Foundation Movements Associated with Shrinkage of Highly Plastic Clay Soils
By James A. "Buck" Durham, P.E. Structural Engineer - Durham and Wehrman Engineering,
Phone 256-353-0504.
A home-buyer transferring into the Huntsville metropolitan area once asked me, "What
is the most common residential foundation problem that you see in your home
inspection business?" I answered, "Differential foundation settlements (movements)
due to expansive clay soils." "Not so much expansion," I explained, "but the opposite
actually-or what we commonly call "shrinkage."
The residential foundation problems are related to volume changes in the clay soils
which underlie the house. These particular clay soils have what engineers call a " high
plasticity". This is a term that is used to describe a clayey soil which remains "plastic"
(or in a "moldable" state), neither turning liquid nor crumbling apart, over a wide range
of moisture contents. This is unlike other fine-grained soils (mixtures of sands, silts
and clays) which either liquify or crumble apart at the extreme ends of a fairly small
range of plasticity (moisture contents.) Liquifying or fluid flow occurs at the wet end of
the moisture scale, whereas crumbling occurs at the dry end. Many of you might
compare a highly plastic clay soil to the popular child's toy, "Playdough®" in that highly
plastic clays can be molded into various shapes or, when worked in between the
thumb and index finger, one can form a long, thin "ribbon" of clay.
The reason why these soils remain plastic over such a wide range of moisture
contents, is because they have an "affinity" or attraction for water. Actually, they have
an attraction for the H2O water molecule. In order to understand this, you must
understand that unlike silts or sands, clay particles are microscopic in size. We cannot
see them with our naked eye. Walk along the beach and grab a handful of sand and you
can easily sort out individual sand particles. You can't do this with clay. The clay
particles (actually platelets) are microscopic in size. They are, in essence, ions or
molecules and they bond together (to one another). Most clay ions also chemically
react or interact with the H20 water molecule. They usually want to bring in (attach)
water molecules to their crystalline lattice (or molecular structure/chain).
Clay soils usually evolve from the weathering of minerals and, therefore, take on the
mineral's crystalline structure or chemical make-up. Typical clay minerals in our area
are -silicates of either iron, magnesium or aluminum. Some clay soil minerals have a
very high attraction for water. Instead of wanting to add one or two water molecules to
their structure, they want to add many more. These are the expansive clays. When this
happens, the individual clay ion's crystalline structure or lattice dramatically grows in
size. When all the millions and billions of clay ions/molecules underlying a foundation
footing absorb large numbers of water molecules, the soil dramatically grows in size. In
the real world, where we live, this volume change can be significant. The reverse is
also true. And here is where the foundation problems seem to arise. Whenever the
clay ions give up attached water molecules (from evaporation or plant root uptake)
they shrink in volume. Extreme examples of this type of shrinkage are the formation of
large cracks in the ground surface and/or the ground pulling away from a
home/foundation during the hot, dry summer months or during droughts. Examples of
foundation problems are differential settlements caused by shrinkage of the clay soil.
A simple analogy for this phenomena would be, "clay soils are like sponges".
Whenever sponges soak up water they swell in volume. When left on the counter to
dry, however, they shrink back down to a very small size...only to puff-up again when
placed into contact with water. Clay soils are also like sponges in that when they
become wet, they become soft or softer. When dry, they become very hard. Place a
heavy load on dry clay soil and it goes nowhere or settles very little. The ground will
support a sky-scraper! When wet or saturated however, don't even think about driving
a riding lawn mower across the ground. It will sink up to its axle.
Hence, when living in a house constructed on highly plastic clay soils (particularly
swelling clay soils) it is very important that the owner make sure that the drainage
conditions around the home are monitored and maintained annually, so that surface
water runoff always flows away from the home and never stands or ponds beside it. If
this ever occurs, and the ground alongside and beneath a home (foundation footing)
becomes wet or saturated during heavy rains or throughout the rainy season (due to
poor drainage conditions, overflowing gutters, lack of gutters, etc.), then the house
may slowly settle into the ground in the areas where the clay soil has become softest.
This, alone, can lead to differential (or uneven) foundation settlement. But another,
and often more serious problem occurs during the following summer or fall after the
hot, dry weather we typically experience in the North Alabama geographic area. The
soils that stayed extremely wet throughout the year, and swelled accordingly, shrunk
dramatically by the time July and August rolled around. The portion of the house
resting on this formerly wet ground also settles (dramatically) along with the shrinking
clay.
