Retaining Wall Construction: Design, Build and Building Regulations

Quick Answer: Any retaining wall holding back more than 600mm of earth requires structural engineer input under Approved Document A. Walls under 600mm can be built by a competent tradesperson using gravity wall principles — 1/3 of total height as base width for masonry. All retaining walls need drainage behind them: unremedied hydrostatic pressure is the leading cause of retaining wall failure in the UK.

Summary

Retaining walls hold back earth on sloped or terraced sites. They look simple — a wall is a wall — but the engineering forces involved are fundamentally different from a garden boundary wall. A boundary wall carries only its own weight and wind loading. A retaining wall must resist the lateral pressure of the retained earth, the hydrostatic pressure of water in the retained soil, and any surcharge loading from vehicles, buildings, or stored materials above.

These forces are significant. A 1-metre-high retaining wall retaining saturated clay soil has to resist lateral pressures equivalent to several tonnes per linear metre. Get the design, drainage, or construction wrong, and retaining walls fail catastrophically — often suddenly, without warning.

For groundworkers, retaining walls are regular work: garden terracing, driveway cuts, basement walls, highway retaining structures. Understanding the basics of retaining wall design — even if the final sizing is done by an engineer — makes the difference between a wall that lasts 50 years and one that pushes over within 5.

Key Facts

• 600mm threshold — retaining walls holding back over 600mm of earth generally require structural engineer design; Building Control may require calculations even below this for certain soil types or surcharge conditions
• Gravity walls — resist lateral earth pressure through sheer mass; suitable to approximately 600–900mm retained height; require adequate base width (minimum 1/3–1/2 of total height in masonry)
• Cantilever walls — reinforced concrete walls with a footing projecting behind the wall (into the retained soil); more material-efficient for heights above 1.2m
• Gabion walls — wire mesh cages filled with stone; gravity type; high permeability naturally addresses drainage; often used for informal or landscape applications
• Lateral earth pressure — increases with soil weight, soil friction angle, and retained height squared; saturated soil exerts significantly more pressure than dry
• Hydrostatic pressure — water-saturated retained soil can double or triple the lateral pressure on a wall; drainage behind ALL retaining walls is non-negotiable
• Surcharge loading — a vehicle parked behind a retaining wall adds equivalent pressure to approximately 10–15kN/m² (0.6m additional equivalent earth height); must be accounted for in design
• Building Control — retaining walls over 600mm typically require Building Control notification and approval under Building Regulations Part A
• Party Wall Act — excavations within 3m of a neighbour's building (for a wall below the building's foundations) may require a Party Wall Notice under Section 6 of the Party Wall etc. Act 1996
• Drainage material — free-draining granular backfill (20mm clean stone or Type 3 granular sub-base) behind all retaining walls; land drain pipe at the base directed to a safe outfall
• Wall ties — masonry retaining walls over 600mm need consideration of bond patterns and cross-ties to resist overturning
• Weep holes — 75–100mm diameter openings at 1.5–2m centres at wall base for water relief; critical in masonry walls

Quick Reference Table — Retaining Wall Types

Detailed Guidance

Lateral Earth Pressure — Understanding the Forces

The lateral pressure a retained mass of earth exerts on a wall is calculated using Rankine's or Coulomb's earth pressure theory. The key equation (simplified):

Active earth pressure (Pa) = Ka × γ × h

Where:

• Ka = coefficient of active earth pressure (approximately 0.33 for sand, 0.4–0.5 for clay)
• γ = unit weight of soil (typically 18–21 kN/m³ for dense soil, 20–22 kN/m³ saturated)
• h = retained height (m)

For a 1.2m wall retaining moist sand: Pa ≈ 0.33 × 20 × 1.2 = 7.9 kPa at the base; resultant force approximately 5.7 kN/m run of wall. This sounds manageable — but if the drainage fails and the soil becomes saturated, hydrostatic pressure adds approximately 12 kPa, more than doubling the wall loading.

The message for tradespeople: drainage is not an optional extra. It is structural.

Foundation Design for Retaining Walls

Retaining wall foundations must resist both vertical load (wall self-weight) and horizontal shear (from lateral earth pressure). The foundation must not slide forward or overturn.

For a gravity masonry wall:

• Base width — minimum 40–50% of total wall height (wall + footing below ground)
• Foundation depth — minimum 450mm in clay, 300mm in granular soil, to be below frost depth and reach undisturbed ground
• Foundation concrete — minimum GEN3 (C20/25), 150–225mm thick
• Toe — foundation should project in front of the wall face by 100–150mm to improve overturning resistance

For a reinforced concrete cantilever wall, the foundation heel extends back into the retained soil — the weight of soil on the heel resists overturning. This is why cantilever walls are more material-efficient: the retained soil itself contributes to stability.

Drainage Behind Retaining Walls

This is the single most important construction detail. Without drainage:

1. Hydrostatic pressure builds up after rain events
2. Lateral pressure can double or triple
3. The wall cracks, leans, or fails

Required drainage:

• Granular backfill — 300mm minimum of free-draining 20mm clean stone or BS Type 3 granular fill directly behind the wall; do NOT backfill with the excavated clay
• Land drain pipe — 80–100mm perforated pipe at the base of the granular fill, bedded on clean stone, with geotextile sock to prevent silting; laid to fall (minimum 1:200) to daylight at a suitable outfall
• Weep holes — in masonry walls, leave unmortered perpend joints (or install 75mm PVC pipes) at 1.5–2m horizontal centres and at least two courses above finished ground level

Where the site drainage cannot discharge to a watercourse, drain, or soakaway, the land drain must connect to a surface water drain — never to a foul sewer.

