave should be scaled by a factor of N-0.75 in 1-G shaking table tests as suggested by Iai et al. (2005), which approximately corresponds to a 1 Hz and 0.3 g real earthquake wave for N=20 in prototype. 2.5 Test conditions Based on the general experimental details described above, a series of shaking table tests were 7
conducted with different pile configurations, including the cases of a single pile, piles aligned in one row parallel to the direction of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3×3 configuration. The details of the tested cases are summarized in Table 2. 3 General results of the single-pile measurements 3.1 Time histories of various parameters
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A single-pile case is presented as the representative test for illustration of the general results obtained from the experimental measurements. A schematic view of the layout and instrumentation of the
single-pile test is shown in Fig.4. PWP transducers were attached on the two sides of the pile at 14
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different locations, divided into 7 pairs at 5 cm intervals. Fig.5 shows the time histories of the excess
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pore water pressure. A general observation is that the excess pore water pressure increased almost
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simultaneously at different positions immediately after the input acceleration started increasing. The excess PWP climbed rapidly to the peak values and was followed by two types of evolution regimes.
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One regime is for sensors at shallower depths, demonstrating a decrease in the excess PWP at first and increasing again to the stable peak that was maintained constant. The other regime showed no
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significant decrease after the first peak value was reached, which was mainly observed for deeper sensors.
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The dynamic bending moments derived from the calibrated strain gauges were recorded with a time interval of 0.001 s, as shown in Fig. 6 (a). Three stages of the bending moment evolution can be roughly distinguished from the time histories at different pile locations. A rapid increasing period is observed at the beginning of shaking, which lasted for approximately 4 s, followed by the decreasing period lasting for approximately 7 s, and the cyclic fluctuation period lasted for approximately 10 s. 8
To remove the effect of the cyclic component, the monotonic component of the bending moment was calculated by averaging adjacent data points every 1 s. Typical results of the monotonic component of the bending moment are shown in Fig. 6 (b). The monotonic component of the piles provides direct insight into the ground behavior. As shown in Fig. 6 (b), the monotonic component of the bending moment was close to zero after the recording time was over 20 s. At this point in the time history, the sloping ground was under stable shaking, as seen from the time history of the input
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motion in Fig. 3. It is suggested that fully liquefied ground could barely apply any load on the piles,
and only the cyclic component remained due to compulsive displacement under the external seismic loading.
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The lateral slope deformation was obtained by two inclinometers installed upstream and
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downstream of the pile. Fig. 7 shows the time histories of the velocity of slope deformation near the
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ground surface. This corresponded well with the time histories of the excess PWP, as shown in Fig. 5. The peak velocity, which measured approximately 2 cm/s upstream, occurred at approximately
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the first peak of the PWP curves. The time histories of the lateral slope deformation at different depths are summarized in Fig.8. In general, the shallower layer had a larger amount of displacement
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than that of the deeper layer, and the downstream sloped ground had a larger deformation than that upstream. The effect of different pile configurations on the pile behavior is further discussed in the
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following sections.
3.2 Stress state around piles The external load exerted on the piles is closely related to the stress state in the adjacent soils, which is largely controlled by the generation of excess pore water pressure. Fig. 9 shows the example of the time histories of the lateral soil velocity and the pile bending moment compared with the excess 9
pore water pressure in a single-pile test. The deformation of the slope and the generation of excess pore water pressure were almost synchronized. Based on the effective stress theory in soil mechanics, an increasing in the PWP resulted in the loss of the shear strength of the soils and consequently led to slope deformation. The deforming slope exerted external force on the piles, which was measured by the strain gauges. The three stages of the pile bending moment, i.e., rapid increasing, decreasing and cyclic fluctuation demonstrated a close correlation with the time histories of the excess pore
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water generation, as shown in Fig. 9 (b). The maximum bending moment appeared at the very beginning of the shaking, and then it started to drop quickly. After the ground was fully liquefied,
the monotonic component of the bending moment was reduced to almost zero, and only the cyclic
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component of the bending moment was captured. It is therefore suggested that for piles in liquefiable
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soil force on the structure begins to decrease.
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ground, the most critical period is close to the onset of liquefaction, after which the applied lateral
It is also worth noting that a sudden decrease is observed in some of the PWP curves after reaching
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the first peak point. This phenomenon is more evident at shallower depths, as shown in the time histories of the PWP in Fig. 5. From a general point of view, the sudden decrease in the PWP might
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be due to the large deformation caused by soil flow after shaking, which would lead to local stress release due to the loss of the adjacent soil and consequently a reduced PWP. The PWP decrease may
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also be attributed to the different stress conditions for the soils located upstream and downstream of the piles. The deforming velocity of the liquefied ground is much larger than that of the piles. Because of the obstruction of the pile foundation, the soils located upstream of the piles could be blocked. Consequently, the soils located upstream of piles are in the condition of compression, while the soils located downstream of piles are in an extension state, as shown in Fig. 10. This caused 10
different stress states for the soils located upstream and downstream. For the downstream soils, the PWP on the extension side decreased due to the loss of the adjacent local soils in the case of flow deformation. The details of the generation of excess pore water pressure are further displayed in Fig. 11. Four time intervals, i.e., 10~11 s, 12~13 s, 20~21 s, 24~25 s, are enlarged and compared with the input acceleration. The downstream and upstream pore water pressure transducers showed different
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phases of change. At the beginning of shaking, i.e., 10~11 s and 12~13 s, the upstream and
downstream excess pore water pressures are in phase with each other. In contrast, after the soil was
fully liquefied, i.e., 20~21 s and 24~25 s, the upstream and downstream excess pore water pressures
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are out of phase with each other. The existence of the piles changed the local boundary conditions
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of the adjacent soils.
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4 Discussion on the effect of different pile configurations 4.1 Bending moment of the piles
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The effect of the pile configuration on the bending moment of the piles is summarized in Fig. 12. For the piles aligned in one row parallel to the direction of the slope, the maximum bending moment
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for the front pile was similar regardless of different pile spacing. However, the piles located in the inner side of the pile group generally carried lower maximum bending moments than those of the
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front- and rear-row piles. For the center piles, narrower pile spacing led to a more significant reduction in the maximum bending moment. Such a group pile effect disappeared when the pile spacing was 8D. Nevertheless, it should be noted that all these values are significantly lower than that of the single-pile condition, which had a maximum bending moment of 619 N·cm. This observation is consistent with results obtained in a full-scale group pile test performed by Rollins et 11
al. (2005), showing that the group pile leads to a reduction in the lateral load relative to the singlepile condition. For the piles aligned in one row perpendicular to the direction of the slope, the maximum bending moments of the inner piles were lower than those of the side piles when the pile spacing was 2D, as shown in Fig. 13. When the spacing was increased to 2.5D, the center pile showed lower values than those of the nearby piles. However, the two side piles had lower bending moments than those of the
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inner piles. This might be caused by the boundary effect since the distance of the side piles to the
side wall was 1.25D in Test-7. For the 3×3 group pile cases, the condition of the front-row piles was
similar to that of the one-row cases, in which the piles were directly affected by the soil flow,
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therefore, the results of the front-row piles in Test-8 and Test-9 (pile spacing 4D and 3.8D,
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respectively) are also plotted in Fig. 13. For both cases, the difference between the center pile and
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the side piles was not obvious. It is suggested that, in the perpendicular direction, the group pile effect was not evident when the pile spacing was larger than 3.8D. Imamura et al. (2004) concluded
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from centrifuge experiments that the group pile effect is not evident when the pile spacing is more than 3D to 4D, which is similar to the results of the current study. Due to the limited width of the
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soil tank, larger spacing for piles aligned in one row perpendicular to the slope direction was not further explored in this study.
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Pile groups are more commonly used in practical projects by combining several rows and columns of piles together. In this study, piles in the form of the 3×3 pile group were further tested in Test-8, Test-9 and Test-10. Fig. 14 shows the time histories of the bending moments (monotonic component) for the 3×3 group piles. It shows that the front-row piles and the rear-row piles carried much higher bending moments than those of the inner piles. Similar to the piles aligned in one row parallel to the 12
slope direction, the inner piles seem to be protected by the side piles. To further confirm this issue, additional tests were performed by installing 4 protection piles in front of a 3×3 pile group. The time histories of the bending moment are also plotted in Fig. 14. Fig. 15 summarizes the maximum bending moment on piles at different locations with and without the installation of protection piles, and the extent of the reduction for different rows is summarized in Fig. 16. The average maximum bending moment of the front piles decreased dramatically (82.9%) after the installation of the
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protection piles. A similar reduction was also observed in the middle-row piles but with a smaller extent (40.4%). For the piles located in the rear row, the reduction effect was not significant. It is
therefore suggested that for multirow pile groups, both the front-row and the rear-row piles are in
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the more critical conditions than the inner piles and should both be protected if possible.
