Pavement.ppt

7/27/2019 Pavement.ppt

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Pavement Design

SCOPE

Comparison of AASHTO Method withTRRL-ORN 31 Method

Study of Design Example


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This method makes Provision onprobability basis.

takes into account -

traffic variation Performance prediction

It is also required to identify initial and terminallevels of Serviceability.

Even this method is subjective and alsoenvironment specific.


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AASHTO Concept

Assessment of Structural

Number, ie. SNcalculated for a

new pavement on existing roadbed soil (sub grade) for

anticipated traffic using AASTHO

formula.


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Structural coefficient generally Based

on resilient modulus / CBR

Structural Number (SN) = Structural Coff: x Layer Thickness

SN = ai x Di ( for the ith layer)

SN (Pavement structure) = ai x Di

No:19

n

i=1


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TRRL ORN 31

Claims to have Made provision forTropical Environmental condition.

Based on,

Sub grade strength

Traffic Volume


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However lacks Provision for Evaluating the

Performance of Pavement Structure under:

Specific Traffic Volume Environmental condition


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Present analysis done with special reference to

the ORN -31 chart 5 /RDAchart 3

max. thickness = 1000 mm

Below 1000 mm depth structure is

considered as sub grade (road bed soil ).


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Design LifeIn Pavement Design, the Design life is takenas the Period of Service measured by a

specified Number of Passes of standard

axles, at the end of which it becomesnecessary to construct an overlay to restore

the original riding quality.

In the AASHTO method design life is definedasAnalysis Period.


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AASHTO method adopts SN concept

TRRL, ORN 31 adopts

CBR criteria

(No reference is method to Structural

Number, SN in ORN)


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In the evaluation of the TRRL, ORN 31

catalog the following are assumed.

Since SN concept has been developed under fpssystem, 25 mm thickness is taken as equivalent to

1 inch.

The contribution by each layer to PavementStrength is quantified as done in AASHTO method

using strength coefficients. (This is a practicalmeans of evaluation of composite layeredstructure.)

Th C t l h t 5 d T


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The Catalog on chart 5 under T6 ,S1, can be written algebraically asfollows.

SNC(T6 S1(CBR2)) = SNSG(S1)+4a1+8a2+9a3+14a4

Similarly T6 ,S6 can be written as

SNC(T6S6 (CBR>30)) =SNSG(S6)+4a1+8a2

Where,

SNC(Ti Si) = modified SN for Pavement Structure on subgradeSNSG(S1) = SN of sub grade

i = i th Class

C td P t t t


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Contd Pavement structure

T6, S1

T6, S6


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The above factor clearly indicates that

structural Number (SN) would be

identified as the basis for ORN 31

It is possible to evaluate ORN 31

catalogues on a SN basis.


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To proceed with evaluation it is required to

assign values for structural coefficient

(strength coefficient) No:9


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AASHTO method makes use of MR

( Resilient Modulus) to obtain the

Contribution by the Sub Grade.


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TRRL LR No 673 Provides structural

coefficient (ai) based on CBR

Strength Coefficient of,

Granular base material,

a1={29.14(CBR)-0.1997(CBR)2

+0.00045CBR3

} x 10-4

Sub base material,

a2=0.01+0.065 log10 CBR

Contribution of Sub Grade(SNSG),SNSG = 3.51 log10 CBR 0.85 (log10 CBR)

2-1.43


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SG CLASSSN

CVALUES(Avg)

T6 T7 T8

S1

4.52 5.06 5.71

S2

4.64 5.18 5.83

S3

4.59 5.13 5.78

S4

4.75 5.29 5.83

S5 4.76 5.30 5.84

S6

4.51 5.05 5.59


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SG CLASSSN

CVALUES(Max)

T6

T7

T8

S1

4.52 5.06 5.71

S2

4.81 5.35 6.00

S3

4.75 5.29 5.94

S4

4.97 5.51 6.05

S5 4.83 5.37 5.91

S6

4.61 5.15 5.69


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SG CLASSSN

CVALUES(Min)

T6

T7

T8

S1

4.52 5.06 5.71

S2

4.48 5.35 5.67

S3 4.43 4.97 5.62

S4

4.54 5.08 5.62

S5

4.68 5.22 5.76

S6

4.42 4.96 5.50


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SN for each level of Traffic (T1,T2..T6)has practically same value Irrespective of the

Sub Grade classSN varies with the Traffic class (Horizontally)

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