HomeMy WebLinkAboutCDP 01-13; Goetz Residence; Geotechnical; 2001-10-10OWEN ENGINEERING GROUP
Civil, Geotechnical and Structural Engineering
October 10,2001 ' DECEIVED
HAR 0 8 2002Mr. Dean Goetz
603 North Highway 101 ENGINEERING
Solana Beach, CA 92075 DEPARTMENT
Subject: Geotechnical Review and Update of Applicable Geology and Geotechnical
Reports for 5323 Carlsbad Blvd. Carlsbad, CA
Re: Assumption of Geotechnical Responsibility, Updated Geotechnical Report,
Assessor Parcel Number 210-120-30, Carlsbad, CA, by C. J. Randle, P.E.,
Dated March 16, 1998.
Dear Mr. Goetz:
This geotechnical letter is being written to updates the referenced report and previously
referenced reports, relative to the development of your site.
Prior to this update, the referenced report addressed bluff stability and recommended
setbacks relative to the construction of the proposed dwellings at this site. In addition,
said report is applicable to all three parcels which are part of Parcel Map 18236
previously identified as APN 210-120-30.
Applicable criteria for this development are summarized as followed:
Where the design parameters differ from the referenced reports, these values are
updated herein and therefore apply to your proposed residence.
As noted in previous reports by this office, the recommended setback from the
bluff is 40 feet from the top of the existing bluff.
Design criteria is as follows:
Item 1. The lateral pressure evaluation should be based on 32-degrees angle of
internal friction and 100 PSF of cohesion.
Item 2. The unit soil bearing for continuous foundations located a minimum of 18
inches below lowest adjacent grade shall not exceed 2500 PSF. This is a
conservative value based on the Terzaghi Unit Bearing Value method for
1525 Grand Ave, San Marcos, California 92069 Telephone (760) 471 - 6000 Fax (760) 471-6096
14661 Myford Road, Suite C, Tustin, California 92806 Telephone (714) 734 - 7993 Fax (714) 734-9732
5323 Carlsbad Blvd
October 18, 2001 Page 3
Limitations
Professional judgments presented herein are based partly on our evaluation of the
available technical information that was reviewed, partly on our understanding of the
proposed construction, and partly on our general experience in the civil and geotechnical
field.
The recommendations and civil engineering basis of design were based on the
assumption that the soil conditions present at the site do not deviate considerably from
those presented in the referenced reports and investigations performed by others (see
references). If variations or undesirable geotechnical or soil conditions are encountered
during construction, the engineering geologist and civil engineer should be notified
immediately and consulted for further recommendations. We do not guarantee the
performance of the project in any respect.
This firm does not practice or consult in the field of safety engineering. We do not direct
the contractors operations, and cannot be responsible for the safety of other than our own
personnel on the subject site; therefore, the safety of others is the responsibility-of the
contractor. The contractor should notify the owner if he considers any of the
recommended actions presented herein to be safe.
Sincerely,
Owen Engineering
C. J. Randle, P.E.
Principal Engineer
RCE 22096 CA
5323 Carlsbad Blvd
October 18, 2001 Page 4
References:
1. Geotechnical Update, Carlsbad Beach Lot, APN 210-120-30, Carlsbad Drive,
Carlsbad, California; 1996, by Geotechincs, Inc., Project No. 0319-001-00,
dated September 3, 1996.
2. Geotechnical Investigation and Bluff Retreat Study, Parcel No. 210-120-30,
Carlsbad, California; 1991, by ICG, Inc., Job No. 04-8529-001-00-00, Dated
March 28,1991.
3. Geotechnical Investigation and Bluff Retreat Study, Parcel No. 210-120-31,
Carlsbad, California; 1989, by ICG, Inc., Job No. 05-8109-001-00-00, Dated
November 6,1989.
4. California's Battered Coast; 1985 California Coastal Commission Meeting,
Professional Papers and Publications, San Diego, California, September, 1985
San Diego Association of Geologists.
5. Supplemental Geotechnical Investigation, Proposed Coastal Protection
Measures for the 2+1- acre coastal site, Carlsbad, California; 1984, by
Converse Consultants, Project No. 83-2299-02, dated September 20,1984.
