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

• 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 percentN = the number of passages to failureP = equivalent single wheel load, KNQ = tyre inflation pressure, KPaFailure criterion : 75mm rut depth.

EFFECTS OF HIGH WHEEL LOADS AND HIGH TYRE PRESSURES

• 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 percentADT = 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

• 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)/CWhere L = Length of road improved or maintained in kmC = 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 RELATIVE COST/KM

Light grading 1.0

Grading with compaction 1.7

Regravelling 17.5

Improvement/Rehabilitation 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 40mm 19mm

PERCENTAGE PASSING

75mm (3”) 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

2.4mm (No.7) 30 - 50 35 – 60

1.2mm (No.14)

0.60mm (No.25) 10 - 30 15 - 30 15 - 35

0.30mm (No.52)

0.75mm (No.200) 5 - 15 5 - 15 5 - 15

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.