Journal of JSCE, Vol. 1, 307-321, 2013 Special Topic - 2011 Great East Japan Earthquake (Invited Paper) STUDIES ON SOIL LIQUEFACTION AND SETTLEMENT IN THE URAYASU DISTRICT USING EFFECTIVE STRESS ANALYSES FOR THE 2011 OFF THE PACIFIC COAST OF TOHOKU EARTHQUAKE Kiyoshi FUKUTAKE1 and Jiho JANG2 1Member of JSCE, Institute of Technology, SHIMIZU Corporation (Etchujima3-4-17, Koutou-ku, Tokyo, 135-8530, Japan) E-mail: [email protected] 2Material & Energy Research Team, GS E&C Research Institute (Cheoin-gu, Youngin-si, Gyeonggi-do, 449-831, Korea) E-mail:[email protected] The 2011 off the Pacific coast of Tohoku Earthquake caused soil liquefaction over a wide area. In par- ticular, severe soil liquefaction was reported in the northern parts of the reclaimed lands around Tokyo Bay, even though the seismic intensity in this area was only about 5 on the JMA scale with low acceleration. The authors surveyed the residual settlement in the Urayasu district and then conducted effective stress analyses of sites affected and not affected by liquefaction. The analyses results were compared with the acceleration waves monitored with K-NET Urayasu or ground settlements surveyed. They were based on the acceler- ation observed on the seismic bedrocks in earthquake engineering in some other districts adjacent to Urayasu. Much of the settlement was due to the long duration of the earthquake, with further settlement resulting from the aftershocks. The study shows that the effects of aftershocks need to be monitored. The simplified liquefaction prediction methods using the factor of safety, FL, also need improvement. Key Words : The 2011 off the Pacific coast of Tohoku Earthquake, liquefaction, settlement, effective stress analysis, aftershock 1. INTRODUCTION Aerial laser surveying was carried out over Urayasu City before and after the earthquake, and the change The 2011 off the Pacific coast of Tohoku Earth- in the ground surface was indicated by the difference quake (hereafter, Earthquake 3.11) caused liquefac- in the elevation values.2) The errors, however, were tion over a wide area. Focusing on the reclaimed land large and the values did not necessarily correspond to in the northern part of Tokyo Bay, the seismic in- the actual amount of settlement. Konagai et al.21) tensity was about 5 (ground acceleration of about 100 conducted a LiDAR survey and drew subsidence to 150 Gal), thus the accelerations were not very maps of the Tokyo Bay shore areas, and the measured high, but the duration of the seismic motions was results contained the effects of the structures. long, and this caused liquefaction and settlement seen The authors measured the amount of settlement in in many areas. On the other hand, even though the Urayasu, Shin Kiba, and Tatsumi a few days after the ground conditions were similar, some places did not occurrence of the earthquake, and prepared drawings undergo large settlements. As a result, the Japanese of the distribution of the settlement. They then in- Geotechnical Society and the Ministry of Land, In- vestigated the causes of the variation in the distribu- frastructure, Transport and Tourism Kanto Devel- tion of the settlement, the amount of the settlement, opment Bureau jointly summarized the liquefaction and the difference in settlement. and non-liquefaction areas on a map as shown in In addition, focusing on the Urayasu area, typical Fig.1.1) However, indices directly related to damage, ground models were prepared in each area from the such as settlement, etc., were not shown on the maps. N-values and borehole data of ground that liquefied 307 and ground that did not liquefy. Response analysis by The results were compared with the K-NET effective stress analysis was carried out based on the Urayasu acceleration records and the amount of set- acceleration records measured at the engineering tlement measured at the locations of liquefaction. In bedrock near these areas, including the main shock addition, an improvement to the FL method of de- (M = 9.0) and the largest aftershock (M = 7.7), which termining liquefaction in cases where the duration is occurred 29 minutes later. long as in multi-segment earthquakes was proposed. 2. CHARACTERISTICS OF SETTLEMENT IN RECLAIMED GROUND IN THE NORTHERN PORTION OF TOKYO BAY Toky (1) Characteristics of settlement distribution Figures 2 and 3 show the amount of horizontal Tokyo Bay ground surface settlement in the Tatsumi - Shin Kiba Kanagawa Pref. Chiba Pref. - Urayasu area located in the northern part of Tokyo Liquefaction Area Bay shown in Fig.1 within a few days after the Non-Liquefaction Area earthquake. These values were measured using bearing pile structure as a benchmark. The numerical values of the colors are: blue, 0 to 10 cm; orange, 10 to 30 cm; and red, 30 cm or more. Measurement of the amount of settlement was carried out using points Shimizu Institute of Technology that had not moved, such as pile-supported buildings Tatsum where no settlement occurred. Settlement of the whole area did not occur, but places where the set- tlement was large and places where there was almost Shin Kiba Urayasu no settlement were mixed. Figure 4 shows the geological section along the ―Liquefaction ― survey line and the improved areas by preloading and Tokyo Bay Non-Liquefaction 17) ■Liquefaction Area sand drain . ■Non-Liquefaction Area Fig.1 Developmental distribution of liquefaction in Kanto region and north Tokyo Bay in Earthquake 3.111). JR Keiyo Line Metropolitan Expressway Shin Kiba station Rinkai Line 400m Fig.2 Settlement in the Tatsumi - Shin Kiba area (cm). 