GEOTECHNICAL
FEASIBILITY
REPORT
for
PROPOSED RESIDEN
TIAL SUBDIVISION
on
Capitola Drive
N
a
p
a
, California
for
NORTH BAY CIVIL ENGINEERING &
DESIGN
By
KC ENGINEERING COMPANY
Project No. VV4
6 30
1 9
April
201
9 ATTACHMENT 9
ATTACHMENT 9
G
e
otechnical Feasibility
Capitola Drive, Napa
1 9
April 2019
________________________________________________________________________________________________________
KC ENGINEERING COMPANY
Project No. VV4630
Page
3 of
10 Purpose & Scope
KC ENGINEERING COMPANY
has
performed a reconnaissance of the existing property and reviewed available geotechnical and ge
ologic information concerning the site and surrounding
area.
The purpose of our
reconnaissance and
review
was to determine whether the proposed
residential subdivision is geotechnically feasible for construction and to provide preliminary recommendations. In summary, our findings indicate that the proposed residential subdivision is geotechnically feasible f
or construction on the subject site. The following sections describe our review, findings, discussions and recommendations.
Site Location and Description
The su
bject property is
located
at
the southern end of
Capitola
Drive
in
Nap
a
, California as shown
on Figure 1
,
“Aerial Vicinity Map”
included in the Appendix of this report
.
The property
is bounded
on the north by
existing
residences and Capitola Drive, on the east by
existing
resid
ence
s, on the
south by existing residences, and on the west by Silverado Trail. The property currently contains a
number of old residences
and detached sheds
and garages. Asphalt driveways access the structures.
A shallow man
-
made
water feature
is located in the central portion of the site
which receives storm
water from Capitol Drive
. Storm d
rain and sewer pipelines cross the property. A
n artesian
water
well is located behind the central residence
and barn
.
Numerous
mature trees
are scattered across
the site. The ground surface topography generally consists of two low hills
on the west and east
sides of the property. The central area is relatively flat.
The above description is based on a reconnaissance of the site by the Geotechnical
Engineer
,
a
review of a Google aerial ima
ge dated
9/1
/1
8 ,
and
a
review a proposed subdivi
sion layout
p
rovided
by
North Bay Civil Engineering & Design
.
The Google aerial image
was
used as the basis
for our
Figure 1,
“Aerial Vicinity Map”
,
and
the
proposed subdivision layout
was used as our
Figure 2,
“Site Plan” included in the Appendix.
Proposed
Development
The proposed development is planned to consist of
demolishing the existing structures and
improvements followed by
constructing
approximately 21 single
-
family residences within the
prop
osed lots shown on Figure 2,
“
Site Plan
”
. The proposed residences are expected to be
constructed of conventional wood
-
framing and are expected to be one to two stories
in height.
Although specific development plans are not available at this time, s
plit
-
level
and flat pad
construction
is expected due to the varying terrain on the property. Capitola Drive is planned to be extend
ed through the site. Two new
asphalt concrete
roads
an
d
cul
-
de
-
sacs
, along with a parking
lot
are
planned
to provide access
and parking
for
the residential lots. Grading o
f
the site is expecte
d
to consist of various cuts and fills
of about 10 vertical feet or less
to create flat or split
-
level building
ATTACHMENT 9
G
e
otechnical Feasibility
Capitola Drive, Napa
1 9
April 2019
________________________________________________________________________________________________________
KC ENGINEERING COMPANY
Project No. VV4630
Page
4 of
10 pads.
Retaining walls of 10 feet or less are als
o expected.
Addi
t
i
onal site improvements are
expected to consist of
relocating existing storm drain and sewer pipelines, as well as constructing new underground utilities. The
central
man
-
made
water feature
will be
removed and the area filled.
Geotechnical Review
It is pointed out that we recently performed a geotechnical exploration for t he first two residences
located on the northeast co
rner of the subject property as presented in our report “
Geotechnical
Exploration Report
, Proposed Custom Residence & Secondary Dwelling,
808 Capitola Drive, Napa
,
California
”
dated 4/17/19
.
