In the first 2 parts of this post we looked at history of subsidence and the influence of trees; in this final part we’ll cover other contributory factors such as the presence of ground water and variations in climate.
The slope of the site may have a significant effect for a number of reasons.
If the site slopes steeply any buildings on the site will generally have foundations of varying depth. We have all seen houses that are level with the footpath at the front, but have several steps down into the garden at the rear (or vice-versa).
In normal building practice the foundations won’t actually slope to follow the site, they will be constructed level. This means that the foundations will be deeper on one side of the site than the other. The shallower foundation will be more prone to seasonal movement than the deeper foundation and this could well lead to differential movement taking place.
Even if the foundation ‘steps’ down to follow the slope of the site there will be minor variations in depth which could result in differential movement.
Now let’s consider what effect a sloping site will have on how wet or dry the clay soil will be. Time for another simple experiment! Put a shallow tray out in your garden in heavy rain. It soon fills up to brim with water. Now put the same tray out in the rain propped up at an angle. The water runs out before the tray can fill up.
Much the same thing happens on clay soil. If the site is flat, rain will lie on the surface for a while and very gradually soak into the clay causing it to soften and swell. If there is a significant slope on the site, water will run off before it has a chance to soak in and the clay will be much drier. We can see from this that we can have two properties in the same area, with the same weather conditions and even the same degree of vegetation, however the flat site could be much wetter than the sloping site.
This leads us onto another condition that often occurs in clay soils; underground water.
Clay soils are quite notorious for the presence of naturally occurring underground springs and water courses. Because of the way in which clay soil shrinks, swells and cracks, these water courses have a nasty habit of moving slightly from year to year. It’s not unusual to find a soft boggy patch in a garden whilst inspecting a property that the house owner swears wasn’t there last year.
Again, this is a factor that makes it difficult to predict how wet or dry clay soils will be or whether a house is likely to be affected by subsidence or not.
The Consistency of the Subsoil
To put it simply, not all clays are the same and not all clay subsoils are composed of 100% clay.
What we describe as ‘clay’ soils can contain a proportion of other materials, such as sands, silts and organic matter. The purer the clay the more likely it is to be affected by seasonal volume changes due to variations in moisture content. Clay is a cohesive soil and the particles tend to stick together; they also have the ability to absorb moisture. Materials such as sand are not cohesive and water tends to run through it rather than being absorbed.
It follows, therefore, that clay soils containing significant proportions of sand or silt will not behave in the same way as pure clay soils. Different types of clay will have different shrink/swell characteristics.
It is rare that we find sites where the clay is completely consistent across the whole site at all depths. Again, this is a factor that makes it difficult to predict when a building will move and when it won’t.
Variations in Climate
This is probably the most annoying variable that we have when considering movement in clay soils; our climate simply won’t play ball and behave consistently. This makes it very difficult for us experts to predict foundation movement.
We can look at seasonal rainfall charts and say that it ought to be wet in the winter and relatively dry in the summer and then we have a year like the present one when we had a virtual drought in the Spring and rain all through August.
We would normally expect clay soils to be quite wet during the Winter months and then gradually dry out through the Summer before becoming wet again in the Autumn and following Winter.
If this happens we tend to get quite regular and gentle cyclical movements. Buildings may move slightly, but usually not enough to cause any noticeable damage.
It’s when the seasons go out of kilter that problems usually occur. For example, if we get several dry Winters followed by hot Summers (remember those) clay soils shrink, but they do not expand again in the Winter if there is not enough rain. If the soil dries out even further the next Summer we might start to get noticeable movements and its usually after a period like this that we start to get a spate of subsidence claims.
However, these periods don’t last forever and as soon as we revert to a more ‘normal’ weather pattern the degree of movement reduces again.
It’s for this reason that it is wise to take a conservative approach to subsidence. As I keep saying, the degree of damage is rarely very severe in structural terms and conditions will nearly always revert to equilibrium. I think, as we have found in many circumstances, nature usually knows better than us and has a habit of sorting itself out if we leave it to get on with the job.
The point that I am really making here is that it is very difficult to carry what we like to think of as a highly detailed and scientific analysis of a problem when one of the variables is constantly changing and is totally outside of our control. Although we can carry out detailed analysis of the subsoil and tree roots that we find and monitor very accurately the width of cracks we can’t make it rain or stop raining. Without being able to control all aspects of an experiment the results will to some extent be unreliable. Although we can say what could or might happen in certain circumstances we can’t guarantee it because we can’t control the weather.
Insurance companies will often monitor buildings over what they call a ‘full cycle of seasons’. Effectively this means we will monitor for a year and see what happens. (Usually what happens is that the house owner gets fed up waiting and wants the cracks repaired and the problem sorted.)
On a more serious note, the monitoring is often inconclusive and this is quite often because the weather hasn’t behaved as we would like it to. In these cases the insurance company will sometimes want to go on monitoring for longer. I have known cases that have been monitored for 3 or 4 years! In one case the insurer concluded after 4 years that the property had stabilised and repaired the cracks.
