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DAMAGE INVESTIGATION OF A RESIDENTIAL FILL SLOPE DUE TO THE 2011 TOHOKU EARTHQUAKE

Katharina NIGGEMANN1, Shoko KOMAI2, Yoshiya HATA3,

Ken-ichi TOKIDA4 and Hirokazu KADOTA5

INTRODUCTION At 2:46 P.M., March11, 2011, a giant earthquake named ’The 2011 off the Pacific coast of Tohoku Earthquake’ with a moment magnitude of 9.0 hit East Japan resulting in enormous loss of human life and social facilities. The Earthquake and resulting damage is also called ‘the 2011 Great East Japan Earthquake’. Here in after, we will use ‘the 2011 main shock’ for the sake of brevity. The causative fault of the 2011 main shock is located in the Pacific Ocean off East Japan (see Figure 1), the size of the causative mechanisms was 500 km in length and 200 km. The rupture area of the 2011 main shock was so large and the rupture process during earthquake was so complicated that ground motions with different features were observed at different locations. The observed strong motion was prolonged for more than 3 minutes and the peak ground accelerations of the seismogram in the horizontal direction were more than 980 Gal (e.g., Goto et al., 2013). Especially, the very large strong motions with the long duration time are observed (and/or estimated) in Sendai City (e.g., Hata et al., 2013a; 2013b; 2013c; 2013d; 2013e; 2013f; 2014a).

2011 Main Shock2011/03/11 14:46 MW9.0

2005 Main Shock2005/08/16 11:46 MJ7.2

Miyagi Prefecture

Miyagi Prefecture

A slope of interest

Sendai City

Small-TitanNanakita J.H.S.（1853，75，6.1）

Yaotome Sta.（1150，75，5.8）

Small-TitanNankodai

-higashi E.S.（709，71，5.9）

0 500 m

A slopeof interest

（PGA(Gal), SI value, JMA Seismic Intensity）

NN

Figure 1 Location of a slope of interest. Figure 2 Distribution of existing observation stations.

1 Inst. of Geotechnical Eng., RWTH Aachen Univ., Aachen, Germany, katharina.niggemann@rwth-aachen.de 2 Graduate School of Engineering, Osaka University, Suita, Japan, skomai@civil.eng.osaka-u.ac.jp 3 Graduate School of Engineering, Osaka University, Suita, Japan, hata@civil.eng.osaka-u.ac.jp 4 Graduate School of Engineering, Osaka University, Suita, Japan, tokida@civil.eng.osaka-u.ac.jp 5 Dept. of Geotechnical Eng., Pacific Consultants Co., Ltd., Tama, Japan, hirokazu.kadota@os.pacific.co.jp

K.Niggemann, S.Komai, Y.Hata, K.Tokida and H.Kadota

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The damage to housing may be classified into tsunami-related damage, damage due to liquefaction of sandy ground on newly reclaimed land, and that caused by the instability of slope sand fill embankments developed for residential use.

The focus of this paper is the damaged one to housing lots on hillside embankments. Emphasis is placed on the damage and failure of the housing lots on the hillside embankments in Sendai City, a city with a population in excess of one million, making it one of the largest cities in East Japan. In particular, the damage to the hillside embankments in Nankodai District, Izumi Ward, Sendai City is discussed in order to compare the damage during the 2011 main shock with non-damage during the 2005 off Miyagi Prefecture Earthquake (called ‘the 2005 main shock’; see Figure 1). In Nankodai District, a lot of geo-disasters mainly occurred at the banking sites due to the 2011 main shock (Mori et al., 2012a; 2012b, Okimura et al., 2011).

Also in the geo-disasters, in this study, we focused on a residential fill slope at the northernmost end of Nankodai District (called ‘slope of interest’). The slope of interest damaged due to the 2011 main shock, and did not damage during the 2005 main shock. Moreover, in the slope, a serious damage is reported also in the 1978 off Miyagi Prefecture Earthquake (MJ7.4), and various countermeasures are taken after the 1978 main shock (e.g., Hata et al., 2013g). By simulating and analyzing the difference between the 2005 non-damage and the 2011 damage, we clarify the key factor of the slope failure. In particular, not only the damage due to the 2011 main shock but also the non-damage due to the 2005 main shock was simulated based on the Newmark’s Sliding Block Method (Newmark, 1965) as a fundamantal study. Furthermore, a pseudo-simulation was also carried out during the 1978 main shock. Our related discussion and obtained findings will be useful in future evaluation of seismic performance of a residential fill slope.