6. Geotechnical Investigation, Proposed Coastal Protection Measures for Ecke
Site, Carlsbad, California; 1984, by Converse Consultants, Project No. 83-
02299-01, dated February 1,1984.
Attachment:
1. Assumption of Geotechnical Responsibility, Updated Geotechnical Report,
Assessor Parcel Number 210-120-30, Carlsbad, CA, by C. J. Randle, P.E.,
Dated March 16,1998.
Excerpts from:
1. From Reference # 2 above, Pages 14 through 19.
2. From Reference # 3 above, Pages 10,12, & 13 and Figures 2 & 3.
3. Jon Jensen, Tentative Parcel Map
ATTACHMENT 1
March 16, 1998
C. J. Handle, P.E., Civil Engineer
5858 Mt. AHfan Drive, Suite 235
San Diego, CA 92111
Telephone (619) 571-6271
Fax (619) 571-3943
Mr. Jon Jensen
Jon A. Jensen and Associates
451 South Escondido Blvd.
Escondido, CA 92025
Subject: Assumption of Geotechnical Responsibility
Updated Geotechnical Report
Assessor Parcel Number 210-120-30
Carlsbad, California
References:
1. Geotechnical Update, Carlsbad Beach Lot, APN 210-120-30, Carlsbad Drive, Carlsbad,
California; 1996, by Geotechnics, Inc., Project No. 0319-001-00, dated Septembers, 1996.
2. Geotechnical Livestigation and BluffRetreat Study, Parcel No. 210-120-30, Carlsbad,
California; 1991, by ICG, Inc., Job No. 04-8529-001-00-00, dated March 28,1991.
3. Geotechnical Investigation and Bluff Retreat Study, Parcel No. 210-120-31, Carlsbad,
California; 1989, by ICG, Inc., Job No. 05-8109-001-00-00, dated November 6,1989.
4. California's Battered Coast; 1985, California Coastal Commission Meeting, Professional
Papers and Publication, San Diego, California, September, 1985 - San Diego Association
of Geologists.
5. Supplemental Geotechnical Investigation, Proposed Coastal Protection Measures for the 2+1-
acre coastal site, Carlsbad, California; 1984, by Converse Consultants, Project No. 83-2299-
02, dated September 20,1984.
6. Geotechnical Investigation, Proposed Coastal Protection Measures for the Ecke Site,
Carlsbad, California; 1984, by Converse Consultants, Project No. 83-02299-01, dated
February 1,1984.
Page 2
APN 210-120-30
March 16,1998
INTRODUCTION
In accordance with your request, we have completed an updated geotechnical study of the subject
site, APN 210-120-30, in the City of Carlsbad, California. The purpose of this study was to review
geologic and soils engineering data as they relate to future site development, evaluate sea bluff rate
of retreat, and establish building and construction set-backs relative to the top of the sea bluff.
SCOPE OF WORK • • . "'
The scope of our work has included the following tasks: ;
Review available geological reports and data pertinent to the subject site. The list of
referenced and/or review data is included on page 1 of this report.
Field review of the site and adjacent areas including an assessment of the nearby geological
units and conditions, to include existing sea bluff conditions.
Preparation and processing of this report.
In addition, our study is supplemented by our engineering services, subsurface work and
observation services during construction of a stairway access to the beach along the south
property boundary of the subject site placed into the bedrock and constructed on caissons.
PROPOSED DEVELOPMENT
The actual plans for future development at the property are not yet complete, but include provisions
that will preclude any excess drainage from within the site to be channeled over the sea bluff face.
This is proposed to be accomplished through landscaping controls, graded earthen swales and other
devices to collect and divert water away from the sea bluff. Additionally, the surface drainage of
the property is enhanced by the surface drainage provisions, which have been developed in and
around the concrete beach access stairway along the south boundary of the property.
Page 3
APN 210-120-30
March 16,1998
FTTynvr^Y OF REVIEW
Based on our study, references 1,2, 5 and 6 adequately describe the property soil and geological
conditions and the report conclusions and recommendations are still- appropriate and applicable to
development of the property except for those items described within the discussion section of this
report. .
DISCUSSION • " ••'•'.:' .
Irevious reports based sea cliff retreat and construction setbacks upon an "empirical" figure of 25:
ifeet extrapolated over 62 years (ref. 2). However, tie same report indicates "relatively little -
changes" to the bluff top. Reference S.indicates "There has been little retreat of the bluff top in the'...
