BLD-2020-1179 GEOTECHNICAL REPORTGEOTECHNICAL INVESTIGATION
OTIS ACCESSORY DWELLING UNIT
10789 JUNIPER COURT
CUPERTINO, CALIFORNIA 95014
Prepared for
Mr. Jim Otis
1 0789 Juniper Court
Cupertino , California 95014
July 2020
Project No. 51 80-1
Independent Code
Consultants11/09/20
July 24, 2020
5180-1
Mr. Jim Otis
10789 Juniper Court
Cupertino, California 95014
RE: GEOTECHNICAL INVESTIGATION
ACCESSORY DWELLING UNIT
10789 JUNIPER COURT
CUPERTINO, CALIFORNIA
Dear Mr. Otis:
In accordance with your request, we have performed a geotechnical investigation for the
proposed accessory dwelling unit to be constructed at your property located at 10789
Juniper Court in Cupertino, California. The accompanying report summarizes the results
of our subsurface exploration, laboratory testing, and engineering analysis, and presents
geotechnical recommendations for the proposed improvements.
We refer you to the text of our report for specific recommendations.
Thank you for the opportunity to work with you on this project. Please call if you have
questions or comments about site conditions or the findings and recommendations from
our site investigation.
Very truly yours,
ROMIG ENGINEERS, INC.
Coleman K. Ng, P.E.
Copies: Addressee (via email)
Innovative Concepts (via email)
Attn: Mr. Jeff Guinta
Attn: Ms. Sheila Guinta
1390 El Camino Real, Second Floor | San Carlos, CA 94070 | (650) 591-5224 | www.romigengineers.com
GEOTECHNICAL INVESTIGATION
ACCESSORY DWELLING UNIT
10789 JUNIPER COURT
CUPERTINO, CALIFORNIA 95014
PREPARED FOR:
MR. JIM OTIS
10789 JUNIPER COURT
CUPERTINO, CALIFORNIA 95014
PREPARED BY:
ROMIG ENGINEERS, INC.
1390 EL CAMINO REAL, SECOND FLOOR
SAN CARLOS, CALIFORNIA 94070
JULY 2020
TABLE OF CONTENTS
Page No.
Letter of transmittal
Title Page
TABLE OF CONTENTS
INTRODUCTION .........................................................................................................1
Project Description ................................................................................................1
Scope of Work .......................................................................................................1
Limitations .............................................................................................................2
SITE EXPLORATION AND RECONNAISSANCE ...................................................2
Surface Conditions ................................................................................................3
Subsurface Conditions ...........................................................................................3
Ground Water ........................................................................................................3
GEOLOGIC SETTING .................................................................................................4
Faulting and Seismicity .........................................................................................4
Table 1. Earthquake Magnitudes and Historical Earthquakes .....................5
Earthquake Design Parameters ..............................................................................6
Table 2. 2016 CBC Seismic Design Criteria ...............................................6
CONCLUSIONS............................................................................................................6
FOUNDATIONS ...........................................................................................................7
Shallow Foundations .............................................................................................7
Lateral Loads on Footings .....................................................................................7
Settlement ..............................................................................................................8
SLABS-ON-GRADE .....................................................................................................8
General Slab Considerations .................................................................................8
Exterior Flatwork ...................................................................................................8
At-Grade Interior Slabs .........................................................................................8
EARTHWORK ............................................................................................................10
Clearing and Subgrade Preparation .....................................................................10
Material For Fill ..................................................................................................10
Compaction ..........................................................................................................10
Table 3. Compaction Recommendations ...................................................11
Temporary Slopes and Excavations ....................................................................11
Finished Slopes ....................................................................................................12
Surface Drainage .................................................................................................12
FUTURE SERVICES ..................................................................................................12
Plan Review .........................................................................................................12
Construction Observation and Testing ................................................................13
TABLE OF CONTENTS
(Continued)
REFERENCES
FIGURE 1 - VICINITY MAP
FIGURE 2 - SITE PLAN
FIGURE 3 - VICINITY GEOLOGIC MAP
FIGURE 4 - VICINITY HAZARD MAP
FIGURE 5 - REGIONAL FAULTING AND SEISMICITY MAP
APPENDIX A - FIELD INVESTIGATION
Figure A-1 - Key to Exploratory Boring Logs
Exploratory Boring Log EB-1
APPENDIX B - SUMMARY OF LABORATORY TESTS
Figure B-1 - Plasticity Chart
GEOTECHNICAL INVESTIGATION
FOR
ACCESSORY DWELLING UNIT
10789 JUNIPER COURT
CUPERTINO, CALIFORNIA
INTRODUCTION
This report presents the results of our geotechnical investigation for the proposed
accessory dwelling unit (ADU) to be constructed at your property located at 10789
Juniper Court in Cupertino, California. The location of the site is shown on the Vicinity
Map, Figure 1. The purpose of this investigation was to evaluate subsurface conditions at
the site and to provide geotechnical recommendations for the proposed improvements.
