If you need the achievements For Science! and/or For More Science! skip this guide and proceed to those solutions to only have to build out this level once to grab all three achievements.
Select level 21 and build 2 bridges. The first bridge should carry you straight across the gap from where you start to the lower blue portal. When you pop out of the top blue portal have another bridge that takes you up to the top yellow portal, then when you pop out of the lower yellow portal the loop is complete and you should end up dropping back down to the beginning to repeat the process. Let your vehicle loop around this circuit 15 times and quit out.
Please note: Most achievements in this game require you to get to the next cut scene before it unlocks or you can simply quit to main menu to unlock it immediately.