Believe it or not, however, this common problem can always be avoided during
foundation construction if builders would simply admit that these soil conditions exist
and recognize that they need to have each building site evaluated by a professional
soil scientist or geotechnical engineer prior to house construction. If not, it is likely
that the builder will utilize the standard shallow-bearing foundation, and place the
footings or foundation just below or within a few feet of the ground surface. Soil
shrinkage-related settlements or movements will eventually lead to foundation
settlement cracking and/or the formation of large cracks in brittle construction
materials such as brick veneer and sheetrock. This is almost a guaranteed.
It has been my experience that whenever an older home (say 15 to 20 years or more)
that is constructed on highly plastic clay suddenly experiences cracking in brick or
sheetrock, after many years of crack-free service, and there's no other obvious
answer to the problem, it's probably related to drying shrinkage of underlying
expansive clay subsoils. These subsoils probably remained wet each year, throughout
the life of the home. Or, at least, the moisture change that did occur each year in the
underlying massive clay soil was not sufficient to cause enough building distortion to,
in turn, cause building material cracking. If the home suddenly develops cracks,
however, there's probably been some major change in the drainage conditions or
landscape surrounding the home.
For example, if the house originally had gutters and these served their purpose for
many years, then, for some reason (frequent leaf/debris clogging hassles and
subsequent rusting deterioration?), the owner removes them, the moisture conditions
around the house suddenly change. My personal experience suggests that the effects
of water falling beside the home will eventually begin to take its toll by eroding more
and more surface soil-- thus exposing the foundation footing and/or the underlying
plastic clays to more and more solar radiation. At some time, say 20 years after
construction, the effects of normal summertime drying shrinkage may finally cause soil
shrinkage sufficient to create enough foundation distortion to bring about enough
building distortion to in turn cause sudden building material cracking, such as cracks
in brick or sheetrock.
Another problem that I've noticed over the past twelve years of inspecting houses, is
that if a particular part of the crawl space or foundation remains moist or under
standing water during most or part of the year, as evidenced by dark, water/algae
stains on the concrete or concrete block foundation system, then the adverse effects
of a summer drought are most pronounced on the underlying bearing soils in these
wet areas. In other words, these soils go from the extreme wet end of the moisture
scale to the dry end. Imagine: the larger the moisture change, the more the soil
shrinkage and, therefore, the greater potential for foundation settlement.
A third problem is related to large trees. If hardwoods are planted near a home soon
after construction, it is likely that in 20 to 30 years, the trees will overhang the
house-which means that their roots underlie the house. These roots are going to
contribute to soil desiccation during a drought by robbing the soil of all available
moisture. Hence, whenever a hardwood or pine tree overhangs a home/roof built on a
highly plastic clay soil site, the tree should probably be removed.
Regardless of trees and poor drainage conditions, if a house is initially built on a
shallow foundation system which bears on expansive clay soils, then the house will
likely move and distort from day one. Moreover, it is unlikely that this movement will
be uniform because the moisture content around and beneath the home will not be
uniform. It will vary because of different drainage conditions that exist, the varying
degree of solar exposure and the presence of large trees or shrubs. As stated, trees
are especially capable of desiccating (completely drying) soil during drought periods.
In time, the effects of uneven foundation movement, or differential foundation
movement/settlement, or simply building distortion, will take its toll on the home and
cracks will begin to form in the more brittle materials. The cracks form to relieve the
built-up stress of distortion.
What can be done to control the distortion?
The most economical way to deal with expansive clay soils underlying a home is to try
and maintain a uniform moisture content in the ground (clay soil) throughout the year.
This is best accomplished by letting nature takes its course throughout the wet winter,
when rainfall is greatest and evaporation is lowest -- then adding water via irrigation,
as required, during the dry periods of the year. Drip irrigation tubing laid alongside the
foundation footings, covered with thick beds of mulch, typically works well to do this.
By applying water slowly, via the drip tube, the ground can absorb water previously
lost or given up to evaporation and plant root uptake. One has to be careful, however,
not to over-water or saturate the ground because as stated, this can soften the
bearing soils and lead to foundation settlement. Any irrigation contractor should be
able to come up with an automated drip irrigation system. In other words, moisture
detectors can probably be buried below/beside the foundation and utilized to activate
(automate) a drip irrigation system using a conventional lawn irrigation control panel.