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4.2 Lateral soil displacement under different pile configurations
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The lateral slope displacement was captured by the inclinometers installed at the upstream and downstream sides. Fig. 17 summarizes the slope deformation under different conditions.
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Displacement information at 5 depth positions was obtained upstream and at 4 depth positions downstream. In general, the surface layers have a greater amount of displacement than that of the
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deeper layers for all the tested cases. The residual displacement near the slope surface was all normalized by the displacement obtained by the upstream inclinometer of the single-pile case, as
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shown in Fig. 17 (c). Although it is more commonly observed that the testing cases with multipiles had smaller displacement values, there is no obvious regularity in the effect of the different pile configurations. The average residual displacement measured upstream is 10.2 cm and that measured downstream is 9.5 cm. The 3×3 pile group in Test-9 resulted in minimum lateral displacement as recorded on both the upstream and downstream sides. However, such an outcome was not 13
reproduced in the other two cases using the 3×3 pile group, i.e., Test-8 and Test-10. It is therefore suggested that the installation of piles in liquefiable ground has a limited effect on the residual displacement mitigation of nearby ground caused by seismic liquefaction. In the concept of performance-based design for slopes, displacement is often used as the key parameter for the evaluation of slope performance (Conte and Troncone 2018). The results from this study indicate
therefore, additional countermeasures should be implemented.
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that the installed piles have an insufficient effect on the deformation mitigation of nearby ground,
The mechanical properties of liquefied soils are rate dependent, which can be observed by
comparing the time history of the bending moment and lateral soil velocity. As shown in Fig. 18,
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the piles at different locations demonstrated close relationships with the velocity of the soil around
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the piles. Larger soil velocities corresponded to higher values of the bending moment. The
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increasing or decreasing slope deforming velocity also corresponded to an increasing or decreasing bending moment on the piles, respectively. When the ground was fully liquefied, the ground could
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barely bear any force considering its liquid nature, and consequently, the monotonic bending moment decreased to zero regardless of the different pile locations.
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5 Conclusions
A series of shaking table tests were conducted to investigate the seismic performance of piles with
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different configurations subjected to liquefaction-induced slope deformation. The various pile configurations considered were single piles, piles aligned in one row parallel to the direction of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3×3 configuration. Based on the experimental measurements, the following conclusions were drawn. (1) The response of the piles is largely controlled by the generation of excess pore water pressure in 14
the adjacent soils. Almost synchronized behavior among the excess pore water pressure generation, slope deformation velocity and the monotonic component of the bending moment was observed. Three stages of the bending moment evolution can be distinguished from recorded time histories, i.e., the rapid increasing period, the decreasing period and the cyclic fluctuation period. The most critical period is the very beginning of shaking when the bending moment and the slope deformation velocity reached their maximum, after which they began to decrease.
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(2) For piles aligned in one row parallel to the direction of slope, the maximum bending moment
for the front pile was similar regardless of different pile spacing. However, the piles located in the
inner side of the pile group generally carried lower bending moments than those of the front and
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rear-row piles. For center piles, narrower pile spacing showed a more significant reduction in the
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maximum bending moment. The group pile effect disappeared when the pile spacing was 8D.
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(3) For piles aligned in one row perpendicular to the direction of the slope, the maximum bending moments of the inner piles could be lower than those of the side piles. This group pile effect was
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not evident when the pile spacing was 3.8D.
(4) For multirow group piles (3×3), the front-row piles and the rear-row piles carried much higher
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bending moments than those of the middle-row piles. Installing protection piles on the upstream side of the front piles led to the reduction in the average maximum bending moment by 82.9% and
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40.4% for front pile and the middle-row pile, respectively, while reduction was insignificant for the rear-row pile. It is suggested that the protection of both the front and the rear piles would be more preferable in engineering design practice. (5) The deformation of the slope occurred once the shaking initiated, and the peak velocity of the deformation corresponded to the first peak of the PWP curves. The shallower layer had a larger 15
extent of displacement than that of the deeper layer. The rate dependency of slope deformation can be observed by comparing the bending moment and soil velocity time history, which suggests that the liquefied soil acting on the pile behaves like a viscous fluid. In general, lateral deformation of the slope on both the upstream and the downstream sides of the piles did not vary significantly with respect to different pile configurations.
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Conflict of interest The authors declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Acknowledgements
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This work was supported by the National Key R&D Program of China (grant no. 2017YFC1501304),
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and Shanghai Education Commission (Peak Discipline Construction Program, Grant No. 0200121005/052 & 2019010206).
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Su, L., Tang, L., Ling, X., Liu, C., Zhang, X. 2016. Pile response to liquefaction-induced lateral spreading: a shake-table investigation. Soil Dyn. Earthq. Eng. 82, 196-204.
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Takahashi, H., Takahashi, N., Morikawa, Y., Towhata, I., Takano, D. 2016. Efficacy of pile-type improvement against lateral flow of liquefied ground. Géotechnique, 66(8), 617-626.
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Tokimatsu, K., Suzuki, H. 2004. Pore water pressure response around pile and its effects on py behavior during soil liquefaction. Soils Found. 44(6), 101-110.
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Towhata, I. 2008. Geotechnical earthquake engineering. Springer Science & Business Media. Towhata, I., Maruyama, S., Kasuda, K. I., Koseki, J., Wakamatsu, K., Kiku, H., Kiyota, T., Yasuda, S., Taguchi, Y., Aoyama, S., Hayashida, T. 2014. Liquefaction in the Kanto region during the 2011 off the pacific coast of Tohoku earthquake. Soils Found. 54(4), 859-873. Yang, J., Sze, H. Y. 2011. Cyclic behaviour and resistance of saturated sand under non-symmetrical 18
loading conditions. Géotechnique. 61(1), 59-73. Yuan, H., Yang, S. H., Andrus, R. D., Juang, C. H. 2004. Liquefaction-induced ground failure: a study of the Chi-Chi earthquake cases. Eng. Geol. 71(1-2), 141-155. Zhang, X., Tang, L., Ling, X., Chan, A. H. C., Lu, J. 2018). Using peak ground velocity to characterize the response of soil-pile system in liquefying ground. Eng. Geol. 240, 62-73.
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Fig. 1 Schematic layout of the experimental setup.
Fig. 2 (a) Acrylic pile pasted with strain gauges; (b) piles fixed on plate and (c) pore water pressure sensors.
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Fig. 3 Input earthquake motion recorded at the base of the shaking table.
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Fig. 4 Test setup and instrumentation for a single-pile case.
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Fig. 5 Time history of excess pore water pressure for a single-pile case: (a) upstream and (b) downstream.
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Fig. 6 Time history of the pile bending moment for the single-pile case: (a) dynamic bending moment and (b) monotonic bending moment. Note: the strain gauges were pasted on the pile at a
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50 mm intervals, and the deepest gauge was 100 mm from the bottom. Fig. 7 Time history of slope deformation velocity near the surface: single-pile case.
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Fig. 8 Time history of slope deformation of a single-pile case: (a) upstream and (b) downstream. Fig. 9 Comparison of PWP, slope displacement velocity and bending moment of a single-pile test. Fig. 10 Stress conditions of the adjacent soils around the pile. Fig. 11 Excess pore water pressure of upstream and downstream at a depth of 150 mm in case 1. (a) 10~11 s; (b) 12~13 s; (c) 20~21 s and (d) 24~25 s 19
Fig. 12 Maximum monotonic bending moment distribution for piles aligned in one row parallel to the slope direction. Data retrieved from approximately 100 mm from the bottom. Fig. 13 Maximum monotonic bending moment distribution for piles aligned perpendicular to the slope direction. Data retrieved from approximately 100 mm from the bottom. Fig. 14 Time histories of bending moments (monotonic component) for 3×3 group piles with and without protection piles
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Fig. 15 Maximum bending moment (monotonic component) at different pile locations: (a) without protection piles and (b) with protection piles
Fig. 16 Reduction of maximum monotonic bending moment at different pile locations
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and (c) normalized displacement near the slope surface
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Fig. 17 Summary of slope deformation under different conditions: (a) upstream; (b) downstream
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Fig. 18 Relation of lateral soil velocity and bending moment at different pile positions. Data were retrieved from Test-9. PWP data were retrieved from the PWP sensor located approximately 300
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mm from the bottom, and the bending moment data were retrieved from strain gauges located
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approximately 100 mm from the bottom.
