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LOW VOLUME ROAD DESIGN EMPIRICAL APPROACH. WHAT ARE THE DIFFERENCES ??

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Presentation on theme: "LOW VOLUME ROAD DESIGN EMPIRICAL APPROACH. WHAT ARE THE DIFFERENCES ??"— Presentation transcript:

1 LOW VOLUME ROAD DESIGN EMPIRICAL APPROACH

2 WHAT ARE THE DIFFERENCES ??

3 30060 Roman Road

4 STRUCTURAL DESIGN  What do we mean by structural design?  What is the purpose of a road pavement?  What do we mean by road pavement failure?  Does everyone agree?

5 TYPES OF FAILURE  FUNCTIONAL FAILURE  STRUCTURAL FAILURE Do they occur at the same time? How do they differ for low volume roads ? Do all users agree ? How do we combine the different views ?

6 Total or Whole Life Costs  Minimise total costs Need to…..  predict road deterioration  predict the effects of maintenance  calculate road agency costs  predict road user costs Models such as HDM 4 ?? PLUS u calculate social ‘benefits’

7 What structural design factors does road performance, and therefore design, depend ?  strength of subgrade  traffic loading  wheel loads  number of repetitions of wheels  strength of pavement layers  thickness of pavement layers ???? u but also time dependant u but how?

8 try to try to For high traffic roads we control as much as we can to reduce risks of failure to a very low value But for low volume roads we cannot afford to do so

9 Pass/fail criteria - no risk Strength of material Specification for trunk road Number of samples

10 1 Estimate traffic loading in equivalent standard axles 2Multiply traffic by regional factor 3Estimate subgrade strength (now as an elastic modulus) 4Select serviceability loss (maximum level of acceptable deterioration) METHOD THEN RECOMMENDS A STRUCTURAL NUMBER, SN BASIC AASHTO METHOD

11 STRUCTURAL NUMBER (SN) SN = a 1 h 1 + a 2 h 2 + a 3 h Where a 1, a 2, a 3 etc. are strength coefficients for layers 1, 2, 3, etc. and h 1, h 2, h 3, etc. are the thicknesses of layers 1, 2, 3 The strength coefficients are related to normal strength measures such as CBR, unconfined compressive strength, Marshall stability, etc.

12 STRENGTH COEFFICIENT, a 2 FOR GRANULAR BASE MATERIALS Strength coefficient (a 2 ) CBR value a 2 = { (CBR) (CBR) (CBR) 3 } x 10 -4

13 STRENGTH COEFFICIENT, a 3 FOR SUB-BASE MATERIALS Strength coefficient (a 3 ) a 3 = (log 10 CBR) CBR of sub-base

14 WEIGHTED EQUIVALENT ESA APPLICATIONS EQUIVALENT THICKNESS De, INCHES x x x x xxxx xx x xx x x x xxx x x x x x x x x x x xxx x x x xxx x x x x x x AASHO "DESIGN" EQUATION COMPARED WITH DATA

15 AASHTO EQUATION Traffic Structural number Subgrade CBR Allowable deterioration Reliability

16 Effect of ‘Reliability’ For 250,000 esa and subgrade CBR = 7% 95% ReliabilitySN = % ReliabilitySN = 2.29 a difference of 62 mm of sub-base

17 STATE OF ROAD PSI TIME or TRAFFIC ROAD DETERIORATION FROZEN SPRING THAW SPRING THAW

18 REGIONAL FACTOR - R CLIMATIC EFFECTS ARE ALLOWED FOR BY APPLYING A REGIONAL FACTOR (R) TO THE TRAFFIC LOADING NO GUIDANCE WAS GIVEN TO THE SELECTION OF R, THIS USUALLY BEING LEFT TO ‘ENGINEERING JUDGEMENT' CLIMATE R TRAFFIC x ARIDWETAASHO

19 Effect of climate The ‘regional’ factor Illinois in summer SN = 2.54 Dry SN = 2.14 (-100mm of sub base) Wet SN = 2.97 (+110mm of sub base) But no guidance available from the Road Test

20 ROAD NOTE 31 A PAVEMENT DESIGN GUIDE FOR PAVED ROADS IN TROPICAL CLIMATES

21 TAKES ACCOUNT OF…..  Variability in material properties  Uncertainty in traffic estimates  Variability in road performance

22 KEY FACTORS  Influence of tropical climates on the moisture conditions in the subgrade  Influence of tropical climates on the nature of soils and rocks  High axle loads and tyre pressures  Severe conditions imposed on the bituminous surface by tropical climates  Inter relationship between design and maintenance

23 BASIS FOR THE DESIGNS  Full scale design and performance experiments carried out by TRL in tropical countries  Performance studies of as-built networks  Empirically based performance models  (Highway Design Model III)  Theoretical / mechanistic analysis

24 THE DESIGN PROCESS  Estimate traffic  Assess strength of subgrade  Select most economical combination of pavement materials and thicknesses that will provide satisfactory service over the life of the pavement Maintenance is always required

25 ESTIMATING EQUILIBRIUM MOISTURE CONTENT CATEGORY 3 CATEGORY 3 No permanent water table Arid climate Rainfall < 250mm pa

26 ESTIMATING EQUILIBRIUM MOISTURE CONTENT CATEGORY 2 CATEGORY 2 Deep water table but rainfall sufficient to produce seasonal changes under the road Rainfall >250mm pa. per year and seasonal

27 Estimated design subgrade strength class under sealed roads in the presence of a water table Depth of water table * from formation level (metres) Subgrade strength class Non-plastic sand S4 S5 S6 Sandy clay PI=10 S4 S5 Sandy clay PI=20 S2 S3 S4 Silty clay PI=30 S2 S3 Heavy clay PI>40 S1 S2 * The highest seasonal level attained by the water table