Project Description
The project consists of constructing an accessory dwelling unit at your property in
Cupertino. The single-story ADU will be located on the eastern portion of the property,
and will have a footprint of about 730 square feet. The ADU is expected to have a
concrete slab-on-grade floor. The site is relatively flat. Structural loads are expected to
be relatively light as is typical for this type of construction.
Scope of Work
The scope of our work for this investigation was presented in our agreement with you,
dated June 12, 2020. In order to accomplish our investigation, we performed the
following work.
• Review of geologic, geotechnical, and seismic conditions in the vicinity of the site.
• Subsurface exploration consisting of drilling, sampling, and logging of one
exploratory boring near the proposed ADU.
• Laboratory testing of selected samples to aid in soil classification and to help evaluate
the engineering properties of the soils encountered at the site.
Mr. Jim Otis Accessory Dwelling Unit Page 2 of 13
• Engineering analysis and evaluation of the surface and subsurface data to develop
earthwork guidelines and foundation design criteria.
• Preparation of this report presenting our findings and geotechnical recommendations
for the proposed improvements.
Limitations
This report has been prepared for the exclusive use of Mr. Jim Otis for specific
application to developing geotechnical design criteria for the proposed accessory
dwelling unit to be constructed at 10789 Juniper Court in Cupertino, California. We
make no warranty, expressed or implied, for the services we performed for this project.
Our services are performed in accordance with the geotechnical engineering principles
generally accepted at this time and location. This report was prepared to provide
engineering opinions and recommendations only. In the event there are any changes in
the nature, design, or location of the project, or if any future improvements are planned,
the conclusions and recommendations presented in this report should not be considered
valid unless: 1) the project changes are reviewed by us, and; 2) the conclusions and
recommendations presented in this report are modified or verified in writing.
The analysis, conclusions, and recommendations presented in this report are based on site
conditions as they existed at the time of our investigation; the currently proposed
improvements; review of readily available reports relevant to the site conditions; and
laboratory test results. In addition, it should be recognized that certain limitations are
inherent in the evaluation of subsurface conditions, and that certain conditions may not be
detected during an investigation of this type. Changes in the information or data gained
from any of these sources could result in changes in our conclusions or recommendations.
If such changes occur, we should be advised so that we can review our report in light of
those changes.
SITE EXPLORATION AND RECONNAISSANCE
Site reconnaissance and subsurface exploration were performed on July 2, 2020.
Subsurface exploration was performed using portable Minuteman drilling and sampling
equipment. One exploratory boring was advanced to a depth of 13 feet. The
approximate location of the boring is presented on the Site Plan, Figure 2. The boring log
and the results of our laboratory tests are attached in Appendices A and B, respectively.
Mr. Jim Otis Accessory Dwelling Unit Page 3 of 13
Surface Conditions
The site is located in a residential area northeast of the cul-de-sac of Juniper Court. At
the time of our investigation, the site was occupied by a two-story, wood frame residence
which had a stucco siding exterior. An attached three-car garage was located at the
western portion of the residence, where a concrete driveway provided access to Juniper
Court. A swimming pool with a concrete pool deck was located at the northern (rear)
side of the residence. Concrete and flagstone flatwork were located at various places
around the residence. The relatively flat site was landscaped with lawn grass, shrubs and
small trees.
Based on the age of the residence, we expect that it is supported on a shallow foundation
system; however, the depth and width of the foundations are unknown. We did not
observe any obvious signs of significant distress on the exterior perimeter stem wall of
the residence, where visible. The concrete and flagstone flatwork, including the
driveway, were generally observed in adequate condition. Roof downspouts appeared to
discharge into a closed pipe system.
Subsurface Conditions
At the location of our Boring EB-1, we encountered about 2 feet of residual soil
consisting of very stiff sandy lean clay/silt of low plasticity underlain by clayey
sandstone bedrock of the Santa Clara Formation to the maximum depth explored of 13
feet, where sampler refusal conditions were encountered.
A Liquid Limit of 36 and a Plasticity Index of 11 were measured on a sample of near-
surface soil obtained from our Boring EB-1. These test results indicate that the surface
and near-surface soils at the site have low plasticity and a relatively low potential for
expansion.
Ground Water
Free ground water was not encountered in our boring during the field exploration. The
boring was backfilled immediately following drilling; therefore, a stabilized ground water
level was not obtained. Please be cautioned that fluctuations in the level of ground water
can occur due to variations in rainfall, landscaping, underground drainage patterns, and
other factors. It is possible that perched ground water conditions may develop in the soils
and near the surface of the bedrock during and after significant rainfall or due to
landscape watering from the upslope areas.
