Compression Grouting - How it Works and How it Doesn't Work

Pressure Grouting - How It Works, and How It Doesn't Work
One of the more well known ways of repairing concrete slabs on grade is
pressure grouting, or also as it is sometimes called, mud jacking.  The
system is quite simple, it involves pumping a mix of cement, sand, and water
under a foundation or slab under high pressure.  The pressure lifts up the
foundation system, and if done right, stabilizes it.  I have recommended
pressure grouting in a number of applications, but I have also seen it used
improperly.
Generally, I've found pressure grouting useful in two applications - the
repair of settling slabs and filling of voids underground in buried trash pits. 
With pressure grouting, you can easily fill underground openings, which saves
you the trouble of trying to excavate and backfill problem areas.  With
concrete slab repair it is often much cheaper to pressure grout than it is to
put in piers under the slab, or remove and replace the slab, although it does
depend on the situation.  In some cases, pressure grouting is used along
with other repair methods.  In one project I was involved in, there was
extensive settlement of soil under a living room slab in a house.  We
repaired it by supporting the slab on steel slab piers and pressure grouting to
fill the large void under the slab.
The worst abuse of pressure grouting I have seen is to simply void fill under
a slab without doing any other repair to solve the underlying problem. 
When a concrete slab is poured on poor material (typically fill soils that
haven't been compacted), the soil will settle under the slab.  Usually, a
large void will open up before the slab actually starts to settle.  I've
seen openings under slabs on the order of 6" to 8"..  A cheap fix is to
call in a contractor to fill that void with pressure grout.  The slab can
be lifted with the pressure, and it will appear that it is fixed.  However,
simply void filling doesn't fix the problems with the soil underneath.  If
it is uncompacted fill, it's still uncompacted and will continue to settle. 
So, you spend the money to pressure grout under your slab, and it looks nice. 
Then in about 6 months or a year, the soil starts to settle again, and you will
see the slab settling - again.
I dealt with one of these types of "fixes" on a foreclosed house that an
investor was preparing to purchase.  We cored into the slab, and found a
significant amount of grout under the slab, but we didn't find any evidence of
piers, or support of the slab through the soft soil underneath.  The soil
underneath the slab was fill to a depth of about 3 feet, and it was still very
soft.  It was going to continue settling for years, and the grout that had
been pumped under the slab actually added weight.  The repair had to be
repeated all over again.  The investor in this case elected not to purchase
the house because the repair was so expensive.  It is more than likely that
whoever paid for the original improper repair didn't get the job much cheaper
than if it was done right.  The house would have not had the damage, and it
would have sold.
The proper way to pressure grout is to push the pipe that the grout is pumped
in through the weak soil down to more solid ground.  The grout should be
pumped in to form a "bulb" up to the surface.  A number of these should be
pumped under the slab, and these will now act as columns supporting the slab. 
The other way is to support the slab on steel slab piers, and then compression
grout to fill the voids.  Slab piers are steel piers that are inserted
through holes cored in the slab, and they are augered down to soil that provides
proper bearing.  A bracket is placed at the top to support the slab. 
I've recommended either slab piers/compression grouting or compression grouting
by itself depending on the situation.  .
The other issue about pressure grouting is that has the potential to do a lot
of damage if it is done wrong.  If you are grouting inside of a subwall, it
can blow the wall out from the pressure.  It can fill up foundation drain
pipes, and otherwise clog drainage systems.  That makes it all the more
important to hire a qualified pressure grouting contractor.  A good
contractor knows how to avoid collateral damage during the grouting, and can
save you a lot of problems.
Finally, compression grouting is often very useful in filling underground
voids.  Here in Atlanta we don't have underground caverns to fill like some
other areas of the country, but often you find buried trash, trees, and other
debris near houses.  You can't "fix" a buried trash pit completely with
pressure grout.  If you have a house or other structure built on top of
buried trash, you really have no choice to support it with piers drilled through
the pit.  Compression grouting isn't going to fill everything, and the
buried material will continue decomposing and shifting.  The structure will
still move.
However, oftentimes you have buried trash and other debris in close proximity
to foundations.  In such case, excavating and removing the buried items
could cause problems for the adjacent foundations, or be very difficult and
expensive.  In such cases, you can stabilize the buried trash to an
acceptable amount by injecting grout into the in the ground throughout the pit. 
In that type of situation, pressure grouting can save a lot of money and solve a
difficult problem.
In summary, I have found pressure grouting useful in filling underground
voids and repair of concrete slabs.  It has to be used for the right
situation, and has to be done correctly.  It's important to use a qualified
contractor, and have the system designed correctly by a professional experienced
in foundation repair.

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