It is often when monitoring is inconclusive that the insurer will decide to help things along by removing trees and other vegetation. At this point it’s probably worth explaining that by inconclusive we generally mean that the building hasn’t totally stabilised, but that the movements are too small to warrant any major structural works like underpinning. In these cases the insurer may be reluctant to pay for crack repairs at that stage in case the cracks open up again and they are called back to carry out another repair.
Insurers and their advisers will often suggest removing trees and other vegetation to see if this will stop relatively minor movements occurring and allow them to conclude the claim. As I have mentioned, Insurers have the benefit that they can look at one problem in isolation. They only have to consider how best to stop the house moving for long enough to repair it. They don’t have to look at the ‘bigger picture’ or consider the ecological effects of removing vegetation.
Pervious and Impervious Materials
One factor which is often overlooked when carrying out site investigations is the nature of the finishes on the site and what effect this has on the clay subsoil.
In this respect I am talking about previous and impervious materials. It has become common in recent years to pave our front gardens to a greater or lesser degree, partly to provide somewhere to park our cars and partly to reduce maintenance. This does have an impact on the clay soil, however.
Where we have flowerbeds and lawns the clay is able to absorb moisture in the form of rainfall, but where we have paving very little of the rain that falls on the surface will find its way through to the clay subsoil; most of it will run off into the drainage system and the subsoil, protected by its impervious cover, will remain dry.
Clay soils take a long time to absorb moisture once they have dried out and it often takes several weeks, or months, of wet weather in the Autumn and Winter to make up the moisture deficit that has built up in the Summer. Where the Autumn and Winter rains can’t get through paving to the subsoil underneath, it is likely that the clay will remain substantially drier than those areas that consist of lawn and flowerbeds. In the long term this can lead to a moisture imbalance which could cause differential movement.
There are now a number of products on the market that can provide suitable surfaces for parking and that are relatively maintenance free, but will still allow moisture to permeate into the subsoil.
Subsidence in clay soils is a complicated issue with many variable factors and it is not a case of ‘one size fits all’ when it comes to finding the right solution for any particular case.
There are a number of key points that we have to consider, however. Firstly, we have to bear in mind that any clay soil has the potential to shrink and swell and how much it will shrink and swell is dependent on a number of different factors, as I have discussed above.
Secondly, any property on clay soil has the potential to ‘move’ unless its foundations are so deep that they are in a layer of clay that is not susceptible to seasonal volume changes. Many new houses are built on piled foundations and these will generally be stable. The majority of our existing housing stock has relatively shallow foundations, however, and there will always be a risk that a house with shallow foundations on a shrinkable clay soil could be affected by subsidence.
Thirdly, it is likely that the majority of buildings with shallow foundations do actually move, but in most cases the movement is so slight that there is no visible damage.
Some buildings will exhibit cracking to a greater or lesser degree and it is these buildings that generally become the subject of subsidence claims.
As I have discussed in some detail, trees do extract moisture from the soil via their roots and in clay soils this can increase the effect of shrinkage in Summer months. Removal of trees and other vegetation is often considered as one possible way of restoring stability to a property that has suffered subsidence damage.
In my view, whilst the removal of trees and vegetation may be appropriate in some cases, it is not a panacea for all ills and it is always a somewhat experimental process. In some cases it does not work and the property continues to move; in cases where the property stabilises following vegetation control there is no way of knowing whether the property would have stabilised anyway, due to other variations beyond our control, such as the weather.
There will always be a significant risk of heave if mature trees are removed, particularly if they substantially pre-date the buildings. In view the risk of damage from long term heave outweighs the risk of damage due to subsidence. I would always prefer to retain mature trees even if this involves underpinning a property affected by subsidence rather than remove the trees run the risk of long term heave affecting potentially several adjoining properties. In my experience it is easier and cheaper to repair a house affected by subsidence than one affected by heave.
As I have discussed previously, vegetation control is often favoured as a first recourse by insurance companies as it may have the desired effect and if it doesn’t work they haven’t really lost anything.
In my view, however, we must have some regard to the environment. Apart from the obvious risk of heave we should really consider what effect tree removal has on the surrounding environment both in terms of visual amenity and the wildlife that is supported by mature trees.
Against this we have to consider the damage to buildings that can be caused by subsidence. Generally, the damage to low rise domestic buildings caused by clay subsidence is relatively slight. Whilst this is not much of a comfort to those whose houses are affected, it is nevertheless true that clay subsidence, rarely leads to serious structural damage, loss of structural integrity or loss of functionality. In many cases the cracking is of an aesthetic nature.
Even in severe cases it is possible to repair or underpin domestic buildings affected by clay subsidence. It is never possible to effectively replace mature trees that have been cut down.
Peter Barry Surveyors offer a full range of residential surveying services include RICS Homebuyer Reports, Building Surveys and Specific Defect Reports. If you are buying a property or require advice on an issue affecting a property that you already own you are welcome to contact us by email or on 020 7183 2578