The buried sensors tell the irrigation control panel when to turn valves on and off. The
irrigation contractor should work closely with a professional soil scientist to determine
the range of soil moisture content readings that should be utilized to
activate/deactivate the irrigation system.
In some instances, when the amount of differential foundation movement has been so
severe that a foundation component has cracked and faulted, when doors and
windows start to bind, or when the brick veneer has cracked, faulted and/or pulled
away from the home, it might become necessary to underpin the distorted foundation
so that future movements do not make matters worse. Foundation underpinning can
also be used to uplift and/or relevel a settled (distorted) foundation.
Underpinning typically entails digging or drilling a vertical shaft into the ground,
alongside and beneath an isolated part of the foundation, then filling the excavation
with concrete. These shafts are usually spaced several feet apart so as to not
undermine the entire foundation, which would obviously lead to collapse. In some
instances the concrete is simply placed into the shaft until it contacts the footing, thus
preventing future or additional settlement. In other cases, the concrete placed into
the excavation is stopped a foot or more beneath the footing and allowed to harden or
cure for several days. Large hydraulic jacks are then placed beneath the footing, on
top of these concrete piers or shafts, and the footing is carefully jacked upward,
extending the jacks in proportion to the amount of settlement at each point. It's a real
art (and danger) to relevel a settled house/foundation, so this type of repair should be
left only to professionals.
A similar method of foundation repair entails the use of helical augers which are
turned or twisted into the ground and stopped whenever the driving torque reaches a
certain level -- indicating that the blades on the auger have reached a sufficient
bearing capacity with the deeper soil strata to provide a substantial amount of vertical
support. Steel brackets are fitted over the auger shaft and under the foundation. The
auger shaft is usually threaded, so large nuts placed below the bracket are turned in
order to lift the footing upward.
There are many firms which specialize in this type of repair: Foundation and Structural
Renovations (Huntsville), Hollis Kennedy House Moving (Athens), Don Kennedy House
Moving (Huntsville) and Grout Tech (Pelham) are just a few.
A conscientious effort has been made by the authors to provide accurate information;
however, neither the authors nor Alabama Residential Inspection Services, LLC will
assume any liability for its use. Readers are advised to perform additional research,
seek other professional advice, and to act on the information provided, herein, very
carefully.
By James A. "Buck" Durham, P.E. Structural Engineer - Durham and Wehrman Engineering,
Phone 256-353-0504.
A home-buyer transferring into the Huntsville metropolitan area once asked me, "What
is the most common residential foundation problem that you see in your home
inspection business?" I answered, "Differential foundation settlements (movements)
due to expansive clay soils." "Not so much expansion," I explained, "but the opposite
actually-or what we commonly call "shrinkage."
The residential foundation problems are related to volume changes in the clay soils
which underlie the house. These particular clay soils have what engineers call a " high
plasticity". This is a term that is used to describe a clayey soil which remains "plastic"
(or in a "moldable" state), neither turning liquid nor crumbling apart, over a wide range
of moisture contents. This is unlike other fine-grained soils (mixtures of sands, silts
and clays) which either liquify or crumble apart at the extreme ends of a fairly small
range of plasticity (moisture contents.) Liquifying or fluid flow occurs at the wet end of
the moisture scale, whereas crumbling occurs at the dry end. Many of you might
compare a highly plastic clay soil to the popular child's toy, "Playdough®" in that highly
plastic clays can be molded into various shapes or, when worked in between the
thumb and index finger, one can form a long, thin "ribbon" of clay.
The reason why these soils remain plastic over such a wide range of moisture
contents, is because they have an "affinity" or attraction for water. Actually, they have
an attraction for the H2O water molecule. In order to understand this, you must
understand that unlike silts or sands, clay particles are microscopic in size. We cannot
see them with our naked eye. Walk along the beach and grab a handful of sand and you
can easily sort out individual sand particles. You can't do this with clay. The clay
particles (actually platelets) are microscopic in size. They are, in essence, ions or
molecules and they bond together (to one another). Most clay ions also chemically
react or interact with the H20 water molecule. They usually want to bring in (attach)
water molecules to their crystalline lattice (or molecular structure/chain).
