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. Two case studies of the effect of the water table on the damage to houses during past earthquakes have been conducted by one of the authors, and their results are summarized in Table 1. These studies showed that an H1 of 2m is the critical thickness to prevent damage to wooden houses.

Similar studies have been conducted in several cities damaged by the 2011 Great East Japan Earthquake. A technical committee organized by the Urayasu City Government and chaired by Prof. K. Ishihara carried out detailed soil investigations and reported valuable data in December 2011 (Urayasu City, 2012, Yasuda, 2014a). Based on data from 221 existing borings and from 23 additional borings, contour lines of the ground water table were estimated and compared roughly with the damage to wooden houses. Frequency distributions of the depth of the water table at different levels of damage are summarized in Figure 10. The average depths of the water table for completely destroyed, large-scale partially destroyed, partially destroyed, and partially damaged houses were GL-1.38m, GL-1.78m，GL-1.84m，GL-2.10m, respectively. However, these relationships are not exact because the depth of the water table at each damaged house was estimated from the existing data from nearby borings. One of the authors and his colleagues measured the exact depth of the water table at the sites of about 28 damaged houses in the Irifune and Mihama districts of Urayasu City (Ishikawa, Yasuda and Ikarashi, 2014). The measured depths are classified by the level of damage to wooden houses and plotted in Figure11. As 平均値μ 1.378 平均値μ 1.776 標準偏差σ 0.965 標準偏差σ 0.827 （Urayasu City）データ数n 9 データ数n 1336 10 400 μ-σ μ μ+σ Completely 350 μ-σ μ μ+σ 全壊 Large-scale Partially大規模半壊 8 destroyed 300 destroyed

Table 1. Effect of water level 6 250 200 Average 頻度 Average 頻度 on the damage to houses during 4 150 WL=GL-1.78m WL=GL-1.38m 100 2 past earthquakes 50 Number ofNumber samples Number ofNumber samples 0 0 0～0.5 0.5～1 1～1.5 1.5～2 2～2.5 2.5～3 3～3.5 3.5～4 4～4.5 4.5～5 0～0.5 0.5～1 1～1.5 1.5～2 2～2.5 2.5～3 3～3.5 3.5～4 4～4.5 4.5～5 Critical depth to 地下水位(m) 地下水位(m) Name of Depth of water table (m) Depth of water table (m) Earthquake cause damage 平均値μ 1.836 平均値μ 2.007 housing lot 標準偏差σ 0.846 標準偏差σ 0.949 to houses データ数n 1873 データ数n 4305 600 1000 μ-σ μ μ+σ Partially 900 μ-σ μ μ+σ 1893 Araya- About GL-1.5 m 500 半壊 Partially一部損壊 destroyed 800 700 injured Nihonkai- mastumi in to -2.5 m 400 600 Average 300 Average 500

chube Akita City 頻度頻度 WL=GL-1.84m 400 WL=GL==2.01m 200 300 2000 Abehikona About GL-1.6 m 200 100 Number ofNumber samples Tottoriken- in Yonago ofNumber samples 100 0 0 0～0.5 0.5～1 1～1.5 1.5～2 2～2.5 2.5～3 3～3.5 3.5～4 4～4.5 4.5～5 0～0.5 0.5～1 1～1.5 1.5～2 2～2.5 2.5～3 3～3.5 3.5～4 4～4.5 4.5～5 seibu City 地下水位(m) 地下水位(m) Depth of water table (m) Depth of water table (m) Figure 10. Estimated depth of water table at each damage level (Urayasu City, 2012)

Level of damage

No damage

Partially injured Depth of water table (m) Half collapsed

Large-scale half collapsed

Depth of water table (GL) Figure 11. Measured depth of water table at sites of damaged and undamaged houses in two districts of Figure 12. Distribution of the depth of the Urayasu (Ishikawa, Yasuda and water table in the Irifune district of Urayasu Ikarashi, 2014) (Ishikawa, Yasuda and Ikarashi, 2014)

shown in this figure, a water table of about 1.7 to 2.0m below the ground surface was the critical depth to cause damage to houses. The distribution of the depth of the water table estimated from the measurements in Irifune district is shown in Figure 12.

