Kazuma IYO

Experimental Investigation of Compressive Properties of Soft Ground and Numerical Analysis of Long-Term Settlement of Embankments

Satoru OHTSUKA

The Japan Sea Coastal Tohoku Expressway has been subjected to continuous settlement of the embankment over a long period of time, necessitating ongoing maintenance and repair. The large amount of groundwater use causes a drop in the groundwater table, which in turn causes land subsidence. Ground subsidence is caused by the consolidation phenomenon of soft ground, and it is assumed that if groundwater is used stably at the same scale every year, the subsidence will be controlled, but in reality, subsidence is a continuing phenomenon. The purpose of this study is twofold: first, to clarify the consolidation characteristics of the local ground through a one-dimensional consolidation test in which groundwater table fluctuations are taken into account in the stress cycle of loading and unloading of the consolidation load; and second, to investigate the effects of the groundwater table fluctuations on the consolidation characteristics of the local ground through the use of a one-dimensional consolidation test. The second is to determine the analytical parameters from the consolidation test results and to perform an elasto-plastic consolidation analysis of the residual settlement of the embankment. The purpose of this study is to predict the impact and residual settlement of the main line due to the widening work expected in the double-track construction, and to investigate methods to reduce the amount of residual settlement.
Repeated consolidation tests were conducted to determine the consolidation characteristics of the soil under repeated loading caused by fluctuations in the groundwater table. The following is a summary of the test results.
 The decrease in pore ratio with cyclic loading was confirmed.
 The decrease in the pore ratio of the soil with cyclic loading results in an overconsolidation of the stress state, but no change in the normal consolidation curve.
 The amount of strain generated is proportional to the magnitude of the overconsolidation ratio that causes the repetition.
 Plastic deformation due to cyclic loading in the range of large overconsolidation ratios is small.
 Plastic deformation with cyclic loading tends to converge to a constant value, but does not reach convergence.
 The amount of strain generated by cyclic loading is strongly correlated with the plasticity index, natural water content ratio, and liquid limit, and the larger these indices are, the greater the amount of strain generated.
　Elastic-plastic consolidation analysis of residual settlement of embankments was conducted using the tij model and the Sekiguchi-Ohta model. The following is a summary of the results of the analysis.
 Although the tij-model accurately reproduced the shear and consolidation behavior associated with embankment loading, it could not represent the secondary consolidation and could not represent the residual settlement associated with long-term settlement. 
 The Sekiguchi-Ohta model showed the same trend as the measured data after the completion of embankment construction. The Sekiguchi-Ohta model can analyze the behavior of the ground with residual settlement relatively well because secondary consolidation can be taken into account.
 The relationship between the residual settlement and the height of the fill was confirmed: the higher the fill height, the smaller the residual settlement. However, the residual settlement is still more than 1 m even when the height of the embankment is increased to 4 m, which is not very effective in reducing the residual settlement.
 Residual settlement was reduced to 0.002 m by applying lightweight embankment from 6000 days after embankment construction. The lightweight embankment method is effective in areas where soft ground is thickly accumulated, because it can significantly reduce the embankment load.