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Foundation repair/remediation contractors usually provide some form of underpinning as one of their services to repair a failed
foundation. While underpinning is critical to the repair of foundations, it is also crucial that customers, engineers, remodeling contractors
and the general public understand the purpose of underpinning, or foundation shoring, and the limitations of same.
DEFINITION OF UNDERPINNING
Underpinning is the process of modifying an existing foundation system by extending it to or into subsurface strata that is deeper and
more stable than the near surface soil that sup- ports the existing foundation system. This is done to provide vertical support that is not
present in the existing design. Methods of underpinning include the construction of footings, stem walls, driven piling or drilled piers.
PURPOSE OF UNDERPINNING
Many of the houses that forensic engineers and repair contractors are asked to evaluate were constructed with foundations that are
inadequate for the conditions existing on site. Because of the lack of suitable land, homes are often built on marginal land that has
insufficient bearing capacity to support the substantial weight of a structure. In addition, there are many areas of the country where the
near surface soils consist predominantly of expansive clays that shrink and swell as their moisture content changes. As a result,
underpinning is required to extend the foundation support to depths that provide greater bearing capacity and/or are less affected by
climate, soil conditions and/or homeowner's actions. This underpinning, if properly designed and installed, provides the basis to lift the
structure to a more acceptable elevation and provides vertical support to prevent the underpinned area from settling.
In many areas of the country, house foundations consist, wholly or in part, of concrete slabs supported directly by the soil. In some
instances, the slab portion forms the ground or basement floor, which is structurally independent from the perimeter foundation. In other
cases, a similar soil supported floor slab rests on top of, and is partially supported by, the perimeter foundation. In Texas, slabs are
generally cast monolithically with perimeter as well as interior beams that are designed to provide sufficient support for the entire
structure as well as to provide stiffness to resist differential soil movement enough to limit cracking in the foundation and finishes. Texas
slabs are typically reinforced with conventional reinforcing steel (re-bar) and/or post-tensioned cables that are installed throughout both
the slab and beam portions of the foundation.
LIMITATIONS OF SLAB-ON-GRADE FOUNDATIONS
Settlement As A Result Of Poor Pre-Construction Compaction:
Slab-on-grade foundations depend upon the uppermost soil layer( s) to provide sufficient bearing capacity to support the structure and
to keep the foundation stable. If the bearing soil was insufficiently compacted prior to construction, the foundation is subject to
settlement as the supporting soil consolidates.
Settlement As A Result of Poor Pre-Construction Compaction
Foundation Movement Resulting From Soil Moisture Changes
CAUSES AND MODES OF FOUNDATION MOVEMENT
Movement As A Result Of Seasonal Moisture Changes
As mentioned above, foundations that are built directly on expansive soils that are subjected to non-uniform changes in the soil moisture
content can suffer from differential movement. During extended periods of dry weather, the expansive supporting soil shrinks causing
foundation settlement. During extended periods of wet weather, the expansive supporting soil swells causing upward movement of the
foundation (upheaval). Localized site and environmental factors that promote or limit the flow of water into and out of the supporting soil
as well as non-uniform distribution of the expansive soil under the foundation affect the magnitudes of the movement ( either upward or
downward) at different locations of the foundation. It is important to understand that it is differential, not the total movement of the
foundation that causes damage to the structure. In other words, the performance of a foundation that moves up and down uniformly with
the changing seasons is superior to a foundation where the movement is not uniform.
Slab/Foundation Movement Caused By Plumbing Leaks
A slab-on-grade foundation acts as a vapor barrier by resisting soil moisture variations due to evaporative moisture loss and by
shielding the under-slab soil from rainfall. Under optimum conditions, the soil moisture under the slab will achieve a degree of
equilibrium. When a plumbing leak occurs under a slab, the moisture equilibrium is distorted. As moisture is added to the soil from the
leak, soil and foundation movement often result. The type and degree of movement depends upon soil type and expansiveness, soil
density, soil moisture content prior to the leak, the length of time over which the leak has occurred, the quantity of moisture being added
to the soil over a given period of time and a few other factors.
Upheaval to a Slab-on-Grade Caused by a Plumbing Leak
1. If the soils are expansive and were dryer than optimum prior to the leak and have a high density, the foundation/slab will heave (move
upward) in the vicinity of the leak and corresponding damages will be apparent in the structure. In this example, the soil will probably not
contract significantly after the leak is repaired, which will result in a permanent dome in the slab.
2. If the soil is at optimum density and moisture prior to the leak, there is an opportunity for the soil to contract. It is possible, but not
probable, that the slab will regain its original elevation profile because clay soil expansion/ contraction generally does not follow a linear
progression as moisture is added and then reduced. The slab could be permanently left above or below its initial elevation.
3. Should a leak occur under the slab where the soil is of very low density, the additional moisture often lubricates the solid clay particles
and causes consolidation of the support soil prior to leak repair. After the leak is repaired under this example, the slab will often "dish" or
settle (move downward) even more.
