What This Requirement Covers
Retaining walls hold back soil or other materials and must resist lateral earth pressure, hydrostatic pressure, and surcharge loads. Where a retaining wall forms part of a building or is within the curtilage of a building, it falls under Building Regulations. The structural design must comply with BS EN 1997 (Eurocode 7: Geotechnical Design) and the relevant material Eurocode.
Key Requirements
Types of Retaining Wall
- Gravity walls: Rely on their own weight to resist overturning and sliding (mass concrete, stone, gabion baskets)
- Cantilever walls: Reinforced concrete walls that resist earth pressure through bending (most common for walls over 1.5 metres)
- Counterfort/buttress walls: Cantilever walls with additional vertical ribs for economy at greater heights
- Crib walls: Interlocking precast concrete or timber units filled with granular material
- Reinforced earth: Layers of geogrid or steel strips embedded in compacted fill behind a facing panel
Design Considerations
- Lateral earth pressure: Active, passive, and at-rest pressures calculated using the soil's angle of internal friction and unit weight
- Surcharge loads: Additional loads from buildings, vehicles, or stored materials behind the wall
- Hydrostatic pressure: Water pressure if the ground behind the wall is saturated; drainage is essential to reduce this
- Stability checks: Overturning, sliding, bearing capacity, and overall stability must all be verified
- Drainage: A granular drainage layer behind the wall with weep holes or a land drain at the base is critical to prevent water pressure building up
Building Regulations
- Retaining walls over 600 mm in height that are within 2 metres of a building or boundary may require Building Regulations approval
- A structural engineer's design is required for all retaining walls over 1.2 metres in retained height
- The foundation of the retaining wall must be designed to the same standards as building foundations (adequate bearing capacity, frost protection depth)
Practical Compliance Tips
- Always investigate the ground conditions before designing a retaining wall; soil type and water table level determine the design
- Provide adequate drainage behind all retaining walls; lack of drainage is the most common cause of retaining wall failure
- Do not build retaining walls from garden wall blockwork or lightweight materials; they do not have the strength to resist earth pressure
- For walls over 1.2 metres, obtain a structural engineer's design and submit it to Building Control
- Consider the impact of the retaining wall on neighbouring properties; the Party Wall Act may apply if excavation is near a neighbour's foundation
- Construct retaining walls in accordance with the engineer's design, including reinforcement, concrete specification, and drainage details
- Monitor retaining walls for signs of movement, cracking, or drainage failure and address issues promptly