Clay soils usually evolve from the weathering of minerals and, therefore, take on the
mineral's crystalline structure or chemical make-up. Typical clay minerals in our area
are -silicates of either iron, magnesium or aluminum. Some clay soil minerals have a
very high attraction for water. Instead of wanting to add one or two water molecules to
their structure, they want to add many more. These are the expansive clays. When this
happens, the individual clay ion's crystalline structure or lattice dramatically grows in
size. When all the millions and billions of clay ions/molecules underlying a foundation
footing absorb large numbers of water molecules, the soil dramatically grows in size. In
the real world, where we live, this volume change can be significant. The reverse is
also true. And here is where the foundation problems seem to arise. Whenever the
clay ions give up attached water molecules (from evaporation or plant root uptake)
they shrink in volume. Extreme examples of this type of shrinkage are the formation of
large cracks in the ground surface and/or the ground pulling away from a
home/foundation during the hot, dry summer months or during droughts. Examples of
foundation problems are differential settlements caused by shrinkage of the clay soil.
A simple analogy for this phenomena would be, "clay soils are like sponges".
Whenever sponges soak up water they swell in volume. When left on the counter to
dry, however, they shrink back down to a very small size...only to puff-up again when
placed into contact with water. Clay soils are also like sponges in that when they
become wet, they become soft or softer. When dry, they become very hard. Place a
heavy load on dry clay soil and it goes nowhere or settles very little. The ground will
support a sky-scraper! When wet or saturated however, don't even think about driving
a riding lawn mower across the ground. It will sink up to its axle.
Hence, when living in a house constructed on highly plastic clay soils (particularly
swelling clay soils) it is very important that the owner make sure that the drainage
conditions around the home are monitored and maintained annually, so that surface
water runoff always flows away from the home and never stands or ponds beside it. If
this ever occurs, and the ground alongside and beneath a home (foundation footing)
becomes wet or saturated during heavy rains or throughout the rainy season (due to
poor drainage conditions, overflowing gutters, lack of gutters, etc.), then the house
may slowly settle into the ground in the areas where the clay soil has become softest.
This, alone, can lead to differential (or uneven) foundation settlement. But another,
and often more serious problem occurs during the following summer or fall after the
hot, dry weather we typically experience in the North Alabama geographic area. The
soils that stayed extremely wet throughout the year, and swelled accordingly, shrunk
dramatically by the time July and August rolled around. The portion of the house
resting on this formerly wet ground also settles (dramatically) along with the shrinking
clay.
Believe it or not, however, this common problem can always be avoided during
foundation construction if builders would simply admit that these soil conditions exist
and recognize that they need to have each building site evaluated by a professional
soil scientist or geotechnical engineer prior to house construction. If not, it is likely
that the builder will utilize the standard shallow-bearing foundation, and place the
footings or foundation just below or within a few feet of the ground surface. Soil
shrinkage-related settlements or movements will eventually lead to foundation
settlement cracking and/or the formation of large cracks in brittle construction
materials such as brick veneer and sheetrock. This is almost a guaranteed.
It has been my experience that whenever an older home (say 15 to 20 years or more)
that is constructed on highly plastic clay suddenly experiences cracking in brick or
sheetrock, after many years of crack-free service, and there's no other obvious
answer to the problem, it's probably related to drying shrinkage of underlying
expansive clay subsoils. These subsoils probably remained wet each year, throughout
the life of the home. Or, at least, the moisture change that did occur each year in the
underlying massive clay soil was not sufficient to cause enough building distortion to,
in turn, cause building material cracking. If the home suddenly develops cracks,
however, there's probably been some major change in the drainage conditions or
landscape surrounding the home.
For example, if the house originally had gutters and these served their purpose for
many years, then, for some reason (frequent leaf/debris clogging hassles and
subsequent rusting deterioration?), the owner removes them, the moisture conditions
around the house suddenly change. My personal experience suggests that the effects
of water falling beside the home will eventually begin to take its toll by eroding more
and more surface soil-- thus exposing the foundation footing and/or the underlying
plastic clays to more and more solar radiation. At some time, say 20 years after
construction, the effects of normal summertime drying shrinkage may finally cause soil
shrinkage sufficient to create enough foundation distortion to bring about enough
building distortion to in turn cause sudden building material cracking, such as cracks
in brick or sheetrock.
Another problem that I've noticed over the past twelve years of inspecting houses, is
that if a particular part of the crawl space or foundation remains moist or under
standing water during most or part of the year, as evidenced by dark, water/algae
stains on the concrete or concrete block foundation system, then the adverse effects
of a summer drought are most pronounced on the underlying bearing soils in these
wet areas. In other words, these soils go from the extreme wet end of the moisture
scale to the dry end. Imagine: the larger the moisture change, the more the soil
shrinkage and, therefore, the greater potential for foundation settlement.
