specs

Geotechnical Investigation South River Place, Gasser Drive, Napa 29 December 2010
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groundwater and/or localized seepage zones. The mass grading and underground contractor
should be made aware of these conditions so that appropriate dewatering methods can be
employed.
Grading
It is recommended that grading for the proposed site be performed during the drier seasons to
minimize potential compaction problems resulting from excessive moisture, perched water and
ground seepage. Should grading be performed during the wetter months, delays may occur due to
extensive processing and/or chemical modification (lime/cement treatment) may be required. The
grading recommendations presented herein should be followed during the mass grading operations
on the subject site.
We anticipate that the proposed grading will consist of cuts and fills up to 7 vertical feet to achieve
the design building pad and pavement subgrade elevations. However, as noted above, there will
still be stockpile soils left at these grades and the soils will be non-uniform. In addition, the existing
near surface materials consists of firm sandy clays and loose sands which require densification prior
to foundation construction. The densification can be achieved through conventional grading
techniques or by in-place methods such as stone columns or compaction grouting.
The following recommendations are applicable to densification by conventional grading techniques.
In areas where buildings are proposed and where Hartle Court will be widened, the old fill soils
identified in the borings should be over-excavated to undisturbed native ground as observed and
approved by the Soil Engineer. Based on the borings, excavations to elevations ranging from +1
to +5 feet should be expected resulting in over-excavation depths of 5 to 11 feet (not including
the stockpile). Localized dewatering of the excavations may be required by the contractor,
especially near Borings 2 and 3. The excavation should extend a minimum of 5 feet laterally
beyond the building footprint and the area replaced with engineered fill as noted below. We also
note that debris may be encountered in the excavated fill that must be removed from the site.
While we did not encounter any during this investigation, our experience on adjacent projects
indicates there is a possibility of this condition.
Based on our experience on the adjacent construction operations for Hartle Court, Transitional
Housing, Homeless Shelter and Fire Station, the native clay soils that will be exposed will likely
be soft, wet and unstable. Placement of fill on these materials with conventional earthmoving
equipment will likely be difficult. Therefore, as we have performed on the adjacent sites, a
stabilization layer should be anticipated by the contractor at the bottom of the excavations. The
stabilization layer should consist of Tensar BX1200 (or approved equivalent) geogrid overlain by
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