When long-stripe structures used as road embankments are constructed on liquefied sandy soil deposits, several countermeasures against liquefaction using soil improvement methods such as sand compaction pile method or deep mixing method are generally applied to the sandy ground directly under the fill slope in the longitudinal direction of the structures. However, such improvement methods are costly. When the site is not restricted to a narrow area and the land price is low, counterweight fill methods are usually applied to soft clayey soil deposits as a static stabilization countermeasure. In the present study, a series of residual deformation analyses during earthquakes was performed in order to clarify the effects of the counterweight fill on the reduction of settlement of the main embankment constructed on liquefied ground, where the counterweight fill was applied to the liquefied ground as a countermeasure against liquefaction without improvement of soil ground described above. Static analyses using the FEM analysis program ‘ ALID’ were conducted for three sections of counterweight fill models with different heights and without counterweight fill. Consequently, the earthquake-induced residual settlement of the main embankment, corresponding to the roadway part, decreased when the width and height of the counterweight fill was long and high, respectively. Therefore, reduction in residual deformation may be possible due to the application of the counterweight fill without the soil improvement methods.

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About 27,000 wooden houses were damaged due to liquefaction during the 2011 Great East Japan Earthquake. There are four possible patterns to reconstruct damaged houses and/or areas. The author and his colleagues developed a new method by enclosing individual house by sheet piles. Effectiveness of the method was demonstrated by shaking table tests. On the contrary, if all or many settled and tilted houses are temporary repaired by uplifting, the ground in the whole area must be treated by special measures to prevent re-liquefaction. The MLIT established a new project, the “Urban liquefaction countermeasure project”. In this project, a wide existing residential area is treated by an appropriate countermeasure and its costs are shared by government and inhabitants. One of the available countermeasures is lowering the ground water table by drain pipes. The applicability of this method has been studied by in-situ tests, centrifuge tests and analyses. The construction of the drain pipes has started in three cities.

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Three dimensional finite element simulation of 3×5 pile groups in layered liquefiable ground during earthquake was carried out in this paper to investigate the dynamic load distribution within the pile groups. A unified plasticity model for large post-liquefaction shear deformation of sand was used for the appropriate simulation of soil liquefaction. Load distribution over the corner, edge and central piles was studied. The influence of pile spacing on the load distribution was also studied. Numerical results showed the corner piles, the edge piles and the central piles were subjected to varying dynamic loads. The pile spacing had significant impacts on the load distribution of pile groups.

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Significant number of subsurface cavities was found in the liquefied ground after the 2011 Great East Japan earthquake. Using the results of the radar exploration conducted in Urayasu-city, Shinkiba-area and Narashino-city, all of those suffered from damage by the ground liquefaction, characteristics of subsurface cavities are investigated. It was found that cavities tended to form near man-holes and joints in pavement. Size and shape of the cavities are larger and thinner compared to those of cavities observed in the non-liquefied ground. A series of model tests was conducted in order to understand the mechanism of sand eruption and underground cavity formation when liquefaction occurs. Liquefaction and sand boiling was simulated in the model test by the upward seepage flow. With the increase in the hydraulic gradient, sand grains initially moved horizontally and then vertically, causing disturbance and loosening in the ground. The flow rate at the gap to cause sand eruption increased with the increase in the grain size. The hydraulic gradient causing sand eruption was much higher than the critical hydraulic gradient in all the test cases.

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Appropriate countermeasures against residential damage induced by liquefaction under great earthquakes in Inashiki City, Ibaraki were investigated because residences sustained severe damage from the 2011 off the Pacific coast of Tohoku Earthquake of March 11, 2011. Metrics to ascertain the magnitude of objective seismic motions and the effectiveness of countermeasures were established, along with their targeted levels for securing safety. This paper presents outlines of damage features in Inashiki under actually measured seismic motion (259 Gal acceleration, 5-weak seismic intensity, M9) of the 2011 off the Pacific coast of Tohoku Earthquake. Simultaneously, the paper reports results of numerical simulations of liquefaction that damaged residences conducted to validate the model. To assess earthquake-related liquefaction, the repeatability of damage features observed from the great earthquake was improved as explained below. i) Age effects corresponding to the sedimentary time period of sandy deposits are considered when evaluating liquefaction factor FL. ii) Sandy deposits with plasticity index Ip of less than 25 are assumed for liquefiable layers. iii) Ground surface displacement Dcy corrected according to the magnitude of liquefaction factor PL. Subsequently, results from numerical analysis of liquefaction severity for cases with and without countermeasures were compared under the actually measured seismic motion. Then numerical analyses were conducted for cases under the seismic motion (200 Gal and M9.0), which are guidelines by the Ministry of Land, Infrastructure and Transport in Japan. Based on results from the numerical analysis stated above, setting of a fundamental policy was proposed for determination of countermeasures against liquefaction under a great earthquake. Countermeasures should be sought that enable attainment of rank A (H1 > 5 m , H1 is thickness of a non- liquefaction layer which is continuous from the ground surface) and rank B1 (H1 > 3 m and corrected Dcy≦10 cm ) under seismic motion of type 2 (medium shock by great quake: 200 Gal, M9.0) in all regions, except for areas ranked locally as B2 (H1 > 3 m and corrected Dcy > 10 cm).

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Lateral flow or differential settlement caused by liquefaction results in serious damage to pavements on sandy reclaimed land in coast area. In general design procedure of port and airport facilities in Japan, a liquefaction countermeasure is applied if liquefiable sand layers exist in which the layers are thicker than 3 m and located shallower than 20 m. However, these criterions are based on an empirical rule or field investigation of past earthquakes and there is a few physical background. Moreover, it has not been systematically studied that the needs for liquefaction countermeasures when multi-liquefiable layers are existed. In present study, centrifuge modeling was conducted to simulate liquefaction and sand boiling on multi-layered sandy ground. Based on the results, conditions of occurrence of sand boiling and localized settlement were discussed. In addition, liquefaction resistance and drainage process after liquefaction of sand containing silt used in centrifuge tests were evaluated using undrained cyclic triaxial compression tests.

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Numerous structures, including residences and buildings, were damaged severely not only in Tohoku but also in Ibaraki, which is adjacent to Fukushima, one Tohoku prefecture affected by the Great East Japan Earthquake that struck on 11 March 2011. One difficulty presented by the great earthquake is the method used to reconstruct damaged residences. Another difficult issue is how to maintain the stability of existing structures on sandy soil deposits with high ground water level (GWL) against the coming great earthquakes. Those structures are endangered by liquefaction-induced damage, particularly vulnerable are aged structures that experienced the great earthquake. This study specifically examines a proposal for reducing the settlement and deformation of existing structures on sandy ground during earthquakes. Model tests were conducted under normal gravity (1 × g) conditions to examine the effects of groundwater level lowering techniques with and without sheet pile walls installed along the sides of structures. Analysis of the test results showed the following: (1) Severe structural settlement occurred when no sheet pile walls on sands with high water level had been installed. However, the longer the sheet pile walls had been installed, the more they reduced settlement during earthquakes. (2) Furthermore, the combined use of sheet pile wall installations and a decrease in groundwater level engendered the greatest reduction in structural settlement. (3) It is greatly advantageous from a practical perspective that it is not necessary to attach sheet pile walls to the side walls of structures when the groundwater level becomes lower. Therefore, it is strongly recommended that groundwater lowering techniques should be combined with sheet pile installation for reducing the damage to existing residences during earthquakes.

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