LOW VOLUME ROAD DESIGN EMPIRICAL APPROACH

WHAT ARE THE DIFFERENCES ??

Roman Road 30060

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? Bring out the ‘major structural repair’ rather than maintenance Bring out the point about potholes and very small percentage of road that needs to display such deterioration Discuss the view of road users Conclude that there is functional and structural failure

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 ? Use blackboard for Total Cost diagram Agency costs and User costss

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 PLUS calculate social ‘benefits’ Models such as HDM 4 ??

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 ? but also time dependant but how?

But for low volume roads we 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

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

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

STRUCTURAL NUMBER (SN) SN = a1 h1 + a2 h2 + a3 h3 + .... Where a1 , a2 , a3 etc. are strength coefficients for layers 1, 2, 3, etc. and h1 , h2 , h3 , 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.

• • • STRENGTH COEFFICIENT, a2 FOR GRANULAR BASE MATERIALS a2 = {29.14 (CBR) - 0.1977 (CBR)2 + 0.00045 (CBR)3} x 10-4 0.15 • 0.14 • 0.10 0.08 • 0.05 40 50 100 110 150 CBR value STRENGTH COEFFICIENT, a2 FOR GRANULAR BASE MATERIALS

• • • STRENGTH COEFFICIENT, a3 FOR SUB-BASE MATERIALS 0.150 • 0.125 • 0.100 0.075 • 0.050 a3 = 0.01 + 0.065 (log10CBR) 0.025 1 5 10 50 100 200 CBR of sub-base STRENGTH COEFFICIENT, a3 FOR SUB-BASE MATERIALS

AASHO "DESIGN" EQUATION COMPARED WITH DATA EQUIVALENT THICKNESS De, INCHES 45 x 40 x x x x x x x 35 x x x x x x x x x x x x x x x 30 x x x x x x x 25 x x x x x x 20 x x x x x 15 x 10 x x 5 103 104 105 106 107 108 WEIGHTED EQUIVALENT ESA APPLICATIONS AASHO "DESIGN" EQUATION COMPARED WITH DATA

AASHTO EQUATION Traffic Subgrade CBR Structural number Allowable deterioration Reliability

Effect of ‘Reliability’ For 250,000 esa and subgrade CBR = 7% 95% Reliability SN = 2.54 85% Reliability SN = 2.29 a difference of 62 mm of sub-base

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

REGIONAL FACTOR - R CLIMATIC EFFECTS ARE ALLOWED FOR BY APPLYING A REGIONAL FACTOR (R) TO THE TRAFFIC LOADING CLIMATE R TRAFFIC x 106 ARID WET AASHO 0.4 1.0 2.0 5.0 2.0 1.0 NO GUIDANCE WAS GIVEN TO THE SELECTION OF R, THIS USUALLY BEING LEFT TO ‘ENGINEERING JUDGEMENT'

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

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

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

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

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

THE DESIGN PROCESS Maintenance is always required 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

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

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

* The highest seasonal level attained by the water table Estimated design subgrade strength class under sealed roads in the presence of a water table Depth of water table* from formation level (metres) 0.5 1 2 3 Subgrade strength class Non-plastic sand S4 S5 S6 Sandy clay PI=10 Sandy clay PI=20 S2 S3 Silty clay PI=30 Heavy clay PI>40 S1 * The highest seasonal level attained by the water table

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

An example of coping with risk 100 80 Cumulative percentage 60 40 20 4 8 12 CBR (DCP) per cent

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

Subgrade strength classes Range (CBR %) Class S1 S2 S3 S4 S5 S6 2 3-4 5-7 8-14 15-29 30

Range (106 esa) Traffic Classes T1 T2 T3 T4 T5 T6 T7 T8 < 0.3 0.3-0.7 0.7-1.5 1.5-3.0 3.0-6.0 6.0-10 10-17 17-30

Summary of material requirements for the design charts CHART NO REFER TO CHAPTERS SURFACING ROADBASE 1 6 and 9 Double surface dressing T1-T4 use GB1, GB2 or GB3 T5 use GB1,A or GB1B T6 must be GB1 A 2 6, 7 and 8 Double surface dressing T1-T4 use GB1, GB2 or GB3 T5 use GB1 T6, T7, T8 use GB1A 3 6 and 8 'Flexible' asphalt T1-T4 use GB1 or GB2 T5 use GB1 T6 use GB1A

* A cement or lime-stabilised sub-base may also be used CHART 1 GRANULAR ROADBASE / SURFACE DRESSING SD 150 175 300 225 200 250 325 125 275 350 100 * * A cement or lime-stabilised sub-base may also be used

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

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

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

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

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

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

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 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 OPTIMUM OR APPROPRIATE PAVEMENT DESIGN METHODOLOGY CONSTRUCTION Quality control Capacity, training & experience Selection and use of plant Influence of construction traffic

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

THE ROAD DESIGN OR “RISK” ENVIRONMENT TRAFFIC CHARACTERISTICS Axle loading Tyre pressures Seasonality Position Growth projections 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 MAINTENANCE Capacity & skills Funding Programming GEOMETRICS AND CROSS-SECTION PROFILES Road width Crown height Demand of terrain Sealed shoulders CONSTRAINTS OF THE “GREEN” ENVIRONMENT Constrained alignments Access to materials Depletion of resources Terrain stability OPTIMUM OR APPROPRIATE PAVEMENT DESIGN METHODOLOGY OTHER Technology solution labour based Intermediate equip Safety Institutional environment capacity Financing Political pressure Design period Road side activity 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