NOTE: Concrete and steel will often develop a "stress memory" after deformation that will not allow the slab to return to its original
shape. This may be the result of soil or concrete chips filling cracks in the slab, which prevents the slab from "coming back together"
completely. In a post-tensioned slab, stress in the post-tensioning cables may resist the tendency for the slab to move back into place.
In a conventionally reinforced slab, permanent deformation (yielding) of the steel reinforcing bars may prevent the slab from returning to
its original shape.
Foundation Upheaval Caused by Negative or Poor Drainage
Since additional moisture can cause expansive soils to swell, areas of poor drainage near the foundation can cause the soil under the
foundation nearby to swell, resulting in upward movement of the foundation.
LIMITATIONS OF PIER AND BEAM FOUNDATIONS
Foundation Upheaval Caused by Poor Drainage
Although pier and beam foundation systems, if properly designed and constructed, will provide protection against settlement; the
potential for foundation upheaval due to poor drainage is sometimes present. If, for instance, the perimeter and/or interior grade beams
were constructed upon expansive clay soil, without providing avoid under the beam for soil expansion, swelling of the underlying soil may
push the beam upward. Swelling soil can also push the supporting piles or piers upward, if they are not designed and constructed to
adequately resist uplift. As a result, the grade beam will lift causing differential movement and subsequent cosmetic, and potentially
structural, damage. It is, therefore, a good idea to maintain adequate drainage away from any type of foundation, especially where
expansive soils are present.
Foundation Settlement Caused by Inadequate Pier Depth
The piers supporting many older pier and beam foundations may not extend below the zone of expansive soil that is affected by the
climate. During periods of dry weather, these shallow piers may not provide sufficient support to portions of the foundation, which may
result in differential settlement.
LIMITATIONS OF UNDERPINNING
Movement Outside of the Underpinned Area
If a single area of a foundation is underpinned, only that area will resist downward movement. For example, if only one corner of the
foundation is supported by piers, only that corner will resist settlement forces. The rest of the structure will be subject to seasonal
settlement as clay soils shrink during dry periods. Therefore, an area that was originally the low portion of the foundation may now
become the high point of the house during dry periods. It is also possible that cracking will occur at the last pier if the unsupported area
settles and is resisted at this hinge point. It is, therefore, important to carefully evaluate and balance site risk factors against cost
savings when electing to partially underpin a foundation.
When underpinning is installed to a stratum that is competent and capable of supporting the structure, it will stop downward movement
of the area of the foundation that is supported. Underpinning is generally not designed to keep the foundation from moving upward if the
original support clays swell due to an increase in moisture. Plumbing leaks, negative drainage and/or acts of man or nature, can
increase the moisture content of the bearing clays. Subsequent upward movement will often occur, which will result in a distorted
foundation and cracking in the finishes
Interior Floor Instability As A Result Of lnterior Settlement and/or Perimeter Upheaval
It is possible that cracks may occur at doors that are perpendicular to the perimeter walls as the interior slab settles from shrinking of
clay soils. Where the wall is tied in to the ceiling and roof structure, a separation can occur between wall and floor. When the floor is
secured to the slab, there may be separations between wall and ceiling. In the case of a pier and beam foundation, the wood floor can
appear "bouncy" as a result of the floor beams being lifted off the interior piers in response to perimeter upheaval.
Damage From Vegetation
Trees, bushes and other vegetation will draw moisture from under the foundation during times of drought. If the perimeter of a
slab-on-grade foundation is underpinned and trees withdraw moisture from under the slab, the interior bearing soil will dry and shrink in
volume. As a result, the interior slab may settle and cracking will likely occur in the interior of the home. It is also possible that tree roots
under a slab will grow large enough to push the slab upward.
Point Of Contact
Underpinning is only as good as the contact or connection point between pier/pile and the structure. If the grade beam, thickened slab,
or steel beam support is faulty, pier support will not be fully transferred to the foundation and downward movement may occur.
Successful underpinning requires proper design and proper installation of piers/piles. However, even the best design and installation
may not ensure a "permanent fix". Generally, when the underlying soil moves, the structure resting upon the soil will move
correspondingly. Special attention must be given to maintaining a consistent soil moisture level around and beneath the foundation slab,
especially in areas with highly expansive clay soils and/or environmentally induced swings in moisture availability.
Shallower soils are also generally the most
affected by seasonal moisture changes. If the
subjected to changes in moisture content,
differential foundation movement can occur if
wetting and drying of the clays does not occur
uniformly across the entire slab. This differential
movement can result in "dishing" or "doming" of
the foundation, and can become quite
pronounced, especially in areas where the local
climatic conditions include extended seasonal
periods of both hot, dry weather and cooler,
wetter weather. In either case (consolidation or
differential shrink/swell movement), inadequate
design and/or construction of the foundation can
result in unacceptable-performance of the
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