1. Roadbed Instability Problems Chapter 19

2. Roadbeds built years ago  Crude Methods Seasoned  Time and Loading Problems  Sinks  Soft Spots  Water Pockets  Slides  Subsidence

3. Instability leads to—  Increased Maintenance Costs  Slow Orders  Reduction in Track Capacity  Derailments

4. Typically a Soils/Drainage Problem  Increase Strength/Stability of Soil  Lower Moisture Content/Lower Plasticity  Improve Drainage  Separate Trackbed Support Layers

5. Divide Discussion into Three Parts:  Subsidence Problems  Slope Stability Problems  Subgrade Support Problems

6. Subsidence  Vertical Settlement  Soil too Weak  Occurs on Fills Weak Soils  Shear  Compresses

8. Height of Fill  Should not exceed bearing capacity of soil, q d  Normally a problem with fine-grained soils Or

9. Corrective Measures:  Lower the Grade  Excavate and Replace  Allow for Settlement  Use Surcharge  Lower Water Table  Relocate Line

10. (a) Excavation of much to firm ground; (b) overloading fill to accelerate settlement

12. Slope Stability—horizontal component  Shear—upper  Foundation—lower  Sidehill Slides—lubricated surface

13. Shear—Slope of fill too steep Solutions:  Flatten slope  Surcharge toe  Improve drainage

14. Foundation—too weak Solutions:  In-place stabilize foundation  Surcharge toe  Lower ground water table

15. Solutions:  Intercept drainage  Retaining wall  Pile and pole driving Sliding of sidehill fill

19. General Comments:  Saturation  Adjust Slope  Rock Falls in Cuts  Adjacent Disturbances  High Water/Drawdown during Floods

20. Subgrade Support  Too Weak to Support Loads  Ballast not Properly Restrained  Water/Ballast Pockets Develop  Frost Heaving more Pronounced

26. Stabilization Solutions  Improve Drainage Diversion ditches—Intercept Conveyance ditches—lateral and longitudinal French drains Deep ditching—lower water table

27.  Soil Additives Blend in during new line construction Pressure inject in existing lines  Advanages Increase strength—cementing Lower plasticity—base exchange Reduce affinity for water—lower PI

28.  Geotextiles (Filter Blankets) Variable Success Must have drainage outlet Must protect GeoWeb is more effective

29.  Separation mat or layer Asphalt successfully utilized  Separate ballast from subgrade  Confine ballast  Waterproof subballast/subgrade  Distribute reduced pressures  Provide Consistent Support