Similar studies on the effect of the depth of the water table or the thickness of the unliquefied surface layer were conducted in several cities damaged by the earthquake. Based on these studies, the MLIT proposed a new criterion to estimate the liquefaction-induced damage to wooden houses in 2014. Figure 13 shows the ground classifications based on this new criterion, in which the possibility of damage can be estimated by liquefaction potential, PL, or ground surface displacement, Dcy, and the thickness, H1, of the non-liquefied layer overlaying the liquefied layer. In this relationship, 3m is defined as the critical depth to cause damage to wooden houses. Figure 14 shows the authors’ concept of the mechanism of house settlement due to liquefaction. The settlement of a house probably occurs for two reasons i) the lateral flow of foundation ground due to a decrease of the shear modulus of the liquefied layer, as shown in Figure 14 (1), and ii) the densification of the liquefied layer due to the dissipation of excess pore water pressure, as shown in Figure 14 (2). When a liquefied layer is of uniform thickness and the upper non-liquefied layer is thin, houses penetrate into the ground, often at an angle, due to the lateral flow of the liquefied layer. In addition, uniform subsidence occurs due to the densification of the liquefied layer. However, if the non-liquefied layer is thick, penetration settlement and tilting is limited, though uniform subsidence due to the densification of the liquefied layer occurs. For the non-liquefied layer, it must be noted that the water table is not stable but increases during shaking for two reasons: i) the inflow of water from the lower liquefied layer due to liquefaction- induced densification, and ii) the spewing out of water due to excess pore water pressure induced in the liquefied layer. The inflow of water from the lower liquefied layer increases the water level by 1 m or less if the thickness and the volumetric strain of liquefied layer is several meters and about 5%, respectively. The water spewed out from the liquefied layer occasionally reaches a few meters above the ground surface, but it flows through narrow spaces, such as cracks in the ground, and lasts for a short time. Therefore, the ground water level usually increases by only a meter or so. Considering this increase in the water table due to liquefaction, a water table of about 2 to 3m below the ground surface must be the critical depth to prevent damage to a house, as schematically shown in Figure 15.

20 L P 判定結果Class Probability 液状化被害の可能性 of liquefaction-induced damage

L 15 P C B2 C 顕著な被害の可能性が高いHigh 10 A B3 B2 顕著な被害の可能性が比較的高いComparatively low

液状化5 指数 B1

Liquefaction potential, B3 B1 0 A 顕著な被害の可能性が低いLow 0 1 2 3 4 5 6 7 Thickness非液状化層厚 of non-liquefiedH1 layer,( m) H 1

Figure 13. A new method to estimate the liquefaction-induced damage to wooden houses (proposed by MLIT in 2014)

(m) 5 (m) 5 0 0 Settlement Liquefied Settlement Settlement Settlement Liquefied -5 -5 -10 -10

-20 -10 0 10 20 -20 -10 0 10 20 (m) (m) (1) Penetration settlement due to lateral flow (2) Uniform settlement due to densification

Figure 14 Authors’ concept of the mechanism of the settlement of houses due to liquefaction

Water Table: GL-1 m Water Table: GL-2 m Water Table: GL-3 m

Slight penetration settlement No penetration and tilting or no damage settlement and tilting

Water table Spew out water Water table raised due to due to high raised due to consolidation pore water consolidation pressure 1.0 1.0 2.0 2.0

Water table before 3.0 earthquake

m) Water table before ： earthquake m)

： Water table before (unit Liquefied Liquefied earthquake (unit Liquefied (unit:m)

Figure 15. Illustration of the impact of the water table on the liquefaction-induced settlement and tilting of a house

In-situ tests to study appropriate dewatering method In Japan, two districts where many houses had been damaged due to liquefaction during past earthquakes were restored by lowering the water table to prevent re-liquefaction during future earthquakes. One is the Tsukiji district of Amagasaki City, built on reclaimed land that was seriously damaged due to liquefaction during the 1995 Kobe Earthquake. As the ground water table was shallow, only GL- 0m to – 1m, drain pipes were buried to a depth of 2 to 3.5m to lower the water table (Suwa et al., 2012). The other district is the Yamamoto housing area of Kashiwazaki City, where serious damage occurred during the 2007 Niigataken-chuetsu-oki Earthquake. The ground water table was very shallow because the area is a gently sloping sand dune. Thus, the water table was lowered to a depth of GL – 2m using drain pipes (JGS, 2009).

