Liquefaction potential and uplift - WIT PressLiquefaction potential and uplift deformation of underground structure K. Tokida, Y. Ninomiya, T. Azuma Ground Vibration Division, Public

Liquefaction potential and uplift

deformation of underground structure

K. Tokida, Y. Ninomiya, T. Azuma

Ground Vibration Division, Public Works

Research Institute, Ministry of Construction,

1 Asahi Tsukuba-shi, Japan

ABSTRACT

The uplift deformation characteristics of such undergroundstructures as a common utility duct due to soil liquefaction arediscussed experimentally based on shaking table tests using atwo-dimensional ground model and an underground structure modelwith a rectangular section. In this paper, from the view pointthat the uplift deformation of an underground structure is causedby the movement of liquefied soils from the surrounding ground tobelow the underground structure, two factors: the thickness of aliquefiable layer below the underground structure model and thewidth of the underground structure model are aimed as importantparameters controlling the movement of liquefied soils and as aresult influencing the uplift deformation of the undergroundstructure model with a rectangular section. Based on the ex-periments, the effects of the above two parameters for reducingthe uplift deformation of the underground structure model areinvestigated. Furthermore, a countermeasure against soil lique-faction which is called "a barrier wall" is proposed consideringthe movement of liquefied soils to uplift the underground struc-ture model and its effects for reducing the uplift deformation ofthe underground structure model are discussed.

INTRODUCTION

Such an underground structure as a common utility duct and apipeline is damaged to be uplifted by soil liquefaction duringearthquakes. Although it is common to judge the stability of un-derground structures against soil liquefaction using a safety fac-tor for uplift in Japan [1] , the seismic stability of undergroundstructures against soil liquefaction should be judged not basedon the safety factor but also the magnitude of uplift deformationin the future for establishing a practical design method of under-

Transactions on the Built Environment vol 3, © 1993 WIT Press, www.witpress.com, ISSN 1743-3509

366 Soil Dynamics and Earthquake Engineering

ground structures. For this purpose, the fundamental mechanismof the uplift deformation characteristics of underground struc-tures caused by soil liquefaction is discussed experimentally inthis paper.

Shaking table tests using a container were carried out toinvestigate the relation between soil liquefaction potential andthe dynamic behavior of an underground structure model with arectangular section buried in a liquefiable ground model. Inthese tests, the thickness of a liquefiable layer below the under-ground structure model and the width of the underground struc-ture model were varied to discuss their effects for uplift defor-mation characteristics of structure models. Furthermore, an un-derground structure model with two projected plates called "abarrier wall" is also prepared to investigate the effects of barri-er walls for decreasing the uplift deformation of the undergroundstructure model. From experimental results, it is confirmed thatthe barrier walls proposed in this paper are effective as one ofcountermeasures for underground structures against soil lique-faction.

METHOD OF EXPERIMENT

The apparatus used in this experiment is shown in Figure 1 andTable 1. A ground model was constructed in a square containerplaced on a shaking table. This container was 1,800 mm long, 1,100mm high and 600 mm wide. The ground models fundamentally con-sisted of two layers : a lowermost nonliquefiable layer and anuppermost liquefiable layer. An underground structure modelwith a rectangular section was buried in the liquefiable layer atthe center of the container horizontally. The total thickness ofthese two layers and the thickness of the liquefiable layer abovethe underground structure model were set to be constant in eachtest: H = 950 mm and Ha = 200 mm, respectively. Because it is con-sidered that the thickness of the liquefiable layer below the un-derground structure model (Hb) is one of the factors to control themovement of liquefied soils, the factor of Hb was varied in threeand two cases according to the underground structure models: 50mm, 100 mm, 150 mm or 75 mm, 150 mm, 300 mm and 100 mm, 200 mm,respectively. Then the thickness of the nonliquefiable layer(!INL) ranged from 300 mm to 550 mm.

The materials used for the ground models were clean sandscalled "Toyoura Sand" in Japan, and its soil properties are shown inTable 2. As for preparation of the ground models, the lowermostnonliquefiable sand layer was firstly compacted enough in thecontainer not to liquefy during excitation, a certain amount of


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200

100

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thquake Engineering 367



Table 1: Experimental ground models.

Model

No.

1

2

3

4

5

6

7

8

9

10

Laver thickness (mm)

Ha

200

200

200

200

200

200

200

200

200

200

Hm

150

150

150

150

150

150

150

150

150

150

Hb

50

100

150

75

150

300

100

200

100

200

HL

400

450

500

425

500

650

450

550

450

550

hNL

550

500

450

525

450

300

500

400

500

400

H

950

950

950

950

950

950

950

950

950

950

Width

W

150

150

150

450

450

450

150

150

450

450

Remarks

-_

._

__

BW

BW

BW

BWBWiWith barrier wails

Table 2: Materials of ground models: Toyoura Sand.

