Exclude area of section within the outline for volume calculation

Hi everyone, i have aquery if you could help me with, it might be obvious that i am missing it. I am looking to calculate the volume of tarmac on a road construction ( i have as built survey of Existing and Proposed ) where the road have an island which is soft landscaping. How do i select (or exclude) the outline of the road where it is only going to be tarmacked. Thanks