308 Figure 5 shows the distribution of settlement along However, according to local taxi drivers, the noise the survey line indicated in Fig.3. Plots and arrows levels are greater when they drive now compared mean measured values and their ranges. Hatching with before the earthquake; likewise, the vibrations zone covers all measured values. With the Tokyo seen to be greater, thus it is considered that a certain Metro Tozai Line as the origin, there is almost no amount of settlement had occurred. From about evidence of settlement up to 2000 m. K-net URAYASU JR Keiyo Line Reclaimed land Phase I Reclaimed land Phase II Shin Urayasu station Metropolitan Expressway Old Coastline Ito- Hinode Yokado Takasu Maihama station Survey Line Tokyo Disneyland A Minato B Fig.3 Settlement in the Urayasu area (cm). Alluvial lowland Reclaimed land (Phase I ) Reclaimed land (Phase II) Fill preloading (Hb=1.7-4.0m) F Sand drain Fig.4 Geological section along the survey line in Fig.3 and the improved areas by preloading and sand drain17). 309 Tozai Line of Tkyo Metropolitan JR Keiyo Ito‐Yokado Co., Ltd. Metro Expressway Line Seawall 0 10 20 Old 30 Coastline Settlement (cm) 40 50 0 1000 2000 3000 4000 5000 Distance (m) Reclamation Phase I Reclamation Phase II (1971) (1978) Fig.5 Settlement along the survey line in the Urayasu area (cm). τ 0.30.3 2400 m, the settlement becomes larger, and is largest σ’mo at around 4000 m, reaching a maximum of about 50 Silty Sand cm. From that point until the sea wall, the settlement 0.20.2 A becomes smaller, thus overall, the distribution of Loose Sand Liquefiable settlement is shaped like a spoon. The settlement B 0.1 becomes smaller towards the sea wall because the Stress Raio 0.1 σ’mo=100kPa Xl =0.1 ground has been compacted as a mitigation measure γDA=5% Not-liquefiable against liquefaction from about 4500 m to near the 0.00.0 sea wall (near point A in Fig.3). In addition, the 1 10 10 100 1000 1000 construction methods taken to promote consolidation Number of cycles to liquefaction (sand drains + preloading) are considered to have a Fig.6 Liquefaction strength curves and liquefaction certain effect in reducing liquefaction near the sea strength lower limit value Xl. wall.17) The effect of construction methods taken to promote consolidation on reducing liquefaction has also been reported at Port Island and Rokko Island thick, thus the amount of settlement is comparatively after the 1995 Kobe earthquake.3), 4) If this type of small. In addition, small differences in the shear mitigation measure had not been taken, then the stress that acted many times are considered to have amount of settlement in the area reclaimed between had a major effect on the resulting phenomenon. In 1975 and 1978 would have appeared to be a bit larger other words, once liquefaction has occurred, the soil compared with the area reclaimed between 1968 to thereafter has been subjected to many repetitions 1971. However, the correlation between the amount resulting in severe liquefaction. The next section of settlement and the year of reclamation is not very considers the cause of this phenomenon. clear. As can be seen from the settlement distribution in (2) Liquefaction characteristics during cyclic Figs.2, 3 and 5, there is large variation in the amount small amplitude shearing at the element level of settlement. The amount of settlement varies con- The characteristics of liquefaction in this case siderably with 100 m separation, and there is large were caused by comparatively small accelerations variation even within narrow areas. This variation in that had continued for a long period of time, and as a the amount of settlement is due to whether the ground result, small shear stresses were repeated a large has been improved or not as described earlier, and number of times. differences in the surface layer soils and thickness of the reclaimed ground. If the surface layer is a thick clay layer, the settlements tend to be small; for ex- ample, near point B in Fig.3, the surface clay layer is 310 62 cycles 68 cycles 15 10 Loose Sand 5 0 -5 -10 ( kPa ) -15 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 132 cycles 15 145 cycles 10 Silty Sand Shear Stress 5 0 -5 -10 -15 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Shear Strain Fig.7 Stress-strain relationship near the liquefaction strength lower limit value Xl (stress ratio = 0.11).