Volume 5, Issue 1 March 2020

Special Contribution

Contribution of geotechnical engineering towards recovery
from damage caused by the 2011 Tohoku earthquake- Summary of the 2019 Ishihara Lecture in Rome Ikuo Towhata
Visiting Professor
Kanto Gakuin University, Japan
ABSTRACT: The 2011 Tohoku earthquake with the moment magnitude of 9.0 caused unexpected and disastrous damage
over the entire Tohoku and Kanto Regions of Japan. While tsunami damage has been widely known, liquefaction affected
people’s properties as well. The post-earthquake reconstruction of communities addressed many geotechnical issues,and the present paper attempts to introduce them to readers within the limited page allocation.
Keywords: 2011 Tohoku earthquake, damage, tsunami, liquefaction, reconstruction
INTRODUCTION
Mitigation of natural disasters is one of the major missions of civil engineering. This mission is becoming increasingly
more important in the recent decades because people and public sectors have started to desire more safety for thewelfare and continuity of their value. In other words, the value of the modern community after economic development
substantially relies on the capability of individuals and the value of human resources is remarkably more important thanthat under lower level of development in less developed communities. In the latter, individuals are often considered tobe replaceable labor. In this respect, the target of disaster mitigation used to be limited to important structures in premodern times whereas the current target is the general public.
Geotechnical hazards typically occur during earthquakes and heavy rains exemplified by slope failures and collapse of
subsoil as shown by liquefaction. Therein, an entire district or area is uniformly affected and the geotechnical disastermitigation has to address the general public. This nature of geotechnical disaster mitigation accounts for the reason why
it used to attract minor public concern in the 20th Century when the value of people was lower, and why it has become
more valuable among people only in the recent few decades.
The first two decades of the 21st Century experienced several natural disasters that affected the geotechnical bases ofhuman community, resulting in the loss of safe living conditions and local industrial bases. Examples of such events were
the seismically-induced slope failures in Pakistan in 2005 (Mw=7.6) and in China in 2008 (Mw=7.9) that directly destroyed
human settlements and transportation networks while generating flood risks due to breaching of natural dams. Anotherexample is subsoil liquefaction in residential districts underlain by young sandy deposits that occurred in New Zealand in2010 and 2011 together with eastern Japan in 2011. The collapse of foundation ground under existing houses madecomplete loss in values of personal real estate, affecting the continuity of the damaged community.
Tsunami is another kind of natural disaster that profoundly affects the continuity of a human community. In case of
extremeness, it affects entire nation as exemplified by the events in the Indian Ocean (2004) and Tohoku District of Japan(2011). It is possible herein for geotechnical engineering to mitigate the disaster by offering good and appropriatetechnology such as construction of sea walls and refugee embankment.
In 2019, the author was granted with an honorable opportunity to deliver the “Ishihara” Lecture during the 7thInternational Conference on Earthquake Geotechnical Engineering that took place in Rome (Towhata, 2019). The content
of this lecture consisted of the achievements of geotechnical engineering, whether or not the author was involved, that
attempted to help the community recover from the induced seismic damage. Mitigation of future disaster of a similar
kind was another important aim. The present paper is intended to summarize the lecture, while adding some points thathave been found valuable after the lecture. Note that the topics concerning rainfall-induced disaster are excluded fromthis paper due to page limitation although they are important as well for the continuity of human communities.

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SEISMOLOGICAL ASPECTS OF THE 2011 TOHOKU EARTHQUAKEThe 2011 Tohoku earthquake registered the seismic magnitude of Mw=9.0 and caused damage over the Pacific Coastregion of the Tohoku District (Fig. 1). The induced damage was due to strong and elongated ground shaking such as
exemplified by slope instability, collapse of building structures together with their non-structural members, and subsoil
liquefaction as well as those due to tsunami in the coastal region. Noteworthy was the nation-wide loss of logistics due
to destroyed transportation routes and the economic aftermath such as energy shortage.
People’s doubt on safety of nuclear power plants and the consequent closure of most reactors were of substantial
economic influence. Although they are important as well, the present paper cannot touch upon them due to page
limitation. The seismic aftermath was characterized by the long duration of aftershocks (Fig. 2). Because strong
aftershocks continued for many months and there was a fear that another big “one” was coming soon, it was difficult toresume the recovery efforts immediately after the earthquake. Another difficulty came from the long duration time ofstrong shaking (Fig. 3) because of the huge seismic energy (Mw=9.0) and a need for revised design codes was felt. Anotherissue is the coseismic subsidence in the coastal area (Fig. 4). Because the subsidence exceeded 1 meter in the epicentralarea, tsunami height relative to the ground level became higher by the same amount than the original expectation.

