Example 10.3

Benkelman beam deflection studies were carried out on a highway pavement with 50 mm thick bituminous surface course, when the mean pavement surface temperature was 40° C and the field moisture content of sub grade soil was 5.5 percent. The soil is found to be sandy and the annual rainfall of the region is 950 mm. the characteristic deflection value, $D_c$ of the selected sub stretch is found to be 1.32 mm. determine the corrected deflection value after applying the corrections for temperature and seasonal variation in sub-grade moisture. (Appropriate chart for moisture correction may be used).

Solution:

Given $D_c = 1.32 \ mm$, pavement temperature during the study = 40°C , field moisture content of sub-grade soil = 5.5 %, sandy soil, annual rainfall = 950 mm

Correction for pavement temperature:

Temperature correction factor (as per IRC) = 0.01 mm per °C

Temperature correction with respect to standard temperature of 35°C

= 1.32 – 0.05 = 1.27 mm.

Correction for seasonal variation is sub-grade moisture content:

For sandy soil and annual rainfall less than 1300 mm, Chart (a) of Fig. 10.13 may be used.

From the chart, moisture correction factor corresponding to 5.5% moisture = 1.12.

Characteristic deflection after applying temperature correction and moisture correction,

$D_{cm} = 1.27 \times 1.12 = 1.42 \ mm$

Example 10.4

Benkelman beam deflection studies were carried out on a highway pavement with 125 mm thick bituminous binder and surfacing, when the mean pavement surface temperature was 31° C and the field moisture content of sub grade soil was 7.0 percent. The sub-grade soil is clayey with low plasticity (PI less than 15) and the annual rainfall of the region is 1650 mm. The characteristic deflection value, $D_c$ of the selected sub stretch is found to be 1.45 mm. Determine the corrected characteristic deflection value after applying the corrections for temperature and seasonal variation in sub-grade moisture. (Appropriate chart for moisture correction may be used).

Solution:

Given $D_c = 1.45 \ mm$, pavement temperature during the study = 31°C , field moisture content of sub-grade soil = 7.0 %, sub-grade soil clayey with PI less than 15, annual rainfall = 1650 mm

Correction for pavement temperature:

Temperature correction factor (as per IRC) = 0.01 mm per °C

Temperature correction with respect to standard temperature of 35°C

= (35 – 31) x 0.01 = 0.04 mm. (to be added)

Temperature deflection after applying temperature correction,

$C_{ct} = 1.45 + 0.04 = 1.49 \ mm$

Correction for seasonal variation is sub-grade moisture content:

For clayey soil with PI less than 15 and annual rainfall of 1650 mm, (exceeding 1300 mm), Chart (b) of Fig. 10.13 may be used.

From the chart, moisture correction factor corresponding to 7.0% moisture = 1.50. Characteristic deflection after applying temperature correction and moisture correction,

$D_{cm} = 1.49 \times 1.50 = 2.235$, say 2.24 mm.

Example 10.5

The number of commercial vehicles in the year of completing overlay construction is estimated to be 2750 per day with an average growth rate of 6.5 percent per year. If the average VDF value is 5.4, determine the design traffic in terms of CSA for 10 years design period, assuming a lane distribution factor of 0.75.

Solution:

Given: N = 275, r = 6.5 % = 0.065, F = 5.4, D = 0.75, n = 10 years.

Design traffic (using equation 10.6)

$CSA = \frac{365{NFD[1+r]^n – 1}}{r} = 54.857 \times 10^6 = \ say \ 55 \ msa$