Foundation Movements Associated with Shrinkage of Highly Plastic Clay Soils
by James A. "Buck" Durham, P.E.
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.
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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