14100083Geotechnical Investigation
Vallco Town Center
Cupertino, California
8.2.2 Mat Foundation
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770633101
Page 24
The recommended allowable dead plus live bearing pressures and corresponding design moduli
of subgrade reaction for mats are presented in Table 7.
TABLE 7
Mat Foundations
Areal
Allowable Dead Plus Live Bearing
Modulus of Subgrade
Pressure
Reaction
(Psf)
(kcf)
West of N. Wolfe
5,000
60
Road
East of N. Wolfe Road
8,000
100
Notes:
1. Assumes area west of N. Wolfe Road will have excavation depths of approximately 10 to 20 feet bgs and area
east of N. Wolfe Road will have excavation depths of 40 to 60 feet bgs.
The moduli values are representative of the anticipated settlement under the building loads.
After the mat analysis is completed, we should review the computed settlement and bearing
pressure profiles to check that the modulus value is appropriate. Higher bearing pressures than
those presented in Table 7 may be used; however, the corresponding modulus may need to be
revised. The allowable bearing pressure may be increased by one-third for total loads including
wind or seismic.
Resistance to lateral loads can be mobilized by a combination of passive pressure acting against
the vertical faces of the mat and friction along the base of the mat. Passive resistance may be
calculated using lateral pressures corresponding to an equivalent fluid weight of 400 pcf; the
upper foot of soil should be ignored unless confined by a concrete slab or pavement.
If waterproofing is used, the allowable friction factor will depend on the type of waterproofing
used at the base of the foundation. For bentonite -based waterproofing membranes, such as
Paraseal and Voltex, a friction factor of 0.15 should be used. Friction factors for other types of
waterproofing membranes should be provided by the manufacturer. If waterproofing is not
used, frictional resistance should be computed using a base friction coefficient of 0.3. These
values include a factor of safety of about 1.5 and may be used in combination without
reduction.
If weak soil is encountered at the mat excavation bottom, it should be over -excavated and
replaced with engineered fill or lean concrete. The bottom and sides of mat excavations should
be wetted following excavation and maintained in a moist condition until concrete is placed.
LANGAN
Geotechnical Investigation 27 March 2018
Vallco Town Center 770633101
Cupertino, California Page 25
If the foundation soil dries during construction, the foundation will heave when exposed to
moisture, which may result in cracking and distress.
We should observe mat subgrade prior to placement of reinforcing steel. The excavation for
the mat should be free of standing water, debris, and disturbed materials prior to placing
concrete.
8.3 Floor Slab
The subgrade soil for buildings with basements should be very stiff or dense; therefore, we
conclude the basement slabs can be supported on grade. Where soft or loose soil is present at
the basement slab subgrade, the weak soil should be removed and replaced with engineered
fill or lean concrete.
Where slab -on -grade floors are to be cast, the soil subgrade should be scarified to a depth of
six inches, moisture conditioned to near (or above) optimum moisture content, and rolled to
provide a firm, non -yielding surface compacted to at least 90 percent relative compaction. Lime
treated soil should be compacted to at least 90 percent relative compaction. If the subgrade is
disturbed during excavation for footings and utilities, it should be re -rolled. Loose, disturbed
materials should be excavated, removed, and replaced with engineered fill during final subgrade
preparation.
Moisture is likely to condense on the underside of the slabs, even though they will be above
the design groundwater table. Consequently, a moisture barrier should be installed beneath
the slabs if movement of water vapor through the slabs would be detrimental to its intended
use. A moisture barrier is generally not required beneath parking garage slabs, except for areas
beneath mechanical, electrical, and storage rooms. A typical moisture barrier consists of a
capillary moisture break and a water vapor retarder.
The capillary moisture break should consist of at least four inches of clean, free -draining gravel
or crushed rock. The vapor retarder should meet the requirements for Class C vapor retarders
stated in ASTM E1745-97. The vapor retarder should be placed in accordance with the
requirements of ASTM E1643-98. These requirements include overlapping seams by
six inches, taping seams, and sealing penetrations in the vapor retarder. The particle size of the
gravel/crushed rock should meet the gradation requirements presented in Table 8.
L A NGA N
Geotechnical Investigation
Vallco Town Center
Cupertino, California
TABLE 8
Gradation Requirements for Capillary Moisture Break
Sieve Size
Percentage Passing Sieve
Gravel or Crushed Rock
1 inch
90-100
3/4 inch
30-100
1/2 inch
5-25
3/8 inch
0-6
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Concrete mixes with high water/cement (w/c) ratios result in excess water in the concrete,
which increases the cure time and results in excessive vapor transmission through the slab.
Therefore, concrete for the floor slab should have a low w/c ratio - less than 0.45. Water
should not be added in the field. If necessary, workability should be increased by adding
plasticizers. In addition, the slab should be properly cured. Before the floor covering is placed,
the contractor should check that the concrete surface and the moisture emission levels (if
emission testing is required) meet the manufacturer's requirements.
8.4 Permanent Below -Grade Wall Design
To construct the basement walls, the site may be open cut and/or temporarily shored. It is the
responsibility of the contractor to determine the safe excavation slopes; however, we
recommend cuts greater than 4 feet be no steeper than 1.5:1 (horizontal:vertical).
Because the on-site soil is expansive, we recommend designing below grade walls for at -rest
lateral pressures corresponding to equivalent fluid unit weights of 70 pcf and 90 pcf for drained
and undrained conditions, respectively. Because the site is in a seismically active area, the
design should also be checked for seismic conditions. Under seismic loading conditions, there
will be an added seismic increment that should be added to active earth pressures (Sitar et al.
2012). We used the procedures outlined in Sitar et al. (2012) and the peak ground acceleration
based on the DE ground motion level (see Section 8.6) to compute the seismic pressure
increment. Basement walls should be designed for the equivalent fluid weights and pressures
presented in Table 9A.
LANGAN