28 ESTIMATING EQUILIBRIUM MOISTURE CONTENT CATEGORY 1 CATEGORY 1 Water table sufficiently close to the surface to control the subgrade moisture content (This depends on the type of soil) (This depends on the type of soil)

29 CBR (DCP) per cent Cumulative percentage An example of coping with risk

30 Soaked CBR Soil A Soil B Common to area where designs developed Rare 5% 15% 8%300mm ? 300mm CBR at equilibrium moisture content Required pavement thickness

31 Subgrade strength classes Class Range (CBR %) S1 S2 S3 S4 S5 S  30

32 Traffic Classes Range (10 6 esa) T1 T2 T3 T4 T5 T6 T7 T8 <

33 Summary of material requirements for the design charts CHART NO SURFACINGROADBASE REFER TO CHAPTERS Double surface dressing 'Flexible' asphalt T1-T4 use GB1, GB2 or GB3 T5 use GB1,A or GB1B T6 must be GB1 A T1-T4 use GB1, GB2 or GB3 T5 use GB1 T6, T7, T8 use GB1A T1-T4 use GB1 or GB2 T5 use GB1 T6 use GB1A and 9 6, 7 and 8 6 and 8

34 * A cement or lime-stabilised sub-base may also be used

35 Consideration of the Road Design Environment for LVSR’s OPTIMUM OR APPROPRIATE PAVEMENT DESIGN AVAILABLE MATERIALS

36 THE ROAD DESIGN OR “RISK” ENVIRONMENT AVAILABLE MATERIALS Alternative & thin bituminous surfacings Pavement materials Marginal materials Standards Subgrade & road formation Problem soils Moisture sensitivity Stabilisation options and treatments OPTIMUM OR APPROPRIATE PAVEMENT DESIGN METHODOLOGY

37 Pass/fail criteria Strength of material Specification for trunk road Number of samples

38 Consideration of the Road Design Environment for LVSR’s OPTIMUM OR APPROPRIATE PAVEMENT DESIGN PREVAILING CLIMATE PREVAILING CLIMATE AVAILABLE MATERIALS DRAINAGE AND HYDROLOGY

39 THE ROAD DESIGN OR “RISK” ENVIRONMENT PREVAILING CLIMATE Rainfall (intensity, distribution) Temperature (evaporation & diurnal change) Future change or unpredictability DRAINAGE AND HYDROLOGY Ground & surface water flow Hydro-genesis Demand of terrain Modifying influences AVAILABLE MATERIALS Alternative & thin bituminous surfacings Pavement materials Marginal materials Standards Subgrade & road formation Problem soils Moisture sensitivity Stabilisation options and treatments OPTIMUM OR APPROPRIATE PAVEMENT DESIGN METHODOLOGY

40 Consideration of the Road Design Environment for LVSR’s OPTIMUM OR APPROPRIATE PAVEMENT DESIGN PREVAILING CLIMATE PREVAILING CLIMATE AVAILABLE MATERIALS CONSTRUCTION DRAINAGE AND HYDROLOGY

41 THE ROAD DESIGN OR “RISK” ENVIRONMENT PREVAILING CLIMATE Rainfall (intensity, distribution) Temperature (evaporation & diurnal change) Future change or unpredictability DRAINAGE AND HYDROLOGY Ground & surface water flow Hydro-genesis Demand of terrain Modifying influences AVAILABLE MATERIALS Alternative & thin bituminous surfacings Pavement materials Marginal materials Standards Subgrade & road formation Problem soils Moisture sensitivity Stabilisation options and treatments CONSTRUCTION Quality control Capacity, training & experience Selection and use of plant Influence of construction traffic OPTIMUM OR APPROPRIATE PAVEMENT DESIGN METHODOLOGY

42 Consideration of the Road Design Environment for LVSR’s OPTIMUM OR APPROPRIATE PAVEMENT DESIGN PREVAILING CLIMATE PREVAILING CLIMATE AVAILABLE MATERIALS TRAFFIC CHARACTERISTICS CONSTRUCTION CONSTRAINTS OF THE “GREEN” ENVIRONMENT MAINTENANCE OTHERS GEOMETRICS AND CROSS-SECTION PROFILES DRAINAGE AND HYDROLOGY

43 THE ROAD DESIGN OR “RISK” ENVIRONMENT PREVAILING CLIMATE Rainfall (intensity, distribution) Temperature (evaporation & diurnal change) Future change or unpredictability DRAINAGE AND HYDROLOGY Ground & surface water flow Hydro-genesis Demand of terrain Modifying influences AVAILABLE MATERIALS Alternative & thin bituminous surfacings Pavement materials Marginal materials Standards Subgrade & road formation Problem soils Moisture sensitivity Stabilisation options and treatments GEOMETRICS AND CROSS-SECTION PROFILES Road width Crown height Demand of terrain Sealed shoulders TRAFFIC CHARACTERISTICS Axle loading Tyre pressures Seasonality Position Growth projections OTHER Technology solution labour based Intermediate equip Safety Institutional environment capacity Financing Political pressure Design period Road side activity MAINTENANCE Capacity & skills Funding Programming CONSTRUCTION Quality control Capacity, training & experience Selection and use of plant Influence of construction traffic CONSTRAINTS OF THE “GREEN” ENVIRONMENT Constrained alignments Access to materials Depletion of resources Terrain stability OPTIMUM OR APPROPRIATE PAVEMENT DESIGN METHODOLOGY


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