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Retaining walls: types, sizing and construction

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Building on a sloped site or need to hold back a bank after excavation? A retaining wall is the structure that stops the soil sliding and pushing against your house. Get the sizing wrong and it will overturn or crack under earth pressure — and remedial work is extremely costly. This article covers the different types of retaining wall, how to size them, and how to build them yourself where that’s feasible.

CROSS-SECTION OF AN L-SHAPED RETAINING WALL (CANTILEVER) RETAINED SOIL Exposed face Finished ground level BEARING GROUND H L (50 to 70% of H) e 1 Reinforced concrete stem 2 Footing (base slab) 3 Reinforcement 4 Drainage layer 5 Drain + weep hole EARTH PRESSURE Geotextile fabric 20/40 gravel build-yourself-a-house.com

When do you need a retaining wall

A retaining wall is needed any time you have to maintain a level difference between two areas of ground. The most common situations in self-build:

  • Sloped site: cutting to create a level building platform
  • Semi-basement: the basement wall is itself a retaining wall
  • External landscaping: terraced areas, below-grade garage access
  • Boundary situations: holding back a neighbour’s ground (or your own)

The forces involved

A retaining wall is subject to three main forces:

  1. Earth pressure: horizontal force exerted by the retained soil. It increases with height and soil density.
  2. Hydrostatic pressure: if water accumulates behind the wall, the pressure can double. This is the leading cause of failure.
  3. Self-weight of the wall: this is what resists overturning and sliding.

Warning — Earth pressure is proportional to the square of the height. A 2 m wall experiences four times the pressure of a 1 m wall. Never scale up dimensions from a small wall to a large one: above 1.50 m, a structural calculation is essential.

Types of retaining wall

Question

Gravity wall

The principle is simple: the wall is heavy enough to resist earth pressure through its own weight. No complex reinforcement required.

Characteristic Value
Materials Solid concrete block, rubble stone, mass concrete, dry stone
Maximum recommended height 1.50 m for self-build
Base thickness 1/3 to 1/2 of the height
Reinforcement Minimal or none (dry stone)
Difficulty Intermediate

L-shaped wall (cantilever)

The L-shaped wall consists of a horizontal footing (base slab) and a vertical stem wall in reinforced concrete. The weight of soil on the heel of the footing contributes to stability — this is the most common type for medium heights.

Characteristic Value
Materials Reinforced concrete (C25/30 minimum)
Maximum recommended height 3 to 4 m (beyond this: structural engineer required)
Stem wall thickness 200 to 300 mm
Footing Width = 50 to 70% of total height
Reinforcement HA10 to HA16, calculated by structural study
Difficulty Advanced

Gabion wall

Wire mesh cages filled with stones. A flexible, free-draining and attractive solution.

Characteristic Value
Materials Galvanised wire mesh cages + fill stone
Maximum recommended height 2 to 3 m (pyramidal stacking)
Foundation 200 mm compacted gravel bed
Reinforcement None
Difficulty Beginner to intermediate

Shuttering block wall (formwork block)

Hollow blocks dry-stacked then filled with reinforced concrete. This is the best ease/strength compromise for the self-builder.

Characteristic Value
Materials Shuttering blocks (200 or 270 mm) + C25/30 concrete + reinforcement
Maximum recommended height 2.50 to 3 m
Thickness 200 or 270 mm depending on height
Reinforcement Vertical HA10–HA12 every 200–400 mm + horizontal every 2 courses
Difficulty Intermediate to advanced

Tip — For the self-builder, shuttering blocks are often the best choice: they’re laid like standard blocks but produce a reinforced concrete wall. The formwork is built-in, which removes the difficulty of traditional shuttering.