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Direction of shaking Fixing plate
600 mm
30% Toyoura sand
Model pile
2650 mm
(c)
(a)
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(b)
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Fig. 1 Schematic layout of the experimental setup.
Acceleration, g
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Fig. 2 (a) Acrylic pile pasted with strain gauges; (b) piles fixed on plate and (c) pore water pressure sensors.
Base input motion
0.4 0.2 0
-0.2 -0.4
5
10
15
20 25 Time, s
30
35
Fig. 3 Input earthquake motion recorded at the base of the shaking table. 21
Side view
Inclinometer C
DN1 5cm DN2 5cm DN3 5cm DN4 5cm DN5 5cm DN6 5cm DN7 5cm
5% Toyoura sand slope
UP1 UP2 UP3 UP4 UP5 UP6 UP7
500 mm
367 mm
Water level
Inclinometer A
2650 mm
400 mm
Top view
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Slope displacement PWP sensor
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Acc.
0
Excess pore water pressure of Downstream (kPa)
10 15 20 25 30 35 40 45 50 UP1
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5
UP2
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UP3
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0 0.81 0.54 0.27 0.00 1.23 0.82 0.41 0.00 1.68 1.12 0.56 0.00 2.22 1.48 0.74 0.00 2.52 1.68 0.84 0.00 2.94 1.96 0.98 0.00 3.3 2.2 1.1 0.0 0
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Excess pore water pressure of Upstream (kPa)
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Fig. 4 Test setup and instrumentation for a single-pile case.
5
UP4
UP5
UP6
UP7
10 15 20 25 30 35 40 45 50
Time (s)
5
0.9 0.6 0.3 0.0
10 15 20 25 30 35 40 45 50 DN1 Acc.
1.17 0.78 0.39 0.00 1.68 1.12 0.56 0.00
DN2
2.01 1.34 0.67 0.00 2.46 1.64 0.82 0.00 2.91 1.94 0.97 0.00 3 2 1 0 0
DN4
DN3
DN5
DN6
DN7
5
10 15 20 25 30 35 40 45 50
Time (s)
(b)
(a)
Fig. 5 Time history of excess pore water pressure for a single-pile case: (a) upstream and (b) downstream.
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0
5
10
15
20
25
30
35
40
45
50
BM1
BM2
BM3
BM4
BM5
BM6
BM7
BM8
BM9
BM10
10
15
20
25
30
35
40
45
50
0 0.0 -0.5 -1.0 -1.5 2.88 1.92 0.96 0.00 23.7 15.8 7.9 0.0 72 48 24 0 138 92 46 0 225 150 75 0 330 220 110 0 420 280 140 0 510 340 170 0 570 380 190 0 0
5
10
15
20
25
30
35
40
45
50
mBM1 mBM2
mBM3
mBM4
mBM5
mBM6
mBM7
mBM8
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5
Monotonic bending moment (N·cm)
Bending moment (N·cm)
0 6.4 3.2 0.0 -3.2 11.2 5.6 0.0 -5.6 36 18 0 114 76 38 0 195 130 65 0 291 194 97 0 420 280 140 0 510 340 170 0 690 460 230 0 810 540 270 0
mBM9
mBM10
5
10
15
20
25
30
35
40
45
50
Time (s)
Time (s)
(b)
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(a)
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Fig. 6 Time history of the pile bending moment for the single-pile case: (a) dynamic bending moment and (b) monotonic bending moment. Note: the strain gauges were pasted on the pile at a 50 mm intervals, and the deepest gauge was 100 mm from the bottom.
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Downstream Upstream
1.5
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1.0 0.5 0.0
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Relative soil velocity (cm/s)
2.0
5
10
15
20
25
30
35
40
45
Time (s)
Fig. 7 Time history of slope deformation velocity near the surface: single-pile case.
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12
20 A5 A4 A3 A2 A1
8 6
A5 C4 C3 C2 C1
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Displacement (cm)
Displacement (cm)
10
4 2
12
0
8 4 0
0
10
20
30
40
50
0
10
20
30
Time (s)
Time (s)
(a)
(b)
40
50
0.0
-0.5
0.5
-1.0 PWP
0.0 0
10
-1.5
20 30 Time (s)
40
(a)
-2.0 50
800
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1.0
PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm
1.0
0.8 0.6
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Relative lateral velocity of sand upstream 0.5 depth=100 mm
Excess pore water perssure (kPa)
1.0
Relative lateral velocity (cm/s)
1.5
1.5
1.2
0.4
600 400
Mainly cyclic component
200
0.2
0
0.0
-0.2
Bending moment at bottom
0
10
Bending moment (N·cm)
2.0 Relative lateral velocity of sand at downstream depth=100 mm
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Excess pore water perssure (kPa)
2.0
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Fig. 8 Time history of slope deformation of a single-pile case: (a) upstream and (b) downstream.
Monotonic component of bending moment
20 30 Time (s) (b)
40
-200 50
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Fig. 9 Comparison of PWP, slope displacement velocity and bending moment of a single-pile test.
Downstream
Upstream
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Pile deflection
Soil deformation
Soil deformation
Extension (-)
Compression (+) PWP
Fig. 10 Stress conditions of the adjacent soils around the pile.
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0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
1.2
-0.2 11
Time (s) PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm Acceleration history
1.0
(c)
0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
20
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
0.6 0.4 0.2 0.0 -0.2 -0.4 12
Excess pore water perssure (kPa)
-0.2 10
0.8
1.2
(d)
1.0 0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
-0.2
-0.2
-0.4
-0.4
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Time (s)
13
Time (s) PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm Acceleration history
1.0
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
Acceleraction (g)
0.8
(b)
Acceleraction (g)
0.8
PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm Acceleration history
1.0
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1.0 Acceleraction (g)
1.0
1.2
Excess pore water perssure (kPa)
1.2
PWP at Upstream, depth=150 mm (a) PWP at Downstream, depth=150 mm Acceleration history
Acceleraction (g)
Excess pore water perssure (kPa) Excess pore water perssure (kPa)
1.2
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Time (s)
-0.6 25
500
9×1, 2D 5×1, 4D 3×1, 6D Single pile 3×1, 8D case
Rear Piles
Front
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600
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700
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400
Deforming direction
300 200
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100
1D
0
Rear
Center Pile location
Front
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Max monotonic bending moment (N·cm)
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Fig. 11 Excess pore water pressure of upstream and downstream at a depth of 150 mm in case 1. (a) 10~11 s; (b) 12~13 s; (c) 20~21 s and (d) 24~25 s
Fig. 12 Maximum monotonic bending moment distribution for piles aligned in one row parallel to the slope direction. Data retrieved from approximately 100 mm from the bottom.
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Max monotonic bending moment (N·cm)
400
Side Piles Deforming Center direction
1×5, 2D 1×5, 2.5D 3×3, 4D 3×3, 3.8D
350 300
Side 250 200 150
1D Side
Side
Center Pile location
-p re Monotonic bending moment (N·cm)
200 180
P7 P7, protected P8 P8, protected P9 P9, protected
Front row
160 140
lP
120 100 80 60 40 20 0 5
10
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Monotonic bending moment (N·cm)
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Fig. 13 Maximum monotonic bending moment distribution for piles aligned perpendicular to the slope direction. Data retrieved from approximately 100 mm from the bottom.
