If you want a fence on top of a wall, there are some things to consider. Is the wall already built? If it is, then you have to know how the wall was built; did they use geogrid, or is it a gravity wall? If the wall is in the planning stages, you have to decide the placement of the fence and how it will affect the construction of the wall.
There are many ways to build a fence above a wall. One is to have the fence set 3 feet (.9 meters) behind the back of it. This method is preferred because it does not require extra engineering. This requires construction tubes to be placed during the construction of the wall. If the fence is added after the wall is already built, the fence installer could damage the structural integrity of the wall. If the wall was built using geogrid, they could tear it, which will cause the wall to fail. Instead, they will have to dig the post holes by hand. If the wall is constructed using no-fines concrete, the fence installer will not be able to install a fence after the wall is built.