Retained Ground and Wall Base Drainage System

Ground retained by a wall holds water, and water held against a wall pushes on it. This system is the arrangement intended to offer that water a route out rather than let it accumulate: a preferential path down the back of the structure, a filter keeping that path open, a collector at the base to take away what arrives, and an outlet letting it leave. It is a sub-assembly rather than a wall type, and every retaining construction has some version of it.

Its parts are defined by role rather than by form. The preferential path may be permeable backfill or a manufactured drainage composite; the filter may be a geotextile or a graded transition; the collector may be a perforated run in the backfill or a channel formed within the structure. What matters is that every role is present and connected, because each depends on the one after it.

The test against subsoil land drainage is whether the arrangement is set out by a wall or by the ground. This drainage vanishes with the wall and follows it in plan; land drainage follows the fall of the ground and would have a purpose with no building on the site. The term French drain names each of these in different places, and is applied to neither here.

Two familiar cases share this identical physical arrangement: the drain around a basement or foundation with occupied space behind the wall, and the drain behind a free-standing level change in a garden. The distinction usually offered, whether anything on the far side would be spoiled by water, describes the building rather than the drainage, so both appear here as labelled cases of one system with very different consequences of failure.

The core roles form a chain in which each link is useless without the next. A permeable path with no collector delivers water to the base of the wall and leaves it there; a collector with no outlet becomes a reservoir at the worst possible level; a path with no filter closes up and returns the wall to the condition it was drained to avoid. Remove any one and the remainder become storage held against the structure.

The structural design and the drainage make assumptions about each other. A wall designed on the basis that water will not accumulate behind it is relying on this system continuously, across the whole life of the wall; one designed to resist water pressure is not. Which is true of a given wall is answered only by its designer, and that answer changes what a silted drain actually means.

Waterproofing, protection and drainage occupy the same space and are placed in the same operation. A drainage composite shields the membrane behind it while itself being vulnerable to backfill compacted against it, and the filter face keeping the drainage path open is easily blinded by fines during placing. This is where the assembly is most often compromised, and it is buried immediately afterwards.

Surface levels above the wall control the demand on everything below. Water arriving over the head enters the retained ground directly, so interception at the top is a drainage component even though it looks like landscaping, and a later patio, planting bed or raised level at the head of a wall can quietly increase what the system behind it was arranged to handle.