Presentation on theme: "Liangcai He Committee in Charge: Professor Ahmed Elgamal, Chair"— Presentation transcript:
1Liquefaction-Induced Lateral Spreading and its Effects on Pile Foundations Liangcai HeCommittee in Charge:Professor Ahmed Elgamal, ChairProfessor Scott AshfordProfessor J. Enrique LucoProfessor Jean-Bernard MinsterProfessor Hidenori MurakamiDepartment of Structural EngineeringUniversity of California, San DiegoHello everyone, today I am glad that I can present you my dissertation “Liquefaction-induced lateral spreading and its effects on pile foundations”. First of all I would like to thank the professors on my committee. Professor elgamal, professor ashford, professor luco, professor minster and professor murakamiUCSD
2One-g Shake Table Experiments Laminar Soil BoxRigid Soil BoxAfter clarifying the relevance of vertical motion to lateral spreading, I conducted a series of shake table experiments to study pile behavior during lateral spreading. Among the experiments are 3 using a rigid wall soil box, another 3 using a mid-size laminar soil box, and 4 using a large laminar soil box.Laminar Soil Box
3UCSD Shake Table Experiment with a Rigid Soil Box (1) This slide shows the setup of the first experiment using the rigid wall soil box. The soil profile consists of a 1.8 m saturated fine sand. Length and width of the model are 5 and 1.2 m respectively. The slope of the ground surface is about 3 degrees. Relative density of the model is about 40%. A 15 cm diameter concrete pile fixed at the base was tested. Shaking is applied in the direction perpendicular to the longitudinal direction
4UCSD Shake Table Experiment with a Rigid Soil Box (2) This is the setup of the second test using the rigid wall soil box. The soil profile is the same as in the first test. Two piles consisting of 10 cm diameter aluminum tubes were tested . They were installed one behind the other to examine pile shadowing effects.
5UCSD Shake Table Experiment with a Rigid Soil Box (3) In the 3rd experiment, two piles consisting of 10 cm diameter aluminum tubes were installed side by side examine pile group effects. The soil profile is also the same as in the first experiment.
6Model ShakingTime (s)The input motion used in the experiments has an amplitude 0.4 g and a frequency 3 Hz. This is a movie showing the first experiment. Here is the pile at downslope of the slope. Water table just covers soil downslope of the pile. Red grid was painted on the ground surface to monitor displacement. Shaking is applied in the direction perpendicular to the longitudinal direction. During shaking, you will see soil moving toward downslope.
7Three Shake Table Experiments with a Laminar Box This slide shows the setup of the experiments using the mid size laminar box. The soil profile consists of 1.9 m saturated fine sand. Length and width of the model were 4 and 1.8 m respectively. The box was tilted 2 degrees on the shake table to simulate an infinite slope. Relative density of the model is about 40%. A 25 cm diameter pile was installed at the center of the model. In the first experiment the pile consisted of a plastic tube and in the second and third experiments consisted of an aluminum tube.
8Experiment Model Preparation This picture shows the installation of the pile.
9Model Shaking – Top View Time (s)The input motion used in the experiments is a .25g 1 Hz sinusoidal acceleration.Here I would like to show you a movie of the 2nd experiment using the mid-size laminar box. On your right is upslope of the model. The pile was located in the center. During shaking, you will see the model will move toward downslope on your left side.
10Model Shaking – Side View This is a side view movie of the model shaking. Upslope is on your left in this view.
11UCSD-Japan Joint Research This picture shows the large laminar soil box. It’s 12 m long, 6 m high and 3.5 m wide.
126m high, 0.3m diameter Pile, Shake Table Tests Unliquefiable crust over liquefiable layerWhole soil layer is liquefiableTwo soil profiles were tested in this large soil box. In one case, the soil profile consisted of 4 m liquefiable fine sand overlain by 1 m nonliquefiable crust. In the other case, the entire soil layer is liquefiable fine sand. Relative density of the model is about 40%. Two piles with diameter of 30 cm were tested. They consisted of steel tubes. The pile at downslope is relatively stiffer than the pile at upslope. Again the soil box was tilted 2 degrees on the shake table to simulate an infinite slope. Input motion was 0.2 g, 2 Hz sinusoidal acceleration.
13Model Response During Shake Table Experiments Pile momentFree field excess pore pressureHere is typical model response during one of the large soil box experiments. In this experiment, the entire soil layer consisted of liquefiable sand. Two single piles were tested. On the left is measured moment time histories of the stiff pile at various depths. We can see that pile moment increases with depth. On the left are measured free field excess pore pressure time histories at different depths. They indicate that the ground liquefied at the first few cycles of shaking. No strong dilation in the soil was observed from pore pressure time histories.
14Model Response During Shake Table Experiments Free field accelerationFree field displacementThis slide shows free field acceleration and displacement time histories. Near ground surface, acceleration was very small after liquefaction. This is due to the fact that liquefied soil has very low stiffness and strength. Maximum displacement occurred at ground surface. After liquefaction, ground displacements at all depths kept increasing cycle by cycle.
