Structural Design of Pavements Australia

CIVL4121: Pavement Design: 1

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GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING CIVL 4121

Structural Design of Pavements

Lecture 6Louise Adamson

MRWA


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1. AUSTROADS (2008). ‘Guide to Pavement Technology Part 2:Pavement Structural Design’. AUSTROADS Incorporated.

2. A Guide to the Selection and use of Naturally Occurring Materials as Base and Sub-base in Roads in Western Australia. Main Roads Western Australia.

3. Main Roads WA. (2010). ‘Specification 501 Pavements’ Document 04/10110-03 Issue 08/01/10 MAIN ROADS Western Australia.

4. Main Roads WA. (2010) ‘Procedure for the Design of Flexible Pavements’Engineering Road Note 9, MAIN ROADS Western Australia, January 2010

References


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Worked Example 3

Determine 15 and 40 year design traffic (in terms of ESA, SAR5, SAR7, SAR12) for Great Northern Highway (GNH) between West Swan Rd and Apple St. (Note: Traffic growth rate is 0.03; GNH is one carriageway highway with 1 lane in each direction; the 1st design year is 2010)AADT and classification data surveyed in 2007 obtained from MRWAReporting Centre is as follows ;

F3, F4, … F12 at SLK 35 of GNH (Muchea) from ERN9 will be adopted:

2007 Class (ci)1-w

AADT 1 2 3 4 5 6 7 8 9 10 11 12

6170 79.93% 4.88% 4.91% 1.41% 0.29% 0.26% 0.50% 0.51% 2.07% 1.55% 3.70% 0.01%

F3 F4 F5 F6 F7 F8 F9 F10 F11 F12

0.38 2.24 1.91 0.56 0.88 2.36 3.24 4.08 5.31 8.04


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Worked Example 3

For average number of vehicles in 2010Use where i = 4 and the growth rate is 0.03

AADT in 2010 is 6742 ; Cumulative Growth Factor CGF (R)

CGF15 years = (((1+0.03)^15)-1)/0.03 = 18.60;CGF40 years = (((1+0.03)^40-1))/0.03 = 75.40;

Using Method 1, the design traffic ESA can be calculated as follows:

n = 6742R(15) = 18.60 and R(40) = 75.40d = 100ci and Fi as per tables given

15 year design ESA = 365*6742*100*18.60*(4.91*0.38+1.41*2.24+…+0.01*8.04)*10-4 =1.83E+0740 year design ESA = 365*6742*100*75.40*(4.91*0.38+1.41*2.24+…+0.01*8.04)*10-4 =7.43E+07

( ) 11 −+= ii raadtAADT

( ) 412121111101099887766554433 10FcFcFcFcFcFcFcFcFcFcRdn 365N(ESA) −×+++++++++⋅⋅=


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Worked Example 3

Using SAR5/ESA, SAR7/ESA, SAR12/ESA listed Table 8 in ERN9, design SARs can be calculated as follows;15 year design traffic:

SAR5 = ESA*SAR5/ESA = 1.83E+07 *1.1 =2.01E+07; SAR7 = ESA*SAR7/ESA = 1.83E+07*1.6 = 2.93E+07;SAR12 = ESA*SAR12/ESA = 1.83E+07*12 = 2.20E+08;

40 year design traffic:SAR5 = ESA*SAR5/ESA = 7.43E+07 *1.1 =8.17E+07; SAR7 = ESA*SAR7/ESA = 7.43E+07*1.6 = 1.19E+08;SAR12 = ESA*SAR12/ESA = 7.43E+07*12 = 8.92E+09;


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Comparing SAR and ESA

Example:Rural highway, say NDT calculation gives NDT = 5 × 107 (i.e. cumulative HVAG)ESA = NDT × ESA/HVAG = 5 × 107 × 0.9 = 4.5 × 107

For asphalt: SARa = ESA × (SARa/ESA) = 4.5 × 107 × 1.1 = 4.95 × 107

For subgrade: SARs = ESA × (SARs/ESA) = 4.5 × 107 × 1.6 = 7.2 × 107

For cemented layers: SARc = ESA × (SARc/ESA) = 4.5 × 107 × 12 = 5.4 × 108

Because of the different fatigue damage exponents (5, 7, 12) for each material type, for a given total design traffic (given number of HVAG), the equivalent number of repetitions of the standard axle (SAR) is different for each material type. Thus, for 5 × 107 HVAG, have to design cemented layer to take 5.4 × 108 standard axle repetitions, but have to design asphalt to take only 4.95 × 107 standard axle repetitions.


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Design Life

The minimum design life for pavement as per MRWA Engineering Road Note 9: PROCEDURE FOR THE DESIGN OF FLEXIBLE PAVEMENTS is as follows;

Unless specified otherwise by the Principal, asphalt must have the following minimum design life:

Asphalt 60 mm nominal total thickness or less - 15 years fatigue life; Asphalt greater than 60 mm nominal total thickness - 40 years fatigue life.


