Slide 1

Roadbed Instability Problems Chapter 19 Slide 2 Roadbeds built years ago Crude Methods Seasoned Time and Loading Problems Sinks Soft Spots Water Pockets Slides Subsidence Slide 3 Instability leads to Increased Maintenance Costs Slow Orders Reduction in Track Capacity Derailments Slide 4 Typically a Soils/Drainage Problem Increase Strength/Stability of Soil Lower Moisture Content/Lower Plasticity Improve Drainage Separate Trackbed Support Layers Slide 5 Divide Discussion into Three Parts: Subsidence Problems Slope Stability Problems Subgrade Support Problems Slide 6 Subsidence Vertical Settlement Soil too Weak Occurs on Fills Weak Soils Shear Compresses Slide 7 Slide 8 Height of Fill Should not exceed bearing capacity of soil, q d Normally a problem with fine-grained soils Or Slide 9 Corrective Measures: Lower the Grade Excavate and Replace Allow for Settlement Use Surcharge Lower Water Table Relocate Line Slide 10 (a) Excavation of much to firm ground; (b) overloading fill to accelerate settlement Slide 11 Slide 12 Slope Stabilityhorizontal component Shearupper Foundationlower Sidehill Slideslubricated surface Slide 13 ShearSlope of fill too steep Solutions: Flatten slope Surcharge toe Improve drainage Slide 14 Foundationtoo weak Solutions: In-place stabilize foundation Surcharge toe Lower ground water table Slide 15 Solutions: Intercept drainage Retaining wall Pile and pole driving Sliding of sidehill fill Slide 16 Slide 17 Slide 18 Slide 19 General Comments: Saturation Adjust Slope Rock Falls in Cuts Adjacent Disturbances High Water/Drawdown during Floods Slide 20 Subgrade Support Too Weak to Support Loads Ballast not Properly Restrained Water/Ballast Pockets Develop Frost Heaving more Pronounced Slide 21 Slide 22 Slide 23 Slide 24 Slide 25 Slide 26 Stabilization Solutions Improve Drainage Diversion ditchesIntercept Conveyance ditcheslateral and longitudinal French drains Deep ditchinglower water table Slide 27 Soil Additives Blend in during new line construction Pressure inject in existing lines Advanages Increase strengthcementing Lower plasticitybase exchange Reduce affinity for waterlower PI Slide 28 Geotextiles (Filter Blankets) Variable Success Must have drainage outlet Must protect GeoWeb is more effective Slide 29 Separation mat or layer Asphalt successfully utilized Separate ballast from subgrade Confine ballast Waterproof subballast/subgrade Distribute reduced pressures Provide Consistent Support