Licuacion Flac Model Fin

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Determination of Liquefaction Potential

of Soil Using (N1)60 by Numerical

Modeling Method

Gholam Moradi Assistant professor

Department of Civil Engineering, University of Tabriz, Tabriz, Iran

Email: [email protected]

Behnam Rahro Khatibi, Mehdi Hosseinzade Sutubadi Department of Civil Engineering, University of Tabriz, Tabriz, Iran

Email: [email protected]

[email protected]

ABSTRACTOne of the important design considerations for structures situated on sand deposits is the

potential for instability caused by the development of excess pore pressure as a result of

earthquake loading. A build-up of excess pore pressure may lead to initial liquefaction. In the

present study, an alluvium with saturated sand was simulated in FLAC software. Three typesof soil mass with various primary condensation (loose, medium dense and dense) were used.Parameter (N1)60 was defined to the software as a main soil property to calculate the other soil

parameters such as internal friction angle, relative density, etc. As dynamic loading,

acceleration time histories of three earthquakes with various magnitudes (low, medium andhigh) were applied to the model. To examine pore water pressure variation in the soil mass

during the earthquake loading, the parameter r u was defined for the software by a Fish

function. r u is the pore water pressure ratio, which equals the pore water pressure incrementdivided by the primary effective stress in soil. The results showed that FLAC can identify the

regions with liquefaction potential by calculation of generated excess pore pressure during

earthquake loading. Time histories of r u showed that liquefaction potential in the soil mass

changes with depth variations.

KEYWORDS: Liquefaction Potential, FLAC, Standard Penetration Test, Pore Water

Pressure Ratio, Dynamic Loading.

INTRODUCTION

When earthquake loading is applied to saturated sand deposits, pore water pressure in the soil

starts to increase leading that the soil strength is lost. Liquefaction is the ultimate appearance of

this phenomenon (Liyanathirana D.S. and Poulos H.G. 2002). At the initiation of soil



Vol. 16 [2011], Bund. D 408

liquefaction, the effective stress in the soil becomes zero and pore pressure in the soil becomes

equal to the initial confining pressure of the soil mass. During past earthquakes, liquefaction

induced displacements have caused serious damages to marine structures (such as quay walls,

piers, breakwater, etc.). Therefore, the prediction of liquefaction is the main concern for

structures located in medium to high seismicity regions (Jafarian et al. 2010). Field experienceduring past earthquakes (Youd et al. 2001) shows that liquefaction has generally occurred at

depths less than 15 m, and recently, some dynamic centrifuge model testing (Steedman et al.

2000) proposes a depth on the occurrence of liquefaction.

Although simplified methods (Seed and Idriss, 1971) are available in calculating the

liquefaction potential of a soil mass, these methods can’t be applied to all earthquakes with the

same accuracy. In addition, they can’t predict the pore pressure developed in the soil due to

earthquake loading. Therefore, it is necessary to implement a numerical simulation to obtain pore

water pressures and shear stresses in the soil due to earthquake loading. The numerical simulation

of liquefaction is essential in understanding the liquefaction mechanism. To perform the

simulation, an applicable constitutive model of saturated soils and a suitable formulation of

numerical method for the two-phase coupled problem are required.

Numerical simulation performed by Shun-gun Li et al . (2010) shows that the cusp

catastrophe model can simulate increment of excess pore water pressure and occurrence of

liquefaction due to earthquake loading. This model can predict pore water pressure increment and

liquefaction potential. Presented results by Jafarian et al . (2010) show that numerical simulation

of liquefaction can predict both excess pore water pressure and displacements of marinestructures. Therefore, implementation of numerical simulation is necessary for examination of

liquefaction potential. Their numerical analysis was performed by FLAC software.

