Groups won't explode, how do i cut them?

I’m trying to use the intersect tool to remove the extra parts of the pillars going through the walls, i created a plan that shows the section i want to cut off and where. I created those arches and grouped them, (from what i understand i need to explode the arches in order to to cut them) when I select them, right click and select ‘‘explode’’; nothing happens. Thanks for the tips…
I’m new to sketch up btw, any tip helps