15
20
25
30
35
80
Middle row
P4 P4, protected P5 P5, protected P6 P6, protected
60
40
20
0 5
10
15
Time (s)
Rear row
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120 100 80
P1 P1, protected P2 P2, protected P3 P3, protected
60
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Monotonic bending moment (N·cm)
140
20
25
30
35
Time (s)
40 20
Rear
Middle
Front
1
4
7
2
5
8
3
6
9
Protection piles
0
5
10
15
20
25
30
Deforming direction
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Time (s)
Fig. 14 Time histories of bending moments (monotonic component) for 3×3 group piles with and without protection piles
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lP
200
With front protection Without front protection
180 160
Average change with protection piles
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140
120
82.9%
3.13%
100
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80 60 40
40.4%
20
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Max monotonic bending moment (N·cm)
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Fig. 15 Maximum bending moment (monotonic component) at different pile locations: (a) without protection piles and (b) with protection piles
0
Rear
Center Pile location
Front
Fig. 16 Reduction of maximum monotonic bending moment at different pile locations
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Single-pile
50
Single-pile
40
Upstream
35 Test10 Test9 Test8 Test7 Test6 Test5 Test4 Test3 Test2 Test1
35 30 25 20 15 10 5 40
30
Test10 Test9 Test8 Test7 Test6 Test5 Test4 Test3 Test2 Test1
30 25 20 15 10 5
20
10
0
40
Residual lateral displacement (cm)
Test-1 Test-3 Test-5 Test-7 Test-9
3 Normalized displacement
40
Distance from bottom (cm)
Distance from bottom (cm)
45
Downstream
30
20
10
0
Residual lateral displacement (cm)
(a)
(b)
Test-2 Test-4 Test-6 Test-8 Test-10
2 Single-pile
1
Downstream
Upstream
Location of measurement (c)
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Fig. 17 Summary of slope deformation under different conditions: (a) upstream; (b) downstream and (c) normalized displacement near the slope surface
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Front row 200 100
Middle row
lP
Bending moment (N·cm)
0 42 21 0 -21
Rear row
144
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72
0.75 0.50
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0.25 0.00
1.5 1.0 0.5
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Lateral soil velocity Excess PWP (kPa) (cm/s)
0 -72
0.0
-0.5
0
5
10
15
20
25
30
35
40
45
50
Time (s)
Fig. 18 Relation of lateral soil velocity and bending moment at different pile positions. Data were retrieved from Test-9. PWP data were retrieved from the PWP sensor located approximately 300 mm from the bottom, and the bending moment data were retrieved from strain gauges located approximately 100 mm from the bottom.
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List of Table Captions： Table 1 Scaling laws for different parameters in shaking table modeling Table 2 Summary of test conditions
Table 1 Scaling laws for different parameters in shaking table modeling
N 1 N N1.5 N0.75 N-0.75 N0.75 1 N4.5
Table 2 Summary of test conditions
(Prototype
/
Scaling factors in this study (Prototype / Model) 20 1 20 89.4 9.5 0.1 9.5 1 35,777
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Scale Mass density Stress and pressure Displacement Shaking time Frequency Velocity Acceleration EI of pile
factors
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Scaling Model)
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Quality items
Spacing
Pile configuration
Relative density (%)
Amplitude (Gal)
Frequency (Hz)
Duration (s)
Test-1 Test-2 Test-3 Test-4 Test-5 Test-6 Test-7 Test-8 Test-9 Test-10
/ 2D 4D 6D 8D 2D 2.5D 4D×4D 3.8D×3.8D 3.8D×3.8D
Single 9×1 5×1 3×1 3×1 1×5 1×5 3×3 3×3 3×3
30 30 30 30 30 30 30 30 30 30
300 300 300 300 300 300 300 300 300 300
10 10 10 10 10 10 10 10 10 10
25 25 25 25 25 25 25 25 25 25
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Test case
29
PII:
S0013-7952(19)31357-2
DOI:
https://doi.org/10.1016/j.enggeo.2019.105355
Reference:
ENGEO 105355
To appear in: Received Date:
12 July 2019
Revised Date:
15 October 2019
Accepted Date:
19 October 2019
Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Performance of piles with different configurations subjected to slope deformation induced by seismic liquefaction Wuwei Maoa,b, Bangan Liu*c, Rouzbeh Rasoulid, Shogo Aoyamae, Ikuo Towhataf a
Department of Geotechnical Engineering, College of Civil Engineering, Tongji University,
Shanghai 200092, China b
Key Laboratory of Geotechnical and Underground Engineering, Ministry of Education, Tongji
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University, Shanghai 200092, China Email: [email protected] c*
China State Construction Engineering Corporation, Beijing 100000, China
Jacobs, Toronto, Ontario M2J 1R3, Canada
e
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Email:[email protected]
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d
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Email: [email protected]
Kiso - Jiban Consultants Co., Ltd., Tokyo, Japan
f
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E-mail: [email protected]
Department of Civil Engineering, University of Tokyo, Tokyo, Japan
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E-mail: [email protected]
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Highlights
A series of shaking table tests were conducted for investigation of pile performance in a sloping ground subjected to seismic liquefaction.
Abundant information regarding the characteristics of excess water generation, pile bending moment and slope deformation were obtained.
Protection of both front row and rear row pile was suggested for effective reduction of pile 1
bending moment in engineering practice.
Abstract Liquefaction-induced lateral ground deformation has been a common observation during many past earthquakes. Piles are widely used in liquefaction-prone areas, and their performance under such circumstances has become an important issue for researchers and engineers. Previous studies on this
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issue have focused primarily on piles located behind quay walls or in level ground conditions, which have considerably different boundary conditions compared to those of an inclined sloping ground condition that is more common for waterfront structures. This study presents a series of shaking
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table tests modeling the performance of piles subjected to liquefaction-induced slope deformation.
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Different pile configurations, including single piles, piles aligned in one row parallel to the direction
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of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3×3 configuration, were tested. Several aspects regarding the behaviors of excess generation of pore
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water pressure, pile bending moment and lateral slope deformation under different pile configurations were revealed and discussed. The results obtained in this study can be used for
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validation of related numerical approaches, or as benchmarks that can further facilitate performancebased pile designs in liquefaction-prone areas.
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Keywords: Shaking table test; liquefaction; lateral flow; group pile; bending moment;
1 Introduction It is well known that when an earthquake occurs, strong ground motion not only directly damages facilities and structures but also causes serious geological disasters, such as liquefaction, collapse, 2
landslides and mudslides. Among them, seismic-induced ground liquefaction often causes serious damage to structures. Fully liquefied soil will completely lose its strength and result in a fluidized type of large deformation, such as lateral spreading and flow slides. Considerable large deformation induced by seismic liquefaction has been extensively reported during past earthquakes (Hamada et al. 1987, Yuan et al. 2004, Kanıbir et al. 2006, Huang and Jiang 2010, and Towhata et al. 2014), which can cut the service of infrastructures and lifeline systems. In cases of sloping ground, such
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damage could be intensified and lead to even more catastrophic consequences.
Observations from field earthquake surveys have shown that seismic liquefaction is one of the main causes of foundation damage in liquefaction-prone areas. Therefore, piles are widely used for
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buildings and structures to improve the bearing capacities of engineering sites. In the past few
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decades, liquefaction-induced large deformation and its impact on piles have been studied by many
al. 2018, Kheradi et al. 2019).
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researchers through laboratory and field tests (Dobry et al. 2003, Ashour and Helal 2017, Zhang et
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Fundamental issues associated with the performance of piles subjected to seismic liquefaction include pore water pressure generation, lateral ground deformation, bending moment and lateral
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forces exerted on piles. General findings from single-pile studies suggested that the deformation of the pile is much smaller than that of the ground in the cases of lateral spreading or flow sliding, and
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the bending moment in the piles is basically controlled by the lateral soil pressure (Horikoshi et al. 1998, Dobry et al. 2003, Su et al. 2016). Tokimatsu and Suzuki (2004) investigated the pore water pressure generation around the pile and emphasized the difference in local soil conditions regarding the compression and extension side of the pile. In practice, piles usually function in the form of a group, and accordingly, group pile behavior in 3
liquefiable ground has been a topic of great interest. In general, the group pile effect leads to the reduction in the total resistance relative to the superposition of the single pile capacity. Imamura et al. (2004) concluded from centrifuge experiments that when the pile spacing is more than 3 to 4 times the pile diameter, there are no interactions among the piles. Rollins et al. (2015a, b) performed lateral load tests on a full scale pile group with different spacings and found that the group pile effects decreased considerably as the pile spacing increased from 3.3 to 5.65 times the pile diameter.