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Design ApproachCalculate the design traffic in terms of the number of design equivalent standard axles, DESACalculate the design SAR (i.e. DSAR) for each material from DESA, using appropriate factorChoose a pavement structure (number of layers and layer thicknesses)Assign stiffness values to each layer (or sublayer, as shown later)Using layered elastic theory (i.e. the CIRCLY program), calculate the strains at the critical points (tensile strains in bound layers, and vertical compressive strain in the subgrade) under a single static application of the Standard AxleUse these strains to calculate the allowable number of SAR for each material, using the generic formula:

Compare allowable SAR (NSAR) to the design SAR (DSAR)If all design DSAR ≤ allowable NSAR, pavement is OK. If not, change one or more layers until this condition is met. In CIRCLY, this is expressed as the Cumulative Damage Factor (CDF) for each layer:

Optimisation involves choosing layer thickness such that the allowable ESA for each layer is as close as possible to the design ESA.

b

SARKN ⎥⎦⎤

⎢⎣⎡ε

== SAR of number Allowable

SARNDSARCDF =


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Design Approach (Contd.): Pre- and Post-CrackingBecause of the severe “damage exponent” in the fatigue relationship for cemented layers (12), a cemented layer often fails at much lower SAR than the asphalt and subgrade layers. However, this is not the end of the pavement, since the cemented layer, post-cracking, is considered to continue to act as an unbound granular layer, with Ev = 500 MPa, Eh = 0.5Ev and ν = 0.35. (This is not sublayered).The approach is:

use CIRCLY to find the strains for the pavement containing the original cemented layer, and, from this, find the NSAR for all layers, including the cemented layer (this is the ‘1st layout’)repeat the CIRCLY calculation, but now treating the cemented layer as cracked (i.e. unbound), with the above elastic parameters, and again find NSAR for each material in this pavement (this is the ‘2nd layout’)convert all NSAR to NESA using the appropriate multiplier for both of these casescombine the 1st layout and 2nd layout results, to calculate the total fatigue life of the asphalt and subgrade layers, as follows (equation 8.5 in Austroads 2004):

ndSstS

CCSndA

stA

CCA N

NNNNN

NNNN 2

12

11 and 1 ×⎟⎟

⎠

⎞⎜⎜⎝

⎛−+=×⎟⎟

⎠

⎞⎜⎜⎝

⎛−+=

NA and NS are the total allowable ESA for asphalt and subgrade, respectively. NC is the allowable ESA for cemented layer (1st layout), N1stA and N2ndA are the numbers of ESA for the asphalt using the 1st and 2nd

layouts, respectively. Similarly, N1stS and N2ndS are the numbers of ESA for the subgrade using the 1st and 2nd layouts, respectively.

(See Austroads 2008, Appendix K, section K2 for example of application of this procedure)


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Tyre load (P)

Tyre Pressure, q

Contact Pressure

Contact Pressure ≈ Tyre Pressure

0.3 L0.6 L

Actual Area (Ac) = 0.5227 L2

L

Footprint of a single tyre

Actual Area (Ac) = Tyre load/ Tyre Pressure = P / q

Modelling Wheel Loads for Determination of Stresses and Strains

The Standard Axle modelled in CIRCLY as 4 circular uniformly loaded patches, with 750 kPa vertical pressure, and radius 92.1 mm, which equates to 80 kN equally distributed to the four circular patches.


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Assumptions (Boussinesque solution)1. Linear elastic, isotropic, homogeneous half space2. Circular loaded area

Vertical (normal) stressNotations

Stresses and Strains in a Flexible Pavement

Not directly applicable to anisotropic layered materials, but diagram gives a representative picture of vertical stress (normalised by the applied surface pressure q) reducing below the centreline of the load (r/a = 0), and also the variation at different radii away from the centre of the loaded area.


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Design of New Flexible Pavements: Critical Strains

1. Tensile strain at bottom of asphalt2. Tensile strain at bottom of cemented material3. Compressive strain at top of subgrade

Unit stress (750 kPa)

Granular material

Cemented material

Asphalt

Critical locations

1800 mm330 mm 330 mm

33

2

1

Standard Axle


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Sublayering of Granular Material for Damage CalculationThe modulus of granular materials depends not only on the intrinsic characteristics of these materials, but also on the stress level at which they operate and the stiffness of the underlying layers. Because of the varying stress levels within an individual granular (i.e. uncemented) layer, these layers must be divided into 5 sublayers.