Fig. 1. Tohoku Region and location of Sendai City

Fig. 2. Aftershocks in Tohoku Region (data by Japan
Meteorological Agency; intensity>3)

Fig. 3. K-Net acceleration record of Tohoku
earthquake at Ishinomaki

Fig. 4. Lowered ground level due to coseismic
subsidence in Bansekiura near Sendai City

GEOTECHNICAL DAMAGE
Tsunami is a phenomenon of water but mitigation of tsunami damage is closely related with geotechnical engineering.
Fig. 5 demonstrates the shape of a sea wall destroyed by incoming tsunami. Sea water overtopped the wall and scoured
the landside foot of the embankment. After the wall was lost during the first tsunami attack, the following tsunami attacks
were able to flow into land without much resistance. This implies the need for scour-resistant design of sea walls. It isnoteworthy, further, that the former tsunami disaster in Banda Aceh in Sumatra, Indonesia, in 2004 was characterized
conversely by the effects of retreating water whose flow concentrated at the weakest points of sea barriers, as
eyewitnesses told the author during his reconnaissance.
The disaster in 2011 showed a very difficult feature of tsunami disaster mitigation. The coast of Tohoku Region
experienced big tsunami disasters in 1896, 1933 and 1960 prior to 2011 (Fig. 6). The memory of the gigantic event in AD
869 had been gone after many centuries. Following the 1933 disaster, many local communities decided to relocate theirsites to higher places where the risk of tsunami was obviously negligible. Then the problem was the inconvenience of3

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daily life because people were living mainly on fishing in the sea. Thereafter, the national government constructed highsea walls to protect many local communities. Fig. 7 shows one of them in the Tohni-Hongo Township. The height of thiswall was designed by considering the tsunami height in 1896 that was the highest in the known history. Then, people felt
safe and came back to the coastal area behind the new wall. It was unfortunate that the tsunami in 2011 was even higher
than the wall and the community was destroyed (Fig. 8). If the community had remained in the high places, nobody would
have been killed in 2011, although living on fishing would have been inconvenient. This tragedy shows that empiricism
cannot always give us perfect safety and that we have to overcome a dilemma between safety during rare natural disasters
and convenience in daily life.

Fig. 6. History of devastating tsunami in east
coast of Tohoku Region

Fig. 5. Damaged seawall near the
Abukuma River mouth

Fig. 7. Sea wall in Tohni-Hongo that wasconstructed prior the disaster in 2011

Fig. 8. Destroyed community behindseawall (Tohni-Hongo Township)

Subsoil liquefaction was a significant problem in the residential areas and river levees. It happened in young cohesionlessloose and water-saturated subsoil that underwent strong seismic shaking. Such liquefaction-prone soil was distributed in
man-made islands in Tokyo Bay as well as along rivers where abandoned channels and former lakes had been filled withsandy soil for agricultural and residential developments. The significant extent of those damage was in clear contrast withthe reasonably good performance of engineered foundation of important structures. This means that the lack of soil
improvement budget was the cause of liquefaction disaster.
Figure 9 indicates a house that tilted due to subsoil liquefaction and extreme softening. Affected houses were located onrecent reclaimed land and, generally, the responsibility for the poor quality of the land belongs to the owner of the land(owner of the house) or the architect as the representative of the owner. In other words, the legal liability is not with thecontractor or developer. Due to this, the owners who were hardly experts of construction business and soil mechanics
claimed the product liability of developers and contractors to whom, nowadays, more liability is assumed. It is still difficult,however, to assume 100% liability to them because the man-made island can be affected by the behavior of theunderlying natural ground whose deficit is not necessarily the responsibility of the developers/contractors. Anotherpossible problem in this regard is that ground improvement would have raised the land price upon purchase. The dilemma
between safety and price of residential land is yet to be overcome.
Fig. 10 is a typical damage of embedded lifeline that was affected by liquefaction. Noteworthy is that liquefaction damageof lifeline is possible to occur not only in such liquefaction-prone ground as man-made islands but also in terrace
geomorphology which hardly liquefies. Therein, the problem is the quality of backfill soil where compaction is desired
but difficult in practice. See liquefaction in backfill in Fig. 11.
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Considering the significant liquefaction effect, two actions were taken by engineering communities. The first one wasdevelopment of a practical method to assess the liquefaction vulnerability of residential land and it consisted of theestablishment of qualified evaluator of residential land (Towhata and Nakamura, 2015) as well as a new method of hazard
assessment (Towhata et al., 2016a). The second action was through the governmental support to reconstruct liquefactionresistant residential areas. In spite of the principle that recovery from natural disaster damage in personal properties is a
responsibility of the owners and that public fund should not be spent on value increase of personal properties, thenational government decided to provide supports because of the vast extent of damage on reliability of the urban
environment which is public. The requirement was that public space (streets and avenues with lifelines) should be worked
on together with the private space. The idea of “together” comes from the fact that liquefaction occurred equally in bothpublic and private subsoils and future safety is achieved only through the overall ground improvement. Land owners wererequested to shoulder 50% of the cost that was needed to improve their own land. Moreover, to improve both public andprivate lands at the same time, residents of the target area were required to unanimously agree on the project andpayment. This requirement was not easy and many affected areas could not achieve it. As far as the author experienced,
the major difficulties were as what follows;⚫
⚫
⚫