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It should be noted that the number of piles used in the previous studies was relatively small (e.g.,
1×2, 1×3, 2×2, and 3×3) and with limited testing cases. Therefore, further investigation concerning the relevant issue is required.
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Apart from the configuration of the piles, it should be noted that many studies have focused on the
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behavior of piles behind quay walls. Ashford et al. (2006) reported that the rear-row piles (near the
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quay wall) carried a larger load (bending moment) than those of the front-row piles. Such behavior was reproduced by other researchers when testing group pile behavior behind quay walls (Motamed
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and Towhata 2009, 2010). Several studies on group piles with a level ground model, however, obtained completely different results that the front-row piles are subjected to the largest load
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(McVay et al. 1998, Imamura et al. 2004, Rollins et al. 2015a, b). During a level ground model test, the lateral spreading or flow sliding of the ground is usually modeled by lateral loading of the piles.
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Such implementations emphasize that the boundary conditions of the piles are considerably different from those of piles installed behind quay walls, as would be the corresponding deformation and stress properties. For piles located in an inclined sloping ground, the problem becomes different again. Motamed et al. (2010) showed that both front-row piles and the trailing-row piles carried larger loads than those of the inner-row piles. Haeri et al. (2012) reported a shadow effect to explain 4
the lower soil pressure exerted on downslope piles in an inclined sloping ground test. They aimed to reveal the neighboring pile effects on lateral soil pressure, but the number of piles was limited (maximum of 3 piles aligned in one row). Takahashi et al. (2016) compared the efficacy of ground improvement with regular and irregular pile arrangements. To date, there has been a lack of experimental study on the response of piles installed in inclined sloping ground. In particular, the seismic performances of piles with different configurations under such situations have scarcely been
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investigated. There is an ongoing demand to clarify these issues in the practice of engineering design. The main objective of this study is to examine the response of piles located in an inclined slope
subjected to liquefaction-induced slope deformation. A series of shaking table tests were performed
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by considering piles of different configurations, and a sloping ground was made to model the
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waterfront structure that is a common situation as observed in past earthquake investigations.
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Aspects including pore water pressure generation, pile bending moment, and lateral slope deformation are revealed, and several issues regarding group pile behavior are discussed.
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2 Experimental details
2.1 Shaking table device and scaling laws
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The tests presented in this study were performed using the 1-G shaking table at the geotechnical laboratory of the University of Tokyo. The shaking table has dimensions of 3m×2m, biaxial shaking
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direction, a maximum loading capacity of 7 tons, a frequency range of 0~50 Hz, a maximum displacement of ±200 mm and a maximum acceleration of 1000 Gal. The stress level in a 1-G shaking table condition is much lower than that in reality. Therefore, the scaling laws should be considered when the results are intended to be interpreted on prototype conditions. Table 1 lists the general scaling laws for different parameters as suggested by Iai et al. 5
(2005) with a scaling factor of 20. Note that the measured experimental values are presented in the subsequent descriptions. Hence, prototype quantities can be estimated based on the scaling laws as presented in Table 1. Meanwhile, other laws of scaling can also be considered for the prediction of real foundations. 2.2 General arrangements of the experiment Fig. 1 shows the general layout of the experimental setup. The model ground was prepared in a soil
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container with 2.65 m in length, 0.6 m in height and 0.4 m in width. Shock absorbers were attached on two sides of the soil container to reduce wave reflection due to the rigid boundaries. Acrylic tubes were used as the model piles and were fixed on the plate with preset screw holes so that
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different configurations can be realized by connecting the piles to different positions (Fig. 2a, b).
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The pile end was closed to avoid the intrusion of water or sands, and the strain gauges were pasted
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on the surface of the pile to measure the bending moment. The basic properties of the model piles are an outer diameter of 32 mm, an inner diameter of 27 mm, and an EI (the flexural rigidity) of
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68.54 N·m2.
Pore water pressure (PWP) sensors were used to measure the excess pore water pressure generation
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and dissipation during the tests. All the PWP sensors were attached to the piles at designed locations and secured with tape as shown in Fig. 2c. The PWP sensors were set in pairs at each level with
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respect to the upstream and downstream of the pile shaft. It is important to saturate the PWP sensor and remove any air bubbles before it is installed to ensure measurement accuracy. Accelerometers were used to measure the input motion at the shaking table and on the soil container. Inclinometers were installed at two different locations (the upstream and downstream sides of the piles) to measure the displacement of the soil at different heights. 6
2.3 Materials Toyoura sand was used for model ground preparation. The physical and mechanical properties of Toyoura sand have been extensively tested in past studies (Koseki et al. 2005, Yang and Sze 2011). The main properties of Toyoura sand are as follows: the specific gravity is 2.65, the mean particle diameter is 0.21 mm, the maximum void ratio is 0.97 and the minimum void ratio is 0.62. The model ground was constructed using the water sedimentation method. That is, a certain depth
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of deaerated water was first filled in the soil container (10 cm above the ground surface in this study); then, the sand was poured into the water slowly layer by layer; the water level was gradually raised
in this process to maintain constant submerge depth. By repeating the above procedure, the whole
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sample was finally completed. The relative density of the model ground prepared with this method
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was measured to be approximately 30%. In 1-G shaking table tests, a reduced ground density is
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usually used to compensate for lower stress conditions (Towhata 2008). The relative density of the experiment should be added by 20% to give the equivalent prototype relative density, i.e.,
2.4 Input motion
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approximately 50% of the relative density in the prototype was modeled in this study.
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A sine wave with a frequency of 10 Hz and a duration of 25 s was used as the input motion for all the tests performed in this study. A rising and decaying period was designed for the first 7 s and the
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last 7 s respectively, as shown in Fig.3. The frequency of the input wave should be scaled by a factor of N-0.75 in 1-G shaking table tests as suggested by Iai et al. (2005), which approximately corresponds to a 1 Hz and 0.3 g real earthquake wave for N=20 in prototype. 2.5 Test conditions Based on the general experimental details described above, a series of shaking table tests were 7
conducted with different pile configurations, including the cases of a single pile, piles aligned in one row parallel to the direction of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3×3 configuration. The details of the tested cases are summarized in Table 2. 3 General results of the single-pile measurements 3.1 Time histories of various parameters
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A single-pile case is presented as the representative test for illustration of the general results obtained from the experimental measurements. A schematic view of the layout and instrumentation of the
single-pile test is shown in Fig.4. PWP transducers were attached on the two sides of the pile at 14
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different locations, divided into 7 pairs at 5 cm intervals. Fig.5 shows the time histories of the excess
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pore water pressure. A general observation is that the excess pore water pressure increased almost
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simultaneously at different positions immediately after the input acceleration started increasing. The excess PWP climbed rapidly to the peak values and was followed by two types of evolution regimes.
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One regime is for sensors at shallower depths, demonstrating a decrease in the excess PWP at first and increasing again to the stable peak that was maintained constant. The other regime showed no
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significant decrease after the first peak value was reached, which was mainly observed for deeper sensors.
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The dynamic bending moments derived from the calibrated strain gauges were recorded with a time interval of 0.001 s, as shown in Fig. 6 (a). Three stages of the bending moment evolution can be roughly distinguished from the time histories at different pile locations. A rapid increasing period is observed at the beginning of shaking, which lasted for approximately 4 s, followed by the decreasing period lasting for approximately 7 s, and the cyclic fluctuation period lasted for approximately 10 s. 8
To remove the effect of the cyclic component, the monotonic component of the bending moment was calculated by averaging adjacent data points every 1 s. Typical results of the monotonic component of the bending moment are shown in Fig. 6 (b). The monotonic component of the piles provides direct insight into the ground behavior. As shown in Fig. 6 (b), the monotonic component of the bending moment was close to zero after the recording time was over 20 s. At this point in the time history, the sloping ground was under stable shaking, as seen from the time history of the input
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motion in Fig. 3. It is suggested that fully liquefied ground could barely apply any load on the piles,
and only the cyclic component remained due to compulsive displacement under the external seismic loading.
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The lateral slope deformation was obtained by two inclinometers installed upstream and
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downstream of the pile. Fig. 7 shows the time histories of the velocity of slope deformation near the
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ground surface. This corresponded well with the time histories of the excess PWP, as shown in Fig. 5. The peak velocity, which measured approximately 2 cm/s upstream, occurred at approximately
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the first peak of the PWP curves. The time histories of the lateral slope deformation at different depths are summarized in Fig.8. In general, the shallower layer had a larger amount of displacement
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than that of the deeper layer, and the downstream sloped ground had a larger deformation than that upstream. The effect of different pile configurations on the pile behavior is further discussed in the
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following sections.