The “Urban liquefaction countermeasure project” aimed to determine: i) how much to lower the water table, ii) how to lower the water table, iii) how much subsidence occurs accompanying the lowering of the water table, and iv) the cost of each method of lowering the water table. The project decided that a water table of about 3m below the ground surface was appropriate to prevent liquefaction damage to wooden structures in most cities based on a comparison of the water tables where structures had been damaged and the water tables where structures had not been damaged in each city and based on the criterion proposed by MLIT shown in Figure 13. In- situ tests were carried out in eight cities to decide appropriate methods of lowering the watertable, how much subsidence occurs with each method, and the cost of each method. Drain pipes were used to lower the water table in Itako, Kamisu, Kuki, Chiba and Kashima cities and in Tokai Village. Shallow wells to a depth of 3m were used in Abiko City and deep wells to a depth of 15m were used in Urayasu City. Figures 16 and 17 show the plane and the soil cross section at the in-situ test site in Chiba City. The test yard was 43.2m in length and 22.0m in width, and was enclosed by sheet pile walls. Two rows of drain pipes 20 cm in diameter were buried at a depth of GL-3.4 m with a distance of 39 m. The water table was lowered from about GL-1 m to about GL-3 m by pumping up the water collected in two manholes from drain pipes. Manhole No.1 Manhole No.2 W6 Sheet pile walls

8.1m 8.1m Measurements Weight of : Depth of water table house for

22m : Settlement of layers 1 story W3 Weight of W1 W2 house for W4 W5 : Pore water pressure 10.8m 10.8m Weight of 2 stories house for : Level of ground surface 2 stories

Drain pipe, D=20cm : Amount of rain water

Drain pipe, D=20cm 39m

43.2m

Figure 16. Plan of the in-situ test site in Chiba City (Chiba City, 2014)

Original water table before tasting Lowered water table Sheet Trench Trench Sheet piles SPT N SPT N SPT N piles 0 50 0 50 0 50 Level Level (m) (m) 0 0

Drain Drain FSC: Reclaimed sandy soil pipe pipe FC2: Reclaimed clayey soil AS1: Alluvial sandy soil ACS: Alluvial clayey or sandy soil A 2: Alluvial sandy soil -10 -10 S

-20 -20 0 10m

Figure 17. Soil cross section at the in-situ test site in Chiba City (Chiba City, 2014)

Depth Manhole No.1 Ground surface Manhole No.1 (m) W2 W3 W4 0 Water table before testing -1

-2 Water table 20 days after drainage began

-3 Drain 40 days 80 days pipe Drain -4pipe Water table 120 days after drainage began

Figure 18. Change of the water table during test in Chiba City

Test results showed that the ground water table could be lowered to a depth of about GL-3m even in the center between two drain pipes, as shown in Figure 18. The ground settlement due to the consolidation of the alluvial clay layer was very small, about 1cm. Pore water pressures measured before and after dewatering at several depths are plotted in Figure 19 together with the test results in Kamisu, Itako, Abiko, Kuki and Urayasu cities. As shown in the figure, pore water pressure did not decrease at deep layers but decreased at shallow depths only. Moreover, the over consolidation ratios, OCRs, of the soft clay layer were greater than 1.0, as shown in Table 2. The subsidence measured in each city was fairly small, as shown in Table 3. Figure 20 shows the time history of accumulated pumped-up water and rain water from the beginning of the test in Chiba City. Though much water had to be pumped up to lower the water table at the beginning of the test, a small amount of pumped-up water per day became constant after three to four months. It was estimated that the amount of pumped-up water would be about 1/2 to 1/3 of the amount of rain water. Based on the in-situ tests and other analyses, the Chiba City Government estimated that it would cost about US$22 million to treat an area of about 300 m x 250 m in the Isobe district, and decided that each family should pay about US$1,000 to maintain the dewatering system for 30 years.