-vicinity... " of the site based on a study interval from 1929 through 1984.
The reports also indicate a concern relative to the cove area of the site. The cove area is reported
. to have undergone a short tern) interval of episodic and relative rapid erosion. Cove areas are not
uncommon along the coastal area of San Diego County, and even if underlain by more erodible or .
"softer" material once the weaker or more credible material forms the cove feature, that natural set
back is sufficient to create a buffer from the brunt of direct wave attack. Thus, once formed* the top:.
of bluff retreatrate for'the cove area becomes similar to 'that of adjacent tops of bluff to both sides,
ofthecove. •.
Reference 4 (a compilation of professional studies), the. 1985 California Coastal Commission
Meeting Publication at San Diego provides documented studies for sea bluff retreat and erosion of
the San Diego County Coastal area. These documents include both short term and long term
studies, as well as work comparison of similar rates for the base and tops of bluffs.
• •" * •
These studies confirmed an overall rate of retreat for both the upper and, lower parts of the bluffs
at 1 to 3 inches per year. The studies also confirmed that rates tended to decrease slightly in areas
after episodic rapid erosion and/or retreat had occurred in any given area. These studies were based
on the time interval of about 50 to 75 years,
Forthe property, we can assume for the 75 year design period that a rate of retreat of about 6 to 19
feet might occur. Because there was a period of relatively rapid episodic erosion for the site area
in about 1977 through 1983, we can assume that the longer term overall rate will be less and more
likely on the order of about 6 to 12 feet, or less. These'rates will be significantly affected and
reduced in the event that longer'term sea cliff erosion control devices are constructed at the site.
Page 4
APN 2-10-120-30
March 16, 1998
CONCLUSIONS AND RECOMMENDATIONS
Based on our study, the previous soil and geologic findings for the site are still appropriate and
applicable for the site, except for those items outlined in the Discussion section of this report and
updated in the following paragraphs. We assume soil and geologic responsibility for the previous
reports, findings, conclusions and recommendations of the previous reports.
Based on the site conditions and our study, the rate of retreat for the top of bluff at the project site
is established to be on the order of 1 to 2 inches per yearfor the 75 year planned life of the proposed
development of the property.
- ,
The bluff retreat may be addressed with the following dimensions and criteria:
1. A 25 foot set back from the face of the bluff with all foundations to be placed on deep
caissons founded in firm dense to very dense, fonnational sandstone (bedrock). The bedrock
appears to be at maximum depths of elevations of-4 to -6 feet (msl) at elevation +8 and
increases easterly*
2. Bluff setbacks will be established at 30 feet from the face of the bluff for all foundation
systems which conform to the current Uniform Building Code criteria. However,
conventional foundation systems will require development of a deepened grade beam
foundation system which will be a minimum of 36 inches below lowest adjacent grade.
3. Typical slab on grade construction with minimal footing depths may be constructed with a
Set back of 40 feet established from the face of the bluff.. -
Based on our elevation of the recommended setbacks (with relation to the proposed 3 lot Parcel
Map) each lot will easily provide for the necessary building area, without encroaching into the
proposed setback. Similarly, the remaining land area will easily accommodate this proposed
development.
. .
The option of set backs is prudent and will easily address coastal retreat, which essentially will be
mitigated by the restrictions placed on the site drainage controls. The 30 and 40 foot options are
for all intents and purposes a conservative response to the bluff top setbacks. When compared to
the proposery 25 foot setback for the northerly and adjacent vacant parcel.
Page5
APN210-120-30
March 16, 1998
Actual foundation recommendations for future proposed site development and construction will
require a specific analysis study which should address the type of foundations outlined above and
other factors unique to each structure.
LIMITATIONS
Soil and bedrock conditions may vary in character and soil moisture content from those disclosed
during the previous site subsurface studies. We assume no responsibility or liability for work,
testing, or recommendations performed or provided by others.
The conclusions and recommedations contained herein are professional opinions. These opinions
have been derived in accordance with current standards of practice and no warranty is expressed or
jrpWl^Ajl *
Should you have any questions, or require additional service please do not hesitate to contract us.
