Structural damage to houses and buildings induced by liquefaction in the 2016 Kumamoto Earthquake, Japan
© The Author(s). 2017
Received: 10 December 2016
Accepted: 30 March 2017
Published: 5 April 2017
In April 2016, Kumamoto City, Japan, and its surroundings were hit by a sequence of strong and devastating earthquakes including two significant events, one on April 14th, 2016, at 21:26 JST (Mw6.2) and the other on April 16th, 2016, at 01:25 JST (Mw7.0). These disasters caused 120 fatalities (including indirect fatalities), 2337 people injured and 177,914 residential houses were damaged. This paper aims to ascertain the damage to residential houses and buildings caused by liquefaction during this earthquake and suggests possible mitigation methods.
Field reconnaissance was conducted in the target area on May 27th–30th, 2016. The post-earthquake inclination angle and the tilt direction of 68 affected houses and buildings in the liquefied sites in Kumamoto City were measured by using a laser rangefinder (Leica DISTO D 510). Ground structure and condition were also determined from topographic maps, bore data and the calculated liquefaction resistance factor, FL.
Based on this investigation, the inclination angle of the houses in the target area seems to be related to the type of building structure and their foundation as well to the local ground composition. The tilt direction has a tendency to be associated with the location of the nearby river. The results presented could be useful to develop future liquefaction mitigation measures for detached residential houses.
Due to these earthquakes, much damage was triggered by ground liquefaction that occurred in Kumamoto City and surrounding area of the Kumamoto Prefecture, such as rupturing and cracking of the ground surface and ground subsidence which resulted in the settlement of buildings and residential houses. In many major previous earthquakes, the resulting liquefaction often caused damage through differential settlement which eventually led to permanent tilting of buildings and structural damage as seen in the 2011 Great East Japan earthquake disaster. Massive liquefaction-induced damage appeared in the city of Urayasu, where more than 9000 residential houses were affected. This major catastrophe leads to an awareness and the importance of safe housing design, particularly against liquefaction. As a result, research related to liquefaction countermeasures became more intensified. Currently, many companies and research institutes are working on developing liquefaction countermeasure for buildings and for residential houses. Several researches and studies have been conducted related to the impact of liquefaction on residential houses during earthquakes, such as the 2010–2011 Christchurch earthquakes (Cubrinovski et al. 2012), the 2011 Tohoku Pacific earthquake (Tokimatsu & Katsumata 2012) and the much earlier study of the 1990 Luzon earthquake (Tokimatsu et al. 1994). Various liquefaction countermeasure methods had been proposed, for example, implementing shallow ground improvement (Tani et al. 2015), using a sandy soil layer (Koseki et al. 2015), installing sheet-pile walls around the housing foundations (Rasouli et al. 2015) and by using log pile under foundations (Yoshida et al. 2012).
Liquefaction-induced damage also occurred and was a serious problem in certain areas in Kumamoto City during the 2016 Kumamoto earthquake. Building on previous experience, field reconnaissance was conducted in the target area from May 27th to 30th, 2016 to investigate the effects of liquefaction on differential settlement and tilting of buildings and residential housing. In this field investigation, information and data were obtained by measuring the structural inclination angles and their direction. In addition, interviews of affected parties in the southern and eastern region of Kumamoto City, where a lot of liquefaction-induced damage reportedly occurred were also undertaken.
Outline of the survey
Based on the survey results, the relationship between ground geology, building structure and foundation type and the tilt of the structures will be clarified. It is hoped that these findings can be used in the future to develop liquefaction countermeasure methods, particularly for residential houses.
The first location surveyed was Mashima the residential complex in Akitsu, which is located along the riverbank on the south side of the Kiyama River. The ground structure of these areas was also examined by using the site bore data and that of the surrounding area. In addition, using bore data, soil classification and SPT N-value, the liquefaction resistance factor, FL at the designed horizontal seismic intensity of KhgL = 0.3 was calculated to determine the liquefaction potential of the ground.
Chikami and Karikusa area
Figure 13c gives the FL values for Chikami and Karikusa areas. Based on calculated FL value its potential to liquefy is moderately high to a depth of about 12.5 m below the ground surface as it is composed mostly of sand and has FL values of less than 1, specifically near the surface. Generally, the potentially liquefiable ground surface may lead to structural damage related to differential settlement and structure tilting because sand boiling has a high potential to take place.