Our exploration included excavation of three exploratory test pits and laboratory testing.
Our findings revealed moderately to highly expansive sandy clays underlain by dense alluvium and volcanic tuff bedrock.
Recommendations were then provided
for earthwork
g
rading, drainage, foundations and pavements.
Site Geology
According to Geologic Map of the
Napa
7.5 ’
Quadrangle
1 , the geologic
formations
underlying the
property
are mapped as
latest Pleistocene to Holocene alluvium
and stream terrace deposits
.
The hillside areas just east of the site are mapped as the Sonoma Volcanics
comprising tuff
breccia. The alluvium
and stream terrace
deposits co
nsist of
well
-
sorted and bedded sand,
gravel, silt and clay
s
.
A
g
eol
ogic
m
ap
of the property and surrounding areas
is included as Figure
3, “Geologic Map”.
The materials encountered and observed on the site generally correlate with those mapped on the site and vicinity
.
Geo
-
Hazards
&
Sei
s
micity
The site is not located within an
Alquist
-
Priolo Earthquake Fault Zone
2 . There are no known
active or inactive faults crossing the site as mapped and/or recognized by the State of California.
Significant e
arthquake related ground shaking should be expected during the design life of the s
tructures at the site. The California Geological Survey (CGS) has defined an active fault as one that has had surface displacement in the last 11,000 years, or has experienced earthquakes in recorded history.
1 Clahan,
Wagner,
Saucedo, Randolph
-
Loar and Sowers
, 20
04 ,
Geologic Map of the Napa
7.5 ’ Quadrangle,
Napa
County,
California
, California Geological Survey
2 Parish, J.G., 2018
Earthquake Fault Zones,
California Geological Survey, Speci
al Publication 42, Revised 2018.
ATTACHMENT 9
G
e
otechnical Feasibility
Capitola Drive, Napa
1 9
April 2019
________________________________________________________________________________________________________
KC ENGINEERING COMPANY
Project No. VV4630
Page
5 of
10 Based on our review of the Fault Activity M
ap of California
3 and the USGS Fault Database
4 , th
e
nearest active faults are
the
West Napa
Fault,
the
Green Valley
Fault,
the Hunting Creek
-
Berryessa Fault, and the Hayward
-
Rodgers Creek Fault
located approximately
2.3 miles
west
,
6.1 miles
e
a
s
t,
11.6 miles
northeast,
and
14.8 miles
west
of the site, respectively.
The 2016 CBC specifies that the potential for liquefaction and soil st rength loss should be
evaluated
for the Maximum Considered Earthquake Geometric Mean (MCE G
) peak ground
ac
celeration with an adjustment for site class effects in accordance with American Society of Civil Engineer (ASCE 7
-
10)
5 .
Structures at the
site
will need to be
d
esigned to withstand the
anticipated ground accelerations.
Specific
earthquake design values
will be determined during
future specific investigations.
Fault Rupture
The site is not located within an Alquist
-
Priolo Earthquake Fault Zone. Based on our review of
geologic maps, no known active
, or i
nactive
faults cross or project toward the subject site.
In
addition, no evidence of active faulting was visible on the site during our site reconnaissance . It
is
our opinion that there is no
potential for fault
-
related surface rupture at the subject site
.
Landsliding
No obvious signs of slope instability were observed on the site during our reconnaissance.
Geologic mapping
does
not show landsliding in the area.
After site grading,
t
he
pr
oposed
home
site
s
will be located on a flat pad
s
.
In our opinion, the property is not subject to seismically induced
landsliding.
Liquefaction
Soil liquefaction is a phenomenon in which loose and saturated cohesionless soils are subject to a temporary, but essentially total loss of shear strength, due to pore pressure build -
up under the
reversing cyclic shear stresses associated with earthquakes.
Our recent exploration on the
northeastern lots revealed
shallow
volcanic rock and
dense
gravelly sands
that are not subject to
liquefaction.
Further
liquefaction
evaluation will be required
for the remainder of the site during
future site specific geotechnical explorations.