Very truly yours,
tf^-l^
Ernest R. Artim '•"••^i::>--;''
CEG 1084
Distribution: (3) client
Charles J
RCE 22096
EXCERPT 1
M.B. Shores Corporation Job No. 04-8529-001-00-00
March 28, 1991 . Log No. 1-1418
Page 14
8.0 GEOTECHNICAL EVALUATION AND RECOMMENDATIONS
8.1 General Discussion
No geotechnical conditions were apparent during our investigation which would
preclude the site development as planned. The site condition which should most
severely impact the development is the potential for future erosion and slope stability
of the bluff area. The on-site soils are considered non-expansive and should be
suitable to support the proposed structure once the existing fills and loose soils have
been removed or recompacted. . .
The following recommendations are intended to provide a suitable bearing pad for
foundations. We recommend that all loose soils or undocumented fill soils within the
building area be removed and replaced as compacted fill. Once these soils have been
. prepared, the structure can be supported on a conventional shallow foundation system
bearing entirely on bedrock or compacted fill.
The remainder of Section 8.0presents our recommendations for foundation design and
construction in detail. These recommendations are based on empirical and analytical
methods typical of the standard of practice in southern California. If these
recommendations appear not to cover any specific feature of the project, please
contact our office for additions or revisions to our recommendations.
\
8.2 Structural Setback and Slope Stability
A structural setback from the bluff rim is recommended to protect dwellings from
collapse within the economic life of the residence. The setback depicted on the
»•"-.. .
Geotechnical Maps (Plates 1 and 3) is based on slope stability analysis along two cross
sections through the site (Plate 2). Engineering practice uses a factor of safety of 1.5
as the minimum criteria for a stable slope (a factor of safety of less than 1.0 indicates
that a slope should fail). Slope stability calculations indicate that the trial failure
surfaces behind the setback line exhibit a factor of safety greater than 1.5. Trial
M.B. Shores Corporation Job No. 04-8529-001 -00-00
March28, 1991 . Log No. 1-1418
Page 16
is in general accordance with the recommendations of this report. Our
personnel should perform sufficient testing of any fill placed to support our
opinion as to whether compaction recommendations have been complied with.
8.3.2 Site Preparation
! ' • ' '
Site preparation should begin with the removal of all vegetation and
deleterious materials within the area to be graded. In addition all loose soils
and undocumented fill should be completely removed down to firm bedrock
materials. The exposed bedrock should be scarified to a depth o'f 8 to 12
inches and recompacted to 90 percent of maximum density, ASTM D-1S57.
• Native on-site soils can be used as fill material, provided all deleterious
materials have been completely removed.
If during grading a cut/fill transition is created, and deep foundations are not
used in the fill portion, the following recommendation should be implemented.
The cut portion of the pad should be overexcavated to a depth of 5 feet below
finish grade and replaced as a compacted fill. The lateral extent of the
excavation should be 5 feet out side of the building perimeter. Alt fill soils
. should be compacted as recommended in Section 8.3.3 Fill Compaction.
8.3.3 Fill Compaction
\ •
All fin and backfill to be placed in association with site development should
be accomplished at slightly over optimum moisture conditions and using
equipment that is capable of producing a uniformly compacted product. The
minimum relative compaction recommended for fill is 90 percent of maximum
density based on ASTM Dl 557 (modified Proctor). Sufficient observation and
testing should be performed by the geotechnical consultant so that an opinion
can be rendered as to the degree of compaction achieved.
M.B. Shores Corporation Job No. 04-8529-001-00-00
March 28, 1991 Log No. 1-1418
Page 17
Representative samples of imported materials and on site soils should be tested
by the geotechnical consultant in order to evaluate the maximum density,
optimum moisture content, and where appropriate, shear strength,
consolidation, and expansion characteristics of the soil.
During grading operations, soil types other than those analyzed in the
geotechnical reports may be encountered by the contractor. The geotechnical
consultant should be notified to evaluate the suitability of these soils for use
as filj and as finish grade soils.
8.3.4 Trench Backfill
AH trench backfill should be compacted by mechanical means in uniform lifts
of 8 to 12 inches. The backfill should be uniformly compacted to at least 90
percent of ASTM D1557.
