What options are available for
addressing foundation performance issues?
Many houses in the Greater
Houston Area will experience foundation performance issues during their useful
life. The
foundation performance issues experienced can range from hairline cracking that
is difficult, if not impossible, to reliably attribute to foundation movement
(or any other specific cause) to severe cracking in drywall and brick veneer
accompanied by wood framing members being pulled apart.
Such a wide range of performance problems dictate a wide range of options
to address the issue. The options
available include the following:
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Taking no action at all.
This is a
perfectly acceptable option so long as there is no structural damage.
In our experience, this is also the most common response when the
damage due to foundation movement is minor.
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Non-structural remediation measures. Non-structural
remediation measures include vigilantly watering the foundation during dry
periods, making occasional cosmetic repairs, improving and maintaining drainage
around the foundation, removing trees and/or large shrubs and making changes in
the finishing of the house such as changes in wall coverings.
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Structural remediation measures. Structural remediation measures applicable to
slab-on-ground foundations usually include what engineers refer to as structural
underpinning, grouting and mudjacking, crack injection and tendon stressing. |
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What options are available for
improving foundation performance by mitigating moisture changes in the soil supporting the foundation?
There are a number of
potentially applicable options for mitigating changes in the moisture in the
soil supporting the foundation. These
options include soil surface grading, subsurface drains, watering the
foundation, removing and pruning trees, controlling roof water, maintaining
ground covers, and controlling lawn irrigation.
Each of these are briefly discussed below:
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Soil
Surface Grading: This is your first and primary line of defense against
excessive moisture in the supporting soils. It is very important that
the ground adjacent to the foundation be graded so it falls 6-inches in
10-feet or to the property line.
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Subsurface
Drains:
Underground drains are appropriate for playing a supplemental role in preventing
the supporting soils from absorbing too much water.
I say supplemental because it is usually a mistake to rely on an
underground drain system when the ground adjacent to the foundation is not
properly graded. Grading the soil
properly to an underground drain around 10-feet away from the foundation is acceptable. Leaving the ground
surface flat and placing the subsurface drain close to the foundation is asking
for trouble. Underground drains can
clog and they can develop leaks.
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Managing Trees and Large
Shrubs: Trees and large shrubs extract large amounts of water from the soil
every day. When they are close to
the foundation, they can significantly exacerbate the shrinkage of soil when
there is a drought. It is during
extended dry periods that we see most of the damage due to foundation movement;
the houses that exhibit damage during a drought almost always have trees close
to the foundation that are clearly making the situation much worse than it would
be otherwise. Trees and large shrubs
can be managed for improved moisture control and foundation performance.
Large shrubs can be removed. Trees
can also be removed. In many cases,
the roots of trees can be pruned between the tree and the foundation.
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Controlling Roof Water:
Large quantities of water come off the roof of a house. The flow of water off
the roof will always be uneven except for the case of a hip roof.
The best way to control water flow off a roof is with a roof gutter
system. It is important that all
eave areas be guttered. Guttering
some of the eaves while leaving other eaves without gutters makes the problem
worse, not better. It is also
important that the water not be discharged onto the ground within 5-feet of the
foundation. Discharging roof water
close to the foundation has the potential of causing significant foundation
movement. Water discharged close to
the foundation can easily percolate through the backfill to the supporting soil
under the perimeter of the foundation if the backfill is not properly compacted.
Unfortunately the backfill is usually clay and clay can be difficult to
compact properly.
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Maintaining Ground Covers:
There are many homes that have areas around the foundation where it is difficult
to get grass to grow. The grass
simply cannot compete with the trees for moisture and soil nutrients.
The best thing to do, in many cases, is to turn these areas into gardens
and mulch them heavily. The mulch
can be very effective in preventing the soil from losing excessive moisture.
What happens in many cases is that the area is not turned into a garden, but is left as an area that is bare and exposed.
During a dry period, these areas can easily become desicated resulting in
more settlement and damage than would otherwise be the case.
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Maintaining Lawn Sprinkler
Systems: There is nothing wrong with having a lawn sprinkler system.
Having stated that point, I must also point out that most of them are
installed in a way that presents a (potential) problem to the future performance
of the foundation. The two most
important problems with these systems are the following: |
| The supply lines are
usually placed to close to the foundation. These lines are prone to
leakage which can cause swelling of the soil near the areas that are leaking.
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| The control values are prone to leakage.
Since these valves are usually
very close to the foundation, they should be checked frequently for leakage.
Ideally, they should also be located at least 5-feet from the foundation. |
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Are there any practical
limitations on our ability to mitigate moisture changes in the soil?
It should be understood that, from a practical
perspective, there are several fundamental reasons why a homeowner, or an
engineer for that matter, has only a limited ability mitigate moisture changes
in the soil supporting the foundation. One
problem is clearly the weather.
Houston
weather varies from very wet to extended periods of dry weather and even
drought. While this can be
compensated for by watering the ground adjacent to the foundation, the reality
is that it is very difficult to assure uniform wetting of the soil to a
sufficient depth to be effective.
