Retaining wall design

We have experience of retaining wall design for a wide range of private clients and landscaping architects.

So what is a retaining wall?

A retaining wall is a structure designed and constructed to resist the loading (lateral pressure of soil ) when there is a proposed change in ground elevation that exceeds the natural maximum angle the soil hold at (the angle of repose).

Retaining walls often use expensive materials and can be slow to construct. They also can be considered to create a safety hazard, especially around schools. Our usual aim is to see how direct walls can be avoided, by the use of more natural embankments. Space however can be limited.

Some things (not all) to consider in retaining wall design…

A retaining wall supports a “wedge” of soil. The wedge is defined as the soil which extends beyond the failure plane of the soil type present at the wall site, and can be calculated once the soils friction angle is known. As the setback of the wall increases, the size of the sliding wedge is reduced. This reduction lowers the pressure on the retaining wall.

The most important consideration in producing retaining walls designs is to recognise and counteract the tendency of the retained material to move downslope due to gravity. Any groundwater behind the wall that is not dissipated by a drainage system causes hydrostatic pressure on the wall.

Many walls are designed on the basis that they can structurally retain a high water level. This can often be the most appropriate solution.

How is surface or groundwater considered in retaining wall designs?

Granular, piped and special draining geotextiles are sometimes specified in a retaining wall design. A drainage system will reduce the hydrostatic pressure and improve the stability of the material behind the wall. However it can often present far greater complications in the discharging of the water which is then collected. A structural solution then becomes a hydraulic problem.

The UK Water Industry Act does not require Sewerage Authorities to take groundwater. Groundwater can considerably limit the capacity of existing sewers. Consent is usually required and could be withheld.

The ideal is to discharge the water back into the ground at the same site, yet that may not be viable.

It is often more appropriate to consider naturally leaking walls. For example dry stone retaining walls are usually self-draining.

Retaining wall design and discharge mechanisms of vadose water can be closely linked.

More advice on retaining wall design

Retaining walls naturally have a structural, hydraulic and general civil engineering element. The professional advice of a Civil Engineering Consultant is recommended. They should be able to advise on:

  • retaining wall options
  • produce a retaining wall design (and submit for building regulations approval)
  • methods to deal with the hydraulic elements and integrate with the site drainage as applicable.

Naturally Wilsham are able to assist, but naturally there are other competent civil engineering consultants also.