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New Project to Prevent Liquefaction-Induced Damage in a Wide Existing Residential Area by Lowering the Ground Water Table

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New Project to Prevent Liquefaction-Induced Damage in a Wide Existing Residential Area by Lowering the Ground Water Table INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 6th International Conference on Earthquake Geotechnical Engineering 1-4 November 2015 Christchurch, New Zealand New project to prevent liquefaction-induced damage in a wide existing residential area by lowering the ground water table S. Yasuda1 and T. Hashimoto2 ABSTRACT In residential areas where liquefaction occurred during the 2011 Great East Japan Earthquake, houses, roads, water pipes, sewage pipes and gas pipes were damaged, interrupting daily life. Though settled and tilted houses were repaired by uplifting, the ground in the whole area, including around lifelines and roads, must be treated by special measures to prevent liquefaction- induced damage. A project to improve the liquefiable soil of an entire area by lowering the ground water table started in November 2011. Based on case studies at sites of damaged and undamaged houses, a water table of about GL-3m was judged to be appropriate to prevent damage due to liquefaction. In-situ tests clarified that the pore water pressure decreased due to dewatering only at shallow depths and that subsidence was small. Introduction In Japan, many remediation methods against liquefaction have been developed and applied since the 1964 Niigata Earthquake, which caused severe damage to many structures due to liquefaction. As a result, many newer bridges, buildings, tanks, port and harbor facilities, and old structures repaired to prevent liquefaction-induced damage escaped damage during the 2011 Great East Japan Earthquake, even though the earthquake caused liquefaction in many parts of the Tohoku and Kanto regions. However, many wooden houses, flat roads and lifelines in residential areas were seriously damaged due to liquefaction because these structures had not been designed to withstand the effect of liquefaction (Yasuda et al., 2012, 2013). Since that earthquake, many settled and tilted houses have been restored by lifting their superstructures and placing them on new footings. However, some countermeasures against re-liquefaction during future earthquakes must be applied in these areas because earthquakes occur frequently in Japan. The countermeasures must be applicable to ground under existing structures and must not cost too much. The lowering of the ground water table has been applied to several areas of damaged residences, and its effectiveness has been confirmed by in-situ tests. 1Prof., Dept. of Civil & Environmental Eng., Tokyo Denki University, Saitama, Japan, [email protected] 2Dr, Dept. of Disaster Management, Chiyoda Engineering Consultants, Tokyo, Japan, [email protected] Liquefaction-induced damage to residential areas during the 2011 Great East Japan Earthquake Figure 1. Settled and tilted houses in Figure 2. Muddy water boiled onto roads Urayasu City in Urayasu City (Photo by Mr. Ogawa) Figure 3. Boiled sands in Chiba City Figure 4. Thrust of an alley in Urayasu City According to the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), about 27,000 wooden houses in Japan were damaged due to liquefaction by the Great East Japan Earthquake. Figure 1 shows a settled and tilted house in Urayasu City, which is about 20 minutes from Tokyo Station by train. Houses tilt because of non-uniform settlement. The non-uniform settlement of houses is most affected by adjacent houses (Yasuda and Ariyama, 2008). If two houses are close to each other, they tilt inward toward each other, and if four houses are close, they tilt toward their common center. In houses greatly tilted by the Great Ease Japan Earthquake, inhabitants felt giddy, sick and nauseous, and found it difficult to live in their houses after the earthquake. Two months after the earthquake, the Japanese Cabinet announced a new standard for the evaluation of damage to houses based on two factors, settlement and inclination. Houses tilted at angles of more than 50/1,000, of 50/1,000 to 16.7/1,000, and of 16.7/1,000 to 10/1,000 were judged to be totally collapsed, large-scale half collapsed and half collapsed houses, respectively, under the new standard. At many flat roads in residential areas, liquefaction caused several phenomena that interrupted traffic: i) much muddy water boiled onto roads (Figures 2 and 3); ii) road asphalt heaved, was thrusted or was deformed into waves at many sites (Figure 4); and iii) many small road cave-ins occurred several months after the earthquake. The duration of shaking during the 2011 Great East Japan Earthquake was extremely long, and the main shock was soon followed by big aftershocks because the earthquake was a “megathrust earthquake” with extremely large magnitude of Mw=9.0. Shaking continued for a long time after the occurrence of liquefaction. Due to the shaking of the