A third problem is related to large trees. If hardwoods are planted near a home soon
after construction, it is likely that in 20 to 30 years, the trees will overhang the
house-which means that their roots underlie the house. These roots are going to
contribute to soil desiccation during a drought by robbing the soil of all available
moisture. Hence, whenever a hardwood or pine tree overhangs a home/roof built on a
highly plastic clay soil site, the tree should probably be removed.
Regardless of trees and poor drainage conditions, if a house is initially built on a
shallow foundation system which bears on expansive clay soils, then the house will
likely move and distort from day one. Moreover, it is unlikely that this movement will
be uniform because the moisture content around and beneath the home will not be
uniform. It will vary because of different drainage conditions that exist, the varying
degree of solar exposure and the presence of large trees or shrubs. As stated, trees
are especially capable of desiccating (completely drying) soil during drought periods.
In time, the effects of uneven foundation movement, or differential foundation
movement/settlement, or simply building distortion, will take its toll on the home and
cracks will begin to form in the more brittle materials. The cracks form to relieve the
built-up stress of distortion.
What can be done to control the distortion?
The most economical way to deal with expansive clay soils underlying a home is to try
and maintain a uniform moisture content in the ground (clay soil) throughout the year.
This is best accomplished by letting nature takes its course throughout the wet winter,
when rainfall is greatest and evaporation is lowest -- then adding water via irrigation,
as required, during the dry periods of the year. Drip irrigation tubing laid alongside the
foundation footings, covered with thick beds of mulch, typically works well to do this.
By applying water slowly, via the drip tube, the ground can absorb water previously
lost or given up to evaporation and plant root uptake. One has to be careful, however,
not to over-water or saturate the ground because as stated, this can soften the
bearing soils and lead to foundation settlement. Any irrigation contractor should be
able to come up with an automated drip irrigation system. In other words, moisture
detectors can probably be buried below/beside the foundation and utilized to activate
(automate) a drip irrigation system using a conventional lawn irrigation control panel.
The buried sensors tell the irrigation control panel when to turn valves on and off. The
irrigation contractor should work closely with a professional soil scientist to determine
the range of soil moisture content readings that should be utilized to
activate/deactivate the irrigation system.
In some instances, when the amount of differential foundation movement has been so
severe that a foundation component has cracked and faulted, when doors and
windows start to bind, or when the brick veneer has cracked, faulted and/or pulled
away from the home, it might become necessary to underpin the distorted foundation
so that future movements do not make matters worse. Foundation underpinning can
also be used to uplift and/or relevel a settled (distorted) foundation.
Underpinning typically entails digging or drilling a vertical shaft into the ground,
alongside and beneath an isolated part of the foundation, then filling the excavation
with concrete. These shafts are usually spaced several feet apart so as to not
undermine the entire foundation, which would obviously lead to collapse. In some
instances the concrete is simply placed into the shaft until it contacts the footing, thus
preventing future or additional settlement. In other cases, the concrete placed into
the excavation is stopped a foot or more beneath the footing and allowed to harden or
cure for several days. Large hydraulic jacks are then placed beneath the footing, on
top of these concrete piers or shafts, and the footing is carefully jacked upward,
extending the jacks in proportion to the amount of settlement at each point. It's a real
art (and danger) to relevel a settled house/foundation, so this type of repair should be
left only to professionals.
A similar method of foundation repair entails the use of helical augers which are
turned or twisted into the ground and stopped whenever the driving torque reaches a
certain level -- indicating that the blades on the auger have reached a sufficient
bearing capacity with the deeper soil strata to provide a substantial amount of vertical
support. Steel brackets are fitted over the auger shaft and under the foundation. The
auger shaft is usually threaded, so large nuts placed below the bracket are turned in
order to lift the footing upward.
There are many firms which specialize in this type of repair: Foundation and Structural
Renovations (Huntsville), Hollis Kennedy House Moving (Athens), Don Kennedy House
Moving (Huntsville) and Grout Tech (Pelham) are just a few.
A conscientious effort has been made by the authors to provide accurate information;
however, neither the authors nor Alabama Residential Inspection Services, LLC will
assume any liability for its use. Readers are advised to perform additional research,
seek other professional advice, and to act on the information provided, herein, very
carefully.
Foundation Movement
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