For granular materials placed directly on to a stiff cemented sub-base, no sublayering is required. This allows full advantage to be derived from good-quality granular basecourse materials

In case of sublayering the following procedures should be followed:Divide the total thickness of the selected granular layer into 5 equal-thickness sublayersEv of the top sublayer of the granular layer is the lower of E = 10 times the design CBR of the selected granular material and E determined using the following Equation

The ratio of moduli of adjacent (i.e. underlying) sublayers is determined using the following equation

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v top granular sublayer

subgrade

ER

E⎡ ⎤

= ⎢ ⎥⎣ ⎦

Ev top granular sublayer = Ev subgrade x 2 (total granular thickness/125)


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Sublayering of Granular Material ... (Contd.)

The modulus of each sublayer may then be calculated from the modulus of the adjacent underlying sublayer, beginning with the in situ subgrade, the modulus of which is known.For all granular materials, the other stiffness parameters required for each sublayer may be calculated from the following relationships:EH = 0.5 EV

f = EV / (1 + νV).

Note: Sublayering for granular layers is not required if the granular layer rests directly on a cemented subbase layer.

This is one of the big advantages of placing a cemented layer lower in the profile, rather than directly below the asphalt layer. See example later.


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Example:A road is to be constructed in which there will be a granular layer of 475 mm thickness on top of the in situ subgrade. The subgrade has a CBR value of 5. Determine the elastic parameters to be used in design, assuming a high-standard crushed rock for the granular material

Solution:1. Subgrade:

CBR = 5, then Ev = 10 × CBR = 50 MPaEH = 0.5 EV = 25 MPa, νv = νH = 0.45f = Ev/(1+νv) = 34.5 MPa

2. Sub-base: Top granular sublayer (1st of 5 sublayers):Minimum of:a) EV top of base = Ev subgrade × 2 (total granular thickness/125)= (50) (2)(475/125) = 696 MPa

b) EV top of base = 500 MPa (see Table 6.3, Slide 50 in these notes using High Standard Crushed Rock)

Then, Ev minimum = 500 MPaLet us now subdivide the granular material into five layers and calculate the elastic parameters for each layer.

Sublayering of Granular Material for Damage Calculation


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Ratio of modulus of an underlying layer: R = (500/50)(1/5) = 1.585

Depth above subgade (mm)

Thickness mm

Ev (MPa)

Remarks Ev = Ev (below) * R

Fifth (Final) sublayer of granular material 475 95 500

Fourth sublayer of granular material 380 95 316 = 199 * 1.585

Third layer of granular material 285 95 199 = 126* 1.585

Second sublayer of granular material 190 95 126 = 79 * 1.585

First sublayer of granular material 95 95 79 = 50 * 1.585

Subgrade 0 95 50

Sublayering of Granular Material for Damage Calculation15

v top granular sublayer

subgrade

ER

E⎡ ⎤

= ⎢ ⎥⎣ ⎦

Note: One of the big advantages of placing a cemented layer lower in the profile (below a granular layer), is that, in such cases, sublayering of the granular layer is not required.

Thus, in the example above, sublayering forces the reduction of the modulus of a good quality base material in the lower 4 sublayers. However, if this layer rested directly on a cemented layer, sublayering is not required, so that the full thickness of the granular material can be assigned the same value as the top sublayer in a sublayered system – 500 MPa in this case.


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79

126

199

315

500

500

50

100

150

200

250

300

350

400

450

5000 100 200 300 400 500 600

Vertical modulus (MPa)

Dep

th o

f top

of s

ubla

yer

(mm

) fro

m S

ubgr

ade

3

4

5

2

1

Subgrade

Sublayer

Sublayer

Sublayer

Sublayer

Sublayer

Sublayering of Granular Material for Damage Calculation


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Mechanistic Design Procedures: Input Requirements (Table 8.1)1. Select a trial pavement and a desired project reliability

2. Determine the following elastic parameters for the in situ subgrade and selected subgrade materials:

EV ; EH = 0.5 × EV; νV = νH; f = EV / (1 + νV)

3. Determine the elastic parameters (as above) of the top sublayer of the granular layer (if relevant)

4. Determine the elastic parameters and thickness of the other granular sublayers (if relevant)

(The Granular layer should be divided into 5 sublayers and a special interpolation is made for the modulus of each of these sublayers between the subgrade and the top sublayer)

5. Determine the elastic parameters for cemented materials, pre- and post-cracking

6. Determine the elastic parameters for asphalt

7. Use the subgrade fatigue criterion to calculate the allowable NSAR for the subgrade

8. Use the fatigue strain criterion for a cemented layer to calculate the allowable NSAR for this layer

9. Use the fatigue strain criterion for an asphalt layer to calculate the allowable NSAR for the asphalt

10. Determine design number of SAR for each relevant distress mode (i.e. DSARa, DSARc, DSARs for the asphalt, cemented layer, and subgrade, respectively)


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Mechanistic Design Procedures: Analysis (Table 8.2)

11. Approximate the Standard Axle wheel loading as four uniformly loaded circular areas at centre-to-centre spacings of 330 mm, 1470 mm and 330 mm; a vertical load of 20 kN is applied to each circular area at a uniform vertical stress distribution of 750 kPa.

