15. An soundness evaluation index and method for dealing with ballasted track that is contaminated with soil

Ballasted track bed that has degraded over time or that has been contaminated with soil due to flooding and landslide after heavy rains is more likely to subside rapidly under train loads. The ballast is usually replaced in this case, but high repair costs have been an issue.
In particular, there is typically a need to resume operation through the affected section as quickly as possible without ballast replacement, by reducing the train speed.
To address this issue, we assessed the settlement characteristics of ballasted track bed by conducting repeated loading tests on ballast with different fine particles. Using the Fouling Index FI (sum of the proportions of ballast containing particle sizes of 75 μm or less and 4.75 mm or less) as an indicator of the soundness of the ballast, we demonstrated that the risk of rapid subsidence can be determined.
The higher the value of FI, the greater the amount of subsidence (Fig. 1).

To control the subsidence of ballasted track bed with a high soil content, we developed a low-strength stabilization (stabilized to a strength that allows re-tamping repair) that mixes ultra-fast hardening cement and polymeric materials to enable repair by tamping.
We confirmed in full-scale tests that the method is effective in limiting subsidence of ballast with an FI of 20% or more, which is likely to suffer rapid subsidence.
With this method, the ballast can even be re-tamped and repaired again after construction (Fig. 2).
Trial constructions using multiple tie tampers on sections that had degraded over time and using ballast tampers on damaged sections demonstrated the effectiveness of subsidence control (Fig. 3).
This method can reduce construction costs by 90% compared to ballast replacement.
In April 2022, we commercialized a method for evaluating the soundness of ballast and a low-strength stabilization We also created manuals for the methods.