Table 2. Over consolidation ratio, OCR, of alluvial clay

City Soil layer OCR Chiba Fc2 1.5 Ac2(GL-7m) 2.5 Abiko Ac2(GL-16m) 0.8 Ac2(GL-24m) 1.8 Fc 1.11 Urayasu Ac1 1.24 Ac2 2.00

Table 3. Results of in-situ tests of subsidence due to lowering the water

Test result City Time Subsidence (cm) Kamisu 60 days 0.1 to 0.5 Abiko Final About 5 Kuki 30 years 7.8

Depth Soil type Depth of Pore wat er Depth Soil type Depth of Depth Soil type Depth of (m) drain pipe pressure (kPa) (m) water pipe (m) (m) drain pipe 0 50 100 150 0 0 Bk 0 B Dr -1.8 Fs -2.0 Fsc Before test -2.8 -2.5 -3.0 -3.4 -3.5 As1

-5 16 months later -5 Sheetpile

Sheet pile Sheet A ฀฀ -5 Fc2 As -10 Ac -10 As1 Ds -10 Acs Kamisu City -15 As2 -15 -15 As2 Chiba City

-20 -20 Itako City

Depth Soil type Depth of Pore water Depth Soil type Depth of Pore water pressure (kPa) Depth Soil type Depth of Well (m) Well (m) pressure (kPa) (m) well (m) (m) water pipe (m) 0 200 400 600 0 50 100 150 0 0 0 F Bs Fs Bc -2.0 Bs -3.0 -3.0 Before test Measured at -4.0 Sheet pile Sheet the end of test -5 8 days later -5 -5 Fc

Analyed stable Ac1 pile

water table Sheet Sheetpile -10 Sheetpile -10 -10 As1 As1

-15 - 15 - 15 Analyzed at the end of test ial Init (Before est )t -15

-20 Ac2 -20 -20 Ac2

-25 -25 Ac1 -25

-30 -30 -30 As2 Kuki City - 35 -35 As2 Ds Abiko City -40 -40 B, F, Dr : Fill, Reclaimed soil As : Alluvial sand (loose) -45 Ac2 Ac : Alluvial clay(soft)

-50 Urayasu City Ds : Diluvial sand (dense) Ds

Figure 19. Change of pore water pressure during lowering of the water table

Rainfall water ) 3

up water water up Total pump-up - water

Pump-up water at No.1 Pump-up water at No.2 Accumulated pump and rainfall water (m

Month / date

Figure 20. Time history of accumulated pumped-up water and rain water in Chiba City （Chiba City, 2014）

Construction in several cities

The construction of drain pipes has started in Itako City, Tokai Village and Kamisu City. Figure 21 shows the drain pipes with many drain holes used in Tokai Village. Before being buried, the drain pipes are covered by screens. The ground under roads is excavated to a design depth with a width of about 1.3m, as shown in Figure 22, drain pipes are placed in the excavations, which are then filled with gravel and sand. Figures 23 and 24 show layouts of drain pipes at the Midorigaoka district of Tokai Village and the Horiwari to Wanigawa districts of Kamisu City. Drain pipes will be buried under most roads in these districts. In the Midorigaoka district, drain pipes will be buried in an area of about 200 m x 500 m for about US$3 million, and in the Horiwari to Wanigawa districts, drain pipes will be buried in an area of about 1 km x 1 km for about US$50 million.

Figure 21. Drain pipes used in Tokai Figure 22. Placing drain pipes in an excavated trench in Village Kamisu City

: Drain pipe

050 100 200 300 (m)

Figure 23. Layouts of drain pipes in Midorigaoka district of Tokai Village (Hashimoto, Oyama and Yasuda, 2014)

Drain pipe

0 500m

Figure 24. Layouts of drain pipes in Horiwari to Wanigawa districts of Kamisu City (Hashimoto and Takeuchi, 2014)

Conclusions

The applicability of lowering the ground water table to improve the liquefiable soil of an entire area has been studied by in-situ tests and other tests. The following conclusions were derived: (1) An appropriate water table to prevent liquefaction damage to wooden structures is about GL- 3m. (2) Drain pipes and shallow wells placed under roads can lower the water level under houses also. (3) Pore water pressure decreases due to dewatering only at shallow depths. (4) Subsidence due to dewatering is small if the water table is lowered moderately.

