Seven out of the top 15 US cities with most foundation failures are located in Texas [1].
There is a reason why residential foundation failures are more common in this area.
The American Society of Civil Engineers estimates that as many as one in four homes undergo
some kind of foundation damage due to this particular issue.
In this video we will explain why these areas suffer more than others and what are some
mitigation techniques.
Frequent foundation problems in residential units are not limited to Texas.
The UK in particular has struggled with foundation problems for many decades.
The Association of British Insurers estimates that foundation problems cost the insurance
industry over 400 million pounds annually in Britain alone.
Australia, India, and South Africa are other notable examples where foundation issues incur
major construction costs.
ll these regions have one thing in common, that is, a specific soil type underlies a
big chunk of them which is a nightmare to build upon. he soil in question is an expansive
clay with high potential for swell and shrink.
Before we dive into the problems and mitigation techniques we need to understand the behaviour
of this clay.
This video will focus on the Southern United States though the concepts here also apply
to the other regions mentioned earlier in the video.
Expansive clays are commonly found in the Gulf Coastal Plain due to a combination of
geological and environmental factors.
This region was covered by shallow seas in the Cretaceous period more than 65 million
years ago.
Over millions of years, the seas deposited marine sediments and formed sedimentary rocks.
Over time, these rocks underwent chemical weathering where the parent materials breakdown
into smaller particles forming clays.
But not all clays exhibit this problematic swelling behaviour.
A specific combination of pressure, temperature, chemical reactions, as well as volcanic activity
resulted in the formation of a mixture of minerals known as smectite.
Smectite is the culprit of this video responsible for the majority of the residential foundation
damage.
Smectites have a layered structure composed of cations sandwiched by negatively charged
sheets.
This entire assembly is held together by weak electrostatic forces.
When water is added to the soil, the water molecules hydrate the sandwiched cations leading
to a decrease in the electrostatic forces and separation of the plates which is manifested
as expansion of the clays.
If confined, which is often the case when there is a foundation on top, the soil will
exert an uplift pressure on the foundation.
The magnitude of the pressure mostly depends on the amount of water adsorbed and type of
swell minerals present.
The uplift pressure can range from 100 kPa (2000 psf) for soils with relatively low swell
potential to an astounding 800 kPa (17 000 psf) for soils with extremely high swell potential.
Note that a typical residential 2 storey house weighs less than 200 psf or (1000 kg per m2)
[2] so its weight is no match for the uplifting forces of the soil.
But the issue is not solely due to the uplift or subsidence of the soil.
What causes most of the damage is the differential heave of the underlain strata.
This can happen if there is a prolonged period of draught followed by heavy rain.
In that case, the soil on the perimeter of the house gets saturated much faster than
the soil underneath the center of the house.
This causes the outer soil to expand much faster than the soil under the center.
This differential movement generates very large forces and cracks within the slabs,
beams, or walls that are part of the foundation assembly.
The phenomenon is also reversible where quick evaporation and drying can cause faster shrinkage
and settlement of the perimeter of the house.
This is the main reason why foundation failures often originate from the corners of the house
since the soil under the corners undergoes the fastest change in water content due to
it being exposed to the outside environment on two sides.
Unfortunately the issue is not limited to this, vegetation can also contribute by sucking
out water from the soil and leading to shrinkage and subsidence of that part of the yard.
In extreme cases, the shade cast from a tree or the house itself can cause less evaporation
and also contribute to differential settlement [3].
The grading and the drainage system are also crucial; I will talk about them in the mitigation
section near the end.
The issues of expanding soils was first documented near the end of the second world war.
By the late 70s, the understanding of expansive soils was good enough to avoid this phenomenon.
The design usually revolves around one, or more of the following concepts:
Provide a stiff enough foundation to accommodate differential vertical movements without a
failure of the reinforced concrete [4].
Alternatively, the foundation can be designed to isolate the structure from the expansive
soil.
One example of this is a structure supported on piles with a void between the soil and
grade beams which accommodates expansion and shrinkage of the soil over the life cycle
of the house [5].
Alternatively, the soil can be targeted to provide stability and a reduction of its swell
potential.
The design of foundations and floors on expansive soils are more complex than regular foundations.
The added complexity usually leads to a costlier construction and motivates developers to cut
corners.
My personal experience with contractors is that if they believe that their actions are
inconsequential, they are much more willing to take risks for higher profits.
I have seen contractors slack on proper compaction because they do not fully understand its importance.
I can definitely imagine some of these risks re-appearing when dealing with expansive soils.
Though, on some other issues, based on experience, contractors do have a much better feel of
the problem than engineers do.
I am very much interested to hear your experience on this subject.
If you have dealt with foundation problems before, feel free to describe your case in
the comments and how you dealt with the situation.
I read 100% of the comments and try to interact as much as possible.
I am hoping for first hand information and a healthy discussion with a variety of solutions.
Speaking of solutions, let’s see what options are there for dealing with this problem.
Underpinning is a commonly employed technique that consists of installing new foundation
elements that transfer the structural loads to a different, usually deeper and more stable
part of the soil strata.
This is usually achieved by installing piles around the perimeter of the house upon which
the grade beams are later attached upon.
This way a void can be created between the foundation and the expanding soil that will
accommodate any future expansion and contraction.
For this technique it is crucial to minimize the skin friction on the piles.
In other words, the piles need to be isolated from the expanding soil such that they remain
unaffected by any future uplift.
This technique is only possible if the existing foundation elements spanning between the points
of underpinning are strong enough to carry the loads from the superstructure essentially
acting as beams.
In the case of a slab-on-grade foundations the remedial measures are much more difficult,
especially if the slab has failed due to its lack of stiffness.
Underpinning a flexible slab would punch right through the concrete creating even more issues.
In some cases, beams can be constructed under the slab and be used as supporting points
for underpinning but as you can imagine this is a difficult and risky task.
Moisture control techniques can be employed to ease up differential movement of the soil
by reducing the fluctuations in water content and providing a more uniform heave motion.
Horizontal and vertical moisture barriers are often installed for this purpose.
The horizontal barrier consists of an impermeable membrane or a rigid concrete slab installed
around the perimeter of the house to guide the water away from the foundation.
Subdrains are installed at the edge of the barrier to prevent ponding.
Similarly, the vertical barrier consists of an impermeable membrane but this time laid
vertically in an excavated trench.
The purpose of both of these techniques is to extend the seepage path and therefore slow
down the rate of moisturizing and heaving.
Though, it is worth noting that even a perfect impervious moisture barrier will not eliminate
the moisture in the soil.
Water migration due to thermal transfer and capillary rise from underlying soils will
introduce moisture in the soil regardless of any moisture barriers [6].
Furthermore, moisture barriers are not a remedial measure in itself.
Structural repairs must be included to repair any damage that has already been incurred.
Repairs are usually done after the soil movement has stabilized.
However, reaching an equilibrium condition in water content may take several years depending
on the situation.
These are only a few of many potential solutions for this problem.
Unfortunately, none of them have a 100% success ratio.
Local contractors specialized in foundation repairs are usually best skilled to judge
which solution is best suited for a particular case.
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