Back in the 60's when my dad decided to fence our property, he poured a 6" wide concrete strip under the fence.
We dug the trench all around the perimeter and then dug the post holes. Then he set 2 X 4 forms to make the concrete strip 8" wide an 4" thick. He rented a mixer and we went to get the sand, rock and cement.
We set all the poles first. Then, the next Saturday, I ran the mixer making the cement, my brother ran the wheel barrow taking the cement to my dad who was trowling it into the trench.
No weeds in that fence and no dog ever dug under it. Just set the mower wheels on the concrete strip and cut the grass.
We dug the trench all around the perimeter and then dug the post holes. Then he set 2 X 4 forms to make the concrete strip 8" wide an 4" thick. He rented a mixer and we went to get the sand, rock and cement.
We set all the poles first. Then, the next Saturday, I ran the mixer making the cement, my brother ran the wheel barrow taking the cement to my dad who was trowling it into the trench.
No weeds in that fence and no dog ever dug under it. Just set the mower wheels on the concrete strip and cut the grass.