Summary

A retaining wall holds back soil whose natural angle of repose would otherwise slump. The forces on the wall are larger than most tradespeople intuitively realise — even 1m of saturated soil exerts several tonnes per linear metre of horizontal pressure on the wall face. This is why retaining walls fail unexpectedly: a wall that "looks fine" in summer collapses after a wet winter because hydrostatic pressure was never relieved.

UK practice categorises walls by construction type: gravity walls (mass concrete, gabion, stone), reinforced concrete walls, masonry cantilever walls (engineering brick or block on a concrete base), sheet pile walls, and segmental block walls (e.g. Kee block, Allan block, Marshalls Croft). Each has a height range and ground condition where it makes economic sense.

The typical domestic retaining wall — 600 to 1500mm high, holding back garden ground — is built either in engineering brick on a strip footing, or in dense concrete block rendered to finish, with a perforated drainage pipe behind. Walls above 1.2–1.5m start to need reinforcement and a designed footing, and unless the tradesperson has a structural design they should not build above this height.

Key Facts

Quick Reference Table

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Wall Type Practical Height Footing Notes
Single-skin brick (102mm) <600mm Concrete strip 450 × 150mm Retains light garden soil only
Double-skin brick (215mm) 600–1000mm Concrete strip 600 × 200mm Class B engineering brick + M6 mortar
Engineering brick cavity wall 1000–1500mm Designed footing Cavity filled with concrete + reinforcement
Mass concrete gravity 1000–2000mm Toe + heel concrete base Width = ~1/3 height (rule of thumb)
Reinforced concrete cantilever 1500–4000mm Designed reinforced base Engineer's calculations required
Gabion 1000–4000mm Compacted hardcore base Stone 100–200mm, mesh BS EN 10223
Segmental block (Allan/Kee) 1000–2500mm Compacted granular base + geogrid reinforcement Manufacturer's design tables
Sheet pile 1500–6000m+ Driven below excavation level Specialist plant — out of scope of typical groundworker
Sleeper wall (timber) <800mm Concrete posts or driven posts Temporary or low-grade application

Detailed Guidance

Designing the foundation

The footing of a retaining wall is engineered to resist three modes of failure:

Failure Modes (BS 8002 / EC7)
1. Overturning  — wall rotates about toe
   Resistance moment ≥ 2.0 × overturning moment
2. Sliding      — wall slides on its base
   Friction + passive resistance ≥ 1.5 × horizontal force
3. Bearing      — soil under toe fails in compression
   Ultimate bearing ≥ 3.0 × maximum toe pressure

For a domestic wall up to 1m, NHBC Standards Chapter 7.1 gives nominal dimensions: footing width approximately 2/3 wall height, footing depth at least 200mm into firm ground, founded below frost line (450mm in southern England, 600mm in north).

Concrete strip footing for 1m wall (example)

For a 1m double-skin engineering brick wall holding back garden soil:

Drainage behind the wall

This is the single biggest failure mode in domestic retaining walls. Saturated soil exerts hydrostatic pressure on top of normal soil pressure, often doubling the force on the wall. Drainage rules:

  1. Perforated land drain — 100mm PVC-u perforated pipe along the back of the footing, falling 1:200 minimum to an outlet
  2. Outlet — soakaway, surface water drain, or daylight to lower ground
  3. Drainage stone — 20–40mm clean angular stone, 200–300mm wide column behind the wall
  4. Geotextile wrap — non-woven membrane around the stone to prevent fines migration
  5. Weep holes — 75–100mm pipe through the wall every 800–1000mm, 150mm above finished ground level
  6. Backfill in lifts — compact granular backfill in 200mm lifts (don't dump and trample)

Coping and top detail

Water penetration at the top of the wall causes frost damage in winter. Always:

Movement joints

Retaining walls expand and contract with temperature. BS 5628 / BS EN 1996 specifies movement joints in clay brick at 10–12m and in concrete block at 6m. For shorter walls, build the wall in single bays without joints — for longer walls, plan joints at architectural break points.

Reinforced retaining walls

For walls above 1.2–1.5m the bricklayer/builder will be working from an engineer's drawing showing:

Critical points on these walls:

Gabion walls

A gabion wall is a stack of rock-filled wire baskets. Useful for low-grade retaining (highway embankments, garden walls) where the look is acceptable. Key points:

Frequently Asked Questions

Do I need Building Regs for a 1m garden retaining wall?

Probably not if it is a freestanding garden boundary wall not supporting a building or roadway, but planning consent may still be needed if higher than 1m fronting a highway or 2m elsewhere, or in conservation areas / listed building curtilage. Always check with the local planning authority.

Can I build a retaining wall in concrete block?

Yes — dense concrete blocks to BS EN 771-3 (compressive strength 7.3 N/mm² or higher) bedded in (iii) mortar are widely used for retaining walls up to 1.2m. Finish with cement render or a slip-on brick face for appearance. Cellular blocks and lightweight aircrete blocks are not suitable.

What's the minimum cover for reinforcement in a retaining wall?

For exposure class XC2 (buried concrete), minimum cover is 25mm; for XC4 (exposed to wetting and drying), 35mm. UK practice usually specifies 40–50mm on the soil face for retaining walls — the engineer's drawing will state the figure.

Do I need weep holes if I have a land drain?

Yes — they back up the land drain. If the drain blocks (silt, root intrusion), weep holes give a secondary route for water to escape rather than building up behind the wall. They should be 75mm minimum, fitted with a small grille if rodent ingress is a concern, and located at the level of the drainage stone behind.

How long after the footing can I build on it?

Concrete strip footings reach approximately 65–75% of strength at 7 days. For domestic retaining walls, 24 hours is the minimum before laying bricks, but if the wall is heavy or the ground is soft, waiting 3 days reduces the risk of differential settlement showing in the wall above.

Regulations & Standards