STRATEGIC REVIEW OF LOCATION INFORMATION IN THE SLOPE OF INTEREST In Sendai City, besides nationwide networks operated by such organizations as NIED (K-NET and KiK-net), JMA and NILIM, a dense strong motion network is also operated by the local organization, which is now a part of Small-Titan (Kamiyama et al., 2012). Thus, a large amount of strong motion records was obtained in the 2011 main shock and the 2005 main shock. The locations of the strong motion stations around the slope site of interest are shown in Figure 2, with peak ground accelerations (PGAs), SI values (Housner, 1952; 1965) and JMA seismic intensities (Nishimae, 2007). Here, at Yaotome Station, only the indices of the seismic motion (see Figure 2) were recorded (time history data were not recorded) to obtain the seismic strength data for quick and efficient emergency response decisions (Hata et al., 2013h).

For instance, although Yaotome Station site and Small-Titan Nankodai-Higashi Elementary School site are close to each other, the PGAs between these sites are quite different (see Figure 2), which indicates the significant difference of the site effects at these stations and the importance of the consideration of the site effects in estimating strong ground motions.

Figure 3 shows ground conditions around the slope site of interest with the distribution of the observation stations. First, Figure 3(a) is the geological map by National Institute of Advanced Industrial Science and Technology (disclosure map by AIST: Kano, 2000). In Figure 3(a), the slope site of interest and Nankodai-higashi E.S. site are located in the same area of Middle to Late Miocene marine and non-marine sediments. On the other hand, Yotome Station site and Nanakita J.H.S. site are located in Late Pleistocene marine and non-marine sediments.

Next, Figure 3(b) shows superficial geologic map by National Land Information Division, National and Regional Policy Bureau, Ministry of Land, Infrastructure, Transport and Tourism (disclosure map by NLID-MLIT, 1967). In Figure 3(b), the slope site of interest and Yotome Station site are same area which consists of Gravel, Sand and Mud area. However, Nankodai-higashi E.S. site is located in Sandstone area, and Nanakita J.H.S. site is located in Gravel & Mud area.

Moreover, distribution of the 4 sites of interest based on geomorphological land classification map by NLID-MLIT (1967) is shown in Figure 3(c). In Figure 3(c), the slope site of interest and Yotome Station site are same area which consists of Valley plain. However, Nankodai-higshi E.S. site is located in steep slope area. Then, Nanakita J.H.S. site is located in gravelly uplands area.

K.Niggemann, S.Komai, Y.Hata, K.Tokida and H.Kadota

3

Finally, Figure 3(d) is soil map focused on the surface subsoil by NLID-MLIT (1967). In Figure 3(d), the slope site of interest is located between Grayish brown soils area and Brown forest soils area. However, Nankodai-higashi E.S. site is an area which consists of Yellowish brown soils (Fukumuro). Furthermore, Nanakita J.H.S. site is located in Yellowish brown soils (Kitataku) area.

Therefore, based on the above-mentioned consideration result by Figure 3, depending on the legend of the adopted contour map, the classification of the slope site of interest and its circumstance 3 station sites is various. It is suggested that a possibility that the ground shaking characteristics have a significant difference between the slope site of interest and its circumstance 3 station sites.

Small-TitanNanakita J.H.S.

Small-TitanNankodai

-higashi E.S.

0 500 m

Yaotome Sta.

NN

A slopeof interest

Late Pleistocene to Holocene marine and non-marine sediments

Late Pleistocene marine and non-marine sediments

Middle to Late Miocene non-marine sediments

Late Miocene to Pliocene non-marine sediments

Gravel, Sand and Mud

Gravel & Mud

Gravel & Mud Sandstone

Mudstone & Lignite

Small-TitanNanakita J.H.S.

Small-TitanNankodai

-higashi E.S.

Yaotome Sta.

A slopeof interest

(a) Deep underground geological map (b) Superficial geologic map

Gravelly uplands (Higher lower)

Steep slope Gravelly uplands (Lower) Natural levee, Sandy mound,

Valley plain less than valley density 80/Km2

Small-TitanNanakita J.H.S.

Small-TitanNankodai

-higashi E.S.

Yaotome Sta.