Mr. Jim Otis Accessory Dwelling Unit Page 4 of 13
GEOLOGIC SETTING
We have briefly reviewed our local experience and geologic literature pertinent to the
general site area. The information reviewed indicates that the site is located in an area
underlain by Pleistocene- and upper Pliocene-aged weathered bedrock (QTsc) of the
Santa Clara Formation (Brabb, Graymer and Jones, 2000). The Santa Clara Formation is
expected to consist primarily of gray to red-brown poorly indurated conglomerate,
sandstone, and mudstone in irregular and lenticular beds. The conglomerate consists
mainly of sub-angular to sub-rounded cobbles in a sandy matrix but locally includes
pebbles and boulders, while the cobbles and pebbles are mainly chert, greenstone, and
greywacke with some schist, serpentinite, and limestone. The geology of the site vicinity
is shown on the Vicinity Geologic Map, Figure 3.
The lot and immediate site vicinity are located in a relatively level to gently sloping area.
The residence is located at an elevation of about 385 feet above sea level.
Faulting and Seismicity
There are no mapped through-going faults within or adjacent to the site and the site is not
located within a State of California Earthquake Fault Zone (formerly known as a Special
Studies Zone), an area where the potential for fault rupture is considered probable. The
closest active fault is the San Andreas Fault, which is located approximately 3.9 miles
southwest of the property. Thus, the likelihood of surface rupture occurring from active
faulting at the site is low.
The site is located near one of the fault rupture hazard zones along the potentially active
Berrocal/Monte Vista/Shannon fault system on the Santa Clara County Geologic Hazard
Zones map for fault rupture. The Berrocal/Monte Vista/Shannon fault system includes
low angle imbricate structural thrust faults, generally dips toward the south and
southwest, and follows the eastern edge of the Santa Cruz Mountains along the
southwestern edge of the Santa Clara Valley. The site is located approximately 1,000
feet northeast of the fault system, as shown on the Vicinity Hazard Map, Figure 4.
The San Francisco Bay Area is, however, an active seismic region. Earthquakes in the
region result from strain energy constantly accumulating because of the northwestward
movement of the Pacific Plate relative to the North American Plate. On average about
1.6-inches of movement occur per year. Historically, the Bay Area has experienced
large, destructive earthquakes in 1838, 1868, 1906 and 1989.
Mr. Jim Otis Accessory Dwelling Unit Page 5 of 13
The faults considered most likely to produce large earthquakes in the area include the San
Andreas, San Gregorio, Hayward, and Calaveras faults. The San Gregorio fault is
located approximately 17 miles southwest of the site. The Hayward and Calaveras faults
are located approximately 14 and 17 miles northeast of the site, respectively. These
faults and significant earthquakes that have been documented in the Bay Area are listed
in Table 1 below, and are shown on the Regional Fault and Seismicity Map, Figure 4.
Table 1. Earthquake Magnitudes and Historical Earthquakes
Accessory Dwelling Unit
Cupertino, California
Maximum Historical Estimated
Fault Magnitude (Mw) Earthquakes Magnitude
San Andreas 7.9 1989 Loma Prieta 6.9
1906 San Francisco 7.9
1865 N. of 1989 Loma Prieta Earthquake 6.5
1838 San Francisco-Peninsula Segment 6.8
1836 East of Monterey 6.5
Hayward 7.1 1868 Hayward 6.8
1858 Hayward 6.8
Calaveras 6.8 1984 Morgan Hill 6.2
1911 Morgan Hill 6.2
1897 Gilroy 6.3
San Gregorio 7.3 1926 Monterey Bay 6.1
In the future, the subject property will undoubtedly experience severe ground shaking
during moderate and large magnitude earthquakes produced along the San Andreas fault
or other active Bay Area fault zones. Using information from recent earthquakes,
improved mapping of active faults, ground motion prediction modeling, and a new model
for estimating earthquake probabilities, a panel of experts convened by the U.S.G.S. have
concluded there is a 72 percent chance for at least one earthquake of Magnitude 6.7 or
larger in the Bay Area before 2043. The Hayward fault has the highest likelihood of an
earthquake greater than or equal to magnitude 6.7 in the Bay Area, estimated at 33
percent, while the likelihood on the San Andreas and Calaveras faults is estimated at
approximately 22 and 26 percent, respectively (Aagaard et al, 2016).
Mr. Jim Otis Accessory Dwelling Unit Page 6 of 13
Earthquake Design Parameters
The State of California currently requires that buildings and structures be designed in
accordance with the seismic design provisions presented in the 2019 California Building
Code and in ASCE 7-16, “Minimum Design Loads for Buildings and Other Structures.”
Based on site geologic conditions and on information from our subsurface exploration at
the site, the site may be classified as Site Class C, very dense soil and soft rock, in
accordance with Chapter 20 of ASCE 7-10. Spectral Response Acceleration parameters
and site coefficients may be taken directly from the SEAOC/OSHPD website based on
the longitude and latitude of the site. For site latitude (37.3347), longitude (-122.0875)
and Site Class C, design parameters are presented on Table 2.
