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Re: drilled pier foundation

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Andrew,

        I've used these a number of times.  They have a number of useful and
limiting characteristics.

1.)    They can only be used in clay or similar cohesive soil that will not cave
in.

2.)    They provide a large area for end bearing loading and require a small
volume of concrete to fill the hole.

3.)    They are ideal where you have a few feet of clay above some undrillable
soil of only moderate strength.  Examples of such soil are wet sand that would
cave in; and heavy gravel that can not easily be drilled through.  They could
even be set on bedrock but I haven't seen it done.

4.)    They are very good for resisting uplift.

5.)    They are reinforced like normal cast-in-place concrete piles.  I've never
seen, or even heard of, anyone using reinforcing within the bell.

6.)    The top of the bell slopes about 45 degrees and the maximum bell/shaft
diameter is about 3:1 but see the contractor for details.

        Your geotechnical engineer should be able to tell you a lot more.

Regards,

H. Daryl Richardson

"Andrew D. Kester" wrote:

> We are working on a single story retail renovation in the greater Houston,
> TX area. The latest renovation plans indicated drilled pier foundations for
> all support of all columns. The notation says that 12/36 denotes shaft
> diameter/ bell diameter. I have not designed one of these before, but I did
> a little research, and I guess this means the main shaft diam.=12", and then
> it flares out into a "bell" at the bottom to 36". From my research , these
> types of foundations are used in clay or cohesive soils, which I have heard
> mainly from this list is a big problem in Tejas. The plans do not indicate
> an embedment. The continuous footings that support the masonry infill are
> not labeled, and  we are missing that detail sheet that shows the sections.
> So I do not know how they are designed or built. There also are no
> geotechnical or general foundation notes on the plans.
>
> Do you  think they used this found. system because of clay? Can anyone
> theorize what is going on here and why they may have done that?
>
> Our renovations do not affect the loads on the drilled shafts, and we are
> not adding any new columns that will require this type of found. BUT, we are
> adding CMU infill walls and I need to know at least something about the soil
> to design the cont. wall footings, and to properly call out notes on the
> plans. I am more then likely going to ask for geotech on this job just to
> confirm that our cont. wall footings are ok.
>
> Thinking ahead, are non-bearing CMU walls built on cont. ftgs. on this type
> of soil even if the columns are drilled shafts? It appears that was done on
> this job.
>
> TIA
>
> Andrew D. Kester, EI
> Structural Engineer
> Bentley Architects & Engineers
> 665 W. Warren Ave.
> Longwood, FL 32750
> 1-407-331-6116
> andrew(--nospam--at)baeonline.com
> www.baeonline.com
>
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