Geotechnical Aspects of Underground Construction in Soft Ground, Kusakabe, Fujita & Miyazaki (eds) 2000 Balkema, Rotterdam, ISBN 90 5809 1 066

Earth retaining structures under seismic motion of Kobe earthquake

M. Furukawa - Kobe City Housing Supply Corporation, Japan'IITELITIELIIO - Department of Civil Engineering, Osaka Sangyo University Japan

YTD)/OS21Wa - National Institute of Industrial Safety Ministry of Labor, JapanS.T2maka - Ka jima Technical Research Institute, JapanI. Mamma - Japan Institute of Structures and Bridge Corporation, JapanK. Sekiya - K iso J iban Consultants Corporation, Japan

ABSTILACT: Earth retaining structures under construction suffered Kobe Earthquake of January 17,1995_Many seismometers and earth pressure cells were broken by severe shock that yielded the greatest earthquakedisaster since Kanto Earthquake of 1923, but some gauges of earth retaining structures recorded invaluablebehavior before and after earthquake. 80 cases of earth retaining structures before and after Kobe Earthquakewere studied in detail. In this paper, mechanical behavior of earth retaining structures under severe seismicmotion was discussed.

1 INTRODUCTION

In recently years, large and deep constructions atovercrowded city area are increasing. However, thedesign of earthquake-proof retaining structure hasbeen overlooked by following reasons.1.The construction term of the earth retaining

structure is short.2. To strengthen the earth retaining structure for

earthquake increases the cost of construction.3.The earth retaining structure moves with

surrounding underground under seismic motion.4. The mechanical behavior of earth retaining

structure under seismic motion is observedscarcely.

Many earth retaining structures at various areassuffered the severe seismic motions of KobeEarthquake. To study the behavior of earth retainingstructure that suffered severe seismic motion ofearthquake is important for the design of earthquakeproof retaining structures.

In this paper, representative 2 cases from amongthe study of 80 cases, suffered Kobe Earthquake,were studied in detail.

2 EARTH RETAINING STRUCTURE ONRECLAIIVIED LAND

2. 1 Case historyOne suffering case is the earth retaining structure forsewerage treatment plant construction on reclaimedland. This spot is 29 km in the northeast from theseismic epicenter at the earthquake. Steel sheet pile

631

walls suffered a heavy damage due to the severeseismic motion of Kobe Earthquake. At this site, therevetment of reclaimed land leaned to the canal bylateral flow owing to liquefaction, and the building ofsewerage treatment plant was destroyed by seismicmotion.

When the earthquake occurred, the work offoundation piles (cast in place reinforced concretepiles, diameter 1 m, length 32 m) and the work ofbase concrete had been completed. Steel sheet piles(type SL, Z=315O cm", length 12 m-15 m) weresupported by H shaped steel struts with islandmethod. The dimensions were 65.5 in long, 67 mwide and 8_5 m-4_6 m deep. Figure 1 shows the soilcondition at this spot.

The seismic motion from north to south surpassedthe motion from west to east.

The seismometers at 83 m below ground level onKobe Port Island, reclaimed land, 6_5 km in the westfrom this spot, recorded seismic motion as follows:

(1) accelerationhorizontal 679 gal, vertical 187 gal

(2) velocityhorizontal 67 kine, vertical 29 kine

(3) displacementhoriontal 28 cm, vertical 12 cm

2.2 Displacement of earth retaining wallsSteel sheet pile walls were deformed inward in alarge way by super severe seismic as shown in Figure2.

N value 0 60 reinforced concrete

L'5 mm__ 10 Wailing(H500)

_Z-TASE -EE . wide flange beam2 (H400)sunAC 1~ ` _ steelsheet iles

. ' (fYP@ L,H5fI1 ) / /steel sheet pies r r/ y Z(type L, II] ) WSW*l base concrete_ Dsc QFigure 3. H shaped walling and struts (site A).GL-25m "" i

Figure l. Soil condition (Site A).

North

899mn35mm27

517mml

South|

775mm

no _QISouth

Figure 2. Horizontal displacements of earthretaining walls after earthquake.

The damages of steel struts were observed asfollows:

l. H shaped steel struts were distorted anddeformed.

2. H shaped structural steels on base concrete,supported struts, bulked.

3. Bolts that connect each struts and wales werebroken.

The liquefaction and pressure of lateral flow dueto seismic motions severely effected on the steelstruts. The cracks of base concrete supported Hshaped stmts were observed. Maximum opening ofthe cracks was 3mm-5mm around struts as shown inFigure 3.

Corner joints of steel sheet piles were cut off bysevere seismic motion and seawater leaked intoexcavated space from the damaged joints. Thereinforced concrete on the head of walls retainingsteel sheet piles was broken.

The length of earth retaining walls alignment fromnorth to south was shortened and the earth retainingwall of both west and east inclined inward. Seismicmotion on reclaimed land surpassed the allowablestress of H shaped steel (struts and wales).

3 EARTH RETAINING WALLS AT SUPERSEVERE EARTHQUAKE AREA

3.l Case /7f.5`l()l'yAnother case (hereinafter called site B) is theunderground car park that suffered the earthquakeduring constmction. The dimensions were 77 m long,53 m wide and 20 m deep as shown in figure 4. Thegreatest care was required for the safety of railways,which were close by excavation area. The height ofearth retaining wall along the railway side was 7 mabove the ground level. The soil stratum of this spotwas as shown in Figure 6.

