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4. Using the AASHTO flexible design procedure, design a pavement for a rural principal arterial road by answering the questions below. The expected design ESAL is 1.0 X 106. The pavement structure is to consist of asphalt concrete with an elastic modulus of 400,000 psi, a granular base with an elastic modulus of 25,000 psi, and a granular subbase with an elastic modulus of 15,000 psi. The resilient modulus of the subgrade (MR) is 6,000 psi during dry season and 3000 psi during wet season (each six months). Material properties of each layer are given in the figure below. Assume that a reliability level of 80 percent is required and the initial serviceability index is 4.0 and the final serviceability index is 2.0. Use a typical value of standard error (So = 0.45). Use the nomograph provided in the next page to get structural numbers of each layer. Neglect minimum thickness requirement. SN1 SN2 SN D E1 = 400,000 psi a1 = 0.42 Ez = 25,000 psi a2 = 0.14 m2 = 0.8 D2 Es = 15,000 psi a3 = 0.08 m2 = 0.8 D3 MR= 6,000 psi (dry)/3,000 psi (wet) ) (a) What is the required thickness of the asphalt layer (D1), granular base layer (D2), and granular subbase layer (De) to satisfy the above design criteria? (25 points) (b) For a full depth asphalt concrete design (AASHTO method), what is the required thickness of the AC layer for this problem? (5 points)