Specific zravitv of soil panicle Gs

Maximum void ratio emax

Minimum void ratio emin

Coefficient of uniformitv Uc

Maximum size(mm)

2.648

0.940

0.621

1.6

0.85

Table 3: Dry/Saturated/Relative density of ground models.

ModelNo.

12

3

4

5

6

7

8

9

10

Dry densityPt(lf/m*)

1.426

1.441

1.434

1.469

1.444

1.443

1.434

1.433

1.469

1.442

Saturated densityPsat(tf/nv*)

1.887

1.897

1.892

1.914

1.899

1.898

1.892

1.892

1.914

1.897

Relative densityDr<%)

26.0

32.0

29.2

43.0

33.2

32.8

29.2

28.8

43.0

32.4


Soil Dynamics and Earthquake Engineering 369

water was poured, and then the uppermost liquefiable sand layerwas prepared by means of the underwater drop method. The waterground level was set equal to the ground surface level. Thedensities of liquefiable sand layers prepared in each test areshown in Table 3. Although these densities were calculated indi-rectly using the volume of the liquefiable layer and the totalweight of dropped sands which were measured carefully, the rela-tive densities estimated in this experiment ranged from 26 % to 43% because the relative density is affected very sensitively by thethickness of the liquefiable layer. This can be known by compar-ing the range of relative densities: 26 to 43 % and the range ofthe dry density (p t): 1.426 to 1.469 (tf/nr*). However, the groundmodels prepared in this experiment could be considered to beloose and liquefiable layers.

Four types of underground structure models whose conditionare shown in Table 4 were prepared in this experiment. These mod-els were hollow boxes with a rectangular section made usingaluminum plates. The height of the cross section and the lengthof the box were constant to be 150 mm and 590 mm, respectively.Because it is also considered that the width of the undergroundstructure model (W) is another factor to control the movement ofliquefied soils, two types of cross section models changing thewidth (W): 150 mm and 450 m as in Table 4 were prepared to discussits influence on the uplift deformation characteristics of eachstructure model. Furthermore, two underground structure modelswith two projected plates called "a barrier wall" as in Table 4were also prepared to investigate the effects of barrier walls forreducing the uplift deformation of each structure model. The de-tail of the barrier wall model is shown in Figure 1. The barrierwalls aim to disturb the movement of liquefied soils from thesurrounding ground to below the underground structure model.Frames made using soft sponges: the thickness and width of 5 mmand 10 mm, respectively were stuck along the edges of both sur-faces of the structure model facing to the container walls toreduce friction force between the container and the undergroundstructure model.

As shown in Figure 1, pore water pressure meters,accelerometers and bench mark points were installed in or on theground models to measure the time histories of excess pore waterpressure, ground acceleration and displacement of ground sur-face. Furthermore, on the upper-side surface of the undergroundstructure model, four slide wire displacement meters were in-stalled to measure the time histories of uplift/verticaldisplacements of the underground structure model. In eachexcitation, a sinusoidal motion with a frequency of 3 HZ was



Table 4: Underground structure models.

ModelNo.1234

Width(mm)150450150450

Length(mm)590590590590

Height(mm)150150150150

Specificgravity0.940.351.030.89

Remarks

--BWBW

BW:With barrier walls whose length is 100mm.Note:Specific gravity of model 3,4 is one of the

structure models with barrier walls.Model 1,2 and 3,4 are the same modelrespectively except for barrier walls.

Table 5: Input motions.

ModelNo.

1

2

3

4

3

07

8

9

10

Maximum accelerationof input motion

real)80110*17080110*14080no*80110170*23080110*140*80140*20080140*80140*20080140*

Frequencyand

duration

3Hz

20 sec

*:The uplift of a structure model wasobserved first.



employed for 20 seconds (60 cycles). Several levels of maximumacceleration : 80 gal to 230 gal were applied stepwise to theshaking table as input motions. The input maximum accelera-tion amplitudes are shown in Table 5. As in Table 5, same levels ofmaximum acceleration amplitudes were tried to input for comparingeach measured datum under the same input conditions.