The soil cement column piles were used as theearth retaining wall (diameter of northern part550mm). The head of earth retaining walls wasreinforced with concrete. The back ground ofsouthern earth retaining walls from west to east wasreinforced with PTP ( prepacked in place concretepiles).

Surrounding almost houses were destroyedcompletely by earthquake or burnt down in the tireoccurred just after the earthquake. The seismicintensity of this area was ultra 7.

3 .2 Displacement of earth retaining wal/The head of northern earth retaining walls from westto east moved inward by super severe seismicmotion.

North

_station-_ H ` i _A Q

6ED

G 8South

Q continuous inclinometes

inclinomete

Q piezometerFigure 4. Surface plane of retaining structures (siteB).

Nvalue 0 60Banking

Silt EECohesive Soil

Sandy son/G1-.wel b

. , ~siity Soil ~ ~ 7

&% _

Sandy ClaySandy Soil/ Gravel

Gravel _ 0Figure 5. Soil condition (site B).

The maximum displacement of the earth retainingwall at the head was lOcm. The head of southernretaining wall from west to east moved toward theback ground of earth retaining wall. Thedisplacement of earth retaining wall that runs fromnorth to south was relatively small ( as shown inFigure 7 ). The crossing parts of struts ( H shapedwide flange beams ) slipped off by severe earthquakemotion. The station building adjacent to theexcavation area was destroyed completely by seismic

motion. The vertical displacement of earth retainingwalls is as shown in Figure 7.

Severe seismic motion caused large displacementsat these earth retaining walls. But it did not result inthe catastrophic failure. It means it had littleinfluence on the stability of earth retaining structuresin this case.

In this case, an asymmetrical earth pressureresulted from one side of the excavation area beingclose to the railway embankment and principaldirection of seismic motion increased thedeformation inward, and thrust away to the background of earth retaining wall through struts.

4 CHARACTERISTIC MECHANICALBEHAVOUR OF EARTH RETAINTNGWALLS

4.1 Szwerirzg cases Q/`ea1-'th 1'etc1ir1i1'1g wallsThrough the study of 80 cases of earth retainingwalls before and after earthquake, the results are asfollows:

l. Diaphragm wallssuffering none :3cases

2. Soil cement column wallssuffering none :22casesa little suffering 19 cases

3. Steel sheet pilessuffering none 214 casesa little suffering 15 casestremendous suffering 1 Zcases

4. Soldier piles and laggingsuffering none 120 casesa little suffering Q 3cases

5. Totalsuffering none : 60 casesa little suffering 5 l7casestremendous suffering 1 3cases

All tremendous suffering cases were observed at theearth retaining walls on reclaimed land, Kobe city.

4.2 Phenomena of earth retaining .S`fl'IlCIfZI7'.S'

Phenomena of eanh retaining structures due toseismic motion are as follows:

l. Deformation of earth retaining wall.2. Deformation or bending of H shaped steel

struts.3. Crack and leakage of soil cement column wall.4. Joint damages of different rigid steel pipe sheet

piles.

633

( crown of wa1l)_l 18-9 N back ground(-) excavation site(+)T T T ` ` 19.1 B D' 7.7 | After Earthquake i_ _ _| ' (Crown Of wall) +8.7mm beforel '_ J 100 earthquake--- "' "* --__ +14.7rnm34.9 Il I 5_0Before Earthquake LJ' | 7| +43l 6.8 | ` JF 0 mmI `___" l _______ -0.6mm4-ll , 5.7 \ 101.7

| Q l back ground(-) excavation site(+)I f- ' 'I Y* - _ Il I

-T4 "*' :'l'Before Earthquake I 4_9 after( Excavation site ) | _ _ _ L _-DV earthquake 8. -5-7mm\ 13.7 0After Earthquake (Excavation site ) O`3mm mmB DFigure 6. Horizontal displacement of earth retainingWalls before and after the earthquake (site B ).

4.3 Main car./.s'e.s' of Cll"Z/'I /'erairring wal/ '.s'dqjfrrnration

Main causes of earth retaining walls deformation byseismic motion are as follows:

l. Lateral flow due to the movement of revetment.2. Liquefaction.3. Seismic inertia force to double retaining walls.4. Differences of earth retaining walls rigidity

facing each other.5. Differences of improved soil ground at

retaining Walls background.6. Uneven earth and water pressure.7. Pullout of` anchoring steel cable.8. Directivity of seismic motion.

5 CONCLUSIGNS

Through the study of SO earth retaining structuresbefore and after Kobe Earthquake, conclusions are asfollows:

l. There was no catastrophic failure of earth retaining structures (within 80 cases) in the influencedarea of Kobe Earthquake.

2. Earth retaining structures suffered from twistsmotion due to different rigidity of earth retainingwalls and uneven earth pressure due to surroundingundergrounds soil conditions.

3. Water inflow from the damaged corners, dropof struts etc. were observed. To strengthen cornersand joints is necessary for the safety under severeseismic motion.

Figure 7_Vertical displacement of earth retainingWall before and after the earthquake ( site B ).

4. Countermeasure for liquefaction or lateral flowunder seismic rrrotiorr is essential to secure the safetyof workers.

REFERENCE

Technical committee in Japan Geotechnical societyon the plan & works related to the earth retainingstructures. The p/'oceedi/rgs' Qf.sy111p0.s'i1/171 011 1/ve/2/an rf- \1`()/'k.`_/E7/' ea/'//1 retaining .S`ll'1lCfI/l'(!.5` 3125148 (in Japanese ), 1998.

-F

634