liquefied ground, large horizontal displacement, which is a kind of sloshing of liquefied ground, was induced and caused roads to thrust (Yasuda and Ishikawa, 2014). Water, sewage and gas pipes were also severely damaged in residential areas due to liquefaction. The large horizontal displacement of liquefied ground, mentioned above, exerted large cyclic compressional and tensile stress on sewage pipes in the horizontal direction, resulting in the disconnection of pipe joints and the shear failure of manholes, as schematically shown in Figure 5, allowing the influx of muddy water into the pipes and manholes (Yasuda and Ishikawa, 2014). Water pipe and gas pipe joints were also disconnected at many sites. Figure 5. Diagram of damage to sewage pipes and manholes due to liquefaction Patterns of ground improvement in damaged residential areas Soon after the 2011 Great East Japan Earthquake, the repair of tilted houses by lifting their superstructures, reconstructing their footings, and placing the superstructures on the new footings started. Almost all of the settled and tilted houses have been repaired in the past four years. However, this kind of repair does not prevent re-liquefaction during a future earthquake. The ground beneath the houses must be improved to prevent liquefaction-induced damage. There are four patterns to improve the ground in areas where houses have been damaged, as illustrated in Figure 6 and explained below (Yasuda, 2014b): (1) Pattern 1: If many damaged houses in a residential area are demolished, the best option is to improve the ground in the entire area and rebuild houses. (2) Pattern 2: If a damaged house is demolished, an appropriate countermeasure against liquefaction must be applied before reconstruction. Demolish damaged houses Repair houses by jacking them up Road Water pipe, Sewage House Improve the ground to prevent Improve the ground to prevent pipe, Gas pipe liquefaction-induced damage liquefaction-induced damage Rebuild houses (3) One area (2) One house (4) One house Space Pay Public Government (Tax) Private Inhabitants Figure 7. Plan of urban liquefaction Figure 6. Four patterns to strengthen the foundation countermeasure project ground of residential areas against liquefaction (Yasuda, 2914b) Original water table H Lowered water table 1 Drain pipe Drain pipe H2 Liquefiable layer Figure 8. Image of lowering the ground water table (3) Pattern 3: If all or many settled and tilted houses are repaired by uplifting, the ground in the whole area, including lifelines and roads, must be treated. (4) Pattern 4: If a settled and tilted house is repaired by uplifting, the ground beneath it must be treated. Pattern 3 is the most favorable because the ground beneath houses and roads and around buried pipes can be treated simultaneously. The MLIT established a new project eight months after the earthquake, the “Urban liquefaction countermeasure project”. In this project, a wide residential area of more than 3,000m2, including roads, buried pipes and more than 10 houses, is treated by an appropriate countermeasure and its costs are shared by the government and inhabitants, as schematically shown in Figure 7. The project aimed to select effective countermeasures and determine how to share their cost with inhabitants. Countermeasures have been applied in 12 damaged cities in the Kanto Region, and the method of lowering the ground water table, schematically shown in Figure 8, has been selected as the most promising for several cities Tokai Hitachinaka Ibaraki Prefecture Kuki : Liquefied site investigated by Kanto Saitama Itako Kashima Regional Development Bureau of the Prefecture Inashiki Ministry of Land, Infrastructure, Kamisu Transport and Tourism and JGS Katori (Yasuda et al., 2013) Tokyo Narashino Abiko Metropolitan : Cities and towns where applicability Urayasu Chiba Asahi of “Urban liquefaction Kanagawa countermeasure project” or similar Prefecture project has been studied. Chiba Prefecture :Cities and towns where lowering water table method has been studied. 20 0 20 (km) Figure 9. Cities where lowering the water table has been studied marked on Figure 9. By comparing the ground water tables in areas where houses were damaged and in areas where housed were not damaged, the government concluded that a water table of about 3m below the ground surface ensured safety against damage due to liquefaction. Effect of lowering water table on damage to houses According to a study by Ishihara in 1985, liquefaction-induced damage to structures depends upon the maximum surface acceleration induced by an earthquake, the thicknesses of the liquefiable soil layer, H2, and the thickness of the un-liquefiable surface soil layer, H1. He showed that an H1 of 3m is the largest thickness at which damage occurred if H2 is greater than 4 m and the maximum surface acceleration is about 200 gals. This relationship suggests that dewatering to some depth prevents liquefaction-induced damage because the soil layer above the ground water table is unsaturated and un-liquefiable.
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