CIV3703 Transport Engineering
CIV3703 Transport Engineering Module 6 – Part 1 Design of Pavements
Dr. Andreas Nataatmadja
6.1 General Approach to Pavement Design Pavement design: • process of developing most economical combination of pavement layers with respect to thickness and type of material • to protect the soil foundation from the cumulative traffic to be carried during the design life.
Structural Design Pavement design essentially a structural engineering problem necessary to define load conditions, ensure materials can absorb stresses and strains imposed by traffic, without suffering unacceptable deterioration.
Design Approach Define loading and environmental conditions Select materials with appropriate properties Select pavement thickness based on empirical rules or by stress-strain analysis of pavement structure Adjust initial design using different materials until satisfactory design is obtained.
Stress Distribution in a Granular Pavement
6.2 Type of Pavements Flexible Pavements flex under traffic loads, rebound when load removed example - gravel base overlain with bitumen seal
Composite Pavements example – asphalt on cemented base
Rigid Pavements slab action when loaded example - cement concrete pavement
6.3 Design Considerations 1. 2. 3. 4. 5. 6.
Properties of foundation soil (subgrade); Pavement environment; Design life/period; Traffic to be carried; Available materials; and Construction and maintenance factors.
PAVEMENT IS DESIGNED TO FAIL!
6.3.1 Subgrade Evaluation Pavement performance depends on subgrade support. Subgrade support depends on: soil type, density, moisture content Subgrade evaluation: Early methods: index calculated from grading, Atterberg Limits, etc. Now CBR at insitu density & moisture conditions
6.3.2 Pavement Environment Most important environmental factors: moisture temperature. Moisture conditions is a very important issue for subgrade Temperature affects asphalt and shrinkage movements of cemented materials Design must recognise ways by which moisture may enter subgrade and pavement determine measures needed to control moisture. DRAINAGE, DRAINAGE, DRAINAGE!
Moisture Movements
Moisture Changes in Subgrade Effects: up to 1.5 m from edge Change in strength (pore water pressure); Change in volume – shrink-swell (expansive clay). Control: Subgrade drainage; Pavement drains. Design: Soil strength evaluation at the highest m/c. Or at the Equilibrium Moisture Content (EMC)
6.3.3 Design Life/Period Length of time (years) before a pavement requires rehabilitation/reconstruction. The period during which the performance of a pavement is expected to remain acceptable with only routine maintenance. Failure modes: wheel path rutting (but not asphalt rutting) asphalt fatigue cracking, or fatigue cracking of cemented layer (with reflective cracking)
Choice of Design Life Factors: • Subgrade type and environmental effects. • Maintenance strategies for road. • Possibility of reconstruction • Funding • Type and cost of rehabilitation at end of design period.
Design Period Typical: Pavement/Overlay New granular pavements New rigid pavements Asphalt overlays Granular overlays
Period (Years) 20 to 25 20 to 40 10 to 15 10 to 20
6.3.4 Available Materials Previously covered in Module 4. Unbound granular materials Cemented materials Bituminous materials Cement concrete
6.3.5 Construction and Maintenance Factors Extent and type of drainage; Use of boxed construction; Availability of equipment; Use of staged construction; Use of stabilisation; Social considerations; Construction under traffic; Maintenance strategy.
6.3.6 Design Traffic Major features of traffic that influence pavement performance are: Determine fatigue • number of axle passes; • the axle loadings; and Determine strains • the axle configurations. Only heavy vehicles are considered in pavement design
Standard Axle Standard axle is used to represent the traffic. Design traffic = number of repetitions of standard axle load. Standard axle: single axle with dual wheels that carries a load of 80 kN. Design traffic - number of Equivalent Standard Axles (ESAs)
Standard Axle Standard Axle Single axle with dual wheels loaded to a total mass of 8.2 tonne (80 kN).
