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Friday, 3rd February 2017 | by: Peter Barry

In the first part of this post we looked at the history of subsidence as we know it today and how subsidence related insurance claims. Today, we’ll move on to look at exactly what subsidence is and some of the contributory factors.

What is ‘Subsidence in Clay Soils?

In geological terms clay is a relatively modern deposit and usually overlies much older rock.  In London and the South East the clay deposits are many metres thick and it is not practical or economical to construct foundations into the underlying rock.

Clay is a cohesive soil and consists of very small particles that bind together when mixed with water.  Clay soils are able to absorb moisture, which causes them to swell and they are also able to give up moisture which causes them to shrink.

The typical scenario is that clays expand in Winter when the weather is wet and shrink in the Summer months when the weather is drier.  It is the seasonal shrinking and swelling of the soil that causes movement and damage to buildings.

The term ‘subsidence’ is somewhat alarmist, as is paints a picture in most people’s minds of dramatic structural movement and possibly collapse.  This type of dramatic movement is more typical of subsidence in mining areas due to the collapse of underground workings or coastal erosion causing the dramatic collapse of buildings near the edges of cliffs.

In clay soils we usually encounter cyclical movement with cracks opening in the Summer months and closing again in the Winter months when the soil expands. In the vast majority of cases the damage to buildings affected by clay subsidence is very slight in structural terms and almost never results in structural instability or danger of collapse. Indeed, it is very rare for the damage to be classed as more than ‘cosmetic’, which means that we are often dealing with cracks of less than 1mm in width (about the width of a thick pencil line).

We will generally be dealing with the kind of cracks that can easily be filled during the course of normal redecoration.

What role do trees play?

So what role do trees really play in this process? Trees need to take in moisture through their roots to replace the moisture that evaporates through the foliage.  Some trees have higher water demands than others and Willows, Poplars and Oaks are all species that have high water demands.

As we have seen, clay soils will shrink in the Summer when they dry out and this process can be exacerbated by the presence of tree roots as they will extract most moisture during the Spring and Summer months when growth rates are at their highest.

Tree roots can spread many metres in search of moisture and there is really no hard and fast rule as to how far they will spread.  It used to be generally accepted that the spread of tree roots would be about equal to the height of the tree, but cases have been encountered where the root spread has been 1 ½ or even 2 times the height of the tree.

So in order to be safe we should remove all the trees that are anywhere near to buildings?

It isn’t really that simple.  In many cases buildings and trees have happily co-existed for many years without any damage being caused.  If trees have been in place before buildings were constructed it is likely that the subsoil would have been in equilibrium at the time of building.  In these cases taking out the trees would possibly do more harm than good.  If the trees are removed their roots will no longer extract moisture from the soil and it is possible that ‘heave’ could occur.

As we have said ‘subsidence’ in clay soils tends to be cyclical with shrinkage and expansion taking place on a seasonal basis.

Heave is likely to be progressive, as the clay will take up moisture following tree removal until it finds a new equilibrium. This can result in upward movement of foundations over a number of years with the damage resulting to the buildings often being substantially more severe than that resulting from subsidence.

So if trees stay they can cause damage to buildings and if we take them out it could also cause damage.  What’s the solution then?

There is no simple solution and we have to treat each case on its merits. As I said earlier, trees are only part of the equation. We have to look at a number of other factors.

Among the variables that we need to take into account are:-

  • The nature and depth of the building foundations
  • Is the site flat or sloping?
  • Is there evidence of ground water?
  • Is the nature of the subsoil consistent across the site?
  • Variations in climatic conditions
  • Is the ground around the building pervious or impervious?

The above are typical factors that would have to be considered before we decide that tree removal is appropriate or necessary.

Foundation Depth

Clay tends to become firmer and denser the deeper one goes.  This is partly due to the fact the Clay tends to be a deposited over long periods of time and the deeper Clays will have been there for longer and will have become more compacted over time as further deposits build up over them.

The deeper Clays will also become more stable for three reasons; firstly they are denser and this makes them less able to absorb moisture; secondly the further they are below the surface the less susceptible they are to seasonal variations in temperature and moisture anyway; finally below a certain depth tree root activity ceases and deeper clays will not be affected by seasonal water demands from tree roots.

It follows, therefore, that if we construct foundations to an adequate depth they will be unaffected by seasonal volume changes and root activity and the risk if any significant movement due to clay subsidence will be negligible.

Many new houses and flats are now built on concrete piled foundations for this reason. Obviously, this doesn’t help us with existing buildings as much of our housing stock is built on relatively shallow foundations.

From what we have discussed before it would seem that every property with shallow foundations on clay soil must move.  If this is the case why is subsidence not more common?  The simple answer is that most older properties do move on a seasonal basis, but they don’t move enough to cause any significant damage.

If you look at most Victorian terraces you will see that there is often significant distortion, with window cills and lintels being out of level as you look along the terrace.  However, you will see surprisingly little crack damage.  It sounds strange to say this of a building, but older houses are generally quite ‘flexible’ and will distort to a significant degree without actually cracking.

The reason for this flexibility is that they are generally constructed from relatively ‘soft’ bricks held together with lime based mortar.  The foundations, although shallow by modern standards, are also relatively flexible as they often consist of brickwork rather than concrete.

Modern houses, by comparison, tend to be much more rigid in their construction and even relatively minor movements will cause cracking.  Most modern houses will have concrete foundations and denser brickwork bedded in cement mortar which is very susceptible to cracking. Modern materials such as man made concrete blocks and sheet materials such as plasterboard are also very intolerant of even minor movements.

The building owner’s perception of damage is also something that has to be taken into account. Many building owners would accept distortion in a Victorian property provided there is no actual crack damage, but would be on the telephone to their insurers if even a hairline crack appeared in a modern house.

