Slope Stabilisation with Stone Walls

How retaining walls work without mortar

A mortared retaining wall behaves as a rigid structure: it resists soil pressure as a single unit until it fails — often suddenly. A dry stone retaining wall, by contrast, distributes load through friction between individual stones and allows slight movement without collapsing. This makes it more tolerant of ground movement caused by freeze-thaw cycles, seasonal swelling of clay soils, and tree root growth — all common in Polish upland sites.

The critical factor is drainage. Hydrostatic pressure — water building up behind an impermeable wall — is the primary cause of retaining wall failure in wet climates. A dry stone wall, with its open joints, is inherently permeable. Water that accumulates behind the wall finds its way through the structure rather than pushing against it as a static load.

Site assessment before building

Before committing to a retaining wall as a slope stabilisation measure, assess what is causing the instability. A shallow surface slip — the topsoil and subsoil moving over bedrock or a clay layer — can be addressed with a relatively low retaining wall. A deeper rotational slip, where a larger mass of material is moving, requires geotechnical investigation before any structural response.

Look at the slope during and after heavy rain. Note where surface water collects, where it runs, and whether there are existing signs of slow movement: tilted fence posts, cracks in the ground surface, or compressed vegetation at the slope base. These indicate where water is accumulating and where the soil is under the most stress.

Identifying where a wall can help

A retaining wall is most effective at the base of a shallow, actively eroding slope, or at an intermediate level where a terrace can capture surface runoff and allow it to drain laterally rather than cutting downslope. In the Carpathian Flysch zone — the Beskid Niski, Sądecki, and Wyspowy ranges — traditional agricultural terraces built on this principle are still visible in many hillside villages.

Designing the wall for slope conditions

Retaining walls differ from freestanding boundary walls in several key respects. They are built against a bank, with soil or fill pressing against the back face. The wall must resist this horizontal pressure in addition to its own weight.

Batter

The face of a retaining wall should lean back into the slope — the opposite of how the rule is sometimes misunderstood. A front face with an inward batter of roughly 1:6 (one unit horizontal for every six vertical) shifts the wall's centre of gravity toward the bank, increasing resistance to overturning. The back face is usually near-vertical, set directly against the cut bank or against a gravel drainage layer.

Wall height and step-terracing

On a long slope, a series of lower walls creating multiple terraces is generally more stable than a single tall wall. A sequence of walls 0.6–0.8 m high at vertical intervals allows each terrace to drain and settle independently. Traditional hillside agriculture in the Małopolska region uses exactly this pattern, with narrow strips of field between each walled terrace.

Foundation depth

On a sloping site, the foundation trench should be cut into the slope horizontally — not parallel to the surface. The footing must rest on undisturbed subsoil or bedrock, not on fill or topsoil. In the Carpathians, where the water table can be high after snowmelt, setting the foundation below the typical frost depth (around 0.8–1.0 m in mountainous areas) prevents heaving from destabilising the base course.

Backfill and drainage

The material placed behind the wall — between the stone structure and the cut bank — determines how well the wall drains. Avoid placing cohesive clay directly against the back face; it retains water and transfers hydrostatic pressure onto the wall. Instead, use coarse crushed stone, gravel, or broken rock as a drainage layer at least 200–300 mm thick immediately behind the wall. This layer should connect to a drainage outlet at the base of the wall or to a lateral drainage channel that carries water away from the slope without redirecting it onto another unstable area.

In traditional Carpathian terrace construction, the natural stone of the slope itself often provides adequate drainage if placed loosely behind the face wall. Where the local geology produces impermeable material (dense flysch shale, for example), an engineered drainage layer is more important.

Integrating walls into the slope landscape

Dry stone retaining walls in Poland's mountain villages are part of a long vernacular tradition. In areas such as Lanckorona, Zalipie, and parts of the Pieniny, historic walled terraces are recognised as cultural landscape features and may be protected under conservation designations. Before modifying or rebuilding existing stone walls in rural areas, it is worth consulting the local Gmina or the provincial Conservator of Monuments (Konserwator Zabytków), who can advise on whether the structure has listed status.

New walls built for slope stabilisation can follow the same material and visual character — using locally sourced sandstone or limestone — to integrate with the existing landscape rather than introducing alien materials.

References

• Dry Stone Walling Association of Great Britain — technical guidance on retaining wall construction.
• Polish Geological Institute (PIG-PIB) — landslide hazard maps and geological surveys for the Carpathian region.
• Wikimedia Commons — Stone retaining walls — reference photographs of retaining wall construction.

Last updated: May 2026