Table 2. 2016 CBC Seismic Design Criteria
Accessory Dwelling Unit
Cupertino, California
Spectral Response
Acceleration Parameters
Design Value
Mapped Value for Short Period - SS 2.3 27
Mapped Value for 1-sec Period - S1 0.838
Site Coefficient - Fa 1.2
Site Coefficient - Fv 1.4
Adjusted for Site Class - SMS 2.793
Adjusted for Site Class - SM1 1.174
Value for Design Earthquake - SDS 1.862
Value for Design Earthquake - SD1 0.783
CONCLUSIONS
From a geotechnical viewpoint, the site is suitable for the proposed improvements,
provided the recommendations presented in this report are followed during design and
construction. In our opinion, the proposed ADU may be supported on conventional
shallow foundations bearing on undisturbed stiff native soil. Specific geotechnical
recommendations for the project are presented in the following sections of this report.
Because subsurface conditions may vary from those encountered at the location of our
boring, and to observe that our recommendations are properly implemented, we
recommend that we be retained to 1) review the project plans for conformance with our
recommendations; and 2) observe and test during earthwork and foundation construction.
Mr. Jim Otis Accessory Dwelling Unit Page 7 of 13
FOUNDATIONS
Shallow Foundations
In our opinion, the proposed ADU may be supported on conventional shallow spread
footings bearing in undisturbed native soil. Spread footings should have a width of at
least 15 inches and should extend at least 24 inches below exterior grade, at least 18
inches below the bottom of concrete slabs-on-grade, and at least 15 inches below the
interior crawl space grade, whichever is deeper.
Footings with at least these minimum dimensions may be designed for an allowable
bearing pressure of 2,500 pounds per square foot (psf) for dead plus live loads, with a
one-third increase allowed when considering additional short-term wind or seismic
loading. The weight of the footings may be neglected for design purposes.
All footings located adjacent to utility lines should be embedded below a 1:1 plane
extending up from the bottom edge of the utility trench. We recommend that continuous
foundations be reinforced with sufficient top and bottom steel to provide structural
continuity and to permit spanning of local irregularities.
The bottom of all footing excavations should be cleaned of loose and soft soil and debris.
A member of our staff should observe all footing excavations prior to placement of
reinforcing steel to confirm that they expose suitable material, have at least the
recommended minimum dimensions, and have been properly cleaned. If soft or loose
soils or fill are encountered in the foundation excavations, our field representative will
require these materials to be removed and may require a deeper footing embedment depth
before the reinforcing steel and concrete is placed.
Lateral Loads on Footings
Lateral loads will be resisted by friction between the bottom of the footings and the
supporting subgrade. A coefficient of friction of 0.30 may be assumed for design. In
addition to friction, lateral resistance may be provided by passive soil pressure acting
against the sides of foundations cast neat in footing excavations within native soil. We
recommend assuming an equivalent fluid pressure of 350 pounds per cubic foot for
passive soil resistance, where appropriate. The upper 1 foot of passive soil resistance
should be neglected where soil adjacent to the footing is not covered and protected by a
relatively level concrete slab.
Mr. Jim Otis Accessory Dwelling Unit Page 8 of 13
Settlement
Thirty-year post-construction differential settlement due to static loads is not expected to
exceed about 3/4-inch across the proposed ADU supported on spread footing
foundations, provided foundations are designed and constructed as recommended.
SLABS-ON-GRADE
General Slab Considerations
To reduce the potential for movement of the slab subgrade, at least the upper 6 inches of
surface soil should be scarified and compacted at a moisture content slightly above the
laboratory optimum. The native soil subgrade should be kept moist up until the time the
non-expansive fill, crushed rock and vapor barrier, and/or aggregate base is placed. Slab
subgrades and non-expansive fill should be prepared and compacted as recommended in
the section of this report titled “Earthwork.” Exterior flatwork and interior slabs-on-
grade should be underlain by a layer of non-expansive fill as discussed below. The non-
expansive fill should consist of aggregate base rock or a clayey soil with a plasticity
index of 15 or less.
Considering the potential for some movement of the surface soils, we expect that a
reinforced slab will perform better than an unreinforced slab. Consideration should also
be given to using a control joint spacing on the order of 2 feet in each direction for each
inch of slab thickness.
Exterior Flatwork
Concrete walkways and exterior flatwork, should be at least 4 inches thick and should be
constructed on at least 6 inches of Class 2 aggregate base. To improve performance,
exterior slabs-on-grade, such as for patios, may be constructed with a thickened edge to
improve edge stiffness and to reduce the potential for water seepage under the edge of the
slabs and into the underlying base and subgrade. In our opinion, the thickened edges
should be at least 8 inches wide and ideally should extend at least 4 inches below the
bottom of the underlying aggregate base layer.
At-Grade Interior Slabs
Concrete slab-on-grade floors should be constructed on a layer of non-expansive fill at
least 6 inches thick that is placed and compacted on a properly prepared and compacted
subgrade. Recycled aggregate base should not be used for non-expansive fill below
interior slabs-on-grade, since adverse vapor could occur from crushed asphalt
components.