In Akitsu area, using the sounding data and the calculated FL value, the ground at a depth of more than 5 m below the ground surface, was categorized as potentially liquefiable. Liquefaction that occurred in this area was mainly affected by the very loose silt and sand layer at a depth of around 6 m below the ground surface, and the existence of the Kiyama river as well. It can be seen on the ground, the lateral movement appeared on the riverside area and also soft ground with high potential of liquefaction was present near the ground surface. As a result, ground subsidence and differential settlement of houses did indeed occur in this location.
In Chikami and Karikusa area, one of the liquefaction occurrences was sand boil. Boiled sand traces remained at the side of the road and around the houses surveyed. This sand boiling took place because the ground at this area is mostly composed of the sand, and from the ground surface to a depth of 12 m was categorized as potentially liquefiable. Furthermore, liquefaction was also affected by the former river on the north–south side of this area. This potentially liquefiable sand on the ground surface may lead to structural damage related to structure tilting due to sand boiling.
Based on this, the liquefaction risk determined using bore data and the value of FL was roughly in agreement with the damage that occurred. Therefore, in order to mitigate liquefaction in the future, it will be necessary to detect the location of weak underground layers. The degree of the damage sustained by houses even within nearby locations depended on the type of building structure and its foundation type. Consequently, in order to select appropriate liquefaction mitigation measures for existing structures such as detached residential houses, it is necessary to consider the characteristics not only of the structure but the geological condition of the ground as well.
In the interview with the residents, many considered moving due to a high risk of re-liquefaction of the ground. It was thought there was little likelihood of applying countermeasures to their houses because of the associated difficulties in doing so and the high cost of the countermeasures. In addition, residents felt uncomfortable with a house tilt angle of 1° or more and even worse, the house became hard to live in if the inclination angle exceeding 2°.
Previously, in the 2011 Great East Japan Earthquake, enormous liquefaction damage occurred in Urayasu City, Chiba Prefecture, in which was about 85% of the city was damaged. Because of the huge area that suffered liquefaction, a large-scale liquefaction countermeasure was undertaken, in the form of underground walls. On the other hand, Kumamoto City, unlike Urayasu City, the occurrence of liquefaction was not centralized in one large area but scattered in small separate regions. Even though some of the buildings were situated in neighboring areas, the damage that occurred due to liquefaction to each building also varied, depending on the factors mentioned earlier, for example, the type of the structure and its foundation type.
Based on the different conditions of liquefaction that occurred in these two cities, the recommended mitigation methods applicable to Kumamoto City and surrounding areas will be different to the methods that employed in Urayasu City. It is thought that will be useful to devise and develop liquefaction countermeasures for existing detached residential houses and buildings, in addition to the large-scale mitigation technique applied in Kumamoto City and nearby areas.
Nowadays, we are doing a research related to liquefaction countermeasure methods that can be utilized for detached residential houses.
In the field reconnaissance, 68 houses were measured and the tilt angle and tilt direction were described on the map presented. Although this survey was in a narrow area, it was found that there was a great variance in the degree of inclination of the buildings within that area.
The extent of the damage was found to relate to such factors as differences in the type of building structure and their foundations, the weight of the structure as well as the ground condition. As a result, the degree of the damage differed from building to building within the same location, for example, while buildings supported by conventional foundations experienced large inclination, the pile-supported building only suffered minor inclination. In addition, it was found that topography affects the direction of inclination which in this case was mostly toward the river location. Furthermore, some of the houses and buildings which experienced large tilt angles were located on the adjacent site. In order to devise and develop a new and effective liquefaction countermeasures for residential houses, it is recommended that a comprehensive investigation be undertaken taking into consideration such factors as implementation methods, costs, material availability as well as structure and foundation type while not forgetting the underlying geological structure of the ground as well the surrounding topography. These considerations may produce liquefaction countermeasure methods suitable for detached residential houses.
This research was financially supported by JSPS KAKENHI Grant Number: 25289135. The authors thank Mr. Naoki Nomura of the Nihonkai Consultant Co. Ltd. and Mr. Kazutaka Shichiroumaru of the Kokudo Kaihatu Center Co. Ltd. for supported in this field reconnaissance. The authors also thank Directorate of Resources for Science, Technology, and Higher Education, Ministry of Research, Technology and Higher Education of Indonesia for the scholarship.
HS, YS, MN, MM, and MY participated in the field reconnaissance and discussed the results of the survey. HS drafted the manuscript. YS and MN prepared the topographic map of Kumamoto City and boring data processing. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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