1. LOW VOLUME ROAD DESIGN
EMPIRICAL APPROACH

2. WHAT ARE THE DIFFERENCES ??

3. Roman Road
30060

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?
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

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

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

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 ?
but also time dependant
but how?

8. 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

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

10. 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

11. STRUCTURAL NUMBER (SN)
SN = a1 h1 + a2 h2 + a3 h
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.

12. • • • STRENGTH COEFFICIENT, a2 FOR GRANULAR BASE MATERIALS
a2 = {29.14 (CBR) (CBR) (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

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

14. 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

15. AASHTO EQUATION Traffic Subgrade CBR Structural number Allowable
deterioration
Reliability

16. 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

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

18. 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'

19. Effect of climate The ‘regional’ factor
Illinois in summer
SN = 2.54
Dry
SN = (-100mm of sub base)
Wet
SN = (+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 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

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

26. 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

27. * 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

28. 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)

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

30. 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

31. Subgrade strength classes
Range (CBR %)
Class
S1 S2 S3 S4 S5 S6
30

32. Range (106 esa)
Traffic Classes
T1 T2 T3 T4 T5 T6 T7 T8
<

33. 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

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

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

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 Number of samples Specification for trunk road
Strength of material

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

39. 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

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

41. 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

42. 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

43. 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