28.9. Numerical Example

Design the pavement for construction of a new bypass with the following data:

1. Two lane carriageway
2. Initial traffic in the year of completion of construction = 400 CVPD (sum of both directions)
3. Traffic growth rate = 7.5 %
4. Design life = 15 years
5. Vehicle damage factor based on axle load survey = 2.5 standard axle per commercial vehicle
6. Design CBR of subgrade soil = 4%.

This chunk outlines the specific conditions and requirements for the pavement design of a new bypass. It includes vital information such as the type of road (two-lane carriageway), the initial traffic volume expected (400 commercial vehicles per day), the anticipated growth in traffic over the years (7.5% per year), the lifespan for which the pavement should be designed (15 years), and the characteristics of the subgrade soil (design CBR of 4%). The vehicle damage factor indicates how different vehicles impact pavement wear.

Imagine planning a two-lane highway in a rural area. You anticipate that in 15 years, traffic will increase due to more people moving into the area and businesses expanding, similar to planting a tree – you need to consider how tall it will grow and what space it will need to thrive.

1. Distribution factor = 0.75
2. N = 365 × (1 + 0.075) ^ 15 × 400 × 0.75 × 2.5 / 0.075 = 7200000 = 7.2 msa

In this section, we calculate the total design traffic over the pavement's lifespan. The distribution factor is a coefficient that helps determine how traffic is spread over the lanes. The calculation uses the initial traffic volume, growth rate, and distribution factor to arrive at the total number of standard axles expected over 15 years, resulting in 7.2 million standard axles (msa). This helps engineers understand the expected load the pavement must support.

Think of it like forecasting the number of visitors to a park over several years. If you expect traffic to increase due to more local families moving into the area, you'll need to calculate how many visitors will come based on past trends, just like predicting the number of axles using growth rates.

3. Total pavement thickness for CBR 4% and traffic 7.2 msa from IRC:37 2001 chart1 = 660 mm.

Once the expected traffic is calculated, the next step is to determine the appropriate thickness of the pavement layers required to withstand that traffic over time. According to the IRC guidelines, for a CBR value of 4% and an estimated traffic load of 7.2 msa, a total pavement thickness of 660 mm is recommended. This thickness is essential for providing the durability and stability needed for the pavement over its expected lifespan.

Imagine building a sturdy shelf to hold heavy books. You need to decide how thick the shelf should be based on how many books you'll store. Similarly, engineers calculate pavement thickness based on expected traffic load, ensuring it can handle the weight without collapsing.

4. Pavement composition can be obtained by interpolation from Pavement Design Catalogue (IRC:37 2001). (a) Bituminous surfacing = 25 mm SDBC + 70 mm DBM (b) Road-base = 250 mm WBM (c) Sub-base = 315 mm granular material of CBR not less than 30 %

The final step in designing the pavement is to determine its composition, which involves the different layers that make up the roadway. This example outlines the specific materials and thicknesses required for each layer: the bituminous surfacing (25 mm of Stone Matrix Asphalt Concrete and 70 mm of Dense Bituminous Macadam), a road-base made of 250 mm of Water Bound Macadam, and a sub-base of 315 mm of granular material with a minimum CBR of 30%. This layered structure is crucial for the longevity and performance of the pavement.

Think of a cake with different layers: the bottom layer might be a thick spongecake (sub-base), the middle could be a rich cream layer (road-base), and the top frosting (bituminous surfacing) ties it all together. Each layer is crucial for ensuring the cake is delicious and doesn't crumble – just like every layer of pavement is vital for its stability and durability.

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