Question $9$

One pavement structure consisting of $2$ pavement layers over subgrade has a required

structural number $(SN)$ of $5\mathrm{.}60.$ The layer coefficient for asphalt surface $(a_1)$ is $0\mathrm{.}44$ and the

layer coefficient for the granular base $(a_2)$ is $0\mathrm{.}14.$ The drainage modifying factor for the

granular base layer is $0\mathrm{.}9.$ If assuming asphalt surface thickness of $6",$ calculate the

thickness of the granular base layer.

$($Round up to the nearest inch and fill in the blank with a number only.$)$