Surcharge load
Also called: surcharge, retaining wall surcharge, surcharge pressure, sloped backfill, load above retaining wall, driveway above retaining wall
Any weight above and behind a retaining wall beyond level soil — a driveway, a parked car, a pool, a slope rising from the top, a building, a fence footing — that adds to the pressure on the wall. Usually triggers a permit and an engineered design at any height.
A retaining wall is designed for the soil it holds: a wedge of earth whose weight and moisture push sideways on the wall, in an amount engineers can calculate from the height and the soil type. A surcharge is anything else pushing on that wedge. A driveway or parking pad adds the weight of vehicles; a slope rising from the top of the wall adds more soil at an angle that pushes harder than level ground; a pool adds water weight; a shed, a deck footing, a fence post or a house foundation within roughly a wall-height of the top adds a concentrated load. Each one increases the lateral pressure on the wall — often by a third to a half — and a wall built to the manufacturer’s level-ground chart is undersized for it.
That’s why the building code and most cities treat surcharge as a separate trigger from height. A 3-foot wall holding a lawn is exempt from permits in most places; a 3-foot wall holding a driveway needs a permit and a stamped design in many of them, because a failure puts a car in the yard below. The engineer’s answer is usually more geogrid, longer grid, a deeper base or a footing, and more drainage — the wall itself may look identical from the front.
The homeowner’s version of the question: what’s above the wall, and what will be? A wall built for a lawn that later gets a patio, a hot tub or a parking spot on top has a surcharge it wasn’t designed for. Terracing — two shorter walls, the upper one set back at least twice the lower one’s height — is the common way to avoid the upper wall surcharging the lower. The cost page draws the DIY line at 4 feet or any surcharge, whichever comes first.