Can a Wall on Strip Foundations Be Used to Retain Ground?

An external wall sitting on strip foundations is designed to carry vertical loads from the structure above and resist horizontal wind pressure from outside. It is not automatically capable of retaining ground — the loads from retained earth are very different in nature and magnitude from wind loads.

Whether an extension wall can also act as a retaining wall depends on the height of retained earth, the wall construction, and the soil type.

Small Retaining Walls: BRE Good Building Guide 27

For small retaining walls (typically garden walls or basement-level retaining below a ground floor extension), the BRE Good Building Guide 27 (GBG 27) provides design charts that allow limited retaining walls to be constructed without bespoke engineering calculations — provided certain conditions are met.

Under GBG 27, a wall may retain ground without bespoke design if the retained height does not exceed approximately 1.725 m and the wall configuration falls within one of the standard design types shown in the guide.

Acceptable Wall Types Under GBG 27

The guide considers:

1. Straight plain masonry walls — checked against chart limits for wall thickness, height, and soil type
2. Staggered walls — walls with plan offsets that increase lateral stiffness
3. Piered walls — walls with buttress piers at intervals
4. Reinforced hollow blockwork — hollow concrete blocks filled with reinforced concrete core, providing significantly increased capacity

Each type has different capacity, and the applicable chart limits depend on the wall height and the soil type (cohesive/clay vs non-cohesive/sandy/gravelly).

Soil Type Matters

The lateral pressure exerted by retained soil depends on its density and angle of internal friction. Clay soils generate higher lateral pressures than granular soils of the same height, and their pressures change with moisture content. Any retained height in clay that exceeds the GBG 27 chart limits requires a bespoke retaining wall design.

When Bespoke Engineering Is Needed

If any of the following apply, a bespoke retaining wall design is required — a standard strip foundation external wall is not sufficient:

– Retained height exceeds the GBG 27 limits for the applicable wall type and soil
– The wall supports a surcharge (e.g. a vehicle parking area or slope above)
– The soil is expansive or of poor quality
– The foundation sits in groundwater
– The existing wall was not designed to retain ground and there are signs of movement

What PorthouseDean Can Do

PorthouseDean can design retaining walls as part of an extension or groundworks project. The design will involve checking the wall stability against overturning and sliding, and verifying the foundation bearing capacity for the combined vertical and horizontal loads.

Contact us to discuss the retaining wall requirements for your project.