Gravity Retaining Wall System

A gravity retaining wall holds ground back by being heavy enough, and wide enough at its base, that the push of the soil can neither slide it forward nor tip it over. Very little in it is asked to bend. Mass concrete, mortared masonry, dry stone, wire baskets filled with rock and interlocking crib elements filled with granular material are all ways of assembling that weight, and the choice alters appearance, drainage behaviour and tolerance of movement rather than the underlying idea.

The boundary against the cantilever-retaining-wall-system entry is what happens when the wall is made slimmer. Thin a gravity wall and it slides or tips as a whole while remaining perfectly sound as masonry; thin a cantilever wall and its stem cracks in bending. The other visible test is whether the wall needs soil standing on its own base to work: a gravity wall does not, and a cantilever wall is designed around exactly that weight.

Gabion, crib and stacked segmental walls sit inside this entry rather than beside it. A gabion wall is a mass wall assembled from filled cages, a crib wall is a mass wall assembled from a lattice filled in place, and both rely on weight and base width in the same way. Where such units are combined with reinforcement layers running back into the retained fill, the structure has stopped being a gravity wall and belongs with the reinforced-soil-retaining-system entry.

The same materials appear in walls that retain nothing at all, and what separates them is invisible from the front: founding below the level at which the ground moves with the seasons, a section wide enough at the base for the height being held, a route by which water behind can escape, and ground underneath able to carry pressure concentrated at the front edge.

Everything a gravity wall does is settled at its base. The mass above resists only because the ground beneath can carry pressure concentrated towards the toe and provide friction against sliding. Soften that ground, found above the depth where it changes with the seasons, or allow water to reach it, and the wall rotates forward while remaining sound as a piece of construction. The wall has not failed; its bearing has, and no amount of extra weight on top helps.

Water behind the wall belongs to the same mechanism rather than to durability alone. Drainage works as a chain: a permeable zone against the retained face, a path down to a collector, a filter arrangement that stops fine soil silting the whole thing, and an outlet that actually discharges somewhere. Break any link, most often the outlet, and the wall quietly carries loads it was never sized for whenever the ground behind saturates.

The coping and the drainage compete for the same water. The coping's role is to direct rain landing on the top out past both faces rather than into the joints and the mass, and how much it turns away follows from the detail and the installed bedding rather than from its presence. Water shed onto the ground behind still has to reach the drainage zone, and paving laid tight against the back of the wall can seal that zone off. Surface levels behind the wall, the coping and the outlet are one decision expressed in separate places by separate trades.

Joints work in the opposite direction to those in a building. A gravity wall settles with the ground it stands on, and where the mass is rigidly jointed and continuous it cracks along a line nobody chose, while dry-laid and cage-built forms absorb distortion by deforming and keep working. A movement joint is where the designer chooses that line, and it has to pass through coping and footing as well as the mass, or it merely relocates the crack it was meant to control.