Question

The following data is obtained from an axle load survey at a site: Average rear axle load = 12000 kg Number of commercial vehicles = 800 per day The pavement at this site would be reconstructed over a period of 5 years from the date of survey. The design life of the reconstructed pavement is 15 years. Use the standard axle load as 8160 kg and the annual average vehicle growth rate as 4.0\%. Assume that Equivalent Wheel Load Factor (EWLF) and Vehicle Damage Factor (VDF) are equal. The cumulative standard axle (in msa) for the pavement design is \_\_\_\_ (rounded off to 2 decimal places).

Hint:

To calculate cumulative standard axles, always ensure to include: Correct axle load equivalency (VDF), Traffic growth factor (CGF), Design period and daily axle repetitions.

Answer 1

Step 1: Determine the Vehicle Damage Factor (VDF). The VDF is calculated using the formula: \[ \text{VDF} = \left( \frac{\text{Axle Load}}{\text{Standard Axle Load}} \right)^4. \] Substituting the given values: \[ \text{VDF} = \left( \frac{12000}{8160} \right)^4 = \left( 1.4706 \right)^4 \approx 4.663. \] Step 2: Calculate the daily standard axle repetitions. The daily standard axle repetitions are given by: \[ \text{Daily Standard Axle Repetitions} = \text{Number of Commercial Vehicles} \times \text{VDF}. \] Substituting the values: \[ \text{Daily Standard Axle Repetitions} = 800 \times 4.663 = 3730.4. \] Step 3: Account for the traffic growth rate. The annual traffic growth rate is \( 4.0\% \), and the cumulative growth factor (CGF) is calculated using the formula: \[ \text{CGF} = \frac{(1 + r)^n - 1}{r}, \] where: \( r = \text{growth rate per annum} = 0.04 \), \( n = \text{design life in years} = 15. Substituting the values: \[ \text{CGF} = \frac{(1 + 0.04)^{15} - 1}{0.04} = \frac{1.8009 - 1}{0.04} = 20.0225. \] Step 4: Compute the cumulative standard axles. The cumulative standard axles (in msa) is given by: \[ \text{Cumulative Standard Axles} = \text{Daily Standard Axle Repetitions} \times 365 \times \text{CGF}. \] Substituting the values: \[ \text{Cumulative Standard Axles} = 3730.4 \times 365 \times 20.0225 = 7.31 \, \text{msa}. \] Conclusion: The cumulative standard axle load for the pavement design is \( 7.31 \, \text{msa} \).