Evaluation of Settlements in Sand Deposits Following Liquefaction During Earthquakes

Correlation between liquefaction resistance and shear wave velocity

The treatment of the joint between new and old roadbed is one of the most concerned problems in the design and construction of highway widening engineering. Laying geogrid is an important treatment method of the joint of new and old roadbed. In this study, the centrifugal model test was used to study the treatment effect of reinforced embankment in the treatment of road widening at the junction of new and old roadbeds from three aspects: the dissipation of pore water pressure, settlement characteristics, and overall stability of the roadbed. It was found that the grid reinforcement treatment can reduce the pore water pressure of the new subgrade and adjust the load stress diffusion and the slope ratio of the uneven settlement at the top of the embankment is reduced by about 0.3%. The tensile strain of the grille near the old road slope is larger, and the maximum value appears at 1/3 length from the left end of the grille, while the tensile strain of the grille near the new road slope is very small. The soft soil layer of the embankment foundation has shear failure under the limit state and is close to the side of the new embankment; the central part of the embankment appears concave instability failure, and the transverse cracks appear at the top of the old road. At the same time, in order to comprehensively analyze the reinforced embankment properties of the widening project, the large-scale finite element software ABAQUS was used to carry out the numerical simulation of the centrifugal model test. The results of the numerical simulation and the centrifugal model test are in good agreement, and the obtained research results can provide scientific and reliable basis for the design and optimization of the increasingly rising highway widening project.

Centrifugal model test of embankment widening with geogrid treatment technique

The pile-supported embankment has been widely adopted in highway widening projects. There is often significant differential settlement between the new and old embankments. Previous studies have not clarified the interaction mechanisms between different parts of the widening subgrade. In this research, a series of centrifuge model tests were carried out on the subgrades widened by pile-supported embankment on the soft soil base. The effect of the old embankment and piles on the subgrade deformation subject to new embankment is analyzed based on the full-field measurement results of displacement. The embankment displacement is mainly vertically downward and basically caused by the deformation of the soft soil base. The old embankment redistributes the vertical deformation of soft soil base by increasing and decreasing the vertical displacement in turn along the direction towards the slope surface. The piles exhibit supporting effect on the embankment above and restrict the vertical movement of the soil base. The piles influence the deformation of soft soil base within a certain area which expands with the increasing number of piles. The boundary of the pile influential area moves towards the subgrade axis with increasing depth. The vertical deformation of soft soil base shows peaks that are nearly linearly distributed along depth in the pile influential area. As the number of piles increases, the verticality degree of such linear distribution changes from the vertical to the slope firstly and then reverts to the vertical. 

Centrifuge modeling of pile-supported embankment on soft soil base for highway widening

When ground liquefaction occurs, settlement is often caused by shear deformation during and consolidation after an earthquake. However, settlement does not occur regularly but usually varies in space. In order to clarify the mechanisms underlying non-uniform settlement caused by liquefaction, 2-dimensional effective stress analyses were performed on a field damaged in the 2011 off-the-Pacific-coast-of-Tohoku earthquake. Distributions of the thickness and resistance of the liquefiable layer obtained from field surveys were considered in the numerical models. Simulated results confirmed that the non-uniform settlement was caused by the spatial distribution of the thickness and resistance of the liquefiable layer.

Prediction Method of Damage of Ground Induced by Liquefaction Based on Geological Structure of Surface Ground

The improvement rate of countermeasures adopted in constructing a second track on an embankment on soft ground is determined empirically due to unknown factors such as the deformation properties of soft ground. In this study, centrifuge model tests were performed to grasp the deformation properties of soft ground for such construction cases. Based on the test result, a new countermeasure was proposed, which has high horizontal support performance and combines deep mixing wall-type ground improvements and cement-mixed gravel. The proposed method having the same improvement rate as deep mixing was confirmed as being able to reduce the deformation of the embankment and the surrounding ground.

Proposal of Constructional Countermeasures for the Widening of Embankments with a Focus on Their High Stability

In the 2011 Great East Japan Earthquake the rail yard embankment constructed by use of coal ash was damaged by severe liquefaction, resulting in the differential settlement of railway track. In the railway the settlement is limited to ensure safety, however the evaluation method for differential settlement is not established. It is thus necessary to establish the evaluation method for the differential settlement of railway track induced by liquefaction. In this study, cyclic undrained triaxial tests for the fill material contained coal ash were performed to validate whether it was liquefied. It was found that the fill material was possibility to be liquefied sufficiently. In addition, field level survey and surface wave exploration method were conducted to examine the effect of the thickness of liquefiable layer and S-wave velocity distribution on differential settlement caused by liquefaction. The results revealed that the differential settlement is concerned with distribution of the S-wave velocity and the thickness of the liquefiable layer.

Investigation of liquefiable property and S-wave velocity distribution focused on coal ash used for rail yard embankments

含水状態の異なる不飽和盛土の模型振動台実験;含水状態の異なる不飽和盛土の模型振動台実験;Model Shaking Table Test of Unsaturated Embankment in Different Water Content

近年，降雨で鉄道盛土に被害が受ける事例が増加しており，列車の運行再開を急ぐ場合には応急復旧が行われ，大型土のうを用いた復旧構造が採用されることが多い．しかしながら，大型土のうでの応急復旧盛土を対象とした安定や沈下に対する検討は十分にはなされていない．本研究では，本構造を対象として，模型盛土に対する降雨散水・載荷試験による列車運行再開時を想定した列車荷重の載荷に対する安定の検討，および繰返し載荷試験よる累積沈下特性に関する検討を行ったところ，土のう直下地盤の支持性能や排水性を確保できれば列車運行再開時の安定を満足できることや，累積沈下の発生を抑制できることがわかった．また，土圧に関するつり合い検討によって土のうの積層に対する安定を評価可能であり，盛土高さや水位・盛土材料の強度特性に応じた応急復旧構造を解析的に示した． 

Study on stability and settlement of emergency reconstructed embankment using sandbag against train load

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