NOTICE FOR EXPERIMENTAL RESULTS

Because the excess pore water pressure in the liquefiable layer isa very Important factor to estimate the stability of uplift defor-mation for the underground structure model, the liquefaction po-tential means the rise of the excess pore water pressure or theexcess pore water pressure ratio in this paper. In this experi-ment, the excess pore water pressures plotted were maximum val-ues recorded in each excitation period, the uplift/verticaldisplacements of underground structure models were calculated inthe time step of 0.33 seconds, and the experimental results arediscussed only about the test cases under the input motions bywhich the underground structure model was uplifted first in eachground model as shown in Table 5. Based on Table 5, the maximuminput acceleration amplitudes compared in this paper were 110 gal170 gal and 140 gal for Model-1 to Model-3 and model-5, Model-4and Model-6 to Model-10, respectively. In the case of Model-4 withthe relative density of 43%, the underground structure model wasnot uplifted by the maximum input acceleration of 110 gal, compar-ing with Model-1 to Model-3 and model-5. However, in the case ofModel-9 with relative density of 43%, the underground structuremodel was uplifted by the maximum input acceleration of 140 galsame with Model-6 to Model-8 and Model-10.

BEHAVIOR OF UPLIFT DEFORMATION

Photographs 1 and 2 show visual behavior of the liquefied groundmodel and the uplift deformation of the underground structuremodel without barrier walls, respectively. From these figures, itcan be seen clearly that the liquefied soils surrounding theunderground structure model move to the lower part of the under-ground structure model when the underground structure model isuplifted.

PORE WATER PRESSURE FOR UPLIFT DEFORMATION

Figures 2(a) and (b) show the relation between the uplift/verticaldisplacements of the underground structure models and the excesspore water pressures in the liquefiable layer for the undergroundstructure models with the width of 150 mm and 450 ram, respective-



(a) Model-2 : W = 150 mm, Hb = 100 mm. Structure Model-1,Elapsed Time = 20 sec, Input motion = 110 gal.

(b) Model-4 : W = 450 mm, Hb = 75 mm, Structure Model-4,Elapsed Time = 20sec, Input Motion = 170 gal.

Photograph 1: Visual deformation of underground structure modelwithout barrier walls and liquefied ground model.



(a) Model-8 : W = 150 mm, Hb = 200 mm, Structure Model-3,Elapsed Time = 11 sec, Input Motion = 140 gal.

(b) Model-10 : W = 450 mm, Hb = 200 mm, Structure Model-4,Elapsed Time = 9sec, Input Motion = 140 gal.

Photograph 2: Visual deformation of underground structure modelwith barrier walls and liquefied ground model.



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0 10 20 30 40 50

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Figure 2: Relation between vertical displacements of undergroundstructure models and excess pore water pressure at the point P5in the liquefiable layer.



ly. The pore water pressures discussed in these figures were mea-sured at the point P5 shown in Figure 1 whose depth was the same asthe bottom of underground structure model and which was assumednot to be affected by the underground structure models.

It can be seen that some pore water pressures, for example, inthe cases of Model-8 and Model-10 as in Figure 2, increase largelymore than the initial effective overburden pressure: about 32gf/cm^ which is estimated indirectly using the initial saturateddensity (p a.*) and the buried depth of the point P5 of 350 mm,according to the progress of soil liquefaction. This indicatesthat the pore water pressure meters are assumed to be settledwhen the soils are liquefied, because the pore water pressuremeters were not fixed at the same depth but buried in theliquefiable layer. The following can be indicated from Figure 2:DThe underground structure models without barrier walls: thewidth of 150 mm and 450 mm begins to be uplifted when the pore wa-ter pressure ratio in the surrounding liquefiable soils are about0.73 to 0.80 and 0.79 to 0.83, respectively.2)The underground structure models with barrier walls: the widthof 150 mm and 450 mm begins to be uplifted when the pore waterpressure ratio in the surrounding liquefiable soils are about 0.90and 0.92 to 0.94, respectively.3)From 1) and 2), it can be indicated that the excess pore waterpressure ratio in the surrounding liquefiable soils for beggingthe uplift deformation of underground structure model increasesaccording to the increase of the width (W) of the structure.4)It can also be indicated that the excess pore water pressureratio in the surrounding liquefiable soils for begging the upliftdeformation of the underground structure model is increased bythe barrier walls.

TIME HISTORIES OF UPLIFT DEFORMATION

The time histories of the uplift/vertical displacement of the un-derground structure models are shown in Figure 3 to 5, includingthe time histories after stopping excitation during 20 to 50 sec-onds elapsed.

Effects of thickness of liquefiable layer below structureFigure 3 shows the time histories of the uplift/verticaldisplacements of the underground structure model with the widthof 150 mm for Model-1, 2 and 3 to discuss the effects of the thick-ness (Hb) of the liquefiable layer below the underground struc-ture model for reducing the uplift deformation. This figure indi-cates that the underground structure model begins to upliftearlier and the uplift velocity of the structure expressed by the



20 30Elapsed time(sec)

Figure 3: Time histories of vertical displacements of undergroundstructure models without barrier walls for Model-1, 2 and 3: W =150 mm.

