What a Retaining Wall Actually Retains
A retaining wall on a hillside holds back the weight of soil pressing against it from above and behind. In wet conditions, that soil also contains water, which adds hydraulic pressure. A dry-stone wall manages these forces differently from a mortared or concrete wall: it has no tensile strength and cannot resist bending, so it must instead be shaped so that the forces passing through it remain compressive — directed downward into the ground rather than outward toward failure.
This requirement determines almost every aspect of how Ligurian muri a secco are proportioned and built.
Batter: The Inward Lean
A vertical retaining wall on a slope is structurally disadvantaged: it presents its full face to horizontal soil pressure and has no geometry helping it resist that load. A battered wall — one that leans into the hillside — converts part of the horizontal force into a downward component that adds to the wall's own weight and increases friction at each stone joint.
In the Ligurian tradition, the batter of a retaining wall varies with height and soil conditions:
- Low walls (under 60 cm) retaining well-drained sandy or rocky soil: batter of 1:8 to 1:10
- Medium walls (60 cm to 120 cm) retaining clay-rich soil: batter of 1:5 to 1:6
- Taller walls or those in heavy-rainfall zones: batter of 1:4, sometimes steeper on the uphill face
These ratios are not drawn from written rules but from accumulated practice observed in surviving walls that have remained stable over generations. Modern dry-stone construction training programmes in Liguria use them as starting points, adjusted for specific site conditions.
Wall Thickness and the Two-Face Rule
A competently built dry-stone retaining wall has two distinct faces: an outer face of carefully selected, flat-fronted stones and an inner face that contacts the soil fill. Between the two faces is a core of packed rubble, smaller stones, and angular fragments that would not work as facing material. The two-face rule is that the outer face and the inner face must each be built as independent, coherent structures connected by tie-stones, not as two skins leaning against a pile of rubble.
A wall that is only one stone thick throughout — what builders call a single-leaf wall — is adequate for low garden edging but will not survive as a retaining structure in Ligurian conditions. The minimum useful thickness for a retaining wall over 50 cm tall is roughly one-third of the wall height. A wall 90 cm tall should be at least 30 cm thick at the base.
Drainage: The Most Common Failure Point
When a terrace wall fails — and many do, especially after decades of neglect — the most frequent cause is not that the stones were poorly laid but that water built up behind the wall until the hydrostatic pressure exceeded the wall's resistance. This happens when drainage at the wall base is inadequate, when fill soil has been replaced with less permeable material over time, or when weep holes are blocked by sediment or root growth.
Traditional Ligurian construction addresses this at three levels:
- Foundation drainage — a bed of angular stone rubble laid in the trench before the wall begins, extending from the foot of the wall to the undisturbed subsoil
- Core drainage — the rubble fill between the wall faces is kept loose and angular rather than packed with fine material
- Weep holes — gaps at the base of the wall face, created deliberately by omitting mortar between two adjacent stones or by spanning a small gap with a flat lintel stone
In wetter zones, additional horizontal drainage channels cut into the slope behind the wall direct runoff to collection points rather than letting it pond against the wall face.
Failure Patterns and Assessment
A wall that is beginning to fail typically shows one of several signs before it collapses:
- Bulging — a convex section on the outer face indicates that the core material behind it is no longer properly supported, often due to loss of tie-stone integrity
- Longitudinal cracking — a crack running horizontally through the face suggests that the wall has separated into two layers moving independently
- Settlement at one end — the wall has tilted along its length, usually because the foundation at one end has been undermined by water or root movement
- Displaced coping — coping stones that have moved outward often signal that the top course below is no longer level, caused by gradual outward rotation of the entire structure
Assessment before repair involves removing a small section to inspect the internal condition: whether tie-stones are still intact, whether core fill has been washed out, and whether the foundation course is still level and well-seated.