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Our real estate inspections frequently reveal conditions that could
damage (and sometimes already have damaged) a building's foundation.
Water and insufficient soil compaction are two of the main culprits.
Here are a few ways to prevent foundation damage:
Backfill
If you're having a new home or commercial structure built, ask your
builder about his or her soil compaction standards. This is important
because insufficiently compacted soil may later "give", causing your
building's foundation to crack or buckle. Ask the builder if your site
contains potentially expansive or consolidating soils. If so, measures
should be taken to prevent the soil from being wetted during and after
construction.
Generally this can be accomplished by ensuring that the backfill placed
around the foundation walls will not settle after construction
completion and that the backfill material is relatively impervious. For
proper compaction, water may need to be added to backfill material, but
do not puddle or saturate.
Backfill should be mechanically compacted to at least 90% of Standard
Proctor (your builder should be familiar with this term.) All plumbing
and utility trenches should be compacted to 95% of Standard Proctor
beneath all structures and 90% elsewhere. Compaction measurements
should be verified with field tests by a structural engineer.
Surface drainage
Make sure that the grade around your building slopes away from the
foundation wall on all sides. A minimum of 12" in the first ten feet is
recommended.
Downspouts and sillcocks should discharge into splashblocks extending
beyond the limits of the backfill. Even better, use long downspout
extensions instead of splashblocks.
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Gravel fill should be placed in all window wells to the level of the basement window sill.
Sump pumps
Ideally installed during construction, sump pumps discharge water that
has collected in a subsurface drain and route it away at least ten feet
from a building. Make sure your builder is following sump pump
standards -- a 1-1/4" discharge line fitted with a backflow preventer
about 30" above the pump. The pump should be wired permanently to a
power supply and the pit or sump should be constructed of solid
material and fitted with a removable lid. The drain itself should be a
perforated rigid pipe at least 4" in diameter placed in a gravel bed
with all joints securely fastened.
Surface water or groundwater?
If you have noticed water in your crawlspace or basement, don't
automatically assume that it is surface water such as rain or sprinkler
system runoff. It may be groundwater!
Groundwater is simply water that has percolated down through the soil
from melting snow and rain. Where it collects underground is the "zone
of saturation", the top of which is the water table. The collection
formation is called an aquifer. Aquifers occur at different depths
(dependent on geologic conditions), but most groundwater is found
within 2,500 feet of the earth's crust. Generally, groundwater is found
in slowly flowing, tightly packed sand and gravel, although some forms
in channels, lava tubes and igneous rock crevices. Since these
formations can be present at varying levels, it's possible for
groundwater channels to lie close to basements and crawlspaces.
A building inspector with appropriate geotechnical and engineering
expertise can help you identify the source of your water problems. If
they are caused by groundwater then perimeter drainage, interceptor
trenches, subterranean liners and soil/aggregate layering around your
foundation are possible fixes.
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