| ACI; an acronym for the American Concrete Institute; this is
the engineering association for the design and construction of
structural concrete; since slab-on-ground foundations are not structural
concrete foundations their publications are not generally specifically
directed to slab-on-ground design or construction; the leading
engineering organization that publishes technical guidelines and
specifications regarding slab-on-ground construction is the
Post-Tensioning Institute or PTI |
| |
| active area; the active area is the area under a
slab-on-ground foundation that extends from the edge of the slab to a
line approximately 8-feet to the interior of the slab; the soil
supporting this area of the foundation is subject to shrinking and swelling
due to seasonal variations in soil moisture content; the area further to
the interior of the slab is called the dormant area as it is usually not
significantly affected by soil distortion due to seasonal variations in soil moisture
content |
|
|
|
| active crack; when used in relation to
slab-on-ground construction, the term active crack refers to a crack that is
visibly growing in width or length |
|
| active soil; a soil that expands or swells in apparent volume
when it becomes wet and shrinks when it dries; the same as “expansive
soil” |
| |
| active soil zone; this is the depth to which seasonal changes in
the moisture content of the soil occur; the deeper the soil, the less
change in soil moisture content occurs as a result of seasonal moisture
changes; the specific depth for a given site depends in part on the
engineering characteristics of the soil at the site |
| |
| adequate watering; during dry periods the soil around the perimeter
should be watered with enough water to replace water that is lost from
evaporation and transpiration; such watering is adequate watering |
| |
| backfill; with respect to slab-on-ground foundations the
term “backfill” is usually used to refer to the soil that is removed
from the foundation pad to form the trenches for the stiffening beams;
much of this material typically winds up laying against the forms; in an
expansive soil area the backfill material is normally clay; clay soil is
usually very lumpy and hard to compact; if the backfill is not
compacted, it can allow water to percolate to the bottom of the
perimeter stiffening beam where it can result in unnecessary foundation
movement |
|
| bearing capacity of soil; the pressure that a soil sample can sustain
without failing |
| |
| clay; clay soil particles are very the smallest soil
particles and are capable of large changes in apparent volume when
exposed to moisture; some clays will measurably change volume due merely
to changes in the relative humidity of the air |
| |
| cohesive soil; soil
in which the individual soil particles adhere to each other; clay is a
cohesive soil; sand is a non-cohesive soil
|
| |
| construction tolerance; with respect to the levelness of a slab-on-ground
foundation surface, standard ACI construction tolerances are given in
terms of what are called Face numbers or F numbers; the local (worst
case) Face number implies a construction tolerance of
in terms of slope of 1.25 inches over 10-feet; slab-on-ground
foundation surface construction tolerances are not used as a design or
construction control in residential construction; the consensus in the
engineering community seems to be that the surface levelness of most slab-on-ground
foundations are well within
the published ACI construction tolerances, but since surface levelness
is not used as a construction control, it is quite possible for a
slab-on-ground foundation to be constructed out of tolerance |
| |
| deflection curve; engineers use mathematical models to understand
engineering phenomena such as the bending of beams; a deflection curve
is a mathematical description the change in shape a beam undergoes when
it is deflected |
| |
| deflection surface; engineers use mathematical models to understand
engineering phenomena such as the bending of plates, a deflection
surface curve
is a mathematical description the change in shape a plate undergoes when
it is deflected |
| |
| differential settlement; differential settlement refers to the a situation
in which the slab-on-ground foundation does not settle uniformly; when
differential settlement occurs, some portions of the foundation settle
more than other portions; differential settlement is the cause of most
distress caused by foundation movement seen in the Greater Houston Area |
| |
| distortion; any post-construction change in shape
such as in the shape of a foundation |
| |
| distortion mode: slab-on-ground foundations exhibit
characteristic distortion modes; the distortion modes are a function of
the soil characteristics and the geometry of the foundation; two common
distortion modes are called "edge lift" and "edge
fall"; the distortion of the slab can cause distinctive patterns of
distress in the house; the patterns of distress are a function of the
distortion mode of the slab, the building geometry and the engineering
characteristics of the building materials |
|
| dormant area; the central area of a slab-on-ground foundation
is called the dormant area because the supporting soil in this area
usually experiences little or no seasonal changes in moisture content |
|
| edge fall; sometimes described in engineering
literature as "center lift", edge fall is a characteristic distortion mode for slab-on-ground
foundations on expansive soils where the perimeter of the foundation
falls relative to the central area of the foundation, usually due the
drying of the supporting soil around the perimeter or edge of the
foundation |
|
| edge lift; a recognized distortion mode for slab-on-ground
foundations on expansive soils where the perimeter of the foundation
lifts up relative to the central area of the foundation, usually due the
wetting up of the supporting soil around the perimeter or edge of the
foundation |
|
| edge moisture variation distance; the soil under the slab is not directly affected