In 2014, the MLIT published a guide to apply this new remediation concept to many cities based on the experience gained from the “Urban liquefaction countermeasure project”. It is hoped that, in the near future, the liquefiable areas in many cities will be treated by lowering the ground water table or by other methods to prevent liquefaction induced damage.

References

Abiko City: Report of Technical Committee on Measures against Liquefaction, 2014. (in Japanese) Chiba City: Report of Technical Committee on Urban Liquefaction Countermeasure Project, 2014. (in Japanese) Hashimoto, T. and Takeuchi, H.: Experimental validation of liquefaction countermeasure by groundwater lowering method in Kamisu City, Proc. of Special Symposium – Overcome Great East Japan Earthquake- , JGS, pp.588-596, 2014. (in Japanese) Hashimoto, T., Oyama, J. and Yasuda, S.: Analysis of residential land damage to Minamidai - Midorigaoka in Tokai Village caused by the 2011 Great East Japan Earthquake, and measures for the restoration, Proc. of Special Symposium – Overcome Great East Japan Earthquake- , JGS, pp.366-374, 2014. (in Japanese) Ishihara, K.: Stability of natural deposits during earthquakes, Proc. of the 11th I.C.SMFE., Vol.1, pp.321-376, 1985. Ishikawa, K., Yasuda, S. and Ikarashi, S.: The effect of groundwater level on damage of wooden houses due to liquefaction and the 2011 Great East Japan Earthquake, Proc. of Special Symposium – Overcome Great East Japan Earthquake- , JGS, pp.523-534, 2014. (in Japanese) Itako City: Report on Urban Liquefaction Countermeasure Project at Hinode, 2013. (in Japanese) JGS: Reconnaissance Report on the 2007 Niigataken-chuetsu-oki Earthquake, 2009. (in Japanese) Kamisu City, 2013, Report of Technical Committee on Measures against Liquefaction. (in Japanese) Komatsu, N., Chino, K., Uno, H., Ishii, I., Akimoto, T. and Hiradate, R.: In-situ tests and pre-study on the method for lowering water table in Urayasu, Proc. of the 68th Annual Conf., JSCE, pp.173-174, 2013. (in Japanese) Kuki City: Report of Technical Committee on Measures against Liquefaction in Kuki City, 2014. (in Japanese) Suwa, S., Fukuda, M., Hamada, A. and Hongo, T.: Application of lowering water table as a countermeasure against liquefaction, Proc. of the 10th Symposium on Soil Improvement, pp.213-220, 2012. (in Japanese) Urayasu City: Report on measures against liquefaction in Urayasu City, 2012. (in Japanese). Yasuda, S. and Ariyama, Y.: Study on the mechanism of the liquefaction-induced differential settlement of timber houses occurred during the 2000 Totoriken-seibu earthquake, Proc. of the 14th World Conference on Earthquake Engineering, Paper No.S26-021, 2008. Yasuda, S., Harada, K., Ishikawa, K. and Kanemaru, Y.: Characteristics of the liquefaction in Tokyo Bay Area by the 2011 Great East Japan Earthquake: Soils and Foundations, Vol. 52, No.5, pp. 793-810, 2012. Yasuda, S., Towhata, I, Ishi, I, Sato, S. and Uchimura, T.: Liquefaction-induced damage to structures during the 2011 Great East Japan Earthquake: J. of JSCE, Vol. 1, pp. 181-193, 2013. Yasuda, S. and Ishikawa, K.: Damage to sewage and gas facilities induced by the 2011 Great East Japan Earthquake: Proc. of the 2nd European Conference on Earthquake Engineering and Seismology, Paper No.1029, 2014,. Yasuda, S.: Allowable settlement and inclination of houses defined after the 2011 Tohoku - Pacific Ocean Earthquake in Japan, Geotechnical, Geological and Earthquake Engineering, Springer, Vol. 28, pp.141~157,2014a. Yasuda, S.: New liquefaction countermeasures for wooden houses, Soil Liquefaction during Recent Large-Scale Earthquakes, CRC Press, Taylor & Francis Group, A Balkema Book, pp.167-179, 2014b.