The initial design of northbound direction on a highway consists of a two-lane flexible pavement that has a SN of 5 and is designed for a subgrade CBR of 6, an overall standard deviation of 0.35, and a reliability of 90%. The initial PSI is 4.6 and the final PSI is 2.5. The pavement is designed for 3000 passes per day of a truck that has a 12,000 lb single axle and two 32,000 lb tandem axles. If the pavement is conservatively designed, how many years longer will the pavement last if a third lane is added.
SOLUTION Note: Open boxes in equations “ ” are to be completed by the reader
This problem shows the effect that the number of lanes can have on the design life of a pavement. Although the number of axle loads may not change when lanes are added or subtracted, the proportion of the axle loads in the design lane will change because axle loads will be distributed over more lanes and fewer will be in the design lanes.
To solve the problem, we must first determine the design-lane W18 used in the initial design of the pavement by applying(or by using Fig. 4.5). To apply the soil resilient modulus is determined as,
MR = 1500 × CBR = 1500 × = , the ΔPSI is determined as,
ΔPSI = initial PSI − final PSI (TSI) = − = , and ZR is determined to be with R = 90% (given in the problem). With these values, and the other values given in the problem, solving for W18 (which is the design-lane value used in the existing pavement design)