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1. PASSABILITY CRITERIA FOR UNPAVED ROADS(The experience of an operational road engineer guided by research findings)PRESENTATION BY MARTIN HMENSA DEPARTMENT OF FEEDER ROADS, GHANA
2. OUTLINE OF PRESENTATION Assumed degree of engineering of unpaved roads.
Classification of materials as earth or gravel
HDM III Design criteria for earth roads
Effects of high wheel loads and high tyre pressures
Criteria to ensure wet weather, passability
U.S Corps of Engineers criteria for the thickness of gravel surface material
Economic analysis of alternative interventions to ensure passability
3. ASSUMED DEGREE OF ENGINEERING Road formed with adequate camber and side drains
Bridges and culverts provided
Low-lying sections raised in waterlogged areas
Road is unpaved (no bituminous seal)
4. CLASSIFICATION OF MATERIALS AS EARTH or GRAVEL Earth:
= 30% of material passes the 75m sieve.
CBR = 10% (?)
<30% of material passes the 75m sieve.
CBR >10% (?)
CBR > 35% (Ghanaian specification)
Materials with 10% < CBR< 35% - Underestimated strength.
5. HDM III DESIGN CRITERIA FOR EARTH ROADS Model developed by Greenstein and Livneh (1981) with data from Thailand and Ecuador
C2 = 0.0138 (N 0.175 )(P 0.580 )(Q0.490 )
C2 = soaked CBR of roadbed soil, in percent
N = the number of passages to failure
P = equivalent single wheel load, KN
Q = tyre inflation pressure, KPa
Failure criterion : 75mm rut depth.