REWARDS - Driving School and Race Tournaments unlock
It seems some construction workers to the north have been disrespecting your sister. You will need to destroy six trailers and kill the foreman. You have the choice of 3 construction tools to aid you, which are a bulldozer, a crane and some explosive barrels. The trailers are marked on your radar with the green blips. As soon you enter a vehicle or destroy the first trailer, you will have 3 minutes to destroy all 6 trailers.
Once all 6 are destroyed, the timer disappears and the foreman comes out and hides back in the porta-john. Grab a bulldozer and push the toilet into the hole nearby. Once you do that, get into the nearby cement truck and park the back end near the hole in the red marker. You may want to kill any workers to make sure they don't disrupt you. Once you do that, the hole will automatically fill with cement and your mission is complete.