specs

Geotechnical Investigation South River Place, Gasser Drive, Napa 29 December 2010
________________________________________________________________________________________________________
It is our opinion that the in-place ground improvement technique most suitable would be to
construct stone columns or rammed aggregate piers (also known locally as Geopier soil
reinforcement). Alternative ground improvement methods may be considered but must be
provided to our firm for review and approval. Rammed aggregate piers are a method of
densifying the soils by drilling pier holes and mechanically ramming well-graded aggregate into
the holes in thin lifts. The high impact energy not only densifies the aggregate, but also
prestrains and prestresses the surrounding soil laterally. This increases the soil’s ability to
withstand higher vertical and lateral stresses. The rammed aggregate piers must be designed and
constructed in accordance with the Geopier Foundation Company recommendations. Hole
casing or alternative methods should be anticipated as was required on the adjacent Homeless
Shelter site. The Geopier design will provide the project Structural Engineer with an increased
allowable bearing capacity, a decreased differential settlement potential and the other foundation
related criteria. If this option is selected, the Foundation Plans and Geopier design documents
should be provided to us for review prior to construction.
In addition, another geotechnical concern for the site is the presence of near-surface, low to
highly expansive clays, the existing stockpile and fill soils beneath the site. The near surface soil
is prone to heave and shrink movements with changes in moisture content and, consequently,
must be carefully considered in the design of grading, foundations, drainage, and landscaping.
The recommendations provided in the following sections will minimize the effects of the noted
concerns.
As previously discussed, the central portion of the site is covered by a variable height stockpile
which is underlain by old dredge fills. Based on the nearby Hartle Court (new), transitional
housing, homeless shelter and fire station improvement grades, we anticipate that the soils at the
proposed pad grade (approximately elev. +15 ft.) will be variable ranging from sandy clay to silt
and sand. Furthermore, portions of the stockpile may remain after cutting to the pad grade.
Therefore, it is our opinion that commercial building structures may be satisfactorily supported
on a deepened spread footing foundation system in conjunction with an interior concrete slab-ongrade
floor provided that the fill materials are mitigated by grading as noted below, or by other
approved ground densification methods. In addition, any additional import soils should meet the
requirements as noted below. Specific recommendations for the spread footing foundation
systems are given later under the heading Foundations.
Groundwater was encountered at elevations ranging from -6.0 to +11.0 feet. The groundwater
table is not expected to impact the future performance of the proposed buildings. However, any
grading near or below these levels and any utilities below these elevations will likely encounter
________________________________________________________________________________________________________
KC ENGINEERING COMPANY Project No. VV3200 Page 15 of 123