Standard Axle
Load Equivalence
N passes of a single axle carrying a load P
Ns passes of a standard axle Ps
Exponent “a” Exponent “a” varies with pavement materials. Higher “a” for stiffer material. Usually value is assumed to be about 4 for overall pavement damage. For a = 4 this relationship is known as the FOURTH POWER LAW
Implications of Fourth Power Law Useful for looking at life of road pavement. Examples: * If P = Ps then N = Ns * If axle load P is doubled from Ps to 2Ps: N= 1/16 Ns e.g. life reduced from 20 years to 1.5 years. * Cars - axle load of 0.5 t compared standard 5.4t (0.5/5.4) to fourth power = 0.0001 i.e. 1 standard axle truck = 10,000 cars * Truck overloading by 20% - halves pavement life.
Axle Groups Many large vehicles use axle groups (2, 3 axles in close proximity) rather than single axles. The following axle & tyre configurations cause the same damage as an ESA:
Austroads Vehicle Classes
Truck Types and ESA Typical ESAs per vehicle: Two axle trucks - about 0.6 ESA Six axle truck - about 3.6 ESA Freight haulage efficiency: Two axle truck - about 2 t of freight per ESA Six axle trucks - about 5.5 to 7.5 t of freight per ESA Bigger trucks contribute more ESAs but cause less damage per tonne of freight moved.
Trucks of Many Kind
ESA Comparison
Notice that cars are insignificant and thus usually ignored in pavement design
ESA Calculation
Methods of Calculation of Design Traffic Method depends on: accuracy required resources available, including finance presence of specialised loadings unusual axle group configurations
Ideal data: actual loading of each axle/group In practice: assume ‘average’ or ‘typical’ loadings.
Step 1 – AADT at Opening Year Many pavement are designed to replace existing road pavements Determine average number of vehicles per day. Growth factor, g = (1 + 0.01i )n AADT (year of opening) = g x AADT (data year) Note: AADT give average annual daily traffic in both directions.
Step 2 – Average ESA/day in Design Lane at Opening Year Estimate Average percentage of heavy/commercial vehicles in the design stream (%HV) -Usually between 5 and 30% Directional factor (AADT covers both directions) Lane distribution factor (LDF) ESA per HV (see Table 6.3) Opening ESA/day = (AADT x DF) x (%HV/100) x LDF x (ESA per HV)
Carriageway
Divided highway
Direction Factor (DF) Direction Factor is the proportion of the two-way AADT travelling in the direction of the design lane. Example: Design lane
Two way AADT = 1000 vpd If DF = 0.6 then the design lane would carry 0.6 x 1000 = 600 veh/day, while the other lane carries 400 vpd. Design lane is normally the lane with the highest traffic.
Lane Distribution Factor (LDF) Calculate distribution of traffic between lanes. If one lane, LDF = 1
Step 3 – Cumulative ESA over Design Life of the Lane Cumulative growth factor (CGF) CGF = ((1 + 0.01i)n – 1)/ 0.01i or CGF = n where: i – Heavy vehicle annual growth rate in percentage n – Design life (in years) Design traffic, NDT = 365 x Opening ESA/day x CGF
i>0 i=0
CGF Calculate Cumulative Growth Factor (CGF) which occurs between the present traffic and the opening of the road when the traffic is growing at i % per annum. {Table 6.4} Heavy Vehicle Annual Growth rates (i) varies with locations and developments. More than one growth rates may be used during the design life to reflect changes in land development, business activities etc. Example: For 20 year Design Life - Growth 3% CGF = 27 - Growth 6% CGF = 37
Example (p. 238): Given • • • • •
Two-lane major urban road (one lane each direction) AADT = 2000 vpd (two years before year of opening) with growth rate of 3% (all vehicles) Annual growth rate of heavy vehicles = 3% Percentage of heavy vehicles = 7% Design period = 20 years
Assume •
The road is urban arterial road
Solution AADT (year of opening)
= 2000 (1 + 0.03)^2 = 2000 x 1.1 = 2200 vpd
Average ESA in design lane in the year of opening Opening ESA/day = (AADT x DF) x (%HV/100) x LDF x(ESA per HV)
= 2200 x 0.5 x 0.07 x 1.0 x 1.8 = 139 ESA Table 6.3
Determine cumulative ESA over design life of the lane NDT = (opening ESA per day)x 365 x CGF
NDT = 139 x 365 x 27 = 1.4 x 106 ESA
Table 6.4, i = 3%
End of Module 6, Part 1
Dr. Andreas Nataatmadja
6.1 General Approach to Pavement Design Pavement design: • process of developing most economical combination of pavement layers with respect to thickness and type of material • to protect the soil foundation from the cumulative traffic to be carried during the design life.