Very senior people are not interested in spending money on safety during future natural disaster. They prefer tokeep their money for their descendants.
Some people have retired and are living on a limited amount of pension money. They cannot afford the project.
Others have already improved their home land by paying a substantial cost for such technology as compaction
grouting and formation of micro piles under houses. Naturally, they do not wish to spend further money on
repetition of ground improvement.

Accordingly, in Urayasu City, which the author was working for as a technical advisor, only 3 districts with approximately
470 families in total were able to achieve the unanimous agreement.

Fig. 9. Tilting of house situated on liquefied
subsoil in Kashima City

Fig. 10. Floating damage ofwater pipeline in Itako City

Fig. 11. Backfill liquefaction effects on embedded
pipeline; note that no liquefaction happened in
the farm land (Naganuma City in Fukushima)

Fig. 12. Effects of soil age on liquefaction
susceptibility of subsoil; example in Urayasu City

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One of the important novel findings after the liquefaction reconnaissance was that the age of sandy soil significantly
affected the proneness to liquefaction. Fig. 12 illustrates the distribution of liquefaction in Urayasu City. This city started
its history as a small fishermen’s village that was situated on a small sandbar (peninsula) at a river mouth. Shown by blue
color in Fig. 12, this part of the city already existed in the 13th Century, according to local legends, and the subsoil hereis probably as old as 1000 years or more. No liquefaction damage is known in this part of the city. In contrast, theremaining parts of the city are resting on very young man-made islands that were constructed after late 1960s. Significant
liquefaction happened there unless soil improvement had been locally practiced; see red part in Fig. 12.
Seeing such age effect at many places, the author and his group worked on many case histories during the earthquake in2011. The factor of safety against liquefaction was calculated at many places by using available bore hole data and nearby
seismic records. It was found that the border of the factor of safety between liquefied and unliquefied sites, which isequal to 1.0 in principle, decreases with increasing soil age (Fig. 13), suggesting that aged soil has additional liquefaction
resistance that is not accounted for in existing design codes. As a consequence, aged soil was less prone to liquefactionin spite of the factor of safety less than unity. Fig. 14 illustrates the increase in liquefaction resistance with soil age. Whiledata from many other literatures are plotted together, the authors’ output is illustrated by shaded rectangles and practice
can increase the liquefaction resistance by 40% if age is 400 years or more (for details, refer to Towhata et al., 2016b).

Fig. 13. Variation of assessed factor of safetywith age of soil

Fig. 14. Effect of soil age on increase in liquefaction
resistance of sand; note that the quantitative trend in
this figure is valid to Japan only (Towhata, 2018)

GROUND IMPROVEMENT TO MITIAGTE LIQUEFACTION RISK IN EXISTING RESIDENATIAL AREAS
To cope with the vast extent of liquefaction damage in residential areas, the national government set up a public support
package by which the cost for ground improvement is supported to individuals. This was an exceptional action but issometimes carried out in the recent times after “big” natural disasters. What is special herein is that public support isprovided to improve the quality (value) of private lands, which is not allowed under normal conditions. From liquefaction
viewpoints, the liquefaction in public space (streets in urban areas) and private land cannot be discriminated; both typesof subsoil liquefy together and affect urban environment. From this perspective, two kinds of subsoil were improved
together with 100% public fund in the public space and 50%-50% sharing between the public and land owners in private land.
The ground improvement projects were carried out under the responsibility of local governments, while cost was partiallypaid by local citizens in the target areas. Payment by citizens meant that citizens who were not necessarily civil engineers
trusted the local governments and provided money. Thus, citizens were clients. Due to this, the projects had the following
special features;
⚫
⚫
⚫
⚫
⚫

