Labor-saving tracks are being introduced on existing lines as an effective means to reduce the costs of track maintenance. However, the cost reduction effect is not always as high as expected with lines on soft roadbeds (i.e. mainly water-containing cohesive soil roadbeds), as the roadbed soil in this ground type tends to flow out. To reduce track maintenance costs further, it is desirable to solve the problem of roadbed degradation so that labor-saving tracks can be laid effectively on soft roadbeds.
      To study the degradation of labor-saving tracks on existing lines on water-containing cohesive soil roadbeds, the RTRI performed element and full-size model tests using various soil qualities and boundary conditions. Through these tests, the RTRI clarified the mechanism of roadbed degradation. In particular, (1) changes in water pressure between the grouting layer and the roadbed soften the roadbed; (2) the softened roadbed soil is pushed out of the grouting layer by the pressure of the water (Fig. 1), forming water channels on the roadbed surface; (3) the water channels spread inside the roadbed, softening the entire roadbed surface further.
      Based on these results, the RTRI developed two methods of preventing cohesive soil roadbed degradation without performing large-scale roadbed improvement work. One involves the use of a permeable layer on the roadbed surface to disperse the surface water pressure, and the other is to protect the roadbed surface with a bentonite layer to prevent weakening of the roadbed surface. Mock-up tests were implemented to confirm the effects of these methods in preventing roadbed degradation and track subsidence (Figs. 2 and 3).

Fig. 1 Draining of roadbed soil from an unprotected roadbed

Fig. 2 Effect of roadbed surface protection work

Fig. 3 Changes in the amount of track subsidence over time

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