Also, there
is virtually nothing a homeowner can do about the movement of moisture from
lower soil strata to the soil near the surface supporting the foundation.
The fact is that engineers have a limited, even inadequate understanding,
of how moisture moves through the soil.
From our experience, most engineers practicing in this area believe that aggressive
moisture stabilization measures can cut foundation movement by around 50%.
That is a significant reduction, but it may take 6-months to 3-years to
obtain that result. It is important
to realize that aggressive moisture stabilization results in a wide range of
reduction of foundation movement. Some houses may get a much greater reduction
and some may get a much lower reduction.
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How is the need for structural
foundation repair established?
Unless foundation movement
results in a structural safety issue that can only be corrected by underpinning
the foundation, repairing the foundation structurally should be viewed as an
option, not a need. In the large
majority of situations where there is impaired foundation performance,
structural foundation repair should be viewed as an option that may or may not
be appropriate for a given situation. Deciding
to make structural repairs to a foundation should be made with a clear
understanding of the likely benefits and risks of the proposed repair.
For instance, there are a series
of questions that should be asked and answered before deciding to underpin a
foundation.
| What improvement in foundation
performance can realistically be expected from underpinning? If the damage is not severe, it may be better to make cosmetic repairs
to the house and take other appropriate non-structural remediation measures.
Underpinning is not normally effective or cost efficient unless the
foundation related damage to the house is severe.
No honest foundation repair contractor will guarantee that
underpinning will result in a level foundation.
In fact, studies have shown that the typical foundation repair job, where
the perimeter of foundation is underpinned, can become less level over time
as the central non-underpinned area settles during dry periods while the
perimeter is not allowed to settle due to the presence of piers or pilings.
Foundation repair contractors also cannot guarantee that the
underpinning will prevent future damage to the house.
There is always some residual risk of damage due to foundation
movement after the foundation is underpinned.
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| What means are available to
mitigate soil moisture changes?
Foundation movement in expansive soil areas is normally driven by
changes in moisture regime in the supporting soil.
Even if the foundation related damage to the house is severe,
underpinning is not likely to be effective in the long run unless the causes
of the changes in the moisture regime are removed.
There are several ways homeowners can mitigate the changes in the
soil moisture. They include
maintaining positive drainage away from the foundation.
The normal recommendation is to shape the surface of the ground
adjacent to the foundation so that it falls 6-inches in 10-feet as you move
away from the foundation. No
water should be allowed to discharge within 5-feet of the foundation.
This includes plumbing leaks, air conditioning condensate discharges,
and discharges of water from roof gutters.
Trees, especially hardwoods such as oaks, and large shrubs can remove
large amounts of water from the soil leading to excessive foundation
settlement, especially at the outside corners of the foundation.
All ground within 5-feet of the foundation should have some sort of
ground cover to prevent excessive drying.
Last, but not least, the ground adjacent to the foundation should be
watered when the weather is dry.
Unfortunately,
the migration of moisture through the soil is one of the least understood
aspects of expansive soils. We
know that, in at least some situations, soil moisture migrates from lower soil
strata to the soils supporting the foundation. This
moisture can become trapped under the slab-on-ground foundation resulting in a
center lift distortion mode. |
| What damage to the house is the
repair process likely to cause? Underpinning and grouting and mudjacking can result in unintended
damage the foundation and the house. The
damage is usually minor, but it can be significant.
The author has personally seen drywall cracks as wide as 1.25 inches
that were caused by underpinning. Underpinning
can result in portions of the foundation being forced to act as an
elevated structure instead of a ground-supported structure.
This can result in significantly higher bending moments in the slab
structure that it was intended to experience.
True structural failure can result.
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| Is the foundation movement
excessive and progressive?
Foundation movement is usually considered excessive if the foundation
is deflecting more than an inch for every 360 inches of span.
Thus, a 60-foot (720-inches) wall could defect 2-inches at each end; a
30-foot (360-inches) wall could deflect 1-inch at each end.
Foundation movement is considered progressive if it is clearly
growing over time. This can be
determined only by monitoring foundation movement over a period if time,
usually at least 6-months but possibly lasting as long as 2-years.
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In summary, the decision to underpin or
not to underpin a foundation is a judgment based on an understanding of the type and extent
of damage judged to be caused by foundation movement, the probable benefits and
risks of foundation repair and the costs involved.
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How are foundations
structurally repaired?
In most cases, foundations that
are structurally repaired are underpinned. This
is an engineering term that, in this context, refers to the process of providing
structural support under the existing slab-on-ground foundation.
Underpinning a slab-on-ground foundation is usually done with the intent
to lift some foundation areas to attempt to bring the slab surface to a closer
approximation to the as-constructed condition. The underpinning is also
intended to make the foundation perimeter less susceptible to future settlement.
The traditional way to underpin a slab-on-ground foundation is to place
reinforced concrete piers under the perimeter of the foundation.
The bottoms of the piers are belled out.
This spreads the load transferred to the soil and prevents the pier from
being pushed up when the soil swells.
If you visit the www.dawsonfoundationrepair.com website, you will find an excellent discussion of the
foundation repair process.
