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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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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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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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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Excellent article as they all are. Andy Jones FRICS FBEng
ReplyDeleteA very interesting article, how common are these problems with modern housing developments? Or this this just a sustainability issue for older housing?
ReplyDeleteAnother really interesting and useful article. Thanks! :)
ReplyDeleteGood post with greaf illustrations.
ReplyDelete