一般注記我が国では，頻発する大規模な自然災害を受けて，それらの対策はもちろんのこと，土構造物に対する要求性能が高度化している．その中でも地盤の液状化に関する研究は，地震災害の歴史と共に発展をみせてきた．液状化試験の基礎的研究では，新潟地震が発生した1964年から試験基準が制定された1990年頃にかけて多くの成果が残されており，1984年にとりまとめが行われた「砂地盤の工学的性質の評価法に関する研究委員会」による報告では，測定器具の仕様や設置方法，供試体条件による結果の違いが示され，それらを統一する必要性が示されている．このような基礎的研究は，当該試験の基準化に当たって十分な検討が行われ，その成果を基に議論が進められることが多いが，地盤材料試験に対する需要の変化によって，力学特性の基礎となる供試体条件についても，更に詳細な知見が必要なケースが出ている．特に，同じ作製方法による設定条件の違いについては，静的載荷による力学試験結果と比較すると，繰返し載荷試験による試験結果は大幅に少ないのが現状である．現在，供試体の準備方法として，空中落下法や各種締固め法など多くの供試体作製方法が提案されているが，実務での適用が多いのは主に湿潤締固めによる作製方法である．実務において，湿潤締固め法の需要が高い理由は，試料の粒度組成や含水状態など物理的条件に対する選択の幅が広いこと，使用する器具が簡易であることである．本研究では，この湿潤締固め法に着目し，粒度組成の異なる2種類の砂質土を用いて，異なる作製条件下の供試体を作製し，繰返し非排水三軸試験と2種類の画像解析を実施している．画像解析では，はじめにX線CT画像によって供試体作製時の供試体内部構造を把握し，その後，PIV（Particle Image Velocimetry）解析結果によって繰返し載荷中の供試体表面における軸方向ひずみを計測し，締固め層厚が三軸液状化特性に及ぼす諸要因の影響について明らかにしている．また，2種類の砂質土による結果の比較を行うことで，粒度組成による液状化特性の相違についての考察を行っている．得られた結果では，現行基準の供試体作製方法における問題点・留意点が述べられ，また湿潤締固め法による供試体作製時の適切な締固め層厚（層数）を提案し，本研究を総括している．
Due to a lot of natural disasters in Japan, not only countermeasures against such disasters, but also the required performance of soil structures is becoming more sophisticated. Countermeasures against liquefaction have been also promoted, and along with that, the design have been developed. From 1964, when the Niigata Earthquake occurred, to around 1990, when test standards were enacted, many results were obtained in fundamental research on liquefaction testing reported by the "Research Committee on Evaluation Methods for Measurements". There are also cases where more detailed knowledge is required for test specimen conditions. In particular, regarding the difference in setting conditions for the same preparation method, the investigation cases of cyclic loading tests are significantly smaller than those of monotonic loading tests. It has been well known in laboratory element tests that liquefaction properties are affected by the difference in specimen preparation methods. Among various specimen preparation methods, the moist-tamping method has been often used in laboratory-element tests, which has a high reproducibility because grain size distribution and water content can be widely selected. In specimen preparation by the moist-tamping method, test specimens are generally prepared by compacting soil samples layer by layer. As a result, it has been reported that the non-uniformity in the specimen is induced during the compaction process. In this study, a series of cyclic undrained triaxial tests was performed on two kinds of sandy soils which are fine sand and well-graded sand to investigate the effects of the specimen-preparation methods on liquefaction resistance. In particular, the effects of the differences in grain size distribution and compaction thickness of the compacted specimens on liquefaction properties are discussed.A series of cyclic undrained triaxial tests was conducted under the different conditions of grain size distribution and compaction thickness to reveal the effects of specimen preparation methods on liquefaction properties of two types of sandy soils. The specimens were prepared using the moist-tamping method. The number of compaction layers was either 1, 4, or 10. The results showed that the liquefaction resistance was influenced by the thickness of the compaction layer, and its tendency changed depending on the grain size distribution and the compaction degree, Dc. In addition, it was found that a difference in the initial moisture content had an influence on deformation properties during cyclic loadings. In particular, the deformation behavior of the specimens compacted under lower moisture content was similar to that of loose specimens. Furthermore, in the case of well-graded sand, there was no significant difference in liquefaction resistance between specimens of 10 and 4 layers; however, the liquefaction properties of a 1 layer specimen were significantly different from that of other compaction layers. Based on the results, the points to note in the current standard preparation method are described, and the thickness of compaction layer by moist-tamping method is proposed, summarizing this study.
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受理日（W3CDTF）2023-12-15T22:13:25+09:00
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