Re: [GBlist] Building on Rock?

["David Porter" <dporter@porterarchitects.com>]

> Can anyone out there give me some information on septic systems for land
> with high rock content.   My understanding is that the land in question
has
> failed 'Perc" test in the past and has a relatively consistent layer of
> solid rock 2' -3' below grade .  What other complications might one run
into
> building on this type of ground?  This type of ground is unusual in my
area
> (West Ky) and local builders are unfamiliar with building on it.  I am
> trying to purchase the land for a home site and trying to do a little
> pre-purchase research.

If you want a basement, your costs will be high if you have to go through
the rock.  If you want to put your foundation on the rock and kick the
basement up out of the ground, that should be fairly easy and fairly
standard costs.  You should hire, before buying the parcel, a soils engineer
(also called a geotechnical engineer and sometimes other expletive names by
structural engineers and architects) and do and pay for some due diligence.
The soils engineer can do some on-site testing to see if the rock layer is
deep enough to support the house loads or if it is just a thin layer or a
broken up layer of rock.  Ideally, you would want the rock to either be real
deep and solid, to put the foundations directly onto the rock, or you would
want the rock to be "excavatable" in bite-size pieces so that you could get
rid of it and build down into the ground normally.  If the rock is deep
enough and solid enough to support the building loads, you shouldn't have to
worry about any future settlement, unless, of course, the lot is sitting on
a fault line in an earthquake zone.

As for the septic system working, you might have to put in a
larger-than-normal drainfield in order to spread out the effluent over a
large enough area so that it will perc into the thin layer of dirt you do
have.  Check with the soils engineer to see if he/she can tell you how a
septic system could work on what you have.  You might have to bring in some
fill and mound up at the septic system and drainfield area in order to build
up enough dirt above the rock to get the percolation depth necessary.  You
might find that you will have to do a customized septic system and
drainfield design that can be certified by the engineer.  Check with the
engineer to see if the local septic tank permit issuance body will accept a
non-traditional design that is signed and sealed by an engineer.  The
existing conditions probably don't fit into the neat and tidy confines of
the septic tank rules for your area but that doesn't mean that a system
cannot be designed to work.

You might use the hardship of having to custom design and build a septic
system on the rock as a way to buy the price down on the property.  Most
buyers like purchasing ready-to-build-on plain dirt properties.

David Porter AIA
Palm Beach Gardens, FL USA
dporter@porterarchitects.com
www.porterarchitects.com




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