3.2 Stress state around piles The external load exerted on the piles is closely related to the stress state in the adjacent soils, which is largely controlled by the generation of excess pore water pressure. Fig. 9 shows the example of the time histories of the lateral soil velocity and the pile bending moment compared with the excess 9
pore water pressure in a single-pile test. The deformation of the slope and the generation of excess pore water pressure were almost synchronized. Based on the effective stress theory in soil mechanics, an increasing in the PWP resulted in the loss of the shear strength of the soils and consequently led to slope deformation. The deforming slope exerted external force on the piles, which was measured by the strain gauges. The three stages of the pile bending moment, i.e., rapid increasing, decreasing and cyclic fluctuation demonstrated a close correlation with the time histories of the excess pore
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water generation, as shown in Fig. 9 (b). The maximum bending moment appeared at the very beginning of the shaking, and then it started to drop quickly. After the ground was fully liquefied,
the monotonic component of the bending moment was reduced to almost zero, and only the cyclic
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component of the bending moment was captured. It is therefore suggested that for piles in liquefiable
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soil force on the structure begins to decrease.
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ground, the most critical period is close to the onset of liquefaction, after which the applied lateral
It is also worth noting that a sudden decrease is observed in some of the PWP curves after reaching
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the first peak point. This phenomenon is more evident at shallower depths, as shown in the time histories of the PWP in Fig. 5. From a general point of view, the sudden decrease in the PWP might
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be due to the large deformation caused by soil flow after shaking, which would lead to local stress release due to the loss of the adjacent soil and consequently a reduced PWP. The PWP decrease may
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also be attributed to the different stress conditions for the soils located upstream and downstream of the piles. The deforming velocity of the liquefied ground is much larger than that of the piles. Because of the obstruction of the pile foundation, the soils located upstream of the piles could be blocked. Consequently, the soils located upstream of piles are in the condition of compression, while the soils located downstream of piles are in an extension state, as shown in Fig. 10. This caused 10
different stress states for the soils located upstream and downstream. For the downstream soils, the PWP on the extension side decreased due to the loss of the adjacent local soils in the case of flow deformation. The details of the generation of excess pore water pressure are further displayed in Fig. 11. Four time intervals, i.e., 10~11 s, 12~13 s, 20~21 s, 24~25 s, are enlarged and compared with the input acceleration. The downstream and upstream pore water pressure transducers showed different
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phases of change. At the beginning of shaking, i.e., 10~11 s and 12~13 s, the upstream and
downstream excess pore water pressures are in phase with each other. In contrast, after the soil was
fully liquefied, i.e., 20~21 s and 24~25 s, the upstream and downstream excess pore water pressures
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are out of phase with each other. The existence of the piles changed the local boundary conditions
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of the adjacent soils.
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4 Discussion on the effect of different pile configurations 4.1 Bending moment of the piles
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The effect of the pile configuration on the bending moment of the piles is summarized in Fig. 12. For the piles aligned in one row parallel to the direction of the slope, the maximum bending moment
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for the front pile was similar regardless of different pile spacing. However, the piles located in the inner side of the pile group generally carried lower maximum bending moments than those of the
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front- and rear-row piles. For the center piles, narrower pile spacing led to a more significant reduction in the maximum bending moment. Such a group pile effect disappeared when the pile spacing was 8D. Nevertheless, it should be noted that all these values are significantly lower than that of the single-pile condition, which had a maximum bending moment of 619 N·cm. This observation is consistent with results obtained in a full-scale group pile test performed by Rollins et 11
al. (2005), showing that the group pile leads to a reduction in the lateral load relative to the singlepile condition. For the piles aligned in one row perpendicular to the direction of the slope, the maximum bending moments of the inner piles were lower than those of the side piles when the pile spacing was 2D, as shown in Fig. 13. When the spacing was increased to 2.5D, the center pile showed lower values than those of the nearby piles. However, the two side piles had lower bending moments than those of the
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inner piles. This might be caused by the boundary effect since the distance of the side piles to the
side wall was 1.25D in Test-7. For the 3×3 group pile cases, the condition of the front-row piles was
similar to that of the one-row cases, in which the piles were directly affected by the soil flow,
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therefore, the results of the front-row piles in Test-8 and Test-9 (pile spacing 4D and 3.8D,
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respectively) are also plotted in Fig. 13. For both cases, the difference between the center pile and
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the side piles was not obvious. It is suggested that, in the perpendicular direction, the group pile effect was not evident when the pile spacing was larger than 3.8D. Imamura et al. (2004) concluded
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from centrifuge experiments that the group pile effect is not evident when the pile spacing is more than 3D to 4D, which is similar to the results of the current study. Due to the limited width of the
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soil tank, larger spacing for piles aligned in one row perpendicular to the slope direction was not further explored in this study.
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Pile groups are more commonly used in practical projects by combining several rows and columns of piles together. In this study, piles in the form of the 3×3 pile group were further tested in Test-8, Test-9 and Test-10. Fig. 14 shows the time histories of the bending moments (monotonic component) for the 3×3 group piles. It shows that the front-row piles and the rear-row piles carried much higher bending moments than those of the inner piles. Similar to the piles aligned in one row parallel to the 12
slope direction, the inner piles seem to be protected by the side piles. To further confirm this issue, additional tests were performed by installing 4 protection piles in front of a 3×3 pile group. The time histories of the bending moment are also plotted in Fig. 14. Fig. 15 summarizes the maximum bending moment on piles at different locations with and without the installation of protection piles, and the extent of the reduction for different rows is summarized in Fig. 16. The average maximum bending moment of the front piles decreased dramatically (82.9%) after the installation of the
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protection piles. A similar reduction was also observed in the middle-row piles but with a smaller extent (40.4%). For the piles located in the rear row, the reduction effect was not significant. It is
therefore suggested that for multirow pile groups, both the front-row and the rear-row piles are in
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the more critical conditions than the inner piles and should both be protected if possible.
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4.2 Lateral soil displacement under different pile configurations
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The lateral slope displacement was captured by the inclinometers installed at the upstream and downstream sides. Fig. 17 summarizes the slope deformation under different conditions.
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Displacement information at 5 depth positions was obtained upstream and at 4 depth positions downstream. In general, the surface layers have a greater amount of displacement than that of the
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deeper layers for all the tested cases. The residual displacement near the slope surface was all normalized by the displacement obtained by the upstream inclinometer of the single-pile case, as
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shown in Fig. 17 (c). Although it is more commonly observed that the testing cases with multipiles had smaller displacement values, there is no obvious regularity in the effect of the different pile configurations. The average residual displacement measured upstream is 10.2 cm and that measured downstream is 9.5 cm. The 3×3 pile group in Test-9 resulted in minimum lateral displacement as recorded on both the upstream and downstream sides. However, such an outcome was not 13
reproduced in the other two cases using the 3×3 pile group, i.e., Test-8 and Test-10. It is therefore suggested that the installation of piles in liquefiable ground has a limited effect on the residual displacement mitigation of nearby ground caused by seismic liquefaction. In the concept of performance-based design for slopes, displacement is often used as the key parameter for the evaluation of slope performance (Conte and Troncone 2018). The results from this study indicate
therefore, additional countermeasures should be implemented.
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that the installed piles have an insufficient effect on the deformation mitigation of nearby ground,
The mechanical properties of liquefied soils are rate dependent, which can be observed by
comparing the time history of the bending moment and lateral soil velocity. As shown in Fig. 18,
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the piles at different locations demonstrated close relationships with the velocity of the soil around
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the piles. Larger soil velocities corresponded to higher values of the bending moment. The
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increasing or decreasing slope deforming velocity also corresponded to an increasing or decreasing bending moment on the piles, respectively. When the ground was fully liquefied, the ground could
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barely bear any force considering its liquid nature, and consequently, the monotonic bending moment decreased to zero regardless of the different pile locations.
