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Gravel Roads: Theory and Engineering

Gravel Roads: Theory and Engineering. How Are Roads Built?. Evolution Stock trails Double tracks Gravel road Asphalt road Revolution New construction. Typical Gravel Road Section. Unimproved double track. Roadbed has been shaped but no imported gravel.

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Gravel Roads: Theory and Engineering

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  1. Gravel Roads:Theory and Engineering

  2. How Are Roads Built? • Evolution • Stock trails • Double tracks • Gravel road • Asphalt road • Revolution • New construction

  3. Typical Gravel Road Section

  4. Unimproved double track

  5. Roadbed has been shaped butno imported gravel.

  6. Slightly improved, with some imported gravel and ditches

  7. Gravel is added and the road gets higher.

  8. Vertical alignment improvements

  9. More gravel is added, the road is widened

  10. What Happens to Roads?

  11. Construction Maintenance Rehabilitation Materials Design Traffic Loads Environment Performance

  12. DLS with Excellent Maint Design Traffic Loads Heavier Traffic Loads Very Heavy Traffic Loads

  13. Gravel Roads Basics • Drainage • A wet road is a weak road • Structural Strength • Each layer must be strong enough to resist the shear stresses applied to it • Rideability • Potholes, Washboards, and Ruts • Dust Control • Loose Aggregate • Safety

  14. Structural Properties of Roads • Road Components • Surface, Base, Subgrade • Road Stresses • Road Materials

  15. Base Contaminated by Subgrade

  16. Roads’ Structural Strengths • Tire pressure at 25 to 125 psi • Strength, from high to low • Concrete • Asphalt • Crushed Base • Pit Run Subbase • Borrow Special Excavation • Subgrade • Sandy or Gravelly • Clayey

  17. Shear Stress Compressive Stress Stresses

  18. Shear Stress Compressive Stress Stress Distribution Tire Stresses on Roads

  19. Gravel Road Stresses 100 psi Base 20 psi Subgrade 10 psi Stress Distribution

  20. Materials • Gravel surface materials need several characteristics: • Strength • Rutting Resistance • Cohesion and Moisture Retention • Aggregate Loss Resistance • Washboard Resistance • Dust Control

  21. Gravel Road Materials • Too much sands • Washboarding • Too many silts and clays • Rutting • Dust problems • Not enough larger gravel • Aggregate loss • Dust problems

  22. Dust Suppressants • Magnesium Chloride, MgCl • Chemically bonds moisture with gravel • Keeps dust down • Prevents loss of fines • Common in Sheridan County, rarely in Johnson and Carbon Counties • Make a note in the comment if road appears to have MgCl • Gravel surface looks damp when other gravel roads are dry

  23. Without MgCl Kittering Road, Sheridan County Same Gravel With MgCl

  24. Compaction • Soil must be compacted (densified) to obtain strength/stability • Reduce air voids in soil structure

  25. Crushed v Pit Run Aggregate

  26. Crushed v Pit Run Aggregate

  27. Plasticity Limits • Properties of material passing #40 sieve • Determined mostly by type and percentage of clays • Sometimes referred to as “Atterberg Limits”

  28. Soil Consistency

  29. Drainage • Bases, subbases, and subgrades are weakened substantially when wet • Inevitable during spring thaw and after heavy rains or snowmelts • Must design drainage to keep water away from the pavement structure • Be aware of capillary action drawing moisture into pavement structures from the ground water table

  30. Three Rules of Drainage 1. Keep water out 2. Get water out 3. Use materials not weakened by water

  31. Water Sources

  32. Gravel Road Drainage

  33. Drainage • Drainage Systems • Surface: to remove surface run off • Ditches • Gutters • Cross slope • Subsurface: to remove ground water • Edge drains • Under drains

  34. Gravel Surface: Must Shed Water • Sloped for drainage • Blade to restore proper cross-slope • Add gravel as needed • Have system of drains

  35. Water needs to be handled carefully

  36. Surface Drainage: Cuts

  37. Surface Drainage: Fills

  38. Drainage • Estimate time of drainage

  39. Spring Breakup – Due to Frost

  40. Traffic

  41. Traffic Impacts • This is not a problem for pavement designers • There are no trucks!

  42. Low Volume Rural Roads • May have very inconsistent truck traffic • Logging is intermittent and seasonal • Oil and gas drilling is heavy, then almost non-existent • Heavy mine traffic may avoid high volume times of day, open weigh stations • Ranch and agricultural traffic is highly seasonal • Consider construction traffic

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