Something else that we have to consider is not just the overall depth of the foundation, but whether the foundation depth is consistent across the building.

In the past, when levels of home ownership were relatively low, it was unusual to see many extensions being built.  However, as home ownership increased it became quite common for houses and bungalows to have several extensions.  As Building Regulations changed over the years foundation depths gradually increased and it became quite common to find that extended houses would have foundations of varying depth.

If we take our standard Victorian terraced house, this was probably built with brick foundations that were no more than 600mm (2’ 0”) below ground level.  When the house was bought in the 1970’s the new owners may have decided to add a single storey extension. To comply with the Building Regulations at that time the foundations would probably have been between 1.0m and 1.2m (3’ 3” to 4’ 0”) in depth.

The construction materials would also have been different. The original house probably had relatively ‘soft’ brick walls built in lime mortar whereas the extension was probably built with ‘modern’ cavity walls using hard cement mortar.  So we now have relatively stiff single-storey extension on a deep concrete foundation attached to a relatively soft two-storey Victorian house built on a shallow brick foundation.

What happens when the inevitable seasonal volume changes occur in the clay subsoil? The two parts of the building move at different rates and crack damage occurs. The cracking will almost always occur at the point where the extension joins the original building.

The answer in this case isn’t to remove all the vegetation around the building or even to underpin the house.  The simplest answer is to introduce a properly formed expansion joint between the two different sections of the building and allow them to move without causing any damage to the structure.

Another problem that we have encountered in more recent years is the tendency to add lightweight structures such as porches and conservatories to our houses.  Many small extensions like this are exempt, or at least partly exempt, from Planning and Building Regulation Approval and it is up to the builder or house owner to decide on a suitable form of construction and foundation depth.

Many house owners (and builders) take the view that a lightweight building doesn’t need much of a foundation and they don’t want the inconvenience and expense of digging deep foundations for a porch or conservatory.

Whilst it’s true that a foundation doesn’t need to be deep to support the weight of the building, it does need to be deep in clay soil to prevent seasonal movement due to variations in the moisture content of the clay.

Now we come to an interesting point. A lightweight building on a shallow foundation in clay soil will actually move more than a heavy building on a shallow foundation!

The heavy building will compact the clay that it sits on to a greater degree than the lightweight building.  Much of the distortion that we often see in old buildings is due to initial settlement that occurred soon after the property was built.  Once the clay is compacted its ability to shrink and swell is reduced.  The clay will still shrink and swell to some degree, but as it swells it will have to create enough force to lift a very heavy building.  In the case of say a conservatory or porch, the clay only has to exert enough force as it swells to lift a very light building.

Try a simple experiment.  Take an empty cardboard box and lift it above your head.  Quite easy wasn’t it.  Now fill your cardboard box with bricks and try to lift it above your head.  How many of you managed that?

Now let’s look at how deep a foundation really needs to be in clay soil.  The top 1.0m (3’ 3”) of clay is always going to be susceptible to seasonal changes in volume.  Roots will have very little effect in this layer of soil as it will dry out in the Summer whether roots are present or not.  If we have a building with foundations much less than 1.0m deep removing all the vegetation around it won’t, in my opinion, guarantee that the building will be stable.

Most tree root activity takes place between 1.0m and 3.0m in depth.  Below 3.0m the clay is generally too dense and the conditions too anaerobic for root activity to occur.

If we want to be fairly certain that buildings won’t move then we need to take the foundations down to a depth where the soil is stable and there is no root activity.  In clay soils this will normally be at least 3.0m.  Digging trenches to this depth is very difficult and costly and for this reason many new buildings are constructed on piled foundations.

As I said previously, however, this doesn’t deal with our legacy of old buildings on traditional shallow foundations.  (Which to be honest is most of the housing stock in London and the South East.)  So do we remove all the trees and shrubs and underpin all the houses?

Neither solution is necessary or practical.  Whilst we have to treat each case on its merits, we have to bear in mind the fact that in most cases of reported clay subsidence the damage is only ‘cosmetic’ and there is rarely any significant effect on the structural integrity of the building or its function.

Also, this type of movement tends to be cyclical and long-term monitoring will often show that the problem has resolved itself without any drastic action being taken.

In most cases the repair usually amounts to no more than superficial crack repairs and redecoration.  In some cases, as I mentioned previously, it may be appropriate to introduce expansion joints into the structure to prevent future damage.

Why do insurers always insist on ‘implicated’ trees being removed if this is the case?

Removing trees may help or it may not.  As mentioned before, it could even make the situation worse if heave results.  Insurers are happy to suggest taking trees out because it doesn’t cost them anything and it might cure the problem; if it doesn’t they are no worse off.  The adjoining owners are though, as they are the ones who have to pay for tree removal and the public and the environment are worse off, as they have to live with the results of this deforestation policy.

Why do ‘experts’ recommend tree removal then?  For the same reason as above, it might work, but to be honest no one really knows if it will or not. Tree removal is always somewhat experimental and you can’t guarantee what effect it will have.

What will happen if we don’t remove trees?  Probably not very much; buildings may continue to move cyclically, but the damage is likely to be only superficial.

So, should we never remove trees?  We cannot say that there is never a case for tree removal, but in my view it should be the exception rather than the rule.  There may be cases where the combination of circumstances is such that tree removal might be the only practical solution, but this should only be considered if the damage is actually severe and all other avenues have been explored; it should not be the first port of call.

In the final part of this series we will look at other relevant factors such as sloping sites, the presence of ground water and variations in climate.

Peter Barry offer a full range of residential surveying services. If you require a pre-purchase survey, specific defect report or advice on a party wall matter you are welcome to contact us on 020 7183 2578 or by email.