8.4 §ite Drainage
Foundation and slab performance depends greatly on how well the runoff waters drain
from the site. This is true both during construction and over the entire life of the
structure. The ground surface around structures should be graded so that water flows
rapidly away from the structures without ponding. The surface gradient needed to
achieve this depends on the prevailing landscape. In general, we recommend that
pavement and lawn areas within five feet of buildings slope away at gradients of at
least two percent. Densely vegetated areas should have minimum gradients of at least
five percent away from buildings in the first five feet. Densely vegetated areas are
considered those in which the planting type and spacing is such that the flow of water
is impeded.
Planters should be built so that water from them will not seep into the foundation,
slab, or pavement areas. Site irrigation should be limited to the minimum necessary
to sustain landscaping plants. Should excessive irrigation, water line breaks, or
MB. Shores Corporation Job No. 04-8529-001-00-00
March 28, 1991 . Log No. 1-1418
Page 18
unusually high rainfall occur, saturated zones or "perched" groundwater may develop
in fill soils.
Because of the highly erosion prone nature of the on-site soils additional care should
be taken to direct runoff water away from the top of the bluff. Drainage channels
and swales should be constructed to help channel water away from erosion prone
areas. We recommend that drainage water be carried in a pipe or conduit to the base
of the bedrock bench in the lower portion of the bluff, rather than be allowed to flow
freely over the bluff face.
i
8.5 Foundation Recommendations
8.5.1 General
Special design considerations for soil expansion are not considered necessary.
All foundations should bear entirely on either uniformly compacted fill or on
bedrock. The following paragraphs in Section 8.6 will present our
recommendations for foundation design. Our recommendations are considered
to be generally consistent with the standards of practice. They are based on
both analytical methods and empirical methods derived from experience with
similar geotechnical conditions. Reinforcement recommendations are
considered the minimum necessary for the likely soil conditions and are not
intended to supersede the design of the Structural Engineer or criteria of
governing agencies.
8.5.2 Foundations on Bedrock
If the building area is prepared such that all footings will bear entirely on
bedrock the following foundation design parameters should be applicable.
M.B. Shores Corporation
March 28, 1991
Job No. 04-8529-001-00-00
Log No. 1-1418
Page 19
Allowable Soil Bearing:3000 psf (allow a one-third increase for short-
term wind or seismic loads)
Minimum Footing Width: 12 inches
Minimum Footing Depth: 18 inches
Minimum Reinforcement:1 no. 4 bar at both top and bottom in continuous
footings, or design as simply supported beam capable
of supporting the applied loads over a span of 4 feet,
whichever is greater. Equal resistance to positive and
negative moments should be provided.
8.5.3 Foyqdations on Fill
If all footings will bear entirely on compacted fill soils, the following
foundation design parameters should be applicable.
Allowable Soil Bearing:2000 psf (allow a one-third increase for short-
term wind or seismic loads)
Minimum Footing Width: 12 inches
Minimum Footing Depth: 18 inches
Minimum Reinforcement:1 no. 5 bar at both top and bottom in continuous
footings, or design as simply supported beam capable
of supporting the applied loads over a span of 6 feet,
whichever is greater. Equal resistance to positive and
negative moments should be provided.
8.5.4 Settlement
The anticipated total and differential settlement for the proposed structure
should be within tolerable limits provided that the recommendations of this
report are followed. In general, total settlements are estimated to be less than
1 inch, and differential settlement is expected to be less than } inch. It is
recommended that we review the actual foundation plans to evaluate the
footing configurations and loading conditions.
EXCERPT 2
Julie Hampton . Job No. 05-8109-001-00-00c/ Kroxnmenhoek/McKeown Log No. 9-2248
Uovember 6, 1989 Page 10
during extremely high tides and high surf. Additional
erosion occurs as a result of ground water seepage at the
contact between the terrace deposit and Santiago
Formation bedrock.
The other major factor in bluff stability is the effect
of surface water erosion. The mesa area above the bluff
generally drains to the west. As the sheet flow nears
the bluff top it is concentrated into shallow gullies.
As the gullies coalesce, they become deeper. Where the
gullies dump over the bluff top, the gullies are ,over 5
feet deep. Over time the gullies migrate up-gradient
(eastward) and laterally, effectively moving the top of
the bluff eastward.