flowchart TD A{Wall height?} -->|Less than 80 cm| B[Gravity wall / dry stone
or gabions] A -->|80 cm to 1.50 m| C{Stable and well-draining soil?} C -->|Yes| D[Gravity wall in solid block
or shuttering blocks] C -->|No| E[Shuttering blocks + drainage] A -->|1.50 m to 3 m| F[Shuttering blocks or L-wall
STRUCTURAL ENGINEER REQUIRED] A -->|Over 3 m| G[Reinforced concrete L-wall
STRUCTURAL ENGINEER + CONTRACTOR] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#56C6A9,stroke:#56C6A9,color:#fff style C fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style D fill:#56C6A9,stroke:#56C6A9,color:#fff style E fill:#F58220,stroke:#F58220,color:#fff style F fill:#F58220,stroke:#F58220,color:#fff style G fill:#CD212A,stroke:#CD212A,color:#fff

Sizing: the key rules

Gravity wall (solid block or rubble stone)

  • Base thickness: minimum 1/3 of the retained height (e.g. a 1.20 m wall → 400 mm base)
  • Batter (rearward lean): 10 to 15% recommended — the wall leans slightly towards the retained soil
  • Foundation: concrete footing 400 to 600 mm wide, buried below the frost line

L-shaped wall (cantilever)

Sizing an L-wall requires a structural calculation to Eurocode 7. Indicative dimensions:

Total height (H) Footing (L) Stem (e) Footing depth (h) Heel
1.50 m 1.00 m 200 mm 250 mm 600 mm
2.00 m 1.30 m 200 mm 250 mm 800 mm
2.50 m 1.60 m 250 mm 300 mm 1.00 m
3.00 m 2.00 m 250 mm 300 mm 1.30 m

Warning — These are indicative dimensions for standard ground (sand-gravel, friction angle 30°). Your soil conditions may differ. Always have the sizing validated by a structural engineer for any wall over 1.50 m. The cost of the study (€500 to €1,500) is trivial compared to the cost of a wall that collapses.

Drainage: the key to longevity

A retaining wall without drainage is a wall waiting to fail. Water accumulating behind the wall generates hydrostatic pressure that can cause it to overturn, even if the sizing is correct for earth pressure alone.

Complete drainage system

  1. Agricultural drain (Ø 100 mm perforated pipe) laid at the foot of the wall on the soil side, wrapped in geotextile fabric and 20/40 gravel
  2. Drainage layer: 300 to 500 mm of 20/40 gravel over the full height of the wall, on the soil side
  3. Geotextile fabric: between the gravel and the soil to prevent the drain silting up
  4. Weep holes: Ø 50 mm holes through the wall every 2 to 3 m, near the base, to discharge residual water
  5. Outfall: the drain must discharge somewhere (ditch, surface water drain, soakaway)
DRAINAGE SYSTEM FOR A RETAINING WALL Cross-section — detail of rear drainage CONCRETE WALL SOIL 💧 Exposed face Rain infiltration 1 Waterproofing coat 2 Anti-clogging geotextile fabric 3 20/40 gravel (300 to 500 mm) 4 Perforated drain pipe (diam. 100 mm, 1% fall) 5 Weep holes (every 2-3 m) Without drainage, the pressure on the wall can DOUBLE in heavy rain build-yourself-a-house.com

Conseil

Best practice — Invest as much in drainage as in the wall itself. Good drainage halves the load on the wall and multiplies its service life tenfold. Never cut costs on geotextile fabric and gravel.

Building a shuttering block wall: step by step

Here is the detailed method for the type of wall best suited to self-build.

Materials needed (per m² of wall)

Material Quantity Indicative price
Shuttering blocks 200 mm 10 blocks €15–20
C25/30 concrete 0.10 m³ €15–20 (delivered)
HA10 rebar 6 lm €5–8
HA8 rebar (horizontal) 3 lm €3–5
20/40 drainage gravel 0.15 m³ €5–8
Geotextile fabric 1.5 m² €2–3
Ø100 drain pipe 0.5 lm €2–3
Total / m²   €45–65

Step 1: The footing

The foundation footing must be wider than the wall and buried below frost depth (600 to 900 mm depending on the region).

