Problem Statement: An engineer has determined that a required Structural Number ($SN$) of 4.5 is needed for a new flexible pavement. The design will consist of three layers: an asphalt concrete surface, a crushed stone base, and a granular subbase.

Given the following material properties:

If minimum thickness requirements dictate a 4-inch asphalt surface ($D_1$) and an 8-inch crushed stone base ($D_2$), calculate the minimum required thickness for the granular subbase ($D_3$).

Problem Statement: A flexible pavement is designed with three layers. The surface course has a layer coefficient ($a_1$) of 0.44 and a thickness ($D_1$) of 4 inches. The base course has $a_2 = 0.14$, a drainage coefficient ($m_2$) of 1.0, and a thickness ($D_2$) of 8 inches. The subbase course has $a_3 = 0.11$, $m_3 = 0.8$, and a thickness ($D_3$) of 10 inches. Calculate the total Structural Number (SN) of this pavement.

Problem Statement: A specific truck axle configuration causes exactly twice as much damage to a pavement structure as a standard 18-kip single axle. If a highway lane carries 500 of these trucks per day, how many ESALs are accumulated over a 20-year design life (assume 365 days/year)?