15Model Response During Shake Table Experiments Excess pore pressure downslope the stiff pileExcess pore pressure upslope the stiff pileThis slide shows excess pore pressure around the pile. At downslope of the pile, pore pressure in the soil had large dips, while the pore pressure at upslope had smaller dips. This is because the soil at the downslope side of the pile tried to move away from the pile and water in this area was stretched.
16Model Response During Shake Table Experiments This figure compares pile head displacement of the stiff pile and free field ground surface displacement. We can see after liquefaction, pile displacement gradually decreased and the ground displacement kept increasing as shaking continued. This indicates that soil flowed around the pile after liquefaction.All shake table experiments of this study show similar soil and pile response.
17Model Response Summary Conducted one-g shake table experiments show excellent repeatability in terms of pore pressure, acceleration, displacement, and pile responses. Shaking successfully liquefied the soil and induced lateral spreading.Free field excess pore pressure reached initial effective stress at the first few cycles of shaking, indicating soil liquefied relatively early.Pore pressures at the downslope side of piles showed larger dips due to the fact that soil moved more than the pile.No strong dilation in the soil was observed during the experiments.After liquefaction, soil accelerations near ground surface decreased significantly and ground displacement kept increasing as shaking continued.Pile response gradually increased before soil liquefaction. After liquefaction, the soil failed against the pile and started to flow around the pile. As a result, pile response gradually decreased.These responses are summarized here.
18Maximum Moment and Pressure Profiles Top view of Test SetupMaximum Moment and Pressure ProfilesAnalyses of the experiments focus on lateral soil pressure on the pile at the time step when pile had maximum response. Recall that 3 experiments were conducted using the rigid box. Measured maximum moment profile along the concrete pile during the 1st experiment is shown by the red circles in this figure. The profile along the front pile during the 2nd experiment is shown by the red circles in this figure. The profile along the trailing in this experiment is shown by the plus sign here. Measured maximum moment profiles along the piles in the third experiment are shown here. Based on the measured moment profile, I back-calculated a uniform soil pressure for each pile that can approximately reproduce the measure moment.
19Free field ground surface displacement Back-Calculated Maximum Uniform Soil PressureTestMaximumpile responseFree field ground surface displacementSoil pressure(kPa)PileMmax(kN·m)Pile head deflection(cm)At the same time as MmaxAt end of shakingRigid1Left pile0.63N/A5.5Right pile0.64Rigid2Front pile1.411.0Trailing pile0.534.5Rigid3Single pile1.8611.5UCSD12.654.28.512.57UCSD23.001.62.87.86UCSD32.822.214.171.124Japan1Stiff pile86102722Japan21181281525Japan316614195240Flexible pile18343Japan413211131059521Using the same procedure, I also back calculated a uniform soil pressure for all other piles. They are listed in the last column in this table. We can see that the uniform soil pressure varies from case to case. For long piles in the large soil box experiments, the pressure is much higher than 10 kpa recommended by Dobry. Because the uniform pressure varies from case to case, it would be very difficult for engineers to determine a uniform pressure to design piles against lateral spreading. So we have to find a better way for engineers. Recall that soil failed against the pile and flowed around the pile after liquefaction. This suggests a passive failure mechanism of the upslope soil against the pile. This passive failure mechanisms has been observed during the experiments and past earthquakes.
20Comparison of Various Methods Lateral spreading pressure on pilesPileDobry et al.(2003)Japan Road Association (2002)p=10.3 kPap=0.3gtzThis study shows passive failure of uphill soilGround SurfacezLiquefied SoilLateral SpreadingThis slide illustrates the difference between the proposed method and current design methods for the case of a liquefiable ground without a nonliquefiable crust. Basically, soil pressure from the proposed method is higher than current design methods.p=kpgtz, kp=tan2(45+f/2), and f=3º for liquefied soilRotational stiffness Kr was measured before shaking
22OpenSees Beam Element for Pile Solid-Fluid Fully Coupled Element for SoilSoftware Package
23Soil Constitutive Model Pressure dependent multi-yield surface constitutive model was used to simulate the saturated soil.Conical yield surface in principle stress space and deviatoric planeShear stress, effective confinement, and shear strain relationship
32ConclusionsHorizontal ground motions dominate lateral spreading. The influence of vertical motion on lateral spreading is very small.Pile group and shadowing effects can reduce lateral load on individual piles by about 50%Experimental and case history observations show soil failed passively against the pile.A passive pressure method based on liquefied strength of the soil is proposed to estimate pile response to lateral spreading. This method satisfactorily predicted pile response in all shake table experiments as well as the performance of piles during past earthquakes.Current design methods can satisfactorily predict the response of short piles in shallow liquefied soil layers but significantly underestimate the response of longer piles in deeper liquefied ground.The FEM successfully simulated the one of the shake table experiments. It is found that the piles have apparent reinforcement effects on the ground.
33Recommendations for Future Research Additional shake table experiments can be conducted using a large laminar box and a single pile of different sizes and different levels of stiffness to further study pile pinning effects.It will be very useful to conduct one-g shake table experiments and numerical analysis on the case of a liquefiable ground with a stiff crust .Pile behavior in liquefiable steep slope might be different from liquefiable infinite mildly inclined slope. One-g shake table experiments and numerical study can be conducted to bring valuable insights into this case.After concluding this dissertation, I would like to make the following recommendations for future research