In this paper liquefaction potential of saturated sandy soil is studied. For this purpose, an

alluvium with saturated sand is simulated in FLAC software using Finn constitutive model. The

effect of soil densification and magnitude of earthquake loading (as a dynamic loading) inliquefaction potential of the soil and level (or depth) of liquefied regions in the soil mass is

considered. Furthermore, the effect of depth variation on liquefaction potential is studied by

definition of time histories of r u in various points of the soil mass. Generally, it is tried to simulate

liquefaction phenomenon in FLAC using parameter (N1)60 as a main factor for soil parameters

required to the software and identifying liquefiable regions in the soil mass.

THEORY AND PROCEDURE OF NUMERICALMODELING

For modeling the soil mass with liquefaction potential, the FLAC software is used. The finite

difference method for analysis of soil mass is used in FLAC. Finn constitutive model isconsidered as default in FLAC that facilitates the ability of soil modeling in liquefaction states.

Also, equivalent linear method (Seed and Idriss 1969) and Finn constitutive method are used.

With FLAC, the effects of pore water pressure with or without loss of pore water pressure can

be obtained. Also, the generation of pore water pressure can be calculated by considering the

irrecoverable volumetric strain on basic model Finn with dynamic option. In Finn constitutive

model, it is assumed that void ratio is constant, also it can be calculate as a function of volumetric

strain and other parameters can be defined by void ratio.



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Vol. 16 [2011], Bund. D 410

Then, the values of bulk modulus and water density and gravity are given to the program.

Next, the parameters of Finn model are defined by introducing (N1)60 for all the regions of soil

mass. The software calculates the values of C1 and C2 for all the regions of soil mass.

Also the quantity of (N1)60 is used to calculate the other soil parameters such as internalfriction angle, relative density and also the required parameters for the model to calculate

volumetric strains such as shear modulus and initial bulk modulus. The relationship between

these parameters with (N1)60 is defined to the software by a Fish function. For example, for

determining the relative density (Dr ) with (N1)60, the Boulanger formula (2004) is used.

Also, internal friction angle φ is defined by Eq. (6):

= 15.4×() +18 (6)

Finally, Rayleigh damping is used for the model and relevant coefficients are given to the

software.

For observation of effective stress and pore water pressure variations, special time histories

are defined in some points of the mesh. These time histories are defined to the software by a Fish

function.

For observation of the earthquake magnitude, acceleration time history of three earthquakes

with low magnitude ( Karebass earthquake with maximum acceleration 0.2g), average magnitude

(San Fernando earthquake with maximum acceleration 0.29g) and high magnitude (Bam

earthquake with maximum acceleration 0.42g) are used as dynamic loading for the soil mass.

EXAMINATION OF THE RESULTS

To distinguish between liquefiable and non-liquefiable areas, parameter r u is defined. r u is the

pore water pressure ratio, which equals the pore water pressure increment divided by the primary

effective stress in soil. It is defined to the software by a Fish function.

Theoretically, if r u inclines to 1, effective stress inclines to zero and liquefaction should

occur. But r u=1 is only theoretic definition for liquefaction occurrence. Previously, it is

experienced that liquefaction phenomenon occurred at r u=0.93. Therefore in this research, if r u

reaches a value greater than 0.9 liquefaction happens.

Examination of Liquefaction Potential of Soil Mass withVarious Condensations by Applying Karebass Earthquake

As previously described, to introduce the soil condensation, parameter (N1)60 is used.

According to Table (1) which is presented by Clayton et.al (1993), classification of soil regarding

(Dr ) and (N1)60 is denoted. By use of this table, values of (N1)60 equal to 8,15 and 25 are chosen.



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Vol. 16 [2011], Bund. D 413

According to Figure (4-A), approximately all the regions of the soil mass by applying high

magnitude earthquake is liquefied and on the contrary to Figure (2-A) and Figure (3-A), a specific

region cannot be distinguished as an area with liquefaction potential.

From Figure (4-B) it is seen that by increasing the soil density, liquefaction potentialdecreases from bottom to top of the soil mass. In this figure on the contrary to previous figures,

only one liquefied region is seen. On the other hand, shallow regions of the soil mass have

maximum liquefaction potential.

