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OUTLINE OF PRESENTATION


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.

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OUTLINE OF PRESENTATION

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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


Outline of presentation l.jpg

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


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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)


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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.


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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.


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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.


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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


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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.


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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.


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    RELATIVE COST OF ACTIVITIES


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    GRAVEL MATERIAL CHARACTERISTICS(GHANA SPECIFICATIONS)

    • Liquid Limit : ≤ 35%

    • Plasticity Index : 6 – 12%

    • Soaked CBR : ≥ 35%


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    GRAVEL GRADING SPECIFICATION (GHANA)


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    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.