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Re: Landslide

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John Buchanan wrote:

> 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."

I would think that a PCI post tension or heavily reinforced rib mat could be
designed to handle this criteria but, for a typical small residence may, as you
indicate farther on, be expensive.  Although difficult to tell from the
wording, this may be what the geotech consultant was recommending (grid

This recommendation seems driven by catastroiphic events.  Is this house right
near or on top of a fault?  Is the geotech primarily concerned with the ridge
moving during a seismic event?  Is the staility of the ridge during torrential
rains a concern?

> 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.

Pier and Grade beam systems are used often in my area (Oklahoma) and some
Structural Engineers will request this as a recommendation, even if site
conditions and foundation loads do not require this system.  I've seen several
houses built with this system.  Where you have shallow hard strata and an
overburden of expansive soils (clay), drilled shaft piers with elevated grade
beams and structural floors cast on void boxes can be an economical alternative
to substansial earthwork or soil modification.

> 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!

Good for you!

> John Buchanan