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
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.
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.
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.
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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