SUMMARY:
When a homeowner, buyer or seller faces foundation
problems with the structure, it is common to take estimates from two
foundation repair companies. But how does one make a proper comparison
and know what's best? Foundation repair contrator companies like to
recommend what is best for THEM, and that may not be what's best for
the structure. What I, Cecil Smith do, is provide unbiased LICENSED
ENGINEERS RECOMMENDATIONS as to which piering system is best or how
much piering is actually needed or if another solution woud be better.
Engineers are
not as precise as accountants. In the life of a structure the loads
may change, weather and other external variations occur and there
may be construction difficulties. For this reason engineers use a
FACTOR OF SAFETY (insurance). As an example - the the failure stress
of the reinforcing steel in concrete is 60,000 pounds per square inch.
In design, the structure is specified so that the maximum stress in
the steel is 24,000 psi. Hence the Factor of Safety is 60000/24000
or 2.5. Foundation leveling procedures are: (1) Press down piers where
the maximum resistance force availableis the weight of the foundation
structure, and (2) Study of the soil properties at the site and calculations
of the resistance of a drilled concrete pier with specified diameter
and length of embedment to Factor of Safety.
The
steel or concrete piers requuire first a bracket placed under
the grade beam of the foundation. Piers are then jacked down
against the bracket to whatever depth produces a rise in the
foundation. This capacity is no greater than the present resistance
of the foundation.
To an engineer the FS is therefore only 1.00 and cannot handle
any variation from the conditions on the day at which it was
installed.
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Drilled
piers require one or two exploratory boring to probable depth
of piers in order to obtain disturbed samples for soil particle
characteristic, activity and moisture content. Also undisturbed
samples are used to test for density, and strength. With these
data can be determined the side shear ability of the shaft and
the end bearing capacity of the bottom of the pier. Side shear
and end bearing values are mobilized because of the wet concrete's
full interaction to the sides and bottom of the rough drilled
hole. Using this data the required diameter and depth of the hole
can be calculated by the equations of soil mechanics to support
that section of the foundation with a required factor of safety.
Reinforcing steel and concrete of specified strength are inserted
in the hole; a jacking cap is added and the concrete allowed to
strengthen to the required value. These characteristics can be
included in the report At strength the structure is jacked to
the final elevations and shimmed into place. |
Drilled
piers require a boring under the foundation of the size and
depth determined from actual data at the site. In the hole reinforcing
rodas are placed and wet concrete poured down to in this case
FULL contect with the sides of the hole and the bottom After
this concrete is set the cap is placed and jacking proceeds.
To determine the size and engineering capacity of the piers
one or possibly two exploratory borings are made to probable
depth. From these disturbed samples are obtained for soil particle
characteristics, activity potential and moisture content. Undisturbed
samples are used to obtain density and strength. With these
data are calculated the end bearing and sideshear available
and the pier dimensions are then specified to the required safety
factor These conclusions are verifiable from the report. Depth
and diameter of oles should be checked during construction
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Multiple bids are recommended
from house levelers. I will inspect elevation measurements taken by
the prospective contractor(s) to determine the best course of action.