Interlocking Concrete Block Systems

Precast interlocking systems (Tobermore, Brett, Marshalls Fairstone, etc.) provide gravity retaining walls without mortar. Each manufacturer provides design tables specifying:

• Maximum retained height for each block size
• Maximum batter (lean-back) required
• Geogrid reinforcement requirements for taller walls
• Base preparation specifications

Always use the manufacturer's published design tables. Do not extrapolate beyond the table limits without engineer input. Common installation sequence:

1. Excavate and compact 150mm of MOT Type 1 sub-base as levelling course
2. Place first (base) course below finished ground level per manufacturer's guide
3. Set back each subsequent course per required batter (typically 10–50mm per course)
4. Backfill with granular material behind each course before placing the next
5. Install geogrid reinforcement if required (typically at 1/3 and 2/3 height in taller walls)
6. Cap course with adhesive-fixed capping unit

Masonry Retaining Walls

For a concrete block retaining wall up to 600mm retained height:

• Use 140mm or 190mm dense concrete blocks (minimum 7.3 N/mm² or stronger)
• Lay in English bond (alternate courses of headers and stretchers) for maximum integrity
• Foundation: 450mm wide × 225mm deep concrete, GEN3, set below ground level
• Mortar: 1:1:6 cement:lime:sand (standard masonry mortar) — use sulphate-resisting cement if soil has elevated sulphate levels
• Backfill: 300mm of clean 10–20mm stone; install perforated drain at base; leave weep holes at 1.5m centres
• Apply waterproof render or bituminous paint to the retained face before backfilling

For brick retaining walls, the same principles apply. Note that facing bricks in contact with soil must be of appropriate durability rating (DW or FL designation to BS EN 771-1).

Railway Sleeper Walls

Hardwood railway sleepers (oak, greenheart) are a popular domestic choice for garden terracing. Installation principles:

• For a single-tier wall: stake 150×150mm posts into the ground at 1.2–1.5m centres, attach sleepers horizontally with structural screws (minimum 150mm)
• Stake posts should go at least 300mm below the base of the wall for stability
• For stacked walls over 600mm: deadman anchors (sleepers laid perpendicular into the bank at 2m centres) dramatically improve stability
• Drainage: as per all retaining walls — granular backfill and perforated drain

Do NOT use softwood sleepers — they deteriorate rapidly in soil contact. New hardwood sleepers are preferred over reclaimed railway sleepers (which may contain oil-based preservatives and heavy metals).

Frequently Asked Questions

Does my retaining wall need Building Regulations approval?

Under Approved Document A, any structural work that could affect the stability of a building requires consideration. The 600mm rule of thumb applies — above this, Building Control should be notified. However, even below 600mm, if the wall is adjacent to a building foundation, near a boundary, or the soil conditions are unusual, you should speak to Building Control. Local authorities vary in their approach, so check before starting.

Do I need a structural engineer for a garden retaining wall?

For walls under 600mm retained height in normal ground conditions, a competent groundworker can design and build a gravity wall following established principles. Above 600mm, an engineer should provide calculations. Above 1.5m, an engineer is effectively mandatory. For any wall where failure could impact a building, road, or neighbouring property, engineer involvement is strongly advisable regardless of height.

Can I use a retaining wall to create a lower patio area?

Yes — this is very common. The wall forms the boundary between the upper garden level and the lower patio. Standard retaining wall design rules apply. Take care that the drainage outlet from behind the retaining wall doesn't flood the lower patio — connect the land drain to a gully or surface water drain.

How long does a retaining wall last?

A well-built masonry retaining wall with correct drainage should last 50+ years. Railway sleeper walls in hardwood last 20–40 years depending on timber quality and soil exposure. Precast interlocking block walls effectively last indefinitely if properly installed. The limiting factor in most retaining wall failures is drainage failure — keep outlets clear and inspect weep holes annually.

Regulations & Standards

• Approved Document A (2004, with 2013 amendments) — structural stability requirements for retaining walls over 600mm

• Party Wall etc. Act 1996 — Section 6 applies to excavations within 3m of a neighbouring building; notice may be required

• BS 8002:1994 — Code of practice for earth retaining structures (now withdrawn but widely referenced; replaced by BS EN 1997-1 Eurocode 7)

• BS EN 1997-1 (Eurocode 7) — Geotechnical design: basis of design for retaining structures

• CIRIA Report C580 — Embedded retaining walls: guidance for economic design

• HSE CITB guidance — Trench and excavation safety during retaining wall construction

• CIRIA — Retaining Wall Design Guidance — technical guidance for practitioners

• BRE — Retaining Walls and Drainage — guidance on drainage behind earth-retaining structures

• ICE Manual of Geotechnical Engineering — earth pressure theory and retaining structure design

• retaining wall overview and landscaping context — landscaping perspective on retaining wall options

• soil types and bearing capacity — understanding the retained material and foundation ground

• concrete mix ratios for foundations and retaining walls — mix specification for foundation concrete

• trench and excavation safety — working safely in excavations when forming retaining wall foundations

• Party Wall Act notices and procedures — when excavations near boundaries require Party Wall notices