No technical challenge because clients wished 100% success and were not interested in technological research anddevelopment
Reliability of technology that has been verified through practice
Ground improvement under existing houses
No substantial damage in or disturbance to daily life of residents
Reasonable financial burden to people6Volume 5, Issue 1 March 2020⚫

Unanimous agreement of local communities on the project; public money should not be spent on ground
improvement under single house.

Accordingly, there were only two possible methods of ground improvement; lowering of ground water level (Suwa et al.,2014; Yasuda, 2016) and construction of underground walls of grid shape (Suzuki et al., 1995). The latter method reduces
the cyclic deformation of subsoil during earthquake shaking and thus reduces the possibility of liquefaction; see Fig. 15.
Obviously, the first option is substantially less costly than the second one. About ten municipalities started to achieve
unanimous agreement of residents in target areas. Some of them failed because residents did not wish to spend theirmoney on future safety. Several municipalities achieved agreement and chose ground water lowering that was lessexpensive and did not even need personal expenditures (Fig. 16). Urayasu City unfortunately could not take this option
because the city rests on 40-meter thick soft alluvial clay that caused significant consolidation problem from 1970s to
1990s and even nowadays. It was feared that additional consolidation settlement might be triggered by drainage andground water lowering. Challenge had to be avoided in the framework of the people’s project. Therefore, to carry out thesecond option of underground wall (soil-cement mixing) in a very narrow space between houses (Fig. 17), a small andlight-weighted jet-grouting machine was developed (Fig. 18).

Fig. 15. Mitigation of liquefaction risk by installed
underground walls of grid shape in narrow spaceamong houses

Fig. 16. Ongoing drainage and lowering of groundwater level in Kuki City (photograph by Prof. Koseki)Fig. 17. Narrow space between houses in
Urayasu City

Fig. 18. Newly-developed small and lightweighted jet-grouting machine

Fig. 19. Conceptual plan of installed gridwall in residential block

Fig. 20. Hindrance of jet grouting by
unforeseen buried plastic drain
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Figure 19 shows the idea of grid wall in a residential area. It was thought important that all the families in a target blockof town unanimously agree on the project so that a complete set of grid wall might be installed. If any one family declines,
the grid will be incomplete and its integrity during strong ground shaking will be of question. Noteworthy is thatcompleteness of grid requires jet grouting even in clayey subsoil that may be encountered in a part of the target area.
When preliminary work started in early 2017, clay mass was encountered. As mentioned earlier, the area of Urayasu Cityis situated in a river mouth where different kinds of soil deposited in waterways at different geological times, leading tomottled pattern of soil-type distribution. This situation was made more complicated during the land reclamation works
which put different types of soil at different places at different times. Although construction of liquefaction mitigation
measure in clay did not make sense, the overall stability of grid wall required it. Then two problems occurred;⚫
⚫

Clay deposit did not allow uniform jet grouting; grout flowed away only through favorable channels or came out of
the ground surface directly,
Unforeseen plastic drains, that had been installed during land construction in 1970s or earlier in order to promoteconsolidation, twined around the grouting device and stopped the operation (Fig. 20).

Due to these two problems, the project was forced to stop for half a year and countermeasures were investigated. After
trials and errors in the field, it was decided to take the following options;⚫
⚫
⚫
⚫

Higher jet pressure is applied to remove clay and drain from the device,
Jet grouting is repeated two times to further ensure the successful work,
The device is carefully inspected during jet grouting in order to detect
possible twining of plastic drain,
If drain twines, it is removed from the device and jet grouting is
executed once more.

The improved procedure was adopted and drilled cores collected from thesite were inspected to demonstrate 100% satisfaction (Fig. 21). Only one
problem was the elongated construction period. Residents did not like it andvoting showed that only one community with 33 families still agreed
unanimously on the project. Thus, the ground improvement was finally
executed in a very small scale in spite of the original idea (Fig. 22).

