HomeMy WebLinkAboutCDP 98-05; Jensen Tentative Parcel Map; Coastal Development Permit (CDP) (2)C. J. Randle, P.E., Civil Engineer
5858 MtfAlifan D/ive, Suite 235
San Diego, CA 92111
Telephone (619) 571-6271
Fax (619) 571-3943
™ UK loo* RECEIVEDMarch 16, 1998
APR 0 2
CITY OF
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 2 1 0-120-30, Carlsbad Drive, Carlsbad,
California; 1996,byGeotechnics, Inc., Project No. 0319-001-00, dated Septembers, 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 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.
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APN 210-120-30
March 16,1998
SUMMARY 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
Previous reports based sea cliff retreat and construction setbacks upon an "empirical" figure of 25
feet extrapolated over 62 years (ref. 2). However, the same report indicates "relatively little
changes" to the bluff top. Reference 3 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 erodible 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 retreat rate for the cove area becomes similar to that of adjacent tops of bluff to both sides
of the cove.
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.
For the 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 210-120-30
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 year for 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, formational 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.
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APN 210-120-30
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
im .
Should you have any questions, or require additional service please do not hesitate to contract us.
Very truly yours,
83.?C?i
Ernest R. Artim ' "4-*i".^i::.J." •-;' Charles J. Ran
CEG 1084 RCE 22096
Distribution: (3) client