My front yard landscaping beds are all trench-edged and I have no problem containing the groundcovers.

Make a V cut, but at two different angles. On the grass side, cut straight down perpendicular to the level of the grass.

Then go stand in your plant bed and make an angled cut to form the V. My trenches are about 8 inches wide at the top.

The pachysandra roots will grow toward the exposed soil of the angled cut and turn back toward the soil of the bed.

Try not to fill the trench in with organic mulch. It will eventually break down, creating a growing medium for the pachysandra roots to grow into and you'll have the same problem all over.

This type of edge also makes mowing easy and edge-trimming unecessary.