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5 Conclusions
A series of shaking table tests were conducted to investigate the seismic performance of piles with
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different configurations subjected to liquefaction-induced slope deformation. The various pile configurations considered were single piles, piles aligned in one row parallel to the direction of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3×3 configuration. Based on the experimental measurements, the following conclusions were drawn. (1) The response of the piles is largely controlled by the generation of excess pore water pressure in 14
the adjacent soils. Almost synchronized behavior among the excess pore water pressure generation, slope deformation velocity and the monotonic component of the bending moment was observed. Three stages of the bending moment evolution can be distinguished from recorded time histories, i.e., the rapid increasing period, the decreasing period and the cyclic fluctuation period. The most critical period is the very beginning of shaking when the bending moment and the slope deformation velocity reached their maximum, after which they began to decrease.
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(2) For piles aligned in one row parallel to the direction of slope, the maximum bending moment
for the front pile was similar regardless of different pile spacing. However, the piles located in the
inner side of the pile group generally carried lower bending moments than those of the front and
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rear-row piles. For center piles, narrower pile spacing showed a more significant reduction in the
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maximum bending moment. The group pile effect disappeared when the pile spacing was 8D.
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(3) For piles aligned in one row perpendicular to the direction of the slope, the maximum bending moments of the inner piles could be lower than those of the side piles. This group pile effect was
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not evident when the pile spacing was 3.8D.
(4) For multirow group piles (3×3), the front-row piles and the rear-row piles carried much higher
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bending moments than those of the middle-row piles. Installing protection piles on the upstream side of the front piles led to the reduction in the average maximum bending moment by 82.9% and
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40.4% for front pile and the middle-row pile, respectively, while reduction was insignificant for the rear-row pile. It is suggested that the protection of both the front and the rear piles would be more preferable in engineering design practice. (5) The deformation of the slope occurred once the shaking initiated, and the peak velocity of the deformation corresponded to the first peak of the PWP curves. The shallower layer had a larger 15
extent of displacement than that of the deeper layer. The rate dependency of slope deformation can be observed by comparing the bending moment and soil velocity time history, which suggests that the liquefied soil acting on the pile behaves like a viscous fluid. In general, lateral deformation of the slope on both the upstream and the downstream sides of the piles did not vary significantly with respect to different pile configurations.
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Conflict of interest The authors declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Acknowledgements
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This work was supported by the National Key R&D Program of China (grant no. 2017YFC1501304),
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and Shanghai Education Commission (Peak Discipline Construction Program, Grant No. 0200121005/052 & 2019010206).
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Koseki, J., Yoshida, T., Sato, T. 2005. Liquefaction properties of Toyoura sand in cyclic tortional shear tests under low confining stress. Soils Found. 45(5), 103-113.
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subjected to lateral spreading. J. Geotech. Geoenviron. Eng. 136(3), 477-489. Motamed, R., Towhata, I. 2010. Mitigation measures for pile groups behind quay walls subjected to lateral flow of liquefied soil: Shake table model tests. Soil Dyn. Earthq. Eng. 30(10), 10431060. Motamed, R., Sesov, V., Towhata, I., Anh, N.T. 2010. Experimental modeling of large pile groups in sloping ground subjected to liquefaction-induced lateral flow: 1-G shaking table tests. Soils
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Fig. 1 Schematic layout of the experimental setup.
Fig. 2 (a) Acrylic pile pasted with strain gauges; (b) piles fixed on plate and (c) pore water pressure sensors.
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Fig. 3 Input earthquake motion recorded at the base of the shaking table.
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Fig. 4 Test setup and instrumentation for a single-pile case.
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Fig. 5 Time history of excess pore water pressure for a single-pile case: (a) upstream and (b) downstream.
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Fig. 6 Time history of the pile bending moment for the single-pile case: (a) dynamic bending moment and (b) monotonic bending moment. Note: the strain gauges were pasted on the pile at a
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50 mm intervals, and the deepest gauge was 100 mm from the bottom. Fig. 7 Time history of slope deformation velocity near the surface: single-pile case.
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Fig. 8 Time history of slope deformation of a single-pile case: (a) upstream and (b) downstream. Fig. 9 Comparison of PWP, slope displacement velocity and bending moment of a single-pile test. Fig. 10 Stress conditions of the adjacent soils around the pile. Fig. 11 Excess pore water pressure of upstream and downstream at a depth of 150 mm in case 1. (a) 10~11 s; (b) 12~13 s; (c) 20~21 s and (d) 24~25 s 19
Fig. 12 Maximum monotonic bending moment distribution for piles aligned in one row parallel to the slope direction. Data retrieved from approximately 100 mm from the bottom. Fig. 13 Maximum monotonic bending moment distribution for piles aligned perpendicular to the slope direction. Data retrieved from approximately 100 mm from the bottom. Fig. 14 Time histories of bending moments (monotonic component) for 3×3 group piles with and without protection piles
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Fig. 15 Maximum bending moment (monotonic component) at different pile locations: (a) without protection piles and (b) with protection piles
Fig. 16 Reduction of maximum monotonic bending moment at different pile locations
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and (c) normalized displacement near the slope surface
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Fig. 17 Summary of slope deformation under different conditions: (a) upstream; (b) downstream
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Fig. 18 Relation of lateral soil velocity and bending moment at different pile positions. Data were retrieved from Test-9. PWP data were retrieved from the PWP sensor located approximately 300
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mm from the bottom, and the bending moment data were retrieved from strain gauges located
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approximately 100 mm from the bottom.
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Direction of shaking Fixing plate
600 mm
30% Toyoura sand
Model pile
2650 mm
(c)
(a)
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(b)
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Fig. 1 Schematic layout of the experimental setup.
Acceleration, g
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Fig. 2 (a) Acrylic pile pasted with strain gauges; (b) piles fixed on plate and (c) pore water pressure sensors.
Base input motion
0.4 0.2 0
-0.2 -0.4
5
10
15
20 25 Time, s
30
35
Fig. 3 Input earthquake motion recorded at the base of the shaking table. 21
Side view
Inclinometer C
DN1 5cm DN2 5cm DN3 5cm DN4 5cm DN5 5cm DN6 5cm DN7 5cm
5% Toyoura sand slope
UP1 UP2 UP3 UP4 UP5 UP6 UP7
500 mm
367 mm
Water level
Inclinometer A
2650 mm
400 mm
Top view
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Slope displacement PWP sensor
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Acc.
0
Excess pore water pressure of Downstream (kPa)
10 15 20 25 30 35 40 45 50 UP1
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5
UP2
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UP3
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0 0.81 0.54 0.27 0.00 1.23 0.82 0.41 0.00 1.68 1.12 0.56 0.00 2.22 1.48 0.74 0.00 2.52 1.68 0.84 0.00 2.94 1.96 0.98 0.00 3.3 2.2 1.1 0.0 0
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Excess pore water pressure of Upstream (kPa)
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Fig. 4 Test setup and instrumentation for a single-pile case.
5
UP4
UP5
UP6
UP7
10 15 20 25 30 35 40 45 50
Time (s)
5
0.9 0.6 0.3 0.0
10 15 20 25 30 35 40 45 50 DN1 Acc.
1.17 0.78 0.39 0.00 1.68 1.12 0.56 0.00
DN2
2.01 1.34 0.67 0.00 2.46 1.64 0.82 0.00 2.91 1.94 0.97 0.00 3 2 1 0 0
DN4
DN3
DN5
DN6
DN7
5
10 15 20 25 30 35 40 45 50
Time (s)
(b)
(a)
Fig. 5 Time history of excess pore water pressure for a single-pile case: (a) upstream and (b) downstream.
22
0
5
10
15
20
25
30
35
40
45
50
BM1
BM2
BM3
BM4
BM5
BM6
BM7
BM8
BM9
BM10
10
15
20
25
30
35
40
45
50
0 0.0 -0.5 -1.0 -1.5 2.88 1.92 0.96 0.00 23.7 15.8 7.9 0.0 72 48 24 0 138 92 46 0 225 150 75 0 330 220 110 0 420 280 140 0 510 340 170 0 570 380 190 0 0
5
10
15
20
25
30
35
40
45
50
mBM1 mBM2
mBM3
mBM4
mBM5
mBM6
mBM7
mBM8
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5
Monotonic bending moment (N·cm)
Bending moment (N·cm)
0 6.4 3.2 0.0 -3.2 11.2 5.6 0.0 -5.6 36 18 0 114 76 38 0 195 130 65 0 291 194 97 0 420 280 140 0 510 340 170 0 690 460 230 0 810 540 270 0
mBM9
mBM10
5
10
15
20
25
30
35
40
45
50
Time (s)
Time (s)
(b)
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(a)
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Fig. 6 Time history of the pile bending moment for the single-pile case: (a) dynamic bending moment and (b) monotonic bending moment. Note: the strain gauges were pasted on the pile at a 50 mm intervals, and the deepest gauge was 100 mm from the bottom.
