Roadbed Instability Problems Chapter 19

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 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

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

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