A slopeof interest

Gray soils (Takarada)

Yellowish brown soils (Kitataku)

Brown forest soils (Takadate-1) Yellowish brown soils (Fukumuro)

Grayish brown soils (Tatara)

Gray soils (Saga)

Brown forest soils (takadate-2)

Grayish yellow soils (Oyaji-1)

Grayish brown soils (Zentsuji) Grayish yellow soils (Oyaji-2)

Small-TitanNanakita J.H.S.

Small-TitanNankodai

-higashi E.S.

Yaotome Sta.

A slopeof interest

(c) Geomorphological land classification map (d) Soil map

Figure 3 Ground conditions around the slope site of interest with distributions of the observation stations.

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SUMMARY OF STRONG MOTION ESTIMATION Based on the obtained findings in the previous chapter, we have already estimated the ground motions at the slope site of interest during the 1978, 2005, 2011 main shocks (Hata et al., 2014b; 2014c). In this chapter, we summarize the ground motion estimation. First, temporary aftershock observation site was created near the toe of slope site of interest. The observation continued about 3 months to investigate the ground shaking characteristics (Hata et al., 2013g). The observed earthquake events were moderate ones, and the observed motions were used for the evaluation of the site ground shaking characteristics at the slope site of interest. The spectral ratio method (Japan Port and Harbor Association, 2007) was applied to evaluate the ground shaking characteristics at the slope site of interest. Figure 4 shows the comparison of the spectral ratios for the Small-Titan Nanakita J.H.S. site and the slope site of interest. As shown in Figure 4, the spectral ratios are similar between before the 2011 main shock case and after the 2011 main shock case at the Small-Titan Nanakita J.H.S. site. It suggests that use of the spectral ratio based on the moderate earthquake observation records is effective in strong motion estimation due to the large events. In Figure 4, the characteristics of the spectral ratio are not similar between the Small-Titan Nanakita J.H.S. site and the slope site of interest. It suggests that records of the 2011 main shock and 2005 main shock at the station sites of Small-Titan Nankodai-Higashi E.S. and Small-Titan Nanakita J.H.S cannot be used directly as the estimated ground motion at the slope site of interest for the 2011 main shock and the 2005 main shock.

Next, Figure 5 shows the framework of strong motion estimation at target sites using the Maruyama’s Technique (Maruyama et al., 2000). The method is simply composed of 3 steps as shown in Figure 5. Figure 6(a) and Figure 6(b) shows the estimation results of acceleration waveforms in the main sliding direction of slope site of interest during the 2011 and 2005 main shocks at the slope site of interest. In comparison of Figure 6(a) and Figure 6(b), between the 2011 main shock and the 2005 main shock, we can confirm a large difference in the peak ground accelerations and the duration time of the estimated ground motion.

During the 1978 main shock, finally, to evaluate strong ground motions based on a characterized source model, Kowada’s Method (Kowada et al., 1998, Nozu et al., 2009) was used, which takes into account the effect of sediments on both the Fourier amplitude and Fourier phase of strong ground motions. Here, adoption of the characterized source model (SPGA model: Nozu et al., 2008) is because the observed ground motion in the 1978 main shock is not recorded near the slope site of interest. The construction of the characterized source model is focused on the reproducibility of the observed waveforms and the Fourier spectra. Strong ground motions were calculated based on this source model and Kowada’s method. The estimated acceleration waveform in the main sliding direction of slope site of interest during the 1978 main shock at the slope site of interest is illustrated in Figure 6(c). In Figure 6(b) and Figure 6(c), we can confirmed that the similarity of waveforms between the 1978 main shock and the 2005 main shock. It suggests that the slope of interest did not damage, supposing the 2011 main shock was the same scale as the 1978 main shock.

SUMMARY OF DAMAGE CONDITION DUE TO THE 2011 MAIN SHOCK A 2-dimensional embankment model at the slope site of interest is shown in Figure 7. In Figure 7, shape of slope, soil layer, groundwater levels and so on were based on disclosure data of the seismic deformation mechanism and its countermeasures in the damaged residential areas by Sendai City Committee (2012). The disclosure data are based on the results of the disaster investigations.

In this chapter, the disaster conditions at the slope site of interest in Nankodai District were summarized. Nankodai District is the large-scale housing area developed from 1962 to 1985. Due to the 2011 main shock, a lot of serious damage was confirmed by several researchers (e.g., Mori et al., 2012a; 2012b, Okimura et al., 2011) as shown in Figure 8. In Figure 8, the damages were concentrated in the banking area and/or boundary area between the banking and the cutting. The slope of interest is located in an embankment area of northernmost end of Nankodai District as shown in Figure 8. Figure 9 shows the seismic damage condition using bird’s-eye view at the site of the residential fill slope of interest. The distribution of ground crack occurrence (Okimura et al., 2011) at

0.1 1 100.1

1

10

Nanakita J.H.S.before the 2011 main shock

Nanakita J.H.S.during the 2011 main shock

A slope site of interestafter the 2011 main shockS

pec

tral

rat

io a

gai

nst

Nan

ko

dai

-hig

ash

i E

.S.

