Loss of Serviceability ∆PSI Resilient modulus M

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4.6 Calculation of Structural Capacity

To calculate the structural capacity of road pavement, as represented by structural number SN, it is necessary to determine several parameters as follow.

4.5.1 Loss of Serviceability ∆PSI

The loss of serviceability can be determined by following the procedure below. a. Average IRI was calculated from the existing data for 8 station of each direction. The IRI for all stations can be seen in Tables 4.8 and 4.9 below. Table 4.8: Loss of Serviceability for Demak – Trengguli Direction Station IRI mkm SV X p o ∆PSI 26+900 3.06 21.21 1.35 2.23 0.23 28+350 2.45 13.57 1.16 2.58 0.58 30+000 3.30 24.78 1.41 2.12 0.12 32+100 4.17 39.48 1.61 2.00 0.00 33+300 3.96 35.63 1.56 2.00 0.00 34+700 2.98 20.19 1.33 2.27 0.27 35+700 3.26 24.06 1.40 2.14 0.14 36+300 3.26 24.06 1.40 2.14 0.14 Average 0.185 Table 4.9: Loss of Serviceability for Trengguli – Demak Direction Station IRI mkm SV X p o ∆PSI 26+900 3.29 24.51 1.41 2.12 0.12 28+350 3.34 25.33 1.42 2.10 0.10 30+000 3.07 21.35 1.35 2.23 0.23 32+100 3.97 35.81 1.57 2.00 0.00 33+300 2.72 16.80 1.25 2.41 0.41 34+700 3.03 20.89 1.34 2.25 0.25 35+700 3.30 24.72 1.41 2.12 0.12 36+300 3.30 24.72 1.41 2.12 0.12 Average 0.169 b. PSI in this case, PSI was referred to initial serviceability, p o can be obtained by using the relationship between PSI and IRI, as follows. 2.6 37 where: 2.7 2.8 c. Loss of serviceability could be calculated using the following equation. ∆PSI = p o - p t 4.6 where p o is initial serviceability index calculated by using equation 4.8 above and p t is terminal serviceability index. In this study, the terminal serviceability used equals to 2. The use of p t = 2 is caused by the minimum terminal serviceabil ity provided by AASHTO’s axle load equivalency factors tables equals to 2. The calculation result ∆PSI for two directions are depicted in Table 4.8 and 4.9. In Tables 4.8 and 4.9, there are road sections having high IRI values that cause the initial serviceability p o is less than terminal serviceability pt. To overcome this problem, all initial serviceability; that was less than two, was equated to two.

4.5.2 Resilient modulus M

R The value of resilient modulus could be measured according to AASHTO procedure or based on relationship with other parameter, such as California Bearing Ratio CBR. This relationship is represented by the following equation. M R psi = 1500 x CBR 2.9 The CBR for every single station on the road and its corresponding M R is shown in Table 4.10. It was assumed that the resilient modulus is similar for the subgrade of both directions. 38 Table 4.10: Modulus Resilient of Subgrade STA Subgrade CBR MR psi 26+900 4.5 6750 28+350 4.7 7050 30+000 6.3 9450 32+100 3.4 5100 33+300 6.8 10200 34+700 6.9 10350 35+700 7.2 10800 36+300 5.7 8550

4.5.3 Calculation of Structural Number SN and Layer Thickness D