Transcript PASSABILITY CRITERIA FOR UNPAVED ROADS

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
OUTLINE OF PRESENTATION
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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
Recommendations
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)
CLASSIFICATION OF MATERIALS AS
EARTH or GRAVEL
Earth:
• ≥ 30% of material passes the 75µm sieve.
CBR ≤ 10% (?)
Gravel:
• <30% of material passes the 75µm sieve.
CBR >10% (?)
CBR > 35% (Ghanaian specification)
Materials with 10% < CBR< 35% - Underestimated
strength.
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 )
where
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.
EFFECTS OF HIGH WHEEL LOADS AND
HIGH TYRE PRESSURES
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Shown in graphs
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.
MATERIAL SELECTION CRITERIA FOR
WET WEATHER PASSABILITY
• Based on empirical studies by Visser (1981)
SFCBR ≥ 8.25 + 3.75 * log (ADT)
where
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
US CORPS OF ENGINEERS DESIGN CRITERIA FOR
THE THICKNESS OF SURFACE MATERIAL OF
GRAVEL ROADS
• Metricated model developed by Hammitt (1970) and
later by Barber, Odom and Patrick is as follows:
Log HG = 1.40 + 12.3 CI –0.466 C2– 0.142 NE 0.124 RDC – 0. 5
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Where HG = thickness of gravel surfacing, in mm;
C1 = soaked CBR of surfacing material, in percent,
C2 = soaked CBR of roadbed soil, in percent;
NE = design number of cumulative equivalent 40KN
single wheel loads at 550kPa tyre pressure;
• RDC = maximum allowable mean rut depth, in mm.
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.
ECONOMIC ANALYSIS OF THE PASSABILITY OF
UNPAVED ROADS
• Minimise the use of cost-benefit analysis in
developing countries.
– Scarce reliable data
– Existing communities require basic access
• Use cost effective analysis.
• Proposed cost effectiveness model for comparing
activities and interventions.
• Cost effectiveness, CE = (L * T)/C
Where
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.
RELATIVE COST OF ACTIVITIES
ACTIVITY
Light grading
Grading with compaction
Regravelling
Improvement/Rehabilitation
RELATIVE
COST/KM
1.0
1.7
17.5
29.0
Reconstruction
60.0
Note: The Relative cost of a single 900mm Ø
pipe culvert of length 7.0m is 6.8
GRAVEL MATERIAL
CHARACTERISTICS
(GHANA SPECIFICATIONS)
• Liquid Limit : ≤ 35%
• Plasticity Index : 6 – 12%
• Soaked CBR : ≥ 35%
GRAVEL GRADING SPECIFICATION (GHANA)
NOMINAL MAX
SIEVE
75mm (3”)
75mm
40mm
19mm
PERCENTAGE PASSING
100
38mm (1½”)
80 – 100
100
19mm (¾”)
60 – 80
80 - 100
100
9.5mm (3/8”)
45 – 65
55 - 80
80 – 100
4.76mm (3/16”)
30 - 50
40 - 60
50 – 75
30 - 50
35 – 60
10 - 30
15 - 30
15 - 35
5 - 15
5 - 15
5 - 15
2.4mm (No.7)
1.2mm (No.14)
0.60mm (No.25)
0.30mm (No.52)
0.75mm (No.200)
RECOMMENDATIONS
• Assess the strength of in situ materials
• Give priority to spot improvement of critical
sections
• Gravel sections that require strengthening only
• For low volume roads, sections with
20% <CBR< 40% do not require gravelling
• Sections with CBR> 40% do not require
regravelling except where corrugations are to be
minimised.
• Give priority to the repair of deficiencies that
render roads impassable during wet seasons.