Stone Retaining Walls for Sloped Gardens

When a retaining wall is needed

A slope steeper than approximately 1:3 (33%) becomes difficult to maintain as open ground — mowing is dangerous, erosion accelerates after heavy rain and topsoil gradually migrates to the bottom of the plot. At this gradient, a retaining wall — or a series of terraced walls — is worth considering.

Key indicators that a retaining wall may be needed:

• Soil visibly slumping or eroding at a slope edge after rain.
• A slope change that exceeds 40–50 cm in vertical height over 1–2 m of horizontal distance.
• A driveway or path at the base of a slope that accumulates washed-down material.
• Desire to create level planting beds or a flat lawn on a terraced hillside plot.

Forces acting on a retaining wall

A retaining wall resists two primary forces: the lateral pressure of the retained soil and the hydrostatic pressure from groundwater or rainwater accumulating behind the wall. Of these, water pressure is often the more dangerous — a waterlogged soil exerts far more force than dry soil. This is why drainage is the most important design element of any retaining wall, often more important than the wall's thickness or the stone quality.

Lateral earth pressure depends on soil type. Sandy, granular soil exerts roughly half the pressure of clay for the same retained height. A wall retaining 60 cm of saturated clay on a slope must be sized considerably more conservatively than a wall retaining 60 cm of gravel.

Wall sizing — practical guidelines

For garden retaining walls up to 1.0 m in retained height (measured from the base of the wall to the level ground above), the following approximate proportions apply for mortared fieldstone:

• Retained height 30–50 cm: base width 30–40 cm; no engineering required in most cases.
• Retained height 50–80 cm: base width 40–55 cm; concrete strip footing recommended.
• Retained height 80 cm – 1.0 m: base width 55–70 cm; concrete strip footing required; drainage system behind the wall is mandatory.
• Retained height over 1.0 m: structural engineering assessment required under PN-EN 1997-1; a building permit may be needed.

These proportions assume well-drained granular backfill. For clay-heavy soils, increase the base width by 20–30% and prioritise drainage installation.

Drainage — the critical detail

Every retaining wall must have a path for water to escape from the soil behind it. Without drainage, water pressure builds up after heavy rain and eventually pushes the wall outward. The standard solution for garden walls:

1. Lay a 150 mm perforated drainage pipe (sleeve-wrapped in geotextile) at the base of the wall's rear face, running along the full length of the wall and discharging at one or both ends to a lower point.
2. Place a 200–300 mm layer of clean washed gravel (16–32 mm) behind the wall from footing level to approximately 100 mm below finished grade. Cover the gravel with a strip of geotextile before adding topsoil backfill, to prevent fine particles washing into the gravel over years.
3. In mortared stone walls, leave weep holes — gaps in one mortar joint per 1.5 m of wall length at or just above ground level on the front face — to allow any water that penetrates past the gravel to escape.

Geotextile placement

Geotextile is a non-woven fabric that separates different materials without blocking water flow. In a retaining wall context, it prevents clay or silt from the backfill migrating into the drainage gravel and clogging it over time. Fold the geotextile at the gravel-topsoil boundary like an envelope: run it up the slope face, over the top of the gravel and tuck it into the topsoil layer. A minimum weight of 150 g/m² is appropriate; heavier fabrics (200–250 g/m²) are used where the backfill contains a high proportion of fines.

Foundation for a retaining wall

Retaining walls carry more load than ordinary fences and sit in moister ground conditions — both reasons to use a concrete strip footing rather than relying on gravel alone.

Footing design for a mortared fieldstone retaining wall retaining 60–80 cm of soil:

• Trench depth: 0.9–1.0 m below finished grade on the low (front) side, to get below the frost line.
• Footing dimensions: 60–70 cm wide, 25–30 cm deep concrete strip (C20/25 grade, reinforced with two 10 mm rebar bars longitudinally if retaining over 70 cm).
• Footing must be level across its width, even if the wall top will follow the slope of the ground.

Constructing the wall

Stone laying follows the same principles as other mortared fieldstone work: break all joints, use tie-stones regularly and fill the core tightly. For retaining walls, additional considerations:

• Batter the wall face 1:10 to 1:6 toward the retained soil — this slightly overbuilds the wall toward the direction of loading, adding inherent stability.
• Use the largest, most regular stones in the first two courses above the footing, where bending stress is highest.
• Do not use hollow or porous stones in the wall body at or below ground level on the soil-side face — they absorb moisture and deteriorate quickly with freeze-thaw cycling.
• Build the gravel drainage layer simultaneously with the wall, course by course. Trying to add drainage behind a completed wall is very difficult and usually ineffective.

Terracing with multiple walls

When the slope change exceeds 1.0 m, a series of shorter retaining walls creating terraces is preferable to a single tall wall. Each terrace wall handles a small retained height, keeps individual wall proportions manageable and creates horizontal planting beds between walls. Standard terrace spacing: the horizontal distance between two wall faces should be at least twice the height of the lower wall — this allows enough root space for plants and prevents the upper wall from overloading the retained soil behind the lower one.

For a slope dropping 2.0 m over 6 m: three walls each retaining 65–70 cm, set 1.5–2.0 m back from each other horizontally, is a practical and proportionate solution.

Sources: Instytut Techniki Budowlanej · Eurocode 7 – PN-EN 1997-1 · PN-81/B-03020 (Ground freeze depth in Poland)