A long-term delayed consolidation settlement of naturally deposited soil and its prediction and improvement methods

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In a naturally deposited clay foundation subject to embankment loading, a delayed settlement can often be observed after the embankment's completion, sometimes also showing acceleration over time. The settlement cannot be always predicted from the laboratory test results even with the undisturbed sample. The prediction of the settlement and the long-term period were always underestimated.In the present study, the compression index ratio Cd Ccr was defined by the ratio of the compression index of the undisturbed sample Cc to remolded sample Ccr, and the in-situ settlement data and the laboratory test results on the about 50 expressways are estimated and classified by the compression index ratio Cd Ccr and sensitivity index St. The mechanism of the large delayed settlement is discussed described by using a soil-water coupled finite deformation analysis with the Supre/subloading Yield Surface Cam-clay model (SYS Cam-clay model). The new findings are as follows :1)The clay, which exhibits a long-term large settlement beyond the prediction, can have high sensitivity St, the value of which is more than 8.0 and high the compression index ratio Cd Ccr, more than 1.5. Through the simulation by the SYS Cam-clay model, the clay is a high degree of initial structure and show fast decay of the structure.2)The in-situ compression behavior was predicted from the laboratory test assuming that both the consolidation yield stresses are the same, and the mechanism of the long-term large settlement was discussed taking the sample disturbance into consideration.3)The occurrence of in-situ secondary/delayed consolidation of clay foundation in Nagashima can be described by using a soil-water coupled finite deformation analysis with the SYS Cam-clay model, and it provide that it is necessary to determine the in-situ initial condition from the laboratory test taking the sample disturbance into consideration.