Mr. Jim Otis Accessory Dwelling Unit Page 9 of 13
Due to the potential for differential movement/settlement, we recommend that slab-on-
grade floors be at least 5 inches thick, and be reinforced with sufficient typical steel
reinforcement to span across local irregularities.
In areas where dampness of concrete floor slabs would be undesirable, such as within
building interiors, concrete slabs should be underlain by at least 4 inches of clean, free-
draining gravel, such as ½-inch to ¾-inch clean crushed rock with no more than 5 percent
passing the ASTM No. 200 sieve. Pea gravel should not be used. The crushed rock layer
should be densified and leveled with vibratory equipment, and may be considered as the
upper portion of the non-expansive fill recommended above.
To reduce vapor transmission up through concrete floor slabs or the basement mat, the
crushed rock section should be covered with a high quality vapor barrier conforming to
the requirements of ASTM E 1745 Class A, with a water vapor transmission rate less
than or equal to 0.01 perms (such as 15-mil thick “Stego Wrap Class A”) should be used.
The vapor barrier should be placed directly below the concrete slab and mat. Sand above
the vapor barrier is not recommended. The vapor barrier should be installed in
accordance with ASTM E 1643. All seams and penetrations of the vapor barrier should
be sealed in accordance with manufacturer’s recommendations.
The permeability of concrete is affected significantly by the water:cement ratio of the
mix, with lower water:cement ratios producing more damp-resistant slabs and higher
strength. Where moisture protection is important and/or where the concrete will be
placed directly on the vapor barrier, the water:cement ratio should be 0.45 or less. To
increase the workability of the concrete, mid-range plasticizers may be added to the mix.
Water should not be added to the mix unless the slump is less than specified and the
water:cement ratio will not exceed 0.45. Other steps that may be taken to reduce
moisture transmission through concrete slabs-on-grade include moist curing for 5 to 7
days and allowing the slab to dry for a period of two months or longer prior to placing
floor coverings. Prior to installation of floor coverings, it may be appropriate to test the
slab moisture content for adherence to the manufacturer’s requirements to determine
whether a longer drying time is necessary.
Mr. Jim Otis Accessory Dwelling Unit Page 10 of 13
EARTHWORK
Clearing and Subgrade Preparation
All deleterious materials, such as slabs, existing foundations and utilities to be
abandoned, soft or loose soils, overly-moist or wet soil, vegetation, root systems, surface
fill and topsoil, should be cleared from areas of the site to be built on. The actual
stripping depth should be established by us at the time of construction. Excavations that
extend below finish grade should be backfilled with structural fill that is water-
conditioned, placed, and compacted as recommended in the section titled “Compaction.”
After the site has been properly cleared, stripped, and excavated to the required grades,
exposed soil surfaces in areas to receive structural fill or slabs-on-grade should be
scarified to a depth of 6 inches, moisture conditioned, and compacted as recommended
for structural fill in the section titled "Compaction."
To help reduce the potential for soil movement due to moisture fluctuation, exterior
flatwork, slabs and pavement subgrades, footing and utility trench excavations should be
kept in a moist condition throughout the construction period.
Material For Fill
All on-site soil containing less than 3 percent organic material by weight (ASTM D2974)
should be suitable for use as structural fill. Structural fill should not contain rocks or
pieces larger than 6 inches in greatest dimension and no more than 15 percent larger than
2.5 inches. Imported non-expansive fill should have a Plasticity Index no greater than 15,
should be predominately granular, and should have sufficient binder so as not to slough
or cave into foundation excavations and utility trenches. Recycled aggregate base should
not be used for non-expansive fill at building interior. A member of our staff should
approve proposed import materials prior to their delivery to the site.
Compaction
Scarified soil surfaces and all structural fill should be placed and compacted in uniform
lifts no thicker than 8 inches in pre-compacted thickness, conditioned to the appropriate
moisture content, and compacted as recommended for structural fill in Table 3. The
relative compaction and moisture content recommended in Table 3 is relative to ASTM
Test D1557, latest edition.
Mr. Jim Otis Accessory Dwelling Unit Page 11 of 13
Table 3. Compaction Recommendations
Accessory Dwelling Unit
Cupertino, California
General Min. Relative Compaction* Moisture Content*
• Scarified subgrade in areas 90 percent Above optimum
to receive structural fill
• Structural fill composed 90 percent Above optimum
of native soil.
• Structural fill composed 90 percent Above optimum
of non-expansive fill.
• Structural fill below a 93 percent Above optimum
depth of 4 feet.
Utility Trench Backfill
• On-site soil. 90 percent Above optimum
• Imported sand 95 percent Near optimum
* Relative to ASTM Test D1557, latest edition.