  1. Dig a trench 500 to 800 mm wide and at least 300 mm deep below finished ground level
  2. Pour a 50 mm blinding concrete layer
  3. Place the footing reinforcement: 4 × HA10 longitudinal bars + HA6 links every 200 mm
  4. Leave vertical starter bars projecting (HA10, length = wall height + 500 mm lap)
  5. Pour C25/30 concrete and vibrate

Tip — Build a 1 to 2% fall into the top of the footing sloping rearward (towards the soil side) to direct water towards the drain. This simple detail significantly improves drainage.

Step 2: Laying the blocks

  1. Bed the first course of shuttering blocks on a levelling mortar bed
  2. Thread the blocks down over the vertical starter bars
  3. Check level and plumb at every block
  4. Stack subsequent courses dry (the blocks interlock with tongue-and-groove joints)
  5. Place horizontal bars (HA8 or HA10) every 2 courses in the dedicated grooves
  6. Do not stack more than 4 courses before pouring (risk of blow-out under fresh concrete pressure)

Step 3: Pouring the concrete

  1. Pour C25/30 concrete in lifts of 4 courses maximum
  2. Vibrate with a poker vibrator (Ø 25 mm) — the cells are narrow
  3. Allow 24 hours before laying the next courses
  4. Continue course by course up to the final height

Step 4: The ring beam at the top

Finish the wall with a horizontal ring beam at the head:

  • U-shaped lintel blocks on the top course
  • 4 × HA10 bars + HA6 links every 200 mm
  • Pour C25/30 concrete

Step 5: Drainage and backfilling

  1. Apply a waterproofing coat (bitumen or resin) to the buried face of the wall
  2. Lay geotextile fabric against the wall
  3. Install the drain at the foot of the wall (1% fall towards the outfall)
  4. Fill with 20/40 gravel to 300–500 mm thickness
  5. Fold the geotextile fabric back over the gravel
  6. Backfill in 300 mm layers, compacting each layer

Warning — Never backfill against a wall whose concrete has not reached its design strength. Wait a minimum of 7 days (ideally 28 days) after the final pour before placing soil. Premature backfill can blow the wall apart.

Common mistakes and failures

Mistake Consequence How to avoid it
No drainage Hydrostatic pressure → overturning Drain + gravel + weep holes
Under-sizing Cracking, overturning Structural study for H > 1.50 m
Inadequate footing Base sliding Reinforced concrete footing buried to frost depth
Uncompacted backfill Settlement → point surcharge Compact in 300 mm layers
No expansion joint Cracking every 10–15 m Flexible joint every 8–10 m
Rebar on wrong face Insufficient resistance → failure Bars on the soil side (in tension)

Warning — The main reinforcement in an L-wall stem must be placed on the soil face (the face in tension), not the exposed face. This is the most serious and most common mistake made by self-builders. A wall with the bars on the wrong side has roughly one-third to one-quarter of the intended resistance.

Regulations and liability

  • Wall on a property boundary: check your local planning rules — some authorities impose maximum heights and set-back distances
  • Planning permission: often required above 2 m height (varies by local authority)
  • Liability: in the event of collapse, you are liable for damage caused to neighbours and third parties
  • Insurance: check that your site public liability insurance covers retaining structures

Checklist: retaining wall

  • Ground investigation carried out (soil type, water table level)
  • Retained height measured and wall type selected
  • Structural study validated if H > 1.50 m
  • Local planning rules checked (height, set-back)
  • Foundation footing sized and reinforced
  • Vertical starter bars in place
  • Shuttering blocks laid in lifts of 4 courses max
  • Concrete vibrated at each lift
  • Ring beam at top poured
  • Waterproofing coat applied on soil face
  • Ø100 drain laid at foot with fall towards outfall
  • 20/40 gravel + geotextile fabric over full height
  • Weep holes formed every 2–3 m
  • Minimum 7-day wait before backfilling
  • Backfill compacted in 300 mm layers
  • Expansion joint every 8–10 m for long walls