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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