Settlement, Subsidence and Heave

and the part clay soils play in this

If you would like further advice on any then please phone 0800 298 5424 for a friendly chat. If you need help and advise with regard to a structural survey (or building survey, as it is now commonly known; structural survey being the old term for it) or a structural problem or an engineers report, or you need a report specifically tailoring to your requirements please do not hesitate to call us for a friendly chat on 0800 298 5424.

The magical properties of clay

Clay has several unique properties. It can both cause problems when it is a clay soil that your house is built upon and be useful when it is used for the bricks that your house is made of.

Clay expands and contracts, depending upon its moisture content. It is at its bulkiest at 40% to 60%, however, it changes form if it gets wetter or dryer. It is this change of its bulk that causes problems. When the clay soil gets too wet the clays bulk becomes larger and almost pushes the property out of the ground. This is known as heave. When the clay dries out it becomes dust like, then we get settlement of foundations and subsidence of the building, as its bulkiness has reduced considerably.

Its ability to change size wouldn't be such a problem if it weren't that most London properties are built on clay, and there are many areas of clay throughout the country, such as Bedfordshire and Peterborough , which, interestingly enough, are also known for brick making.

Finding out if your property is built on clay soil

There are several ways of finding out if your property is built on clay soil.

Cracks

If the property has historic cracks it may be that it is built upon clay. It is best to check before you panic. You can do this by looking at maps geological maps or ringing up your friendly insurance broker, as they have a postcode index as to what areas are considered high risk with clay and which aren't. However, we must add that these maps are quite general and that when we were on a course many years ago we asked how these maps were originally made. We were advised that students were employed during their summer holidays, so the reliability of them, it could be argued, is limited for the purposes of identifying if clay is under your building, as they were originally produced to help farmers.

Clay test

This is what a good surveyor would do if they had come to investigate problems with your foundations and the owner of the property was happy for them to dig up the garden! We take a lump of soil from the garden; this should be approximately the depth of the foundations. If it is Victorian or Edwardian property it may be a lot less, in a modern property it is likely to be 1 metre to 1.2 metres, to even 1.4 metres deep.

On a summer's day you can leave the lump of soil in the garden for it to dry out in the sun. When it dries out if it becomes powdery and much lower in volume then it is clay. Equally, if it rains and it becomes a larger volume then it is clay. Also, if it not warm you can leave it inside on the radiator. We believe (although we would need to check it) that a growth or reduction of approximately 30% (for some reason 28% is ringing bells). This type of clay is known as shrinkable clay.

Not all clay expands and contracts

We would add that not all clay expands and contracts. The deeper clay tends to be the more stable and harder clay and therefore doesn't expand to the same extent.

Clay that is used for brick making

We thought we would just add something about clay that is made for the use of brick making.

Old soft red bricks

The older bricks, were possibly even sun baked rather than fired, used a softer red clay. They have their own oil so they can affectively cook themselves.

You will also notice that it is the softer red clay bricks, even up the post-war era, that tend to be affected first by spalling.

Blue clay

Blue clay is fairly well known. This type of clay tends to be the harder clay and produces harder bricks and you may find bricks made out of this clay. A classic brick, and generally considered the hardest brick, is the Accrington brick and you would have to dig deep for this clay, i.e. normal clay would be dug at about 30 feet, or the metric equivalent!

Yellow or white clay

A yellow or white clay, that is used in a London stock brick, is harder than the soft red bricks but not as hard as the blue clay bricks.

Fletton brick

The one thing that used to confuse us for many years is what is the difference between a Fletton brick, a stock brick and a common brick. A Fletton brick, we believe, has its origins in Fletton near Peterborough , where bricks are produced in such quantities that it became the common name. Stock bricks tend also to be a common brick, but it specifically relates to bricks that don't have their own oils to fire, they would typically have to have methane today and years gone by coal dust to fire them, where as the soft red bricks tend to have their own oil that they can fire in. Therefore, you tend to find many of the older properties have a red brick as they were fired in the “sun. The term common brick is a generic name.

Silicone brick

This isn't a brick at all, but, we believe, is made from concrete.

You may also be interested in these articles regarding cracking in properties:

Structural Cracking

Cracks in my Wall

How, in our experience, Insurance Companies deal with Cracks in Properties

Please see our section on: Specific Defects Reports

If you truly do want an independent expert opinion from a chartered surveyor, and many of us are also chartered builders, with regard to specific defects reports, structural surveys, commercial surveys, building surveys, engineers reports, home buyers reports or any other property matters please contact 0800 298 5424 for a chartered surveyor to give you a call back.

If you have a commercial property, be it leasehold or freehold, then you may wish to look at our Dilapidations Website at www.DilapsHelp.com and for Disputes go to our Disputes Help site www.DisputesHelp.com .

We hope you found the article of use and if you have any experiences that you feel should be added to this article that would benefit others, or you feel that some of the information that we have put is wrong then please do not hesitate to contact us (we are only human).

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