At the start of site grading and earthwork construction, and prior to subgrade preparation
and placement of non-expansive fill, representative samples of on-site soil and import
material will need to be collected in order for a laboratory compaction test to be
performed for use during on-site density testing. Sampling of on-site soil and proposed
import material should be requested by the contractor at least 5 days prior to when our
staff will be needed for density testing to allow time for soil sampling and laboratory
testing to be performed prior to our on-site compaction testing.
Temporary Slopes and Excavations
The contractor should be responsible for the design and construction of all temporary
slopes and any required shoring. Shoring and bracing should be provided in accordance
with all applicable local, state and federal safety regulations, including the current OSHA
excavation and trench safety standards.
Because of the potential for variation of the on-site soils, field modification of temporary
slopes may be required. Unstable materials encountered on slopes and trenches during
and after excavation should be trimmed off even if this requires cutting the slopes back to
a flatter inclination.
Mr. Jim Otis Accessory Dwelling Unit Page 12 of 13
Protection of structures near cuts should also be the responsibility of the contractor. In
our experience, a preconstruction survey is generally performed to document existing
conditions prior to construction, with intermittent monitoring of the structures during
construction.
Finished Slopes
We recommend that new finished slopes be cut or filled to an inclination preferably no
steeper than 2.5:1 (horizontal:vertical). Exposed slopes may be subject to minor
sloughing and erosion that would require periodic maintenance. We recommend that all
slopes and soil surfaces disturbed during construction be planted with erosion resistant
vegetation.
Surface Drainage
Finished grades should be designed to prevent ponding and to drain surface water away
from foundations and edges slabs and pavements, and toward suitable collection and
discharge facilities. Slopes of at least 2 percent are recommended for flatwork and
pavement areas with 5 percent preferred in landscape areas within 8 feet of the structures,
where possible. At a minimum, splash blocks should be provided at the ends of
downspouts to carry surface water away from perimeter foundations. Preferably,
downspout drainage should be collected in a closed pipe system that is routed to a storm
drain system or other suitable discharge outlet.
Drainage facilities should be observed to verify that they are adequate and that no
adjustments need to be made, especially during the first two years following construction.
We recommend preparing an as-built plan showing the locations of surface and
subsurface drain lines and clean-outs. The drainage facilities should be periodically
checked to verify that they are continuing to function properly. It is likely the drainage
facilities will need to be periodically cleaned of silt and debris that may build up in the
lines.
FUTURE SERVICES
Plan Review
Romig Engineers should review the completed grading and foundation plans for
conformance with the recommendations contained in this report. We should be provided
with these plans as soon as possible upon completion in order to limit the potential for
delays in the permitting process that might otherwise be attributed to our review process.
Mr. Jim Otis Accessory Dwelling Unit Page 13 of 13
In addition, it should be noted that many of the local building and planning departments
now require “clean” geotechnical plan review letters prior to acceptance of plans for their
final review. Since our plan reviews typically result in recommendations for
modification of the plans, our generation of a “clean” review letter often requires two
iterations. At a minimum, we recommend that the following note be added to the plans:
“Earthwork, slab subgrade and non-expansive fill preparation, foundation construction,
utility trench backfilling, site drainage and grading should be performed in accordance
with the geotechnical report prepared by Romig Engineers, Inc., dated July 24, 2020.
Romig Engineers should be notified at least 48 hours in advance of any earthwork or
foundation construction and should observe and test during earthwork and foundation
construction as recommended in the geotechnical report. Romig Engineers should be
notified at least 5 days prior to earthwork, trench backfill and subgrade preparation work
to allow time for sampling of on-site soil and laboratory compaction curve testing to be
performed prior to on-site compaction density testing.”
Construction Observation and Testing
The earthwork and foundation phases of construction should be observed and tested by us
to 1) Establish that subsurface conditions are compatible with those used in the analysis
and design; 2) Observe compliance with the design concepts, specifications and
recommendations; and 3) Allow design changes in the event that subsurface conditions
differ from those anticipated. The recommendations in this report are based on a limited
amount of subsurface exploration. The nature and extent of variation across the site may
not become evident until construction. If variations are then exposed, it will be necessary
to re-evaluate our recommendations.
REFERENCES
Aagaard, B.T., Blair, J.L., Boatwright, J., Garcia, S.H., Harris, R.A., Michael, A.J.,
Schwartz, D.P., and DiLeo, J.S., 2016, Earthquake Outlook for the San Francisco Bay
Region 2014-2043 (ver. 1.1, August 2016): U.S. Geological Survey Fact Sheet 2016-3020,
6 p., http.
Al Atik, L., and Sitar, N., 2010, Seismic Earth Pressures on Cantilever Retaining Structures,
Journal of Geotechnical and Geoenvironmental Engineering, ASCE Vol. 136, No. 10.
American Society of Civil Engineers, 2016, Minimum Design Loads for Buildings and
Other Structures, ASCE Standard 7-16.