Frequency (Hz)

Observed seismic waveformat Small-Titan Nanakita J.H.S.

Fourier amplitude and phaseat Small-Titan Nakakita J.H.S.

Correction for thedifference of path effects

between the sites of interestand Small-Titan Nanakita J.H.S.

Fourier amplitudeat the sites of interest

Fourier phaseat the sites of interest

Estimation of seismic waveformat the sites of interest

Correction for the differenceof Fourier amplitude spectrabetween the sites of interest

and Small-Titan Nanakita J.H.S.

Inverse Fourier transformConsideration of causality

( Parzen window )

Figure 4 Spectral ratio due to the earthquakes. Figure 5 Framework of ground motion estimation.

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150-1400

0

1400

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150-1400

0

1400

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150-1400

0

1400

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1500

10

20

30

40

Estimated waveform

Estimated waveform

Estimated waveform

The 2011 off the Pacific coast of Tohoku Earthquake

The 2005 off Miyagi Prefecture EarthquakeThe 1978 off Miyagi Prefecture Earthquake

Threshold Acceleration: 296.5 Gal

at the slope site of interest

at the slope site of interest

at the slope site of interest

The 2011 off the Pacific coast of Tohoku Earthquake

The 2005 off Miyagi Prefecture Earthquake

The 1978 off Miyagi Prefecture Earthquake

Comparison of the residual displacement

(a)

(b)

(c)

(d)

[N30°E]

[N30°E]

[N30°E]

35.8 cm

1.8 cm0.3 cm

PGA= 1,403 Gal

PGA= 434 Gal

PGA= 354 Gal

Acc

. (G

al)

Acc

. (G

al)

Acc

. (G

al)

Sli

din

g D

isp

. (c

m)

Time (s) Figure 6 Computation of the residual displacement of the residential fill slope of interest based on

Newmark’s Sliding Block Method due to the estimated ground motions.

0 10 m

N30°E

Ac

AsNn （Eng. Bedrock）

B1

B2 Ground water level

Estimated slip surface

Retaining wall

Figure 7 2-dimeisional model of the residential fill slope of interest based on the existing examination.

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the slope site of interest and its circumference site is shown in Figure 9(b). The slope of interest is large-scale embankment with a dissected valley, and has confirmed a lot of significant residual deformation at shoulder and toe (see Blue seat area in Figure 9(a)).

Figure 10 shows information of location and direction of photography focused on the slope of interest (e.g., Okimura et al., 2011). The seismic damage condition by author’s reconnaissance results are shown in Photograph 1. Due to the residual deformation, first, in a house where is close to the shoulder, serious damage with collapsed piers was confirmed (see Photograph 1(a), (b) and (c)). The residential house which influenced a great deal of seismic damage is about 10 near the slope of interest. In Photograph 1(d) and (e), next, the blue seat areas due to ground crack occurrence are located in not only shoulder but also banquette of the slope of interest. Finally, Photograph 1(f), (g) and (h) show the seismic damage conditions due to the slope failure at toe of the slope of interest. In Photograph 1(f) and (g), we can confirm that the destruction of retaining walls and stairs in the southeast area (see Figure 10) due to the seismic slope failure. On the other hand, in the northwest area, the difference of the relative displacement between adjacent walls is about 15 cm. That is, we think that the relative displacement is a minimum value of the residual displacement by the slope failure.

DAMAGE AND NON-DAMAGE SIMULATION In this chapter, we evaluated the residual displacement of the residential fill slope of interest based on Newmark’s Sliding Block Method (Newmark, 1965) as a fundamental study. The numerical analysis model at the slope site of interest is shown in Figure 7. In Figure 7, the various factors of the model were considered some of results of in-situ ground investigations (Sendai City Committee, 2012). The characteristics of soil mechanics of the model are shown below. In sandy soil layers of B1 and B2, first, the wet unit weight is 19KN/m3 and the saturated unit weight is 20 KN/m3. Next, we determined the internal friction angle as 25 deg. (in B1 Layer) and 29 deg. (in B2 Layer). Based on the empirical relationship (e.g., Japan Road Association, 1999) between cohesion c and ground depth h；c(kPa)= h(m), finally, we determined the value of cohesion considering the ground depth dependency.

