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Re: Expansive Foundation

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I've done lots of foundations in Dallas, pier and beam as well as PT slabs.  We have extremely expansive soils here.
It may be different elsewhere, but I don't see an "uplift" force on the beams called out in the soil report for pier and beam foundations.  It's really undefinable depending on several factors, and can very easily exceed even 2000 psf. 
What we see in the reports is an uplift force on the surface of the pier in the range of 1500 to 2000 psf.  It acts on the upper, active soil stratum, usually about 8 ft.  That leaves a heck of an anchoring requirement for the pier in lower soil.

>>> On 10/17/2007 at 8:52 AM, <bill(--nospam--at)polhemus.cc> wrote:

IMO, and "uplift pressure value" is kinda dumb. In the first place, it's not the way that expansive soils are typically classified. Also, it doesn't tell you what you need to know for various types of foundation systems.

For instance, if you have a "floating" system such as a post-tensioned slab, you need to know things like edge-lift and center-lift values.

IMO, this geotech is "fobbing off" his responsibility to accurately report the soil conditions on YOU, and YOU are in the position of having to figure out how to design for this without REALLY knowing what you're designing FOR.

Tell him to give you REAL expansive soils parameters such as those recommended by the PTI manual using the VOLFLO technique or similar.

(N.B. I am in the Houston area).

  I've had soils reports in california call for 2000psf uplift

-g

On 10/16/07, Garner, Robert <rgarner(--nospam--at)moffattnichol.com > wrote:

We did a job in the Bayview Texas area.  The geotech recommended an expansive clay uplift value of 1000 psf.  This info is just for a sanity check.

 

Bob Garner, S.E.

 


From: rowyz(--nospam--at)sbcglobal.net [mailto: rowyz(--nospam--at)sbcglobal.net]
Sent: Tuesday, October 16, 2007 2:00 PM


To: seaint(--nospam--at)seaint.org
Subject: Re: Expansive Foundation

 

You should use the parameters given from soil report, and in accordance to PTI's design guide. There was no direction given for "reduced" forces pertaining to your case.

 

thks,

Milo

 

----- Original Message ----
From: Gautam Manandhar <Gautam_Manandhar(--nospam--at)ci.richmond.ca.us>
To: seaint(--nospam--at)seaint.org
Sent: Tuesday, October 16, 2007 12:11:56 PM
Subject: RE: Expansive Foundation

Adjebli, Chuck, & Milo :

 

Thank you for your response.

 

Milo :

 

You indicated you used PT Slab.  I felt that 1500 psf uplift was high – just curious what uplift forces did you design your PT slabs for.

 

Gautam

 


From: rowyz(--nospam--at)sbcglobal.net [mailto: rowyz(--nospam--at)sbcglobal.net]
Sent: Friday, October 12, 2007 7:40 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Expansive Foundation

 

At a residential project I've used a PT slab on grade; design guides and handbooks are available at PTI.

 

Milo Z, PE  

----- Original Message ----
From: Gautam Manandhar <Gautam_Manandhar(--nospam--at)ci.richmond.ca.us>
To: seaint(--nospam--at)seaint.org
Sent: Friday, October 12, 2007 3:13:53 PM
Subject: Expansive Foundation

 

List members

 

I am working on a site that has expansive soil.   The soil report indicates the uplift force from the expansive soil to be 1500 psf.  The foundation consists of piers and grade beams.  I understand there is a compressible material that can be installed between the soil and the underside of the grade beam to reduce the overall uplift load.  Can anyone point me to web site regarding this material or provide specs on this material.

 

Gautam

 

 


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-gm
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