A wall is only as good as what is behind it
When a slope will not stay put, a properly built retaining wall holds the ground and gives you usable space back. When a wall is not built properly, it leans, bulges and eventually fails, and the reason is almost always the same: water got behind it and nothing was there to take it away.
That is why retaining walls sit with erosion control on our list rather than with landscaping. The visible part is block or stone. The part that determines whether it is still straight in ten years is the base preparation, the drainage installed behind the wall, and the geotextile fabric that keeps soil from migrating into that drainage and clogging it. Water behind a wall exerts pressure that the wall was never designed to hold, and hydrostatic pressure has flattened a lot of good looking block.
We evaluate the existing grades and soil conditions first, because soil type changes what the wall has to resist. Then we excavate and prepare a stable base, install drainage behind the wall, construct the wall to the manufacturer's specifications for that block and that height, backfill and compact in lifts rather than all at once, and restore the surrounding area.
Erosion control covers the situations where a wall is not the answer. Rip rap on a bank or a pond edge, slope stabilization, geotextile under stone, and grading that slows water down rather than trying to hold it back. A slope that is washing out does not always need a structure. Sometimes it needs the water that is causing the washout to be intercepted uphill, which is drainage work.
We have built small walls that made a real difference to how a property could be used, and we have told people a wall was not what their slope needed. Both of those are the job.
The visible part is block. The part that decides whether it is still straight in ten years is the base and the drainage behind it.
Why walls fail