3 Jennings, C.W. and Bryant, W.A., 2010,
Fault
Activity Map of California
, California Geological Survey Geologic
Data Map No. 6, scale 1:750,000
4 U.S. Geological Survey, 2008 National Seismic Hazards Maps –
Source Parameters, accessed
4/17
/19
, from USGS
web site:
https://earthquake.usgs.gov/cfusion/h
azfaults_2008_search/query_main.cfm
5 American Soci
ety of Civil Engineer (ASCE), 2010, Minimum Design Loads for Buildings and Other Structures, Standard 7
-
10. ATTACHMENT 9
G
e
otechnical Feasibility
Capitola Drive, Napa
1 9
April 2019
________________________________________________________________________________________________________
KC ENGINEERING COMPANY
Project No. VV4630
Page
6 of
10 Conclusions
, Discussions
and Recommendations
Based on our review,
recent exploration on the site,
experience in the
Napa
area and site
reconnaissance
,
it is
the
opinion
of
KC ENGINEERING COMPANY
that the proposed
residential
subdivision
is geotechnically feasible
for construction
and can be safely developed
on the subject
site.
Due to the presence of
existing structures
, the man
-
made water feature
and
expected
variable
soil conditions,
we anticipate that
conventional earthwork operations inclu
ding demolition,
dewatering, subdrain installation, over
-
excavation, site processing and compaction will be
required to construct the proposed building pads
and streets.
The primary geotechnical
a
n
d
g
e
o
l
o
g
i
c
hazards identified
on the property consist of expansi
ve
surface soils
and
the potential for differential fill thickness
and soil conditions
under proposed
structure locations
.
The site soils are prone to heave and shrink movements with changes in moisture content and, consequently, must be carefully considered in
the design of grading,
foundations, and drainage. Considering the varying depths of materials and the highly expansive nature of the soils, differential foundation movements can be anticipated if the structure is not adequately supported. Therefore, it
is the opinion of
KC ENGINEERING COMPANY
that the
proposed structures
will need to
be supported on either
thickened
post
-
tensioned slab
foundation system
s
,
or a
well
-
reinforced and deepened
pier and grade beam foundation with a
raised wood floor
foundatio
n system
.
Conventional
spread footings
may also be
utilized where
shallow bedrock conditions are present.
Once
the project receive
s
planning department approval,
we recommend that a
site specific
geotechnical subsurface ex
ploration and laboratory testing
program be performed
to determine
the actual soil conditions and foundation design parameters
as required in
the 2016 California
Building Code
,
Chapter 18 “Soils and Foundations”,
Section 1803
.
Lot
s
pecific geotechnical
recommendations can then be provided for site grading, foundation type and design, retaining walls, slabs, drainage improvements and driveway sections.
In our opinion, the site should be
investigated by a combination of conventional exploratory drilling and sampling with associa ted
laboratory testing, as well as
explor
atory test pits
to evaluate the underlying soil deposits and
their geotechnical characteristics.
ATTACHMENT 9
APPENDIX
Aerial Vicinity Map
Site Plan
Geologic Map
ATTACHMENT 9
Project No. VV4630
Proposed Residential Subdivision
Capitola Drive, Napa, CA
Figure 1
–
AERIAL VICINITY MAP
KC ENGINEERING COMPANY
865 Cotting Lane, Suite A
Vacaville, CA 95688
707.447.4025
Silverado Trail
Capitola Drive
ATTACHMENT 9
KC ENGINEERING COMPANY
865 Cotting Lane, Suite A
Vacaville, CA 95688
707
-
447
-
4025
Project No. VV4630
Propos
ed
Residen
tial Subdivision
Capitola Drive, Napa
, California
Figure 2
–
SITE P
LAN
ATTACHMENT 9
Project No. VV
4630
Proposed
Residential Subdivision
Capitola Drive, Napa
,
California
Figure
3 –
GEOLOGIC MAP
KC E
NGINEERING
C
OMPANY
865 Cotting Lane, Suite A
Vacaville, CA 95688
(707)
447
-
4025
N
Project Site
ATTACHMENT 9