In Figure (4-C) it is obviously seen that in depth of 12m there is maximum liquefaction

potential. However, by comparison with previous corresponding figures, larger area of the soil

mass is in the liquefaction boundaries.

General Results From Above Figures

- In Figure (2) and Figure (3) which are related to low and medium magnitude earthquakesrespectively, the soil mass has maximum potential of liquefaction in two separated regions.

According to these figures, when soil densification increases, the area of liquefied region

decreases in depth of 30m.

- According to Figure (4), on the contrary to Figure (2) and Figure (3) in the soil mass with

applied high magnitude earthquake, only one specific region with liquefaction potential is seen.

- Comparing Figure (2) and Figure (3), shows no particular difference between liquefied

region area. It is due to the small difference between maximum accelerations of two earthquakes,

which are applied.

-All figures show that maximum potential of liquefaction in the soil mass is between depthsof 10 and 15m. This range of depth is similar to results of Seed and Idriss method (1971).

The Effect of Depth Variations on Liquefaction Potential ofSoil Mass

To examine the modeling exactness, several time histories are defined in various points of the

model mesh to score variations of soil parameters during earthquake loading (such as r u, effective

stress and pore water pressure). For example in this section, time history of r u in middle region of

the soil mass in depth of 10 and 20m is studied.



ol. 16 [20

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11], Bund.

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20m related

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Vol. 16 [2011], Bund. D 415

Both of two figures show that the soil mass has experienced maximum value of r u in tenth

second of earthquake loading. Maximum value and variations slope of r u in Figure (5) are more

than corresponding values in figure (6). It shows that liquefaction potential in low depth is more

than liquefaction potential in high depth. The maximum value of r u in Figure (5) is 0.93, it means

that soil mass in that point is liquefied, while maximum value of r u in figure (6) is 0.87 and thesoil mass is not liquefied in that point.

At both states, the final value of r u is 0.83 after peak point. In Figure (5), value of r u after tenth

second suddenly falls and it inclines to 0.83 with a lightly ascending slope. In figure (6), value of

r u after tenth second inclines to 0.83 with lightly descending slope.

Generally, if depth increases, less reduction occurs in effective stress and there will be less

increase in r u as well. On the other hand, minimum effective stress in depth of 20m is more than

corresponding value in depth of 10m. It shows that when depth increases, liquefaction potential

decreases.

CONCLUSION

Based on numerical results the following conclusion can be obtained.

1. Results show that Finn constitutive model can calculate excess pore pressure during

earthquake loading by measurement of irrecoverable volumetric strain. By introducing r u as a

liquefaction potential index, the liquefied regions are specified.

2. In the loose and medium dense sand with applied low and medium magnitude earthquake

loading, two separated regions with liquefaction potential are seen in depth of 12 and 30m. The

volume of liquefied region located in approximate depth of 30m is less than volume of liquefied

region in approximate depth of 12m.

3. In all types of soils (with various densities) by applying high magnitude earthquake

loading, only one area with liquefaction potential is seen. When soil condensation becomes high,

the liquefied region moves to the surface. This result shows that the effect of high magnitude

earthquake shows its effect in low depth of the soil mass.

4. During earthquake loading, in all types of the soils with various densities, liquefaction

potential in low depth is more than liquefaction potential in high depth.

5. Results show that variations slope of r u (or pore water pressure) in lower depth (10m) is

more than variation slope of r u in higher depth (20m). On the other hand, the soil mass near the

surface reaches sooner to the liquefaction state.

6. Schema of variations of r u in depth of 10m is different from variations of r u in depth of

20m. In depth of 10m, the value of r u after tenth second suddenly falls and it inclines to constant

value (0.83) with lightly ascending slope. In depth of 20m, value of r u after tenth second inclines

to constant value (0.83) with lightly descending slope.



Vol. 16 [2011], Bund. D 416

REFERENCES

1.

