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A Case Study from 2011 Great East Japan Earthquake: Chiba – Japan

On my previous post, I mainly discussed about the liquefaction phenomenon based on scientific theory point of view. Now, I would like to discuss it in the more applicative ways in our daily lives.

An example of geotechnical phenomenon, which I would like to discuss, was spotted during the post Great East Japan Earthquake 2011 in the residential areas around Tone River. For the information, Tone River is the longest river in Japan starting from Gunma Prefecture (North-West Tokyo) heading through the Pacific Ocean at Ibaraki Prefecture as shown in Fig. 1. In the post 2011 Great East Japan Earthquake, the downstream areas, as shown in Fig. 2, suffered various geotechnical damage related to the earthquake forces. At the time it was spotted, there was no single conclusion on what may cause the random geotechnical damage on these areas other than strong earthquake motion. Around these areas, some houses suffered heavy damage while other houses that located side-by-side with those heavy damaged areas didn’t suffer any damage at all. Most of the damage can be easily recognized as liquefaction-induced damage such as large soil settlement, soil flows at the footing of embankment and the uplift of manholes as shown in Fig. 3 and Fig. 4. Liquefaction induced damage in these areas were already suspected by the researchers because these areas are located around the very end of the downstream Tone River. It is widely known that the soil material in the downstream area of river is very weak against liquefaction. The type of soil material in this area mostly contains grainy sandy soil, which is eroded from the upstream river and accumulated to the downstream part, so called young deposit material. It is called young deposit material because this material can be illustrated as a new (eroded) material that different from their origin. Rocks and soils beneath the ground surface are compressed with various stresses such as overburden stress and other physical pressures. Pressured material over long period of time would have additional shear strength due to aging effect. For instance, the material with the same physical properties could have different strength against loads/forces depending on their aging effects. Materials that experience more pressure in the past naturally behave stronger than materials experience less. This analogy can be applied during the lab testing of the undisturbed sample and reconstitute sample on the same site. Naturally, the shear strength capacity of undisturbed sample is higher than the reconstituted one.

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Fig. 1: Tone River in Japan

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Fig. 2: Investigation site (Abiko City – Chiba)

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Fig. 3: Tilting of the house caused by large differential settlement.

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Fig. 4: The uplift of a man hole due to liquefaction.

Figure 5 shows one case of the side-by-side residence block area separated by a street in the middle. The area located on the right side of this figure suffered with heavy soil settlement, tilted houses, while the opposite areas didn’t suffer anything at all. This peculiar phenomenon became interesting subject to study by researchers. In order to fully understand this behavior, soil investigation were carried out by digging several boreholes at different sites. The borehole results clearly show that liquefiable soil was found in the heavily damaged area, while the non-liquefiable soil was found in others. However, the question still remains, why the liquefied sites appeared to be mixed to each other with the non-liquefied sites? The current aerial views of these locations, as shown in Fig. 6, confirmed no clear boundary between the liquefied areas and the non-liquefied areas.

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Fig. 5: Side-by-side of damaged and non-damaged areas.

Researchers went to look another clue to solve this phenomenon, which was the cross examination on old topography map. The old aerial view (taken at 1947) of these areas is shown in Fig. 7, while the older map of the sites (taken at 1928) can be seen in Fig. 8. By cross-examining the current aerial view and the old topography map of these areas, we are able to understand their history and their development. The darker/grey part on Fig. 8 shows part of the area covered with water which was actually pond. It was found that these ponds were later reclaimed for the development of residential areas. Since these areas located around the river side, the material actually used to reclaim the ponds was the material taken from the bottom of the river by pumping. By overlapping the liquefaction sites on the current map to the old map, we were able to indentify their locations in the old map.

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Fig. 6: Current aerial view of damaged sites

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Fig. 7: Aerial view taken at 1947.

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Fig. 8: Old map of damaged sites (taken at 1928)

The result was stunning. It was revealed that all the liquefied sites in these areas were all located on the reclaimed lands, which previously used to be the ponds. From this evidence, it’s very clear that the untreated reclaimed soil showed weaker response against liquefaction. In the most occasions, the reclaimed land for residential areas is not treated well such as soil compaction. The reason is simple: Money. Most developers do not want to spend extra money for treating that particular soil. In addition, the customer who purchases the land on these reclaimed ponds usually doesn’t know the history of these areas. Most developers are also would not inform their customers in advance. If the reports were disclose to the customers, the land located on the reclaimed land would see their prices falling down.

In summary, we can pull two lessons from this case either as an engineer or as a customer. As an engineer, having answered the question of “HOW THING WORKS” has a huge impact in our practical solutions. However, having answered the question “WHY THING WORKS” takes everything to whole new level. The question of “WHY” is the most basic question, yet in most cases are the most difficult to answer. From this case, we understand from the borehole test that some particular characteristics of soil are prone to liquefy. However, it doesn’t tell us why the location of liquefy and non-liquefy soil look mixed to each other. By asking a deeper question of “WHY”, the engineer would investigate more in detail and finally lead to the conclusions. The second lesson would be about us, as a customer. I would like to suggest for those who live in the Earthquake prone disaster areas/countries should do a bit of research. First, we can compare the current aerial view (We can get easily from the Google earth) with the old map of those areas. In some countries such as Japan, this old map is well documented and can be found in the local city office. However, most other countries may not. Second possible solution is to ask the locals where you plan to purchase the land. Small interview turn out to be one of the easiest way to do effective research.


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