OUTLINE OF PRESENTATION . Assumed degree of engineering of unpaved roads.Classification of materials as earth or gravelHDM III Design criteria for earth roadsEffects of high wheel loads and high tyre pressuresCriteria to ensure wet weather, passabilityU.S Corps of Engineers criteria for the thi
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PASSABILITY CRITERIA FOR UNPAVED ROADS(The experience of an operational road engineer guided by research findings)PRESENTATION BY MARTIN HMENSADEPARTMENT OF FEEDER ROADS, GHANA
CBR ≤ 10% (?)
CBR >10% (?)
CBR > 35% (Ghanaian specification)
Materials with 10% < CBR< 35% - Underestimated strength.
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.
Education of heavy truck drivers required:
I. To check deterioration
ii. To reduce road maintenance cost
iii.To avoid blockage of roads
32% reduction in tyre pressure of heavy trucks can increase the life of an earth road by 3 times.
SFCBR ≥ 8.25 + 3.75 * log (ADT)
SFCBR = the soaked CBR at standard Proctor Laboratory compaction (600KJ/M3) in percent
ADT = the average daily vehicular traffic in
both directions, in vehicles per day
Log is logarithm to base 10
Log HG = 1.40 + 12.3 CI –0.466 C2– 0.142 NE 0.124 RDC – 0.5
Replace the co-efficient 12.3 by 0.856 P 0. 235Q0.285 where the wheel load P and the tyre pressure Q are significantly different.
L = Length of road improved or maintained in km
C = the cost of an activity in appropriate currency.
T = Time period over which the road remains improved or satisfies a minimum specified criterion.
20% <CBR< 40% do not require gravelling