Fig. 21. Collected specimen ofsolidified clayey soil after modified
working procedure

This chapter takes example of Rikuzen-Takata City in Iwate Prefecture thatwas inundated by tsunami up to the elevation of 10 to 17 meters.
Considering this extent of inundation, the reconstruction project decided toconstruct a new sea wall of 12.5-meter height associated with elevating the
ground surface level to 7-12 meters by earth filling as well. The volume offilled soil and the area of thus elevated ground are 114.2 million cubic
meters and 3.03 square km, respectively (Figs. 23 and 24). One fear is that
Fig. 22. Ongoing ground improvement
nine years have passed after the disaster and some people have moved out
by small jet grouting device in a veryof the city, while others have got new houses at the top of hills (Fig. 25).
narrow space among houses
Those people may not come to live in the newly elevated land. Because thepopulation has decreased, it is not clear whether or not the newly elevatedcity will be fully occupied by residents in near future. Last and not least, it should be borne in mind that the ground levelis likely to settle down by more than 1 meter during the fault rupturing (Fig. 26). Hence, the tsunami, which comes later,
becomes higher than the expectation by this amount relative to the sunken ground.

Fig. 23. Ongoing reconstruction of
Rikuzen-Takata City

Fig. 24. Vast size of elevated ground in
Rikuzen-Takata City
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Fig. 25. New residential area at hill top

Fig. 26. Tectonic subsidence in Onagawa Harbornear Sendai where ground level is lower thanthe previous level by more than one meter

CONCLUSION
This paper attempts to summarize the author’s “Ishihara Lecture” that was delivered in Rome in June 2019 during the
7th International Conference on Earthquake Geotechnical Engineering under the auspices of ISSMGE TechnicalCommittee 203 on earthquake problems. While the lecture covered many topics such as tsunami disaster, subsoil
liquefaction, coseismic subsidence of earth crust, model tests on improvement of seismic resistance of infrastructures
and the nuclear accident of the Fukushima No.1 Power Plant in addition to the contents of this paper, the author limits
his scope herein to geotechnical issues. It may be understood in this paper that substantial damage occurred to people’s
lives and properties and responses to the disaster required new geotechnical efforts.
REFERENCES
Suwa, S. and Fukuda, M. (2014) Case Study on Countermeasure to Liquefaction by Dewatering Method, Journal of theSociety of Materials Science, 63(1): 21-27 (in Japanese).
Suzuki, Y., Saito, S., Kimura, T., Kibayashi, M. and Hosomi, H. (1995) Seismic performance of building foundation improvedby grouting in square grid shape, Foundation Engineering & Equipment Monthly, 23(10): 54-58 (in Japanese).
Towhata, I. and Nakamura, H. (2015) Qualified evaluator of subsoil quality for safety of residential land, International
Conference on Soft Ground Engineering (ICSGE2015), Singapore, Paper No. 180.
Towhata, I., Yasuda, S., Yoshida, K., Motohashi, A., Sato, S. and Arai, M. (2016a) Qualification of residential land from theviewpoint of liquefaction vulnerability, Soil Dynamics and Earthquake Engineering Journal, 91: 260-271.
Towhata, I., Taguchi, Y., Hayashida, T., Goto, S., Shintaku, Y., Hamada, Y. & Aoyama, S. 2016b. Liquefaction perspective ofsoil ageing, Geotechnique, 67(6): 467-478.
Towhata, I. (2018) Progress of Liquefaction Ageing in Seismically Active Japan, Geotechnical Earthquake Engineeringand Soil Dynamics V, Texas Austin, GSP 290, 473-483.
Towhata, I. (2019) Summarizing geotechnical activities after the 2011 Tohoku earthquake of Japan, 7th Int. Conf.
Earthquake Geotechnical Engineering, Ishihara Lecture, Rome.
Yasuda, S. (2016) Liquefaction mitigation in urban developed areas, Bulletin of JAEE, 28: 18-23 (in Japanese).

◆ A brief CV of Prof. Ikuo Towhata
Ikuo Towhata obtained his doctoral degree from the Civil Engineering Department of
the University of Tokyo in 1982. Currently, he is a professor emeritus of that school andis a visiting professor at Kanto Gakuin University in Yokohama. He has been majoring
mitigation of geotechnical disasters caused by earthquakes and slope failures throughlaboratory tests, shaking model test, mathematical/numerical analyses and field
investigation. He is also interested in microscopic observation of soil mechanics
phenomena such as development of shear band and ageing. In addition to academia,
he is working for an architectural office and a geotechnical consultant firm. He is theauthor of an “encyclopedia” book entitled Geotechnical Earthquake Engineering
published by Springer in 2008.

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