Structural Design Pavement design essentially a structural engineering problem necessary to define load conditions, ensure materials can absorb stresses and strains imposed by traffic, without suffering unacceptable deterioration.
Design Approach Define loading and environmental conditions Select materials with appropriate properties Select pavement thickness based on empirical rules or by stress-strain analysis of pavement structure Adjust initial design using different materials until satisfactory design is obtained.
Stress Distribution in a Granular Pavement
6.2 Type of Pavements Flexible Pavements flex under traffic loads, rebound when load removed example - gravel base overlain with bitumen seal
Composite Pavements example – asphalt on cemented base
Rigid Pavements slab action when loaded example - cement concrete pavement
6.3 Design Considerations 1. 2. 3. 4. 5. 6.
Properties of foundation soil (subgrade); Pavement environment; Design life/period; Traffic to be carried; Available materials; and Construction and maintenance factors.
PAVEMENT IS DESIGNED TO FAIL!
6.3.1 Subgrade Evaluation Pavement performance depends on subgrade support. Subgrade support depends on: soil type, density, moisture content Subgrade evaluation: Early methods: index calculated from grading, Atterberg Limits, etc. Now CBR at insitu density & moisture conditions
6.3.2 Pavement Environment Most important environmental factors: moisture temperature. Moisture conditions is a very important issue for subgrade Temperature affects asphalt and shrinkage movements of cemented materials Design must recognise ways by which moisture may enter subgrade and pavement determine measures needed to control moisture. DRAINAGE, DRAINAGE, DRAINAGE!
Moisture Movements
Moisture Changes in Subgrade Effects: up to 1.5 m from edge Change in strength (pore water pressure); Change in volume – shrink-swell (expansive clay). Control: Subgrade drainage; Pavement drains. Design: Soil strength evaluation at the highest m/c. Or at the Equilibrium Moisture Content (EMC)
6.3.3 Design Life/Period Length of time (years) before a pavement requires rehabilitation/reconstruction. The period during which the performance of a pavement is expected to remain acceptable with only routine maintenance. Failure modes: wheel path rutting (but not asphalt rutting) asphalt fatigue cracking, or fatigue cracking of cemented layer (with reflective cracking)
Choice of Design Life Factors: • Subgrade type and environmental effects. • Maintenance strategies for road. • Possibility of reconstruction • Funding • Type and cost of rehabilitation at end of design period.
Design Period Typical: Pavement/Overlay New granular pavements New rigid pavements Asphalt overlays Granular overlays
Period (Years) 20 to 25 20 to 40 10 to 15 10 to 20
6.3.4 Available Materials Previously covered in Module 4. Unbound granular materials Cemented materials Bituminous materials Cement concrete
6.3.5 Construction and Maintenance Factors Extent and type of drainage; Use of boxed construction; Availability of equipment; Use of staged construction; Use of stabilisation; Social considerations; Construction under traffic; Maintenance strategy.
6.3.6 Design Traffic Major features of traffic that influence pavement performance are: Determine fatigue • number of axle passes; • the axle loadings; and Determine strains • the axle configurations. Only heavy vehicles are considered in pavement design
Standard Axle Standard axle is used to represent the traffic. Design traffic = number of repetitions of standard axle load. Standard axle: single axle with dual wheels that carries a load of 80 kN. Design traffic - number of Equivalent Standard Axles (ESAs)
Standard Axle Standard Axle Single axle with dual wheels loaded to a total mass of 8.2 tonne (80 kN).