Since a form of the slip surface was a non-circle according to the examination in Sendai City Committee (2012), this study also adopted the slip surface form (see red trace in Figure 7). Based on the determined conditions of the model, we calculated threshold acceleration as 296.5 Gal. Here, the threshold acceleration At is equivalent to the horizontal seismic intensity kH (=At / 980Gal) from which the value of safety factor of the slope failure based on Janbu Method (Janbu, 1955) is almost 1.0. Moreover, the estimated strong ground motions by authors (Hata et al., 2014b; 2014c) during the 2011, 2005 and 1978 main chocks (see Figures 6(a), 6(b) and 6(c)) were used for input earthquake motions.

As shown in Figure 6(d), we computed the time histories of sliding displacements for the 3 events based on the concept of the Newmark’s Sliding Block Method. As a result, the slope of interest did not damage during the false 1978 main shock, supposing the 2011 Tohoku Earthquake was the same scale as the 1978 main shock. On the other hand, in Figure 6(d), the residual displacements are 35.8 cm (for the 2011 main shock) and 1.8 cm (for the 2005 main shock). This comparison result of the estimated residual displacements between the 2011 main shock and the 2005 main shock coincides with the above-mentioned actual results of damage and non-damage in the residential fill slope of interest. We were able to evaluate the existence of the seismic damage during the 2011 main shock and the 2005 main shock based on the Newmark’s Sliding Block Method which is a very simple and practical technique. However, according to the author’s reconnaissance, the actual result of the residual displacement in the horizontal direction at the shoulder along the section of the 2-dimensional embankment model (N30°E: Figure 9(a)) is from 70 cm to 90 cm. The value of the actual result is not similar with that of the estimation result. We can consider the following 2 points as this inharmonious reason. One is that the original Newmark’s Method cannot be considering the amplification effect of the ground motion by seismic response of the embankment model. Another is that since the groundwater level was comparatively high in the embankment model, an excess pore water pressure may have occurred during the earthquakes. However, the original Newmark’s Method cannot be considering the increase effect of the excess pore water pressure. These special issues suggest that the

Slope failure of residential embankment（Slope of interest in this study）

Residual deformation of retaining wall of a water channel

Partial ground subsidencenear the underground facilities

Partial ground subsidencenear the several residential houses

Boundary

Ban

kin

g Cu

tting

Serious damages occurredin embankment area

NN

Figure 8 Relationship between geographical condition and damage sites in Nankodai area.

0 50 m

Shoulder

Toe

NN

0 50 m

NN

(a) Bird's-eye view photograph (b) Ground crack occurrence Figure 9 Seismic damage condition at a site of the residential fill slope of interest.

0 50 m

NN

Shoulder

Toe

Photo. 1Photo. 2

Photo. 3Photo. 4Photo. 5

Photo. 6Photo. 7Photo. 8

Figure 10 Location information of photography at a site of the residential fill slope of interest.

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(a) Damage of the residential house (b) Damage of the residential house near shoulder

(c) Slope failure at shoulder site of the residential fill slope of interest

(d) Slope failure at gradient slope site of the residential fill slope of interest Photograph 1 Seismic damage condition by author’s reconnaissance results.

(e) Focused slope failure at gradient slope site of the residential fill slope of interest

(f) Slope failure at toe site of the residential fill slope of interest

0 50 cm (g) Damage condition in the southeast area (h) Damage condition in the northwest area

Photograph 1 Seismic damage condition by author’s reconnaissance results (Cont.).

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necessity for execution of seismic response analysis of the embankment model considering the excess pore water pressure.

CONCLUSION We computed the residual displacements of the residential fill slope at Nankodai landslide site during the 2011 off the Pacific coast of Tohoku Earthquake, the 2005 off Miyagi Prefecture Earthquake and the 1978 off Miyagi Prefecture Earthquake based on the concept of Newmark’s Sliding Block Method. We were able to evaluate the existence of the seismic damage at the slope site of interest during the 2011 main shock and the 2005 main shock based on the Newmark Method which is a very simple and practical technique. However, since the observed horizontal deformation and the estimated residual displacement are not in quite agreement, we need to perform the seismic response calculation considering the excess pore water pressure as a future study.

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