30060

 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 ?

 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 ??

 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?

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

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

SN = a
1 h
1
+ a
2 h
2
+ a
3 h
3
+ ....
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.

Strength coefficient
(a
2
)
0.15
0.14
a
2
= { 29.14 (CBR) - 0.1977 (CBR)
2
+ 0.00045 (CBR)
3 } x 10
-4
•
•
0.10
0.08
•
0.05
0 40 50
CBR value
100 110 150
STRENGTH COEFFICIENT, a
2
FOR GRANULAR BASE MATERIALS

Strength coefficient
(a
3
)
0.150
0.125
0.100
0.075
0.050
0.025
1
•
•
•
5 10 a
3
= 0.01 + 0.065 (log
10
CBR)
50 100 200
CBR of sub-base
STRENGTH COEFFICIENT, a
3
FOR SUB-BASE MATERIALS

15
10
5
45
EQUIVALENT THICKNESS De, INCHES
40
35
30
25
20 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
0
10
3
10
4
10
5
10
6
10
7
WEIGHTED EQUIVALENT ESA APPLICATIONS
10
8
AASHO "DESIGN" EQUATION COMPARED WITH DATA

Log W
8 .
2

Z .
S
0

9 .
36 .
Log
10

SN

1


Log
10


PSI
0

PSI
4 .
2

1 .
5 f
0 .
4


SN
1094

1

5 .
19



2 .
32 .
Log
10
M
R

7 .
87
Traffic
Structural number
Allowable deterioration
Subgrade
CBR
Reliability

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

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

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

 Variability in material properties
 Uncertainty in traffic estimates
 Variability in road performance

 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

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

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)

40
20
0
4
100
An example of coping with risk
80
60
8
CBR (DCP) per cent
12

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

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

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

Number of samples
Specification for trunk road
Strength of material

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

THE ROAD DESIGN OR “RISK” ENVIRONMENT
DRAINAGE AND
HYDROLOGY
Ground & surface water flow
Hydro-genesis
Demand of terrain
Modifying influences
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
OPTIMUM OR
APPROPRIATE
PAVEMENT
DESIGN
METHODOLOGY

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

THE ROAD DESIGN OR “RISK” ENVIRONMENT
DRAINAGE AND
HYDROLOGY
Ground & surface water flow
Hydro-genesis
Demand of terrain
Modifying influences
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
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
DRAINAGE AND
HYDROLOGY
PREVAILING
CLIMATE
AVAILABLE
MATERIALS
GEOMETRICS AND
CROSS-SECTION
PROFILES
OTHERS
OPTIMUM OR
APPROPRIATE
PAVEMENT
DESIGN
TRAFFIC
CHARACTERISTICS
MAINTENANCE
CONSTRUCTION
CONSTRAINTS OF THE
“GREEN”
ENVIRONMENT

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