Brabb, E.E., Graymer, R.W., and Jones, D.L., 2000, Geology of the Palo Alto 30 x 60
Minute Quadrangle, California: U.S. Geological Survey Miscellaneous Field Studies Map
MF-2332.
California Building Standards Commission, and International Code Council, 2019
California Building Code, California Code of Regulations, Title 24, Part 2.
California Department of Conservation, Division of Mines and Geology (DMG), 1994,
Fault-Rupture Hazard Zones in California, Special Publication 42.
California Geological Survey, 2011, Probabilistic Seismic Hazards Mapping Ground
Motion Page, http://redirect.conservation.ca.gov/cgs/rghm/pshamap/pshamap.asp/
California Geological Survey, 2002, Seismic Hazard Zones Map of the Cupertino
Quadrangle.
Cotton, Shires & Associates, Inc., 2003, Geologic and Seismic Hazards Map of the City of
Cupertino.
SEAOC/OSHPD, 2019, Seismic Design Maps, https://seismicmaps.org/
Insert map here and add line around picture - size 1 in black
Scale: 1 inch = 2000 feet
Base is United States Geological Survey Cupertino 7.5 Minute Quadrangle, dated 1991.
VICINITY MAP FIGURE 1
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
SITE
LEGEND
EB-1 Approximate Locations of Exploratory Boring.
Approximate Scale: 1 inch = 10 feet.
Base is site plan prepared by Innovative Concepts, dated June 5, 2020.
SITE PLAN FIGURE 2
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
JUNIPER COURT
EB-1
Proposed ADU
at East Corner
of the Property
Insert map here and add line around picture - size 1 in black
Base is Geologic Map of Palo Alto 30 x 60 Minute Quadrangle (Brabb, Graymer, and Jones, 2000).
VICINITY GEOLOGIC MAP FIGURE 3
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
LEGEND
Contact: dashed where approximately
located, dotted where concealed.
Reverse or thrust fault.
Strike and dip of bedding.
SITE
Qhaf
Qpaf
Santa Clara Formation
Alluvial fan deposits
Alluvial fan and fluvial deposits
Sheared Rock
Monterey Formation
Greenstone
Scale: 1 inch = 1000 feet
Base is Geologic and Seismic Hazards Map of the City of Cupertino (Cotton, Shires, & Associates, Inc., 2003).
VICINITY HAZARD MAP FIGURE 4
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
LEGEND
SLOPE INSTABILITY
FAULT RUPTURE
LIQUEFACTION/INUNDATION
HILLSIDE
SITE
Insert map here and add line around picture - size 1 in black
Earthquakes with M5+ from 1900 to 1980, M2.5+ from 1980 to January 2015. Faults with activity in last 15,000 years.
Based on data sources from Northern California Earthquake Data Center and USGS Quaternary Fault and Fold
Database, accessed May 2015.
REGIONAL FAULT AND SEISMICITY MAP FIGURE 5
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
SITE
Magnitude Year
0 3 6 12 miles
APPENDIX A
FIELD INVESTIGATION
The soils encountered during drilling were logged by our representative and samples
were obtained at depths appropriate to the investigation. The samples were taken to our
laboratory where they were examined and classified in accordance with the Unified Soil
Classification System. The log of our boring, as well as a summary of the soil
classification system (Figure A-1) used on the log, are attached.
Several tests were performed in the field during drilling. The standard penetration test
resistance was determined by dropping a 140-pound hammer through a 30-inch free fall,
and recording the blows required to drive the 2-inch (outside diameter) sampler 18
inches. The standard penetration test (SPT) resistance is the number of blows required to
drive the sampler the last 12 inches, and is recorded on the boring log at the appropriate
depths. The results of these field tests are also presented on the boring log. Soil samples
were also collected using 2.5-inch and 3-inch O.D. drive samplers. The blow counts
shown on the log for these larger diameter samplers do not represent SPT values and
have not been corrected in any way.
The location of the boring was established by pacing using the undated site plan provided
to us. The location of the boring should be considered accurate only to the degree
implied by the method used.
The boring log and related information depict our interpretation of subsurface conditions
only at the specific location and time indicated. Subsurface conditions and ground water
levels at other locations may differ from conditions at the location where sampling was
conducted. The passage of time may also result in changes in the subsurface conditions.
USCS SOIL CLASSIFICATION
SOIL
TYPE
CLEAN GRAVEL GW Well graded gravel, gravel-sand mixtures, little or no fines.
COARSE GRAVEL (< 5% Fines) GP Poorly graded gravel or gravel-sand mixtures, little or no fines.
GRAINED GRAVEL with GM Silty gravels, gravel-sand-silt mixtures, non-plastic fines.
SOILS FINES GC Clayey gravels, gravel-sand-clay mixtures, plastic fines.
(< 50 % Fines) CLEAN SAND SW Well graded sands, gravelly sands, little or no fines.
SAND (< 5% Fines) SP Poorly graded sands or gravelly sands, little or no fines.