Model 10(Hb=200tJLl=450u)

Model 7(Hb=100u.l*l50u)

Model 9(Kb=100mm.;=450mm)


Figure 4: Time histories of vertical displacements of undergroundstructures with barrier walls for Model-7, 8, 9 and 10: W = 450 mm.



gradient of the time history drawn in Figure 3 and the accumulativevertical displacement during excitation becomes large according tothe increase of Hb. Furthermore, it should be noticed that theuplift deformation of the underground structure models stopsjust after stopping excitation of a shaking table whether thepore water pressure in the liquefied soils keeps high level.

Effects of width of underground structure modelFigure 4 shows the time histories of the uplift/verticaldisplacements of the underground structure models with the bar-rier walls for Model-7, 8, 9 and 10 to discuss the effects of thewidth (W) of the underground structure for decreasing the upliftdeformation. It can be indicated that the uplift velocity of theunderground structure models with the width of 150 mm andaccumulative vertical displacement during excitation is largerthan that with the width of 450 mm. Thus, it can be indicated thatthe uplift velocity and accumulative vertical displacement of theunderground structure model during excitation becomes small ac-cording to the increase of the width (W). Furthermore, theeffects of the thickness (Hb) can also be seen in this figure asindicated in Figure 3.

Effects of barrier wallsFigures 5(a) and (b) show the time histories of the uplift/verticaldisplacements of the underground structure models to discuss theeffects of the barrier walls for reducing the uplift deformation.From Figure 5(a), it can be seen that the uplift velocity and accu-mulative vertical displacement of the underground structure mod-el with barrier walls during excitation for Model-7: Hb = 100 mmare smaller than that without barrier walls for Model-2: Hb = 100mm. From Figure 5(b), it can be seen that the uplift velocity andaccumulative vertical displacement of the underground structuremodel with barrier walls during excitation for Model-9: Hb = 100mm are smaller than that without barrier walls for Model-4: Hb =75 mm. Therefore, it can be indicated that the barrier walls workeffectively to reduce the uplift velocity and accumulative verti-cal displacement of the underground structure models.

CONCLUSIONS

The relation between the uplift deformation of an undergroundstructure with a rectangular section and liquefaction potential isdiscussed experimentally using a shaking table. The following canbe concluded:

1) An excess pore water pressure ratio in the surrounding liquefiedsoils for begging the uplift deformation of an underground struc-




(a) Model-1, 2, 3 and 7 : W = 150 ram.

Vertical d

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Model 10(Bb=200mm)Tith barrier walls

Model 9(Hb= 100mm)With barrier walls

1

(b) Model-4, 5, 6, 9 and 10 : W = 450 mm.

Figure 5: Time histories of vertical displacements of undergroundstructures with and without barrier walls.



ture model increases according to the increase of the width of theunderground structure model (W) and by barrier walls.2) The uplift deformation of an underground structure model stopsjust after stopping excitation of a shaking table.3) An underground structure model begins to uplift earlier andboth uplift velocity and accumulative vertical displacement of anunderground structure model during excitation become small ac-cording to the decrease of the thickness of a liquefiable layerbelow the underground structure model: Hb and/or the increase ofthe width of the underground structure model: W.4) The reason why the factors of Hb and/or W are effective for re-ducing the uplift deformation of an underground structure modelis based on the simplicity of the movement of liquefied soils fromthe surrounding ground to the lower part of the undergroundstructure model.5) It is important to pay attention to the thickness of aliquefiable layer below an underground structure and the widthof the underground structure, when we discuss the seismic designmethod and/or countermeasures for the underground structureagainst the uplift deformation due to soil liquefaction.6) The barrier walls projected at an underground structure modelare effective to reduce the uplift velocity and accumulativevertical displacement of the underground structure model duringexcitation.

Further investigations should be advanced to establish apractical design method and countermeasures for undergroundstructures in the future.

ACKNOWLEDGEMENT

The authors wish to express their sincere gratitude to Mr. Hideoviatsumoto, Deputy Head of Planning Division of PWRI for valuablecomments to conduct the experiment.

REFERENCES

L. Japan Road Association: "Design Guidelines for Common UtilityDuct, March, 1986. (in Japanese)2. Tokida, K. and Ninomiya, Y., "Experimental Study on the UpliftDeformation of Underground Structures Induced by Soil Liquefac-tion", Fourth Japan-U.S. Workshop on Earthquake Resistant Designof Lifeline Facilities and Countermeasures Against Soil Liquefac-tion, Honolulu, May 1992.


Documents

Liquefaction potential and uplift - WIT PressLiquefaction potential and uplift deformation of underground structure K. Tokida, Y. Ninomiya, T. Azuma Ground Vibration Division, Public