* Radius of each loaded area R = 2523p –0.5 (about 92.1 mm for highway traffic), where R = radius (mm) and p = vertical stress (kPa).

12. Determine critical locations in the pavement for the calculation of strains as follows:ً * bottom of each asphalt or cemented layer

* top of in situ subgrade and top of selected subgrade materials.* on vertical axes through the centre of an inner wheel load and through a point midway between the

two wheel loads at a centre-to-centre spacing of 330 mm.

13. Input the above values into CIRCLY and determine the maximum vertical compressive strain at the top of the subgrade and top of selected subgrade materials and the maximum horizontal tensile strainat the bottom of each cemented and/or asphalt layer.

If the post-fatigue cracking phase of cemented materials life is being considered, the calculation is repeated, using the post-cracked material parameters for the cemented layer (Ev = 500 MPa = 2Eh; ν= 0.35)


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Mechanistic Design Procedures: Interpretation (Table 8.3)

14. Determine using the criteria selected in Steps 7, 8 and 9 the allowable number of SAR (Standard Axle Repetitions) for each of the relevant distress modes.

If the post-cracking phase of cemented materials life is being considered, calculate the total allowable loading of the pre-cracking and post-cracking phases of life.

15. For each distress mode, compare allowable number of SAR (NSAR) with the design number of SAR (DSAR)

16. If, for all distress modes, the allowable number of SAR exceeds the design number of SAR (DSARa, DSARs, DSARc) the pavement is acceptable. If not, it is unacceptable.

(In CIRCLY, this is expressed as Cumulative Damage Factor, CDF, for each layer: if CDF ≤1, that layer is OK.

17. If the pavement is unacceptable or additional pavement configurations are required for comparison, select a new trial pavement, return to Step 1 and repeat Steps 1 to 16.

18. Compare alternative acceptable designs for economic assessment.


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Main Roads in WAWhat ratios of MRWA pavement construction are of the following types (I am assuming that practically 100% is flexible pavement):

(a) full pavement where the asphalt is part of the structural system ~ 0.05%

(b) full pavement where there is minimum asphalt cover (40 mm or less ?) ~5% almost all in the metro area, some in major urban centres such as Albany and other sites are steep grades and intersections subject to heavy vehicles turning movements

(c) full pavement where there is only a spray seal / chipseal, but no asphalt ~92%

(d) unsealed pavement ~3% of the 37,445 lane km

Is it the case that that (a) is only used for major urban freeways/highways, or is it also used for major rural highways?

Rural asphalt is typically only short lengths of 30mm or 40mm thickness at intersections - type (b) above. Type (a) would be very rare in regional areas. The attraction is not so much the structural properties or long life, it is the rapid construction time that reduces the traffic control costs and other fixed overheads typically incurred with granular construction. Processes such as mixing, surface finishing and trimming, drying back layers before placing the next layer or applying a prime seal are all take time and drying back layers is only effective in warm dry conditions. Rain events can prolong the time required.

Do you have a notional cost per m2 of these pavement types (I know this depends on many factors, including pavement thickness, so just some general figures would be fine - or even figures that were in terms of thickness)?

The average cost of metropolitan pavements is $1.6M / lane km and regional costs would be in the order of $300, 000 / lane km.

How commonly do you use a cemented layer somewhere in the pavement (that is, apart from an asphalt layer)? It it's used, is it always separated from the asphalt layer by an unbound granular layer, or is it directly below the asphalt layer (or seal)?

We do routinely stabilise pavement materials with cement and lime to improve the properties of marginal material (usually after failure but sometimes at initial construction) or basecourse that will be used in poor drainage conditions. Those pavements are still considered as unbound layers with UCS values < 1MPa and modulus < about 1000MPa. Rural pavement repairs typically comprise in situ stabilisation of 200mm of pavement with 2% LH cement.

We have very recently constructed a short length of bound pavement under 175mm of asphalt at Leach /Orrong intersection as a construction expedience to allow traffic to be shifted and structural works to proceed. It is very much the exception. 175mm is the minimum thickness of asphalt over a bound layer recommended by Austroads.

(Thanks to Ross Keeley, MRWA, for supplying this information in response to my questions)


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Road Statistics for WA (National, MRWA, and Local Authority)

Note:

• Most roads (in terms of total length) are local authority roads

• Most main roads are sealed

• Most local roads (in terms of km) are unsealed

• The average age of main road pavements is 29 years, with the seals having average age of 11 years


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HIPAVE: Version of CIRCLY for Heavy Industrial Pavements


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APSDS: Version of CIRCLY for Airport Pavement Design

Structural Design of Pavements Australia

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