SAND SM Silty sands, sand-silt mixtures, non-plastic fines.
WITH FINES SC Clayey sands, sand-clay mixtures, plastic fines.
ML Inorganic silts and very fine sands, with slight plasticity.
FINE SILT AND CLAY CL Inorganic clays of low to medium plasticity, lean clays.
GRAINED Liquid limit < 50% OL Organic silts and organic clays of low plasticity.
SOILS MH Inorganic silt, micaceous or diatomaceous fine sandy or silty soil.
(> 50 % Fines) SILT AND CLAY CH Inorganic clays of high plasticity, fat clays.
Liquid limit > 50% OH Organic clays of medium to high plasticity, organic silts.
HIGHLY ORGANIC SOILS Pt Peat and other highly organic soils.
BEDROCK BR Weathered bedrock.
RELATIVE DENSITY CONSISTENCY
SAND & GRAVEL BLOWS/FOOT* SILT & CLAY STRENGTH^ BLOWS/FOOT*
VERY LOOSE 0 to 4 VERY SOFT 0 to 0.25 0 to 2
LOOSE 4 to 10 SOFT 0.25 to 0.5 2 to 4
MEDIUM DENSE 10 to 30 FIRM 0.5 to 1 4 to 8
DENSE 30 to 50 STIFF 1 to 2 8 to 16
VERY DENSE OVER 50 VERY STIFF 2 to 4 16 to 32
HARD OVER 4 OVER 32
GRAIN SIZES
BOULDERS COBBLES GRAVEL SAND SILT & CLAY
COARSE FINE COARSE MEDIUM FINE
12 " 3" 0.75" 4 10 40 200
SIEVE OPENINGS U.S. STANDARD SERIES SIEVE
Classification is based on the Unified Soil Classification System; fines refer to soil passing a No. 200 sieve.
* Standard Penetration Test (SPT) resistance, using a 140 pound hammer falling 30 inches on a 2 inch O.D. split spoon
sampler; blow counts not corrected for larger diameter samplers.
^ Unconfined Compressive strength in tons/sq. ft. as estimated by SPT resistance, field and laboratory tests, and/or
visual observation.
KEY TO SAMPLERS
z Modified California Sampler (3-inch O.D.)
y Mid-size Sampler (2.5-inch O.D.)
x Standard Penetration Test Sampler (2-inch O.D.)
KEY TO EXPLORATORY BORING LOGS FIGURE A-1
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
SECONDARY DIVISIONS
PRIMARY DIVISIONS
DRILL TYPE: Minuteman with 3-1/4" Continuous Flight Auger LOGGED BY: WZ
DEPTH TO GROUND WATER: Not Encountered SURFACE ELEVATION: NA DATE DRILLED: 7/2/2020
CLASSIFICATION AND DESCRIPTION
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CL/0 z
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10 x 69 19
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x 40 15
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x 87 15
15
20
EXPLORATORY BORING LOG EB-1 BORING EB-1
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
Stiff
Very
Soft
Residual Soil: Red-brown, Sandy Lean Clay/Silt, moist, fine
to coarse grained sand, low plasticity.
n Liquid Limit = 36, Plasticity Index = 11.
Santa Clara Formation: Red-brown, Clayey Sandstone,
moist, fine to coarse grained, low to moderate plasticity fines,
trace fine subangular gravel, very severely weathered.
Bottom of Boring at 13 feet.
Note: The stratification lines represent the approximate
boundary between soil and rock types, the actual
transition may be gradual.
*Measured using Torvane and Pocket Penetrometer devices.
APPENDIX B
LABORATORY TESTS
Samples from subsurface exploration were selected for tests to help evaluate the physical
and engineering properties of the soils that were encountered. The tests that were
performed are briefly described below.
The natural moisture content was determined in accordance with ASTM D2216 on nearly
all of the samples recovered from the boring. This test determines the moisture content,
representative of field conditions, at the time the samples were collected. The results are
presented on the boring log at the appropriate sample depths.
The Atterberg Limits were determined on one sample of soil in accordance with ASTM
D4318. The Atterberg limits are the moisture content within which the soil is workable
or plastic. The results of this test are presented in Figure B-1 and on log of Boring EB-1
at the appropriate sample depth.
Passing USCS
Chart Boring Sample Water Liquid Plasticity Liquidity No. 200 Soil
Symbol Number Depth Content Limit Index Index Sieve Classification
(feet) (percent) (percent) (percent) (percent) (percent)
EB-1 0-1 23 36 11 CL/ML
PLASTICITY CHART FIGURE B-1
OTIS ACCESSORY DWELLING UNIT JULY 2020
CUPERTINO, CALIFORNIA PROJECT NO. 5180-1
ROMIG ENGINEERS, INC.
1390 El Camino Real, 2 nd Floor
San Carlos, California 94070
Phone: (650) 591 -5224
www.romigengineers.com