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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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Presentation Transcript
how are roads built
How Are Roads Built?
  • Evolution
    • Stock trails
    • Double tracks
    • Gravel road
    • Asphalt road
  • Revolution
    • New construction






Traffic Loads



dls with excellent maint
DLS with Excellent Maint

Design Traffic Loads

Heavier Traffic Loads

Very Heavy Traffic Loads

gravel roads basics
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
structural properties of roads
Structural Properties of Roads
  • Road Components
    • Surface, Base, Subgrade
  • Road Stresses
  • Road Materials
roads structural strengths
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

Shear Stress

Compressive Stress

tire stresses on roads

Shear Stress

Compressive Stress

Stress Distribution

Tire Stresses on Roads
gravel road stresses
Gravel Road Stresses

100 psi


20 psi


10 psi

Stress Distribution

  • Gravel surface materials need several characteristics:
    • Strength
      • Rutting Resistance
    • Cohesion and Moisture Retention
      • Aggregate Loss Resistance
      • Washboard Resistance
      • Dust Control
gravel road materials
Gravel Road Materials
  • Too much sands
    • Washboarding
  • Too many silts and clays
    • Rutting
    • Dust problems
  • Not enough larger gravel
    • Aggregate loss
    • Dust problems
dust suppressants
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
  • Soil must be compacted (densified) to obtain strength/stability
  • Reduce air voids in soil structure
plasticity limits
Plasticity Limits
  • Properties of material passing #40 sieve
  • Determined mostly by type and percentage of clays
  • Sometimes referred to as “Atterberg Limits”
  • 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
Three Rules of Drainage

1. Keep water out

2. Get water out

3. Use materials not weakened by water

  • Drainage Systems
    • Surface: to remove surface run off
      • Ditches
      • Gutters
      • Cross slope
    • Subsurface: to remove ground water
      • Edge drains
      • Under drains
gravel surface must shed water
Gravel Surface: Must Shed Water
  • Sloped for drainage
    • Blade to restore proper cross-slope
    • Add gravel as needed
  • Have system of drains
  • Estimate time of drainage
traffic impacts
Traffic Impacts
  • This is not a problem for pavement designers
  • There are no trucks!
low volume rural roads
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