Subsidence – Part 2 – Factors that contribute to subsidence and what to look for

As with tree roots, a drainage system is buried therefore not obviously visible at the time of an inspection.  It always amazes me how people tend to ignore the condition of the below ground drainage system when purchasing property and do not seem to see this as important

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In last week’s article I gave an example of the consequences of building subsidence, which can be extremely disruptive and expensive to deal with, however I also emphasised that the vast majority of subsidence damage is less serious and can be rectified reasonably easily.  I also identified that to the average householder the mere mention of the word subsidence strikes fear and panic into them as there is a perception that subsidence damage is always serious. As you would expect and as I have mentioned many times before, if you are thinking of purchasing a property it is always advisable to have a professional, such as a Building Surveyor, inspect the building before you commit to buy.  The Surveyor’s report will identify any issues that are present and inform you if any are serious.  A Building Surveyor will also highlight factors that may contribute to subsidence in the future and not just focus on the here and now.  There are a number of factors that could lead to subsidence and some examples are discussed below:

Clay Soils

To support a building it is essential that the load bearing capacity of the ground is capable of supporting the dead load of the building (the building’s self weight) as well as any imposed load (furniture, fitting, people, snow etc.), once completed and occupied.  The type of ground is essential to a building’s stability as this will determine the most appropriate as well as the depth of the foundation that should be used.  When siting a building, clay soils are particularly problematic compared to most other types of soil because clay has the ability for significant volumetric change depending on how much water/moisture it contains at any particular time.  When clay is wet it will swell and therefore expand, however when the ground starts to dry out all of this moisture is slowly removed and the clay will shrink.  Think about this process happening with a building on it!  If the ground is constantly expanding and then shrinking, then it is inevitable that the weight of a building will eventually be effected by these changes and cause the building to move.  Having said the above there is no reason why a building cannot be constructed on clay as long as this is established through ground investigations and appropriately catered for in the design.  This may involve deeper foundations, as well as the inclusion of root barriers where trees and vegetation may be in close proximity to the building.

Trees

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Whilst inspecting a property, as well as focussing on the building itself I would always look very closely at the surrounding environment and in particular the size and location of trees.  If not managed, trees and in particularly their roots have the ability to undermine foundations, damage drains and cause significant damage to a building.  The problem with tree roots is that you often cannot see the extent of the root growth or proximity to the building because they are buried.  This however does not mean that they should be ignored and where trees are deemed the pose a threat to a building then the services of a tree expert (Arboriculturist), should be called upon.  This is necessary because different species of tree will exhibit different characteristic in terms of size, growth rate, root spread etc. in addition to advice that can be provided in respect of the condition of trees and any recommended remedial action.

Tree roots do a number of things when in the ground.  Firstly they take up large amounts of water from the ground.  Given what has been discussed above in respect of clay soils you can easily see that in continued spells of warm weather and high temperatures that clay soil and tree roots are not a good combination and together this will significantly increase the potential for subsidence.  Secondly, as the roots grow they have the ability to physically impact on soils, particularly soft/granular types which can undermine stability especially when they have a foundation and a building siting upon them.  Also, as can be seen below tree roots have the ability to damage below ground drainage.

Drainage

Although it is possible to make a broad assessment of a drainage system during an inspection, by lifting manhole/inspection chamber covers this is limited to a small number of access points only and does not identify the condition of the vast majority of the drainage system around a building.  As with tree roots, a drainage system is buried therefore not obviously visible at the time of an inspection.  It always amazes me how people tend to ignore the condition of the below ground drainage system when purchasing property and do not seem to see this as important.  Even if there is no visible indication of any issues with a drainage system it is still worth considering a CCTV inspection of the system is carried out.

Below ground drainage is quite vulnerable and can become damaged in a number of ways.  Ground movement, even subtle movement can result in drains becoming displaced and fractured, particularly around the joints.  Tree roots can also damage below ground drains and find their way into the system.   If this type of damage does occur then the surface and foul water, which is usually heading toward a sewer, will actually start to discharge at the point/s where the drainage is affected.  If left undetected for a period of time then vast amounts of foul and surface water can be discharged into the ground around a building, which over time will start to influence the stability of the soil, and eventually lead to ground movement.  The lesson here is always establish the condition of the below ground drainage system and deal with any problems quickly, before they become much more serious.

Adjacent Excavations

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A building could sit quite happily for many years on stable ground without any problems and will only be affected if for some reason the ground conditions change.  One way this could happen is works being carried out in close proximity to a building that requires excavations.  If excavations are carried out to a depth and distance that could undermine or influence the stability of another building then this can cause movement, sometimes, sudden movement.  This should be considered in design where it may be necessary to provide temporary support. I have encountered this on numerous occasions where ground movement has been caused by a neighbour excavating (usually foundations) and usually through ignorance has not considered the stability of their neighbours building.

Leaking Rainwater Goods

Even simple repair and maintenance tasks, if left unattended over a period of time can introduce large amounts of water into the ground, which can affect the soil and undermine foundations which can cause ground movement. Rainwater gutter and downpipe repairs are usually inexpensive however this is one of the most common defects that a Surveyor will encounter when carrying out inspection. 

The article above provides a quick overview of some of the factors that could contribute to subsidence.  The points raised are not exhaustive (there are others) and you will note that no attempt has been made to discuss mining subsidence, which is a subject in its own right, perhaps for a future post. 

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