Standard Axle
Load Equivalence
N passes of a single axle carrying a load P
Ns passes of a standard axle Ps
Exponent “a” Exponent “a” varies with pavement materials. Higher “a” for stiffer material. Usually value is assumed to be about 4 for overall pavement damage. For a = 4 this relationship is known as the FOURTH POWER LAW
Implications of Fourth Power Law Useful for looking at life of road pavement. Examples: * If P = Ps then N = Ns * If axle load P is doubled from Ps to 2Ps: N= 1/16 Ns e.g. life reduced from 20 years to 1.5 years. * Cars - axle load of 0.5 t compared standard 5.4t (0.5/5.4) to fourth power = 0.0001 i.e. 1 standard axle truck = 10,000 cars * Truck overloading by 20% - halves pavement life.
Axle Groups Many large vehicles use axle groups (2, 3 axles in close proximity) rather than single axles. The following axle & tyre configurations cause the same damage as an ESA:
Austroads Vehicle Classes
Truck Types and ESA Typical ESAs per vehicle: Two axle trucks - about 0.6 ESA Six axle truck - about 3.6 ESA Freight haulage efficiency: Two axle truck - about 2 t of freight per ESA Six axle trucks - about 5.5 to 7.5 t of freight per ESA Bigger trucks contribute more ESAs but cause less damage per tonne of freight moved.
Trucks of Many Kind
ESA Comparison
Notice that cars are insignificant and thus usually ignored in pavement design
ESA Calculation
Methods of Calculation of Design Traffic Method depends on: accuracy required resources available, including finance presence of specialised loadings unusual axle group configurations
Ideal data: actual loading of each axle/group In practice: assume ‘average’ or ‘typical’ loadings.
Step 1 – AADT at Opening Year Many pavement are designed to replace existing road pavements Determine average number of vehicles per day. Growth factor, g = (1 + 0.01i )n AADT (year of opening) = g x AADT (data year) Note: AADT give average annual daily traffic in both directions.
Step 2 – Average ESA/day in Design Lane at Opening Year Estimate Average percentage of heavy/commercial vehicles in the design stream (%HV) -Usually between 5 and 30% Directional factor (AADT covers both directions) Lane distribution factor (LDF) ESA per HV (see Table 6.3) Opening ESA/day = (AADT x DF) x (%HV/100) x LDF x (ESA per HV)
Carriageway
Divided highway
Direction Factor (DF) Direction Factor is the proportion of the two-way AADT travelling in the direction of the design lane. Example: Design lane
Two way AADT = 1000 vpd If DF = 0.6 then the design lane would carry 0.6 x 1000 = 600 veh/day, while the other lane carries 400 vpd. Design lane is normally the lane with the highest traffic.
Lane Distribution Factor (LDF) Calculate distribution of traffic between lanes. If one lane, LDF = 1
Step 3 – Cumulative ESA over Design Life of the Lane Cumulative growth factor (CGF) CGF = ((1 + 0.01i)n – 1)/ 0.01i or CGF = n where: i – Heavy vehicle annual growth rate in percentage n – Design life (in years) Design traffic, NDT = 365 x Opening ESA/day x CGF
i>0 i=0
CGF Calculate Cumulative Growth Factor (CGF) which occurs between the present traffic and the opening of the road when the traffic is growing at i % per annum. {Table 6.4} Heavy Vehicle Annual Growth rates (i) varies with locations and developments. More than one growth rates may be used during the design life to reflect changes in land development, business activities etc. Example: For 20 year Design Life - Growth 3% CGF = 27 - Growth 6% CGF = 37
Example (p. 238): Given • • • • •
Two-lane major urban road (one lane each direction) AADT = 2000 vpd (two years before year of opening) with growth rate of 3% (all vehicles) Annual growth rate of heavy vehicles = 3% Percentage of heavy vehicles = 7% Design period = 20 years
Assume •
The road is urban arterial road
Solution AADT (year of opening)
= 2000 (1 + 0.03)^2 = 2000 x 1.1 = 2200 vpd
Average ESA in design lane in the year of opening Opening ESA/day = (AADT x DF) x (%HV/100) x LDF x(ESA per HV)
= 2200 x 0.5 x 0.07 x 1.0 x 1.8 = 139 ESA Table 6.3
Determine cumulative ESA over design life of the lane NDT = (opening ESA per day)x 365 x CGF
NDT = 139 x 365 x 27 = 1.4 x 106 ESA
Table 6.4, i = 3%
End of Module 6, Part 1