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Downstream Upstream
1.5
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1.0 0.5 0.0
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Relative soil velocity (cm/s)
2.0
5
10
15
20
25
30
35
40
45
Time (s)
Fig. 7 Time history of slope deformation velocity near the surface: single-pile case.
23
12
20 A5 A4 A3 A2 A1
8 6
A5 C4 C3 C2 C1
16
Displacement (cm)
Displacement (cm)
10
4 2
12
0
8 4 0
0
10
20
30
40
50
0
10
20
30
Time (s)
Time (s)
(a)
(b)
40
50
0.0
-0.5
0.5
-1.0 PWP
0.0 0
10
-1.5
20 30 Time (s)
40
(a)
-2.0 50
800
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1.0
PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm
1.0
0.8 0.6
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Relative lateral velocity of sand upstream 0.5 depth=100 mm
Excess pore water perssure (kPa)
1.0
Relative lateral velocity (cm/s)
1.5
1.5
1.2
0.4
600 400
Mainly cyclic component
200
0.2
0
0.0
-0.2
Bending moment at bottom
0
10
Bending moment (N·cm)
2.0 Relative lateral velocity of sand at downstream depth=100 mm
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Excess pore water perssure (kPa)
2.0
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Fig. 8 Time history of slope deformation of a single-pile case: (a) upstream and (b) downstream.
Monotonic component of bending moment
20 30 Time (s) (b)
40
-200 50
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Fig. 9 Comparison of PWP, slope displacement velocity and bending moment of a single-pile test.
Downstream
Upstream
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Pile deflection
Soil deformation
Soil deformation
Extension (-)
Compression (+) PWP
Fig. 10 Stress conditions of the adjacent soils around the pile.
24
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
1.2
-0.2 11
Time (s) PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm Acceleration history
1.0
(c)
0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
20
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
0.6 0.4 0.2 0.0 -0.2 -0.4 12
Excess pore water perssure (kPa)
-0.2 10
0.8
1.2
(d)
1.0 0.8
0.8
0.6
0.6
0.4
0.4
0.2
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-0.2
-0.2
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21
Time (s)
13
Time (s) PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm Acceleration history
1.0
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
Acceleraction (g)
0.8
(b)
Acceleraction (g)
0.8
PWP at Upstream, depth=150 mm PWP at Downstream, depth=150 mm Acceleration history
1.0
ro of
1.0 Acceleraction (g)
1.0
1.2
Excess pore water perssure (kPa)
1.2
PWP at Upstream, depth=150 mm (a) PWP at Downstream, depth=150 mm Acceleration history
Acceleraction (g)
Excess pore water perssure (kPa) Excess pore water perssure (kPa)
1.2
24
Time (s)
-0.6 25
500
9×1, 2D 5×1, 4D 3×1, 6D Single pile 3×1, 8D case
Rear Piles
Front
lP
600
re
700
na
400
Deforming direction
300 200
ur
100
1D
0
Rear
Center Pile location
Front
Jo
Max monotonic bending moment (N·cm)
-p
Fig. 11 Excess pore water pressure of upstream and downstream at a depth of 150 mm in case 1. (a) 10~11 s; (b) 12~13 s; (c) 20~21 s and (d) 24~25 s
Fig. 12 Maximum monotonic bending moment distribution for piles aligned in one row parallel to the slope direction. Data retrieved from approximately 100 mm from the bottom.
25
Max monotonic bending moment (N·cm)
400
Side Piles Deforming Center direction
1×5, 2D 1×5, 2.5D 3×3, 4D 3×3, 3.8D
350 300
Side 250 200 150
1D Side
Side
Center Pile location
-p re Monotonic bending moment (N·cm)
200 180
P7 P7, protected P8 P8, protected P9 P9, protected
Front row
160 140
lP
120 100 80 60 40 20 0 5
10
na
Monotonic bending moment (N·cm)
ro of
Fig. 13 Maximum monotonic bending moment distribution for piles aligned perpendicular to the slope direction. Data retrieved from approximately 100 mm from the bottom.
15
20
25
30
35
80
Middle row
P4 P4, protected P5 P5, protected P6 P6, protected
60
40
20
0 5
10
15
Time (s)
Rear row
ur
120 100 80
P1 P1, protected P2 P2, protected P3 P3, protected
60
Jo
Monotonic bending moment (N·cm)
140
20
25
30
35
Time (s)
40 20
Rear
Middle
Front
1
4
7
2
5
8
3
6
9
Protection piles
0
5
10
15
20
25
30
Deforming direction
35
Time (s)
Fig. 14 Time histories of bending moments (monotonic component) for 3×3 group piles with and without protection piles
26
lP
200
With front protection Without front protection
180 160
Average change with protection piles
na
140
120
82.9%
3.13%
100
ur
80 60 40
40.4%
20
Jo
Max monotonic bending moment (N·cm)
re
-p
ro of
Fig. 15 Maximum bending moment (monotonic component) at different pile locations: (a) without protection piles and (b) with protection piles
0
Rear
Center Pile location
Front
Fig. 16 Reduction of maximum monotonic bending moment at different pile locations
27
Single-pile
50
Single-pile
40
Upstream
35 Test10 Test9 Test8 Test7 Test6 Test5 Test4 Test3 Test2 Test1
35 30 25 20 15 10 5 40
30
Test10 Test9 Test8 Test7 Test6 Test5 Test4 Test3 Test2 Test1
30 25 20 15 10 5
20
10
0
40
Residual lateral displacement (cm)
Test-1 Test-3 Test-5 Test-7 Test-9
3 Normalized displacement
40
Distance from bottom (cm)
Distance from bottom (cm)
45
Downstream
30
20
10
0
Residual lateral displacement (cm)
(a)
(b)
Test-2 Test-4 Test-6 Test-8 Test-10
2 Single-pile
1
Downstream
Upstream
Location of measurement (c)
-p
ro of
Fig. 17 Summary of slope deformation under different conditions: (a) upstream; (b) downstream and (c) normalized displacement near the slope surface
re
Front row 200 100
Middle row
lP
Bending moment (N·cm)
0 42 21 0 -21
Rear row
144
na
72
0.75 0.50
ur
0.25 0.00
1.5 1.0 0.5
Jo
Lateral soil velocity Excess PWP (kPa) (cm/s)
0 -72
0.0
-0.5
0
5
10
15
20
25
30
35
40
45
50
Time (s)
Fig. 18 Relation of lateral soil velocity and bending moment at different pile positions. Data were retrieved from Test-9. PWP data were retrieved from the PWP sensor located approximately 300 mm from the bottom, and the bending moment data were retrieved from strain gauges located approximately 100 mm from the bottom.
28
List of Table Captions： Table 1 Scaling laws for different parameters in shaking table modeling Table 2 Summary of test conditions
Table 1 Scaling laws for different parameters in shaking table modeling
N 1 N N1.5 N0.75 N-0.75 N0.75 1 N4.5
Table 2 Summary of test conditions
(Prototype
/
Scaling factors in this study (Prototype / Model) 20 1 20 89.4 9.5 0.1 9.5 1 35,777
ro of
Scale Mass density Stress and pressure Displacement Shaking time Frequency Velocity Acceleration EI of pile
factors
-p
Scaling Model)
re
Quality items
Spacing
Pile configuration
Relative density (%)
Amplitude (Gal)
Frequency (Hz)
Duration (s)
Test-1 Test-2 Test-3 Test-4 Test-5 Test-6 Test-7 Test-8 Test-9 Test-10
/ 2D 4D 6D 8D 2D 2.5D 4D×4D 3.8D×3.8D 3.8D×3.8D
Single 9×1 5×1 3×1 3×1 1×5 1×5 3×3 3×3 3×3
30 30 30 30 30 30 30 30 30 30
300 300 300 300 300 300 300 300 300 300
10 10 10 10 10 10 10 10 10 10
25 25 25 25 25 25 25 25 25 25
Jo
ur
na
lP
Test case
29