by the weather; during rainy weather, the ground around the perimeter of
the slab will be wetter than the soil under the edge of the slab and
where the weather is dry, the ground around the perimeter of the slab
will be dryer than the soil under the edge of the slab; in wet weather
soil moisture moves from the soil adjacent to the slab to the soil under
the edge of the slab and in dry weather soil moisture moves from under
the edge of the slab to the soil adjacent to the slab; the distance
under the slab over which the moisture moves is called he edge moisture
variation distance; the two fundamental types of slab distortion occur
due to movement of moisture over the edge moisture variation distance |
|
| elevated slab; an elevated slab is a slab structure that does
not rely on ground support; an example would be a bridge |
|
| elevation survey; the term "elevation
survey" refers to a survey that shows the elevation of various
locations of the slab surface or, in the case of a finished house, the
finish floor; the use of a single elevation survey is problematic at
best and can be very misleading; elevation surveys taken at different
times, say 6-months apart, can be very useful in foundation performance
evaluation |
|
| elevations; measurements of the relative heights or
elevations, of a finish floor surface; if it can be assumed that a
slab-on-ground foundation was cast flat and level such measurements are
a reliable basis for judging the post-construction deformation of the
foundation; absent the ability to verify such an assumption, a single
elevation survey may be more misleading than helpful |
|
| expansive soil; a soil that expands or swells in apparent volume
when it becomes wet and shrinks when it dries; the same as “active
soil” |
|
| foundation; a slab-on-ground foundation consists of the
concrete slab cast integrally with the concrete stiffening beams and the
soil in which and on which the foundation sits |
|
| foundation failure; most engineers are hesitant to use the term
“foundation failure” without a precise definition; normally in
structural engineering the term failure is confined to situations where
an elevated structure fractures leading to significant loss of load
transfer capability or collapse; since slab-on-ground foundations are
ground-supported structures they do not transfer loads and cannot
collapse; sometimes the term “foundation failure” is used to mean
substandard performance of the foundation |
|
| foundation pad; the soil in which and on which the slab-on-ground
foundation sits |
|
| foundation performance; the structural function of a slab-on-ground
foundation is to act as a buffer that mitigates the differential
distortions between the supporting soil and house supported on the
foundation; slab-on-ground foundations do this by resisting the
moisture-induced distortion of the supporting soil and by spanning over
moisture-induced distortions in the supporting soil; the performance of
a slab-on-ground foundation is normally judged by how well the
foundation mitigates distortions in the house structure such as drywall
cracking, brick veneer cracking and door frame distortion |
|
| geotechnical engineer; geotechnical engineers
specialize in soil
mechanics; geotechnical engineers perform engineering tests to determine
the engineering properties of soils to be used to support slab-on-ground
foundations; an older term is soils engineer |
|
| grade; the slope of the ground surface adjacent to the
foundation; the grade of the soil adjacent to a slab-on-ground
foundation should slope away from the foundation so that the ground
surface drops 6-inches over 10-feet |
|
| ground-supported; a ground-supported structure such as a
slab-on-ground foundation is intended to be continuously supported by
the ground; ground-supported structure is distinguished from elevated
structures such as a bridge; elevated structures can fail by collapsing
while ground-supported structures cannot collapse |
|
| gumbo; a
local term for a highly plastic clay found mostly south of I-10; the
gumbo south of I-10 is frequently referred to as “black gumbo” since
it is usually black in color |
|
| heave potential; a measure of how much as soil sample will swell
when exposed to moisture |
|
| licensed engineer; a licensed engineer is
licensed by the Texas Board of Professional Engineers to practice the
profession of engineering |
|
| licensed real estate inspector; a real estate inspector that is licensed by the
Texas Real Estate Commission (TREC) |
|
| liquid limit; the liquid limit is a measure of how much
moisture a clay soil can take on before the soil behaves like a liquid;
the liquid limit is determined by standard geotechnical engineering
tests performed in a laboratory; the valve determined should be
understood as an approximation since the engineer must make a subjective
judgment as to at what point the clay soil behaves like a liquid |
|
| mudjacking; the process of pumping a slurry (usually a water
and soil/cement mix) under the slab in order to cause the slab to lift;
the process must be done carefully by experienced technicians under
qualified supervision to avoid damage to the foundation |
|
| noncohesive soil; a soil in which the soil particles do not stick
together such as sand |
|
| perched water table; a perched water table is a lens of water that is
trapped by an impervious soil strata such as a clay above the true water
table; the impervious soil strata prevents the perched water from
percolating downward to the actual water table; shallow perched water
tables are common north of I-10 in the Greater Houston Area and can play
havoc with slab-on-ground foundations |
|
| plastic limit; the plastic limit is a measure of how much
moisture a clay soil can take on before the soil becomes moldable like a
plastic; the plastic limit is determined by standard geotechnical
engineering tests performed in a laboratory; the valve determined should
be understood as an approximation since the engineer must make a
