* Superintending Engineer & Regional officer, M/o RT&H, Regional Office (C), Bengaluru (India) –560001. Email – [email protected] , [email protected] 3. FINAL PHASE OF EMPIRICAL METHOD IN INDIA – IRC:37–1984 [4] : 3.1. The empirical design method for flexible pavement, proposed in 1970, was continued for design traffic volume up to 1500 CVD. However, the modified CBR curves for 10.2 tones single axle legal limits were used instead of 8.16 tones and consequently, the pavement thickness was increased by 10 to 20%. 3.2. Recognizing the fact that the structural damage caused by a vehicle depends on the axle load it imposed on the road, the equivalent axle load concept was introduced in India also similar to other countries of the world to handle the large spectrum of axle loads actually applied to a pavement. Design traffic (N x ) carried by pavement during its design life was considered in terms of cumulative number of standard axles in the lane carrying maximum traffic and evaluated as under: = ∗ ∗ ∗ …(2) where, N x = cumulative number of standard axles to be catered for design, (expressed in terms of million (10 6 ) standard axles or msa) A = initial traffic, in the year of completion of construction in CVD, as modified for lane distribution, r = annual growth rate of commercial traffic, taken as 7.5%, x = design life in years, taken as 10 to 15 years. F = vehicle damage factor. 3.3. Design curves relating pavement thickness to the cumulative number of standard axles for different subgrade strengths (assessed in terms of CBR value) were evolved. Pavement composition (thickness of component layers) was therefore might be decided by the designer subject to the minimum thickness as determined from the thickness combination block given in the IRC:37–1984. Example 2: Given that, subgrade CBR = 5%, and traffic after construction, A = 730 CVD. Design flexible pavement for 10 years for two lane NH in plain terrain as per (a) IRC:37–1970 and (b) IRC:37–1984. Solution: Let, total pavement thickness = T, mm (a) Pavement design as per IRC:37–1970, and using equation (1), we get A D = 730 (1+.075) 10 = 1505, and therefore, T = 475 mm from IRC:37–1970. This thickness is applicable for A D varies from 1501 CVD to 4500 CVD. (b) Pavement design as per IRC:37–1984, and using equation (2), we get N x = [365 x (730*0.75) * {(1+.075) 10 – 1}* 2.75/0.075] = 7.78 msa therefore, T = 540 mm from IRC:37–1984. As per IRC:37–1984, surfacing should be 25 mm SDC or BC with binder course 75 mm DBM while base course should have a minimum thickness of 250 mm with material of 100% CBR. Therefore, thickness of sub-base = 540 – (25+75+250) = 190 mm > 150 mm, O.K. Provide sub-base of 200 mm with material of 30 % CBR. In base course, either (1 x 100 + 2 x 75 = 250 mm) of WBM in three layers or two layers each of 75 mm thick of WBM + one layer of 75 mm thick BM (2x75+75*1.5=262.5 mm) can be provided. Later option is preferable. Comments: As per IRC:37–1984, for A = 2183 CVD, N x = 23.25 msa and T = 635 mm. As per IRC:37–1970, pavement thickness is constant for traffic volume

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Flexible PAVEMENT DESIGN BY IRC:37

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* Superintending Engineer & Regional officer, M/o RT&H, Regional Office (C), Bengaluru (India) –560001. Email – [email protected], [email protected]

3. FINAL PHASE OF EMPIRICAL METHOD IN INDIA – IRC:3 7–1984[4]:

3.1. The empirical design method for flexible pavement, proposed in 1970, was

continued for design traffic volume up to 1500 CVD. However, the modified CBR curves for 10.2 tones single axle legal limits were used instead of 8.16 tones and consequently, the pavement thickness was increased by 10 to 20%.

3.2. Recognizing the fact that the structural damage caused by a vehicle depends on the axle load it imposed on the road, the equivalent axle load concept was introduced in India also similar to other countries of the world to handle the large spectrum of axle loads actually applied to a pavement. Design traffic (Nx) carried by pavement during its design life was considered in terms of cumulative number of standard axles in the lane carrying maximum traffic and evaluated as under:

�� =��∗ ∗ �� ∗ �

� …(2)

where, Nx = cumulative number of standard axles to be catered for design, (expressed in terms of million (106) standard axles or msa)

A = initial traffic, in the year of completion of construction in CVD, as modified for lane distribution,

r = annual growth rate of commercial traffic, taken as 7.5%, x = design life in years, taken as 10 to 15 years. F = vehicle damage factor.

3.3. Design curves relating pavement thickness to the cumulative number of standard axles for different subgrade strengths (assessed in terms of CBR value) were evolved. Pavement composition (thickness of component layers) was therefore might be decided by the designer subject to the minimum thickness as determined from the thickness combination block given in the IRC:37–1984.

Example 2: Given that, subgrade CBR = 5%, and traffic after construction, A = 730

CVD. Design flexible pavement for 10 years for two lane NH in plain terrain as per (a) IRC:37–1970 and (b) IRC:37–1984.

Solution: Let, total pavement thickness = T, mm (a) Pavement design as per IRC:37–1970, and using equation (1), we get

AD = 730 (1+.075)10 = 1505, and therefore, T = 475 mm from IRC:37–1970. This thickness is applicable for AD varies from 1501 CVD to 4500 CVD.

(b) Pavement design as per IRC:37–1984, and using equation (2), we get Nx = [365 x (730*0.75) * {(1+.075)10 – 1}* 2.75/0.075] = 7.78 msa therefore, T = 540 mm from IRC:37–1984. As per IRC:37–1984, surfacing should be 25 mm SDC or BC with binder course 75 mm DBM while base course should have a minimum thickness of 250 mm with material of 100% CBR. Therefore, thickness of sub-base = 540 – (25+75+250) = 190 mm > 150 mm, O.K. Provide sub-base of 200 mm with material of 30 % CBR. In base course, either (1 x 100 + 2 x 75 = 250 mm) of WBM in three layers or two layers each of 75 mm thick of WBM + one layer of 75 mm thick BM (2x75+75*1.5=262.5 mm) can be provided. Later option is preferable.

Comments: As per IRC:37–1984, for A = 2183 CVD, Nx = 23.25 msa and T = 635 mm. As per IRC:37–1970, pavement thickness is constant for traffic volume

after construction varies from 730 CVD to 2183 CVD while as per IRC:37–1984, it varies from 540 mm to 635 mm making the pavement design more responsive to applied traffic volume (loading).