Subsoil Land Drainage System

Water held in the ground moves slowly towards anything offering an easier path. A land drain is that easier path: a trench of permeable fill, separated from the soil so that fines cannot enter it, with a perforated pipe at the bottom to carry away what the fill collects, running to an outfall which has to sit below the level the water is being asked to fall to.

The collecting element is the permeable surround, held apart from the soil by the filter separation, and the conduit at its base is a carrier. Silting, which is how these runs usually end, is therefore an interaction between the soil, that separation and the fill rather than a defect in any one of them. Layout is a separate question again: whether the arrangement intercepts water arriving from upslope or lowers water standing across an area.

Set against retained ground and wall base drainage, the test is whether the arrangement would still have a purpose if the building were removed. Land drainage would, because its route follows the ground and the water within it. Wall base drainage exists only because a wall is there and is set out by that wall. The term French drain is avoided in both entries, since British usage attaches it to this filter drain and North American residential usage attaches it to the wall base arrangement.

Because it is buried and gradual, this system fails quietly. Ground that becomes wet again long after the work was done is usually reporting a silted surround, a collapsed outfall or a drain that was always in the wrong place, and telling those apart needs the layout record more than it needs an excavator.

Soil, filter and fill are a single problem. The fill collects only while its voids stay open; the voids stay open only while the filter holds back the fines the surrounding soil releases; and how much the soil releases depends on its particle sizes and on how the trench was excavated and backfilled. Silting, the characteristic end of a land drain, is a failure of that relationship rather than a defect of the pipe.

Layout and ground behaviour set each other's terms. A drain influences the ground only as far as the soil will release water towards it, so the spacing of runs is a property of the ground rather than a preference. Whether the arrangement is a single interceptor across a slope or a field of laterals depends on whether the water is arriving from somewhere or standing everywhere.

The outfall governs the whole run from its lowest point. Water can only be taken down to the level the outfall permits, so a drain laid faultlessly and connected to an outfall that is higher, drowned or seasonally submerged achieves nothing while looking complete. That is why the outfall is the first thing examined when land wets up again.

Access provision determines whether any of the rest can be recovered. A silted surround cannot be cleaned, but a silted pipe can be rodded if there is somewhere to rod from, and a blocked outfall can be found if there is a catchpit to look into. Without those points, the only remedy left for a buried arrangement is to dig it up and begin again.