A flexible pavement is designed using the AASHTO 1993 method. The required total Structural Number ($SN_{req}$) for the entire pavement structure is calculated to be $4.50$. The proposed materials have the following layer coefficients ($a$) and drainage coefficients ($m$):

The minimum required thickness for the asphalt surface ($D_1$) is $100 \text{ mm}$ ($4 \text{ inches}$), and the minimum required thickness for the base ($D_2$) is $150 \text{ mm}$ ($6 \text{ inches}$). Using these minimums for the upper layers, calculate the required thickness of the subbase layer ($D_3$) in inches. (Note: AASHTO 1993 equations typically use inches).

A two-lane highway has a design life of $20$ years. The estimated Average Annual Daily Traffic (AADT) in the design lane is $2,500$ vehicles per day. The traffic stream consists of:

Assuming no traffic growth over the 20-year period (for simplicity), calculate the total Equivalent Single Axle Loads (ESALs) over the design life of the pavement.

A dual-wheel assembly carries a total load of $40,000 \text{ lbs}$ ($20,000 \text{ lbs}$ per wheel). The center-to-center spacing between the two wheels ($S_d$) is $14 \text{ inches}$, and the clearance between the tires ($d$) is $4 \text{ inches}$. Calculate the Equivalent Single Wheel Load (ESWL) at a pavement depth ($z$) of $20 \text{ inches}$ using Boyd and Foster's approximate method.

A massive quarry truck weighing $100,000 \text{ lbs}$ drives over a dirt road, instantly creating deep ruts because the soil (subgrade) fails under the stress. The road is then paved using a flexible pavement design consisting of a $100\text{ mm}$ asphalt surface, a $200\text{ mm}$ crushed stone base, and a $300\text{ mm}$ gravel subbase. The same truck drives over the paved road without causing any damage. Explain the mechanical principles of the layered system that prevent the subgrade from failing.