Retaining Walls
Retaining walls are something we come across regularly during the course of our inspections. For the most part they are fairly obvious and straightforward to inspect. Sometimes they are hiding in plain sight. This article sets out the common types of retaining wall that we will come across and also shows some that are less obvious.
The diagram above shows the four main types of retaining wall.
(Source: http://en.wikipedia.org/wiki/File:Retaining_Wall_Type_Function.jpg)
Gravity wall – this relies on the wall having sufficient mass to retain the earth behind it – they will generally be large and fairly thick.
Piling wall – this relies on the lateral resistance of a pile to support the wall and retain the earth.
Cantilever wall – this relies on the wall having sufficient mass to retain the earth behind it. It is assisted by the self-weight of the soil – they will generally be large and fairly thick.
Anchored wall – this relies on the resistance of ground anchors to exert sufficient force to retain the earth.
How do they fail?
Retaining walls need to be designed so as avoid excessive movement in any one direction. There are three common ways in which they fail.
Bearing capacity failure – This is where the pressure exerted at the base of the wall exceeds the bearing capacity of the soil. The pressures are rarely uniform and too much soil pressure will cause localised failure of the wall.
Lateral sliding failure – This is where the base of the wall slides.
Wall rotational failure – This is where the force of the soil behind the wall tips the wall over
Some images of retaining walls will help to illustrate the subject.
Typical brick retaining wall in garden
Dry stone retaining wall
We inspect all boundaries to the property as there will be a public liability in the event of failure.
Underground car park
Retaining walls are sometimes hiding in plain sight. The walls to the underground car park on this block of flats are retaining walls. The Bridgewater canal towpath is on the other side of the back wall.
Driveway retaining wall
There is a small retaining wall here to the side of the drive. It doesn’t look too significant at first glance…
That is until you realise that the side wall of the garage is a retaining wall and is holding back the subfloor fill and concrete. Also, when you get inside the garage, it is single skin with brick piers (one such pier visible on the front corner). This means that there is a large force pushing at the base of the wall. Whilst it does not necessarily require remedial work, the original construction has been poorly thought through.