subjective judgment as to at what point the clay soil behaves like a
liquid |
|
| plasticity index; the plasticity index, or PI is an engineering
terms that gives an indication of how expansive a soil is; expansive
soils will usually have a PI of 20 (some engineers consider a soil to be
expansive if it has a PI of 10 or more); in the Greater Houston Area PIs
in the 40s, 50s and even 60s are not uncommon south of I-10; a soil with
a PI of 40 to 60 is considered to be highly expansive; any stated
plasticity index should be understood as an approximation since it is
calculated from the liquid limit and the plastic limit and these values
always rest is part of the subjective engineering judgment of the
engineer making the tests |
|
| PTI; an acronym for the Post-Tensioning Institute: this is
the leading engineering association for the design and construction of
slab-on-ground foundations |
|
| practice of engineering; the practice of
engineering is legally defined as "the performance of or an offer
to perform any public or private service or creative work the adequate
performance of which requires engineering education, training or
experience in applying special knowledge or judgment of the
mathematical, physical, or engineering sciences to that service or
creative work"; with few exceptions only licensed engineers are
allowed to engage in the practice of engineering |
|
| professional engineer; another term for licensed engineer |
|
| registered engineer; another term for licensed engineer |
|
| sand; sands are relatively large soil particles; sands
make good, stable foundations as long as they
are fairly dense |
|
| settlement; the drop of some portion of the foundation below
the original as-constructed elevation; expansive soils can cause
settlement when they become dry, shrink and pull away from the
foundation |
|
| silt; silts are the smallest noncohesisve soil
particles; silty soils do not make good foundations |
|
| slab-on-ground foundation; a reinforced slab that rests directly on the
ground; slab-on-ground foundations are ground-supported slabs as opposed
to elevated slabs |
|
| soil; soil is the loose material in the Earth’s
crust; soil consists of water, air and solid particles that have been
formed over time by the weathering and disintegration of rocks; the
solid particles in soil consist of sands, clays and silts; the coastal
plain on which the Greater Houston Area sits is part of a wedge of
sediment that dumped to the Gulf of Mexico by the Trinity and San
Jacinto Rivers over the past few million years; the sediment that is our
soil comes ultimately from the Rocky Mountains and other interior areas
of North America |
|
| soil refusal; as used in the residential foundation repair
business, soil refusal refers to a situation where segmented piles have
been driven using the weight of the foundation
driving force will not penetrate any further into the ground;
soil refusal is indicated when the slab lifts up instead of the pile
penetrating deeper |
|
| Standards of Practice for Real Estate Inspectors; a
standard of practice for real estate inspectors published by the Texas
Real Estate Commission |
|
| structural damage; the term structural damage, as normally used by a
structural engineer, refers to a situation where a load-carrying
structural member is broken or otherwise damaged so that it cannot carry
or transmit the loads it was intended to carry |
|
| structural engineer; a licensed engineer who is qualified by
education, training and experience to design structures; everyday
examples of structures designed by structural engineers are bridges,
flag poles, beams, columns, buildings and foundations |
|
| transpiration; the removal of soil moisture by vegetation; adult
oak trees can remove 150 to 250-gallons of water per day from the soil |
|
| TREC; an acronym for the Texas Real Estate
Commission; TREC licenses and regulates real estate or home inspectors
in the state of Texas; Licensed Real Estate Inspectors are required to
adhere to the TREC Standards of Practice for Real Estate Inspectors |
|
| underpinning; in the context of slab-on-ground foundation
repair, underpinning refers to the use of piers and pilings to provide
additional support to the concrete slab; underpinning is not only used
for residential foundations but other foundations as well – for
example, the White House foundation has been extensively underpinned |
|
| upheaval; upheaval refers to a situation where swelling
soil lifts the foundation or a portion of it upward; in an expansive
soil situation upheaval is almost always caused by excess moisture |
|
| USACE; see US Army Corps of Engineers below |
|
| US Army Corps of Engineers; the US Army Corps of
Engineers (USACE) is the largest public engineering institution in the
world and arguably one of the most proficient engineering organizations
in existence; the office of Chief Engineer was established by the
Continental Congress in 1775 to construct military fortifications;
virtually all the employees are civilians (34,600 versus 650 military);
the USACE has published numerous publications on expansive soil problems
including the full length book, Foundations in Expansive Soils, which
can be downloaded from the download page of this website; Chapter 9 of
this publication is particularly informative; an extensively annotated
version Chapter 9 can also be downloaded from this website |
|
| void ratio; soil can be thought of as consisting of particles
of soil, such as sand, silt and clay particles, water and air; the void
ratio is a basic soil property that is a ratio of the volume of the
water and air in the soil to the volume taken up by the soil particles |
|
| water leaks; when used in the context of slab-on-ground
foundation performance evaluations, the term “water leaks” refers to
water from any domestic source that can accumulate under or very near
the foundation; water leaks can lead to localized upheaval |
|
| water table; the depth where the soil is saturated in a
permeable soil or rock |
