Specs

Bridgestone/Firestone – Lakewood, CO Addendum June 21, 2012
PSI Project No. 532281 Page 2 of 6
Based on the potential for vertical movement due to the high swelling soils and
Bridgestone/Firestone’s maximum allowable movement requirements (1 inch), it is PSI’s opinion
that the proposed structures be supported on a drilled pier foundation system and that a structurally
supported floor system be utilized. However, as an alternative to the drilled pier foundation system,
PSI recommends that the proposed structures be placed on shallow footing foundations bearing on
non-expansive imported structural fill.
If the shallow footing foundation alternative is selected, PSI recommends that overexcavation
and replacement with structural fill be performed within the building pad area to reduce the
potential movement of the building due to swelling soils. This overexcavation should extend to a
depth of 4 feet below bottom of footings, 7 feet total. It should be noted that these
recommendations are based on a depth of wetting of 7 feet below grade, an average swell
potential of 5 percent for native in-situ soils, an average swell percentage of 2.5 percent for
moisture conditioned and compacted on site soils, and an average swell percentage of less
than 1 percent for moisture conditioned and compacted non-expansive imported structural fill
material.
The engineering design team should perform a cost analysis of the various depths and type of
structural fill and determine the amount of risk that may be tolerated from swelling soils. The
overexcavation and replacement with on-site soils should extend 10 feet horizontally on the
north, south and west sides and should extend 5 feet horizontally on the east side. Additionally,
PSI recommends a 5 foot horizontal zone of inorganic/non-irrigated material around the
proposed structure to reduce the potential infiltration of water to moisture-sensitive soils.
On-Site Structural Fill Material
Based on PSI’s field and laboratory data it is our opinion that on-site overburden soils will NOT be
suitable for re-use as backfill soils and for use as structural fill. Also, the claystone bedrock should
NOT be used as fill material.
Imported Structural Fill
Imported structural fill should be free of organic or other deleterious materials, have a liquid limit
less than 30, a plasticity index less than 10, and meet the following gradation outlined below.
This structural fill criteria is intended as a general guideline. Imported structural fill materials
should have a swell potential of less than 1 percent when compacted to 95 percent of maximum
dry unit weight (MDUW) and at 2 percent below optimum moisture content (OMC) and tested
under a swell test surcharge of 500 psf. The MDUW and OMC should be determined by ASTM
D698 (Standard Proctor).