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Re: Landslide

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To all,

As an example of the soils settlement problem, I have included a portion of
the geotechnical report.

"Considering the seismic setting including the potential for ridge top
shattering, the site
history, soil characteristics, and site preparation recommendations, it is
our opinion that an
appropriate foundation system to support the proposed structures will
consist of an
interconnected reinforced concrete grid system. The grid system should
consist of continuous
exterior spread footings interconnected with interior continuous footings.
Isolated spread
footings fire not recommended for this site due to the possibility of ridge
top shattering.

The grid footings should be embedded into the sandstone/ siltstone bedrock
and below the
lowest adjacent grade not less than 18 inches. Footing widths should be
based on the
allowable bearing value but not less than 12 inches for I story and 15
inches for 2 story
structures. Footing excavations must be observed by the Geotechnical
Engineer before steel is
placed and concrete is poured to insure bedding into proper material. The
footing excavations
should be saturated prior to placing concrete.

The foundation grid system should be designed so that the building moves as
a unit and is
capable of spanning cracks and differential ground movement which may occur
during
ridge top shattering. The grid system should be able to span a void
appearing anywhere under
the building with a diameter of 10 feet. In Addition, the footings of the
grid system should be
able to resist the tensile forces applied to the foundation system from
cracks appearing and
opening beneath the footings. Continuous spread footings should contain
tensile reinforcement
sufficient to resist tensile forces of 1000 pounds per lineal foot of
footing for it distance of 1/1
the footing length. For example, a 20 foot long footing should contain
tensile reinforcement
able to withstand 10,000 pounds of tensile force. As a minimum, footing
should be reinforced
with 4 #4 bars, 2 at the top and 2 at the bottom."

This particular foundation system recommended a "conventional foundation
system". Does any one have any comments regarding this type of system?
These types of recommendations are starting to become more prevalent around
this area. Some times the geotechnical engineers require a pier and grade
beam system.

My problem with these recommendations is how to economically design a
foundation system so the project owners may still be able to afford to
build with out dumping a tremendous amount of money into the foundation.

I do appreciate the comments regarding the drilled piers. I personally do
not use any thing less than an eighteen inch drilled pier and a minimum of
eight number five bars with ties at 10" o.c. I do now I have lost clients
to the other engineers that use 12" piers with one number 6 bar. Just as
well, I refuse to become what is termed a lick and stick engineer!

John Buchanan