JUNE 10 2009 MEETING
Wednesday, June 10, 2009
3:30 - 4:45 pm

WORKSHOP SUMMARY

Trees Next to Foundations

Shear Strength of Shallow Soils

Workshop Speakers:

David Eastwood, P.E. President of Geotech Engineering and Testing, Houston, 713-699-4000

Dean Read, P.E. of Geostructural Tool Kit ), Austin, Tel. 512-835-7000

WORKSHOP SUMMARY

Mr. Eastwood presented a slide show titled, "The Effects of Trees on Foundations". His slides were of roots found next to and beneath foundations.

Mr. Read presented a slide show titled, "Modeling Vegetation (and other) Effects Using VOLFLO Software".

Some highlights of Dean Read's presentation to a room of about 70 at HESS:

• Since January 1, 2009, it is required to use PTI 3rd Edition with both amendments, which replaces older editions. This amended edition does not require that suction tests be run. If this amended procedure is used properly, the foundation designs should come out 10 to 15% heavier than with the 2nd Edition.
  
  
• There is no relationship between the VOLFLO method and the PTI design method except that both are based on Dr. Bob Lytton's unsaturated soil mechanics theory. PTI has always said the engineer can use unsaturated soil mechanics in lieu of their method for determining soil movement parameters.
  
  
• Because the Ym parameter is the vertical movement of soil without a foundation or externally applied loads, Ym should always be greater than the differential movement of a foundation. The foundation would have to perfectly flexible and weightless for the differential movement to equal Ym.
  
  
• Ym is not PVR. Do not use TxDOT 124E PVR method by Chester McDowell for designing foundations. It is an indicator only, one that becomes more inaccurate with depth. There are no equations available to use PVR in slab on grade design.
  
  
• Per PTI, it is reasonable to have a spread of 1.5 pF at the ground surface in the suction plots. For Houston a reasonable design spread is from 3.0 to 4.5 pF. This spread is independent of when the foundation is built and when the geotechnical investigation is done.
  
  
• The geotechnical engineer needs to look at the site and at aerials to check for trees recently removed or about to be removed. Per VOLFLO, a typical Houston clay with PI=56 will yield Ym's of 1.6" and 0.7" for center and edge lift, respectively. If a tree is likely to be removed say at the perimeter of the new slab just prior to construction, the 0.7" Ym will jump to 8.4", meaning a slab on grade is no longer a design option.
  
  
• How long must a builder wait to build a normal slab-on-grade on an expansive site after the trees are felled? Nothing has been published on this but TAMU Professor Bob Lytton has proposed an equation where the reduction in suction (pF) is equal to the square root of the number of years after tree removal divided by the estimated total years until the soil is rehydrated. So  for an assumed 5 year rehydration period (assuming an open site after tree felling), the suction and Ym relationship for the above example would be:

Time (yrs.)  Suction (pF)   Ym (in.)
 0              4.5             8.4
 1              4.0             3.9
 2              3.8             2.4
 3              3.6             1.4
 4              3.5             1.1
 5              3.4             0.7

Thus for this site with a PI=56, the builder would need to wait at least 1-2 years to use a normal slab-on-grade design using Dr. Lytton's proposed computation.

To download Mr. Read's slide presentation, click here

To read a summary of Mr. Read's Dec 08 FPA Presentation, click here