--- title: "IRC CBR-method design: 1200 CVPD, 4% subgrade" subject: "Transportation Engineering II" chapter: 2 key: "semester_7/transportation_engineering_ii:irc-cbr-method-design-1200-cvpd" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/irc-cbr-method-design-1200-cvpd site: BCT Engineering --- # IRC CBR-method design: 1200 CVPD, 4% subgrade Transportation Engineering II · Chapter 2: Highway Pavement - Asked in: 2072 Chaitra, 8 marks - Video: [Numerical on design of flexible pavement using IRC method- IRC 37:2018 code](https://www.youtube.com/watch?v=q0vT3L097As) — Letz Learn - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/irc-cbr-method-design-1200-cvpd - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2 ## Question 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 ## 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. ## Answer #### 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): $A=1200(1.06)^{0.5}=1235$ 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: $240-50=190$ mm; provide **200 mm** (practical minimum 150 mm). - Sub-base (gravel, CBR 20 %) = thickness above compacted soil minus thickness above sub-base: $340-240=100$ 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: $510-340=170$ mm; provide **170 mm** (compacted in layers of 100-150 mm). Check: $50+200+150+170=570$ mm $\ge510$ mm required above the 4 % subgrade. ```text +--------------------------------+ | 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the design traffic class found for this road? A. Use 1200 CVPD directly, class D B. Multiply 1200 by 365 x 10 years to get total vehicles C. Grow 1200 CVPD by 6 % for 0.5 year to about 1235 CVPD, giving class E (450-1500) D. Divide 1200 by two for lane distribution, giving class C Answer: C. Traffic at the end of construction is 1200(1.06)^0.5 = 1235 CVPD; the road is single lane so all of it counts, which falls in class E. (Key point: Traffic at completion and class) 2. How is the thickness of an individual layer found in the CBR method? A. Take the chart value for the layer's own CBR as its thickness B. Divide the total subgrade thickness equally among layers C. Use only the minimum practical thickness for every layer D. Subtract the thickness required above the next better layer from the thickness required above the layer's own CBR Answer: D. Layers are found by differences of thickness above successive CBR values, e.g. compacted soil layer = 510 - 340 = 170 mm. (Key point: Equivalent thickness concept) 3. How was the crushed stone base thickness of 190 mm (provided 200 mm) obtained? A. Thickness above sub-base (240 mm) minus the 50 mm surfacing B. 340 - 240 mm C. 510 - 340 mm D. 240 + 50 mm Answer: A. The total above the CBR 20 % sub-base is 240 mm, of which 50 mm is asphalt concrete, leaving 190 mm for the base. (Key point: Base course thickness) 4. The sub-base calculation gave 340 - 240 = 100 mm, yet 150 mm was provided. Why? A. The chart is always conservative by 50 mm B. Traffic growth requires extra thickness C. The minimum practical thickness for a sub-base is 150 mm D. The subgrade CBR is only 4 % Answer: C. A layer cannot be built thinner than its practical minimum (150 mm), so the calculated 100 mm is raised to 150 mm. (Key point: Minimum practical thickness) ## Related in Transportation Engineering II - [IRC pavement design: 5640 CVPD, CBR 6%](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Airc-design-5640-cvpd-cbr-6) — Transportation Engineering II, chapter 2 - [Asphalt Institute design: gravel road upgrade, 0.8×10³ ESA](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aasphalt-institute-design-0-8e3-esa) — Transportation Engineering II, chapter 2 - [Asphalt Institute design: 1.888×10³ ESAL/day, 10 years](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aasphalt-institute-design-1-888e3-esal) — Transportation Engineering II, chapter 2 - [Plain CC pavement thickness by IRC, 7 m highway](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aplain-cc-pavement-thickness-irc-7m) — Transportation Engineering II, chapter 2 - [Flexible design: 4-lane single carriageway, 2000 CVPD](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-four-lane-single-2000) — Transportation Engineering II, chapter 2 - [Flexible pavement life with 140 cvpd traffic](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-life-years-140-cvpd) — Transportation Engineering II, chapter 2 ## Related in other subjects - [CBR test on subgrade soil](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acbr-test) — Transportation Engineering I, chapter 6 - [Aggregate size; well-graded from grading curve](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aaggregate-size-well-graded) — Concrete Technology and Masonry Structure, chapter 1 - [Why well-graded aggregate is better](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awell-graded-aggregate-better) — Concrete Technology and Masonry Structure, chapter 1