24. Seismic resistance design method considering unsaturated condition of embankments

  • Following severe rainfall, water and air can be found mixed in the interstices of an embankment. Strength tests have shown that the strength of material in an embankment is greater when partially saturated than when fully saturated.
  • A method for designing the antiseismic reinforcement of embankments was therefore proposed, considering the unsaturated condition of an embankment.
  • A manual was produced and the design was applied in trials, which demonstrated that antiseismic reinforcement work costs could be reduced by up to 20%.

Over the past few years, progress has been made in antiseismic reinforcement of embankments, however, existing design methods for antiseismic reinforcement are based on designs using the results of strength tests on saturated embankment materials. Consequently, there is a dearth of seismic evaluations and much work still has to be done in the field of reinforcement.

The water content inside an actual embankment affected by rainfall was therefore measured over a long period of time. These measurements confirmed the unsaturated state of parts of the embankment and that it was possible to reproduce the behavior of the embankment through seepage analysis (Fig. 1).

Strength tests on unsaturated embankment material demonstrated that cohesion was greater than when the embankment was saturated (Fig. 2).

A design method for antiseismic reinforcement considering the unsaturated condition of an embankment was proposed on the basis of this outcome, and a manual was produced setting out the prerequisite conditions, surveying methods and laboratory test and design methods.

Subsequently the proposed method was compared with the existing evaluation method. The outcome showed that the required soil nails could be shortened, reducing the whole cost of work by approximately 20% compared to designs based on the existing method (Fig. 3).

Fig. 1 Distribution of saturation level inside a railway embankment after rain
Fig. 2 Cohesion of unsaturated soil
Fig. 3 Comparison of construction cost