6.3 Bluff Retreat Rates
Bluff collapse or block topples are episodic and
catastrophic. There are long periods when there are no
failures followed by one or two events where tens of feet
of bluff may retreat•. Aerial photographs taken in 1929
and 1953 (figures 2 and 3) were compared with photographs
taken by Gerald Kuhn in 1978 and 1983 and maps prepared
in 1984. The base line for measurements were the
railroad tracks east of the site (Location Map, Figure
1). The photographs indicate that there has been little
retreat of the bluff top in the vicinity with the
exception of the "cove" area described by Converse
Consultants (1984) .
The anomalous retreat in the "cove" area is a result of I*
a lack of a bedrock bench to protect the erodible terrace
deposit in that portion of the bluffs. Additionally,
Julie Hampton Job No. 05-8109-001-00-00e/0 Kroinmenhoek/McKeown Log No. 9-2248
November 6, 1989 Page 11
there is an offshore channel that refracts unbroken waves
into this section of beach. Without the benefit of an
offshore reef to dissipate wave energy, the "cove" is
subject to more violent wave attack.
The bluff top within the subject property does not appear
to have retreated in the past 61 years. The storm in
August, 1983, -did remove roughly 20 feet from the base
of the upper bluff without collapse of the bluff top.
The result was to steepen the bluff above the bedrock
bench. . ,
Future retreat of the bluff at the subject site will be
controlled by several factors:
a. A large reef is located just offshore from the
property. Waves break on the reef rather than on
the beach or against the bluffs thereby reducing
their energy significantly.
b. The bedrock bench has not been undermined and does
not contain open joints or other structural
weaknesses. The bench is 20 feet wide and 5 feet
above the present beach. The bench should protect
, the more credible terrace deposit and fill by acting
like a natural sea wall. Direct wave attack on the
base of the upper bluff would only occur when the
tide and storm surge is in excess of +13 feet of
present mean sea level.
c. The fill present in the old erosional gully and
exposed in the upper bluff face within the property
contains chunks of concrete and asphalt. The fill
Julie Hampton Job No. 05-8109-001-00-00
V Krommenhbek/McKeown Log No. 9-2248
November 6, 1989 Page 12
appears to be draped over the erodible portion of
the terrace deposit. The fill will act, to a
certain extent, as rip-rap. While the fill is
erodible, it appears much less erodible than the
friable sand in the terrace deposit.
d. The subject site is roughly 100 feet north of the
"cove" area. Further erosion in the "cove" will
not likely affect the proposed development for well
beyond the economic life of the residence.
''
A structural -setback has been established for the subject
property based on the historical bluff retreat in the
area and a 75 year economic life of the residence.
Although the bluff top does not appear to have retreated,
the 20 feet of erosion at the base of the upper bluff has
. been extrapolated over the 61 years of documentation to
estimate a average retreat rate of 0.33 feet per year.
A factor of safety of 1.5 is applied and the result is
multiplied by 75 years to arrive at the 40 foot setback
from the bluff top shown on Plate I.
7 • 0 SETSMICITY
7.1 General
The site is considered to be a seismically active area,
as can all of southern California. There are, however,
no known active faults either on or adjacent to the
project site. Figure 4 shows the known active faults
and major earthquake epicenters in the region and their
geographic relationship to the site. Because these
active faults are at a substantial distance, the seismic
, 1929 PHOTO
" 04-8109-001-00-00 DATE: November 1989 FIGURE:
1953 PHOTO
"04-8109-001-00-00 November 1989 FIGURE:
EXCERPT 3
-f .1 I I I I i i I i f i i I i f i. .1 I .f. i. I f i I I i
I • ^*™ i ™ ^" • i i x* I•/•J PROPOSED IriPRCVEMfalTS ' X
^J-
^ifZlZSfVSVSKS
ID M cm or wuaw MK •rouaco no IK ONPMH* woor va KM or <w *s OMUMCO • IK are Kcaion|
or > nrauar WMKO oocw Known MMI. moo. »oia nMunr «• wi H KMIMO >«a»n> louone CHUM •"TOM
rj "K «r into- t«M
•>••» .M« K«MMCMI ran IMC tou PI^«OIma cat. • *a jui«c «acon
u CKncw.rai lie Mmt a racmM nemi «• COKK. ion<cas w K on OF cuuvo. KMCU CMUMI HUU«W> wo «c