Retaining Wall

A retaining wall holds back a bank of earth that would otherwise slump or wash downhill. Cutting into a slope and holding the cut face turns ground too steep to use into a level shelf that can carry a building, a path or a planting. Where the wall is stone or masonry and faces the sun it does a second job at the same time. Dense material absorbs heat through the day and gives it back slowly after dark, the same principle as a mass of rock or water inside a greenhouse, so the space behind the wall stays warmer at night. The shaded side of the same wall runs colder than open ground and behaves as a frost pocket, so each face is planted for what it offers.

What the wall's life rests on is water rather than stone. Soil behind a wall that cannot drain fills up, and the trapped water pushes on the back of the wall harder the deeper it gets. The answer is a layer of coarse, free-draining material packed directly behind the wall and a clear outlet at the base. Both are buried and cannot be fixed later, and an outlet that silts up or fills with roots turns a wall that needed no attention into a failing one without warning. Where winters are hard the same trapped water freezes, expands and works the wall outward a little each year, so the drainage failure and the frost failure are one failure.

The other way a long-standing wall fails suddenly is surcharge, meaning load added above it after it was built: a vehicle track, a shed, a heap of spoil, or a steepened slope behind. A wall is sized for the earth it was built to hold and nothing more. Height decides who may build it, and residential building codes commonly call for stability calculations signed by a licensed engineer once the exposed face reaches about 1.2 m (4 ft). Upkeep is mostly looking: check the outlet still runs, watch for a forward lean or an opening joint, and keep woody roots out of the backfill.