Boulanger, R. W. and Idriss, I. M (2004) “State normalization of penetration resistances and

the effect of overburden stress on liquefaction resistance,” Proc., 11th Int. Conf. on Soil

Dynamics and Earthquake Engineering, and 3rd Int. Conf. on Earthquake Geotechnical

Engineering, D. Doolin et al ., eds., Stallion Press, Vol. 2, 484-491.

2. Byrne, P (1991) “A Cyclic Shear-Volume Coupling and Pore-Pressure Model for Sand,”

Second International Conference on Recent Advances in Geotechnical Earthquake

Engineering and Soil Dynamics (St. Louis, Missouri, March, 1991), No. 1.24, 47-55.

3. Cundall, P. A., H. Hansteen, S. Lacasse and P. B. Selnes (1980) “NESSI — Soil Structure

Interaction. Program for Dynamic and Static Problems,” Norwegian Geotechnical Institute,

Report 51508-9.

4. Clayton, C. R. I., Milititsky, J and Woods, R. T (1993) “Earth Pressure and Earth-Retaining

Structures,” London: Blackie Academic & Professional.

5. Idriss, M., Boulanger, R. W (2004) “Semi-Empirical Procedures For Evaluating Liquefaction

Potential During Earthquakes,” Department of Civil and Environmental Engineering,

University of California, Davis, CA 95616-5924, 11th International Conference on Soil

Dynamics and Earthquake Engineering, (ICSDEE), and The 3rd International Conference on

Earthquake Geotechnical Engineering (ICEGE), Berkeley, California, USA.

6.

Itasca Consulting Group Inc. (2000) FLAC, version 4.0. Itasca Consulting Group Inc.,

Minneapolis, Minn.

7. Jafarian, Y., Alielahi, H., Sadeghi, A. A. and Vakili, R (2010) "Seismic Numerical

Simulation of Breakwater on a Liquefiable Layer: IRAN LNG Port," Electronic Journal of

Geotechnical Engineering, Vol. 15.

8.

Li, S.g., Chai, S.x., Ren, Q.x and Wang, Y.h (2010) "Prediction of Liquefaction Potential and

Pore Water Pressure due to Earthquake," Electronic Journal of Geotechnical Engineering,

Vol. 15.

9.

Liyanathirana D.S. and Poulos H.G (2002) “Numerical simulation of soil liquefaction due to

earthquake loading,” Soil Dynamics and Earthquake Engineering, 22, 511–523

10.

Martin, GR, WDL. Finn and HB. Seed (1975) “Fundamentals of Liquefaction Under CyclicLoading,” J. Geotech. Div. ASCE, 101(GT5), 423-438.

11. Seed, H. B., and I. Idriss (1969) “Influence of Soil Conditions on Ground Motion During

Earthquakes,” J. Soil Mech. Found., Div. ASCE, 95, 99-137.

12. Seed, H.B. and Idriss, I.M (1971) “Simplified procedure for evaluating soil liquefaction

potential,” Jnl GED, ASCE, 97(9), 1249-1273.



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13.

Steedman, R.S., Ledbetter, R.H., and Hynes, M.E (2000) “The influence of high confining

stress on the cyclic behavior of saturated sand,” In Soil Dynamics and Liquefaction 2000:

Proceedings of Sessions of Geo-Denver 2000, Denver, Colo., 5–8 Aug. 2000. Edited by

R.Y.S. Pak and J. Yamamuro. American Society of Civil Engineers, Geotechnical Special

Publication 107, pp. 35–57.

14. Youd, T.L., Idriss, I.M., Andrus, R.D., Arango, I., Castro, G., Christian, J.T., Dobry, R., Finn,

W.D.L., Harder, L.F., Jr., Hynes, M.E., Ishihara, K., Koester, J.P., Liao, S.S.C., Marcuson,

W.F., III, Martin, G.R., Mitchell, J.K., Moriwaki, Y., Power, M.S., Robertson, R.K., Seed,

H.B., and Stokoe, K.H (2001) “Liquefaction resistance of soils: summary report from the

1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of

soils,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 127(10): 817–

833.

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