How is the life of the given pavement found?

Compute the ESALs the section can carry (via SN), then divide by yearly traffic growth to find years

Read the life directly from the CBR

Take the design life as 20 years

Design CBR is taken as 5 %. What subgrade modulus is used?

M_R = 10 x 5 = 50 MPa (7252 psi)

How is SN of the section computed?

Sum of coefficient x thickness in inches for each layer

Thickness in mm times the coefficient

Only the surface layer coefficient

After finding [(1.08)^n - 1]/0.08 = 7.22, how is n obtained?

(1.08)^n = 1 + 0.08 x 7.22, so n = ln(1.577)/ln(1.08), about 5.9 years

2076 Chaitra · 8 marks

Flexible pavement life with 140 cvpd traffic

Short answer

Using AASHTO, subgrade CBR 5 % (90th percentile) and structural number 3.37 give a capacity of 0.79 million ESAL. With vehicle damage factor 1.90, opening traffic 157 cvpd (8 % growth, 1.5-year construction), solving the traffic equation yields a pavement life of 5.9 years.

A proposed flexible pavement design of single lane carriageway consists of 75mm of Asphalt concrete, 120 mm of emulsified stabilized base course and 145mm of granular subbase. The expected commercial traffic volume is 140 cvpd. The expected traffic composition is 30% truck, 30% of mini truck and 40% of bus, whose truck factors are 5.6, 0.6 and 0.1 respectively. The expected annual traffic growth rate is 8% for all vehicles, and 18month required for construction to be completed. The CBR test conducted at 7 locations strating from Ch.0+100 at the interval of 200m distance gave the value of 11,9,7,10,8,6,4. E_sub base=275 MPa, E_base=500MPa and E_surface=2400 MPa. How many years can this pavement last?

Approach

The structure is given, so its capacity (ESAL) is found with the layer-coefficient (structural number) method of AASHTO, then divided by the yearly traffic to get the life. Assumed: reliability 90 % ( $Z_R=-1.282$ ), $S_0=0.5$ , $\Delta PSI=2.0$ , drainage coefficients $m=1$ , single lane so the design lane takes 100 percent of traffic.

Step 1 - Subgrade

CBR values at 7 locations: 11, 9, 7, 10, 8, 6, 4. The 90th percentile value (90 percent equal or greater) lies between 4 % (100 percent) and 6 % (85.7 percent) and is about 5.4 %; adopt CBR = 5 %, $M_R=10\times5=50$ MPa $=7252$ psi.

Step 2 - Layer coefficients (E in psi)

\begin{aligned} a_1&=0.323\log_{10}(2400\times145.04)-1.42=0.370\\ a_2&=0.249\log_{10}(500\times145.04)-0.977=0.233\\ a_3&=0.227\log_{10}(275\times145.04)-0.839=0.205 \end{aligned}

Step 3 - Structural number of the section

SN=0.370\times\frac{75}{25.4}+0.233\times\frac{120}{25.4}+0.205\times\frac{145}{25.4}=1.092+1.102+1.172=3.37

Step 4 - ESALs the pavement can carry

With $SN=3.37$ and $M_R=7252$ psi in the AASHTO-93 equation: $\log_{10}W_{18}=5.896$ , so $W_{18}=786452$ ESAL ( $\approx0.79$ million).

Step 5 - Traffic per year

VDF from truck factors: $0.30\times5.6+0.30\times0.6+0.40\times0.1=1.90$ . Traffic at opening (18 months of construction): $A=140(1.08)^{1.5}=157.1$ CVPD.

W_{18}=365\,A\,F\,\frac{(1.08)^n-1}{0.08}\ \Rightarrow\ \frac{(1.08)^n-1}{0.08}=\frac{786452}{365\times157.1\times1.90}=7.22

(1.08)^n=1+0.08\times7.22=1.577\ \Rightarrow\ n=\frac{\ln1.577}{\ln1.08}=5.92\ \text{years}

Answer: the pavement will last about 5.9 years after opening (CBR 5 %, SN = 3.37, capacity 0.79 million ESAL).

✓ Answer checked by Claude Opus 5.5 (high effort)

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