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How much improvement in
performance can reasonably be expected from foundation repair?
This is a question that is
difficult answer. Most people are surprised that no foundation repair
contractor actually promises to make their foundation work better than it did
prior to the repair work. All they warrant is that after the foundation
repair work the underpinned area will not settle more than a specified amount,
usually an inch of settlement for every 30-feet of underpinned span. The
repair work is almost always done without any definitive evidence that the
foundation has settled more than an inch for every 30-feet of span. Most
homeowners who underpin their foundation have no way of knowing if their
foundation has settled more than an inch for every 30-feet of span. Nor do
they normally have a rational basis for believing that the foundation will or
will not settle more than the warranted amount in the future absent any
underpinning.
In our experience, foundation
underpinning will usually reduce the future settlement by about half. In terms
that the lay person can relate to that generally means that the number of
drywall and stucco cracks will be cut in about half. The number of brick
veneer cracks will usually not change but the average width of the brick veneer
crack will usually be cut in half. The number of sticking doors may also
be cut in half.
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If I buy a house that shows
signs of foundation movement related damage, will I be able to sell the house?
Engineers and real estate
inspectors are in the business of informing potential buyers as to the condition
of the house, not its marketability. However,
there are a couple of points that bear on this question that can best be made by
an engineer or inspector:
| It is unreasonable to not expect some degree of damage
due to foundation movement in resale houses in the Greater Houston Area.
The combination of expansive soils, thin flexible slab-on-ground
foundations and wooded lots virtually guarantees that many houses will
experience some damage due to seasonal foundation movement.
This type of movement rarely affects the structural stability of the
frame structure of the house.
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| There is a wide range of opinions concerning how much
cosmetic damage is acceptable or not acceptable.
Some people find even hairline cracks unacceptable; others find
almost any crack width acceptable so long as there are no structural safety
or stability issues. If the
degree-of-damage shown by a house you are considering buying makes you
uncomfortable, you should probably not buy the house. |
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What are some examples of
foundations in need of concrete repair?
The following are two examples
of concrete repair that are sometimes required:
| Wedge or Corner Cracks:
These are cracks that are frequently formed within 12-inches of the outside
corners of the foundation. They result when the brick veneer expands
when it is heated by the sun. As the brick veneer expands it pushes
outward on the foundation. The frictional forces on the concrete
surface cause the foundation to crack at the corner. Wedge cracks or
corner cracks are not indicative of a structural performance issue.
They may, however, require concrete repair if the cracking is so bad that
the brick veneer begins to fail.
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| Exposed Cable Ends:
In post-tensioned slabs-on-ground, the ends of the post-tensioning cables
may become exposed. When this happens, the anchors that hold the
cables in place can become damaged due to corrosion. To prevent the
damaging corrosion, a concrete repair should be executed that will protect
the anchors. |
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What
is your philosophy of foundation mitigation and repair?
Our philosophy of foundation
evaluation can be summed in the following statements:
| Most houses that show distress due to
foundation movement do not warrant foundation repair.
The reason is that most damage due to foundation movement is
cosmetic. Making cosmetic repair is more economical than
underpinning the foundation and underpinning the foundation is not
likely to be effective in preventing future cosmetic damage.
In our opinion, foundation repair should be reserved for houses that show true
structural damage or severe cosmetic and/or functional damage.
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| The cause of the soil moisture changes
causing the damaging foundation movement should isolated.
This is the key to mitigating the damage to the house caused by the
moisture changes in the soil. The US Army Corps of Engineers
recommends that no underpinning be done until the causes of the soil
moisture changes have been corrected and some period of time
allowed to pass so the effect of the corrections can be evaluated.
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| The work may have to be repeated because of
a failure to isolate the cause of the moisture changes in the
foundation soil. It is not always obvious what the
source of the moisture changes in the supporting soil is. Any
homeowner should be prepared for the possibility that the search for
the source or sources of the moisture changes may take awhile.
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| Only one remedial measure at a time should
be attempted at a time so that its effect on the structure can be
evaluated. The best approach is usually to
attempt one remedial measure at a time. It is important to
allow some time to pass before attempting another remedial measure
so the effect of the first remedial measure can be evaluated.
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| The structure is seldom rebuilt to its
original condition. It is unrealistic to expect
that non-structural remedial measures or foundation
underpinning can make the house like new. Once a house is
damaged by foundation movement, it will always show some distortion.
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| Remedial measures may not be successful. Many
people naively believe that foundation underpinning will
"fix" the foundation. Nothing could be further from
the truth. The fact is that some houses cannot be
"fixed" regardless of any structural repairs or
non-structural remedial measures that are taken.
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| Be prepared to live with what you are
buying. Since it is possible that remedial measures
may not be successful, understand that of you buy the house it will
be yours and you will have to live with it. Do not purchase a
house that shows a level of foundation performance that you are not
comfortable with or are not willing to tolerate. |
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We believe the above approach is
consistent with the Texas Section ASCE publication Guidelines for the
Evaluation and Repair of Residential Foundations and with US Army Corps of Engineers
publication Foundations in Expansive Soils.
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