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IRC CBR-method design: 1200 CVPD, 4% subgrade

Short answer

For 1200 CVPD, 6% growth, 10-year design, subgrade CBR 4%, IRC CBR chart class E gives total thickness 510 mm. Section: asphalt concrete 50 mm, crushed stone base 200 mm (CBR 90%), gravel sub-base 150 mm (CBR 20%), compacted soil 170 mm (CBR 10%); total 570 mm.

A road pavement is to be designed for a stretch of road with the following pavement layers: (i) Minimum thickness of asphalt concrete on the surface course = 50 mm. (ii) Well graded crushed stone aggregate for base course, CBR value = 90% (iii) Fairly graded gravel for sub-base course, CBR value = 20% (iv) Compacted Soil, CBR value = 10% (v) 90th percentile sub grade CBR Value = 4% The road has single lane carriage way & caters present ADT of 1200 commercial vehicle per day with annual growth of 6%. The pavement is to be designed for 10 years period. Design the pavement section using IRC recommendation for CBR method. The road is to be compacted with 6 months from initial traffic count

Method

CBR method of IRC:37 (design charts of total thickness against subgrade CBR for traffic classes A-G, where the class is fixed by commercial vehicles per day at the end of construction). Layers are found by the equivalent thickness concept: the total thickness required above a layer depends on its own CBR.

Step 1 - Design traffic

Traffic at completion (6 months after the count): A=1200(1.06)0.5=1235A=1200(1.06)^{0.5}=1235 CVPD. The road is single lane, so the full traffic is taken for the one lane. This falls in class E (450-1500 CVPD). The growth rate (6 percent for 10 years) is accounted for in the chart; the design is therefore for class E curve.

Step 2 - Thickness above each CBR (class E curve of the IRC chart)

The total thickness of pavement needed above a material of given CBR is read from the class E curve of the IRC CBR chart (approximate readings; use the attached chart if provided):

Material (CBR)Pavement thickness required above it (mm)
Subgrade (4 %)510
Compacted soil (10 %)340
Gravel sub-base (20 %)240

Step 3 - Layer thicknesses

  • Surface: asphalt concrete (minimum) = 50 mm.
  • Base (crushed stone, CBR 90 %) = thickness above sub-base minus surfacing: 240−50=190240-50=190 mm; provide 200 mm (practical minimum 150 mm).
  • Sub-base (gravel, CBR 20 %) = thickness above compacted soil minus thickness above sub-base: 340−240=100340-240=100 mm; the minimum practical thickness is 150 mm, so provide 150 mm.
  • Compacted soil layer (CBR 10 %) = thickness above subgrade minus thickness above compacted soil: 510−340=170510-340=170 mm; provide 170 mm (compacted in layers of 100-150 mm).

Check: 50+200+150+170=57050+200+150+170=570 mm ≥510\ge510 mm required above the 4 % subgrade.

  +--------------------------------+
  | Asphalt concrete       50 mm   |
  | Crushed stone base    200 mm   | CBR 90 %
  | Gravel sub-base       150 mm   | CBR 20 %
  | Compacted soil        170 mm   | CBR 10 %
  +--------------------------------+
  | Subgrade, 90th pct CBR 4 %     |
  +--------------------------------+

Answer: AC 50 mm + base 200 mm + sub-base 150 mm + compacted soil layer 170 mm (total 570 mm) over the 4 % subgrade. Note: the chart readings for class E are approximate; the layer procedure (differences of thickness above successive CBR values) is what the method requires.

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