HomeMy WebLinkAboutCUP 06-11B; PACIFIC RIDGE SCHOOL EXPANSION; GEOTECHNICAL UPDATE LETTER; 2013-05-144
Leighton and Associates, Inc.
A LEIGHTON GROUP COMPANY
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APR 152O1S
LA""" DEVELOPMENT
ENGINEERING
May 14, 2013
Project No. 971009-068
To: Pacific Ridge School
6269 El Fuerte Street
Carlsbad, California 92009
Attention: Mr. Bob Buscher
Subject: Geotechnical Update Letter, Pacific Ridge School Expansion, Bressi
Ranch, PA-13, California
Introduction
In accordance with your request, this letter has been prepared to update our
geotechnical report for Pacific Ridge School Expansion located in Bressi Ranch,
Carlsbad, California (Figure 1). The purpose of our geotechnical update was to observe
and evaluate the existing geotechnical conditions of the subject site, review the project
geotechnical reports, update geotechnical recommendations for the seismic design
parameters per 2010 California Building Code, provide updated geotechnical map and
cross-sections, and provide appropriate conclusions and additional recommendations
for the proposed development of the site.
We understand that the Pacific Ridge School Expansion will include construction of a
three new school buildings including middle school building, a performing arts building,
and theater building. We also anticipate that the proposed development will include
associated improvements such as concrete flatwork, underground utilities, landscaping,
etc.
Existing Site Conditions
Currently, the Pacific Ridge expansion area is occupied by 4 modular administration
and classroom temporary buildings with associated landscaping and a large lawn/sports
field area to the north. The proposed improvements are underlain by the Santiago
Formation and with artificial fills associated with original rough and fine grading at the
site and existing site improvements (see Plate 1).
3934 Murphy Canyon Road, Suite B205 • San Diego, CA 92123-4425
858.292.8030 • Fax 858.292.0771 • www.lelghtongroup.com
971009-068
Conclusions and Recommendations
Based on our review of the project geotechnical report (Leighton, 2008), the
geotechnical conditions of the subject site have not significantly changed since the
completion of our geotechnical report in 2008. Therefore, it is our professional opinion
that the conclusions and recommendations in the referenced geotechnical report are
still considered applicable and should be adhered to during the design and construction
phase of this project unless superseded by recommendations presented below.
The following recommendations should supersede those presented in our referenced
reports (which is attached as Appendix A):
Undocumented landscaping fill should be removed to competent material
throughout the site. The bottom of the removal should be processed (i.e., scarified
a minimum 8 inches, moisture-conditioned to 2 percent above optimum moisture-
content) and recompacted to a minimum 90 percent relative compaction (based on
ASTM Test Method D1557). Expansion and sulfate testing should be performed on
finish grade soils are in accordance with referenced geotechnical report.
Based on the proposed building locations (Hoffman, 2013) and our review of the
as-graded conditions, fills on the order of approximately 2 to 75 feet will underlie
the proposed buildings. As a result, significant fill thickness differentials will be
present below the proposed buildings will need to be mitigated (see Figure 2).
Mitigation measures include utilizing overexcavation of the shallow fill area in order
to reduce the fill differential thickness or a deep foundation system (i.e. piles or
caissons). Recommendations to mitigate differential fill thickness will be provided
at a later date.
The west side of the proposed theater and performing arts buildings will be
underlain by relatively thin thickness of fill soils (on the order of 3 feet). As a
result, cut/fill transition conditions may be present beneath the proposed
buildings that may need to be mitigated by the overexcavation of the shallow fill
and the formational material below. To mitigate cut to fill transition an
overexcavation of 6 feet below finished grade or 4 feet below the foundations,
whichever is deeper, is recommended. The excavations should extend laterally
at least 5 feet outside building foundations. It should be noted that excavations
for the overexcavation at the basement area in the theater will extend up to 17
feet west of building and into existing improvements (i.e. utilities). Shoring may
be required to perform theater building overexcavations in this area. The limits
and depths of overexcavation are provided on the attached updated geotechnical
map and the geological cross-sections (see Plate I and Figure 2).
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Leighton
971009-068
The new storm drain planned between the new proposed buildings will likely make
it extremely difficult to perform the recommended building pad overexcavations
and excavations for the proposed theater basement. Therefore, we recommend
that pad overexcavations and remedial grading for the proposed buildings adjacent
to new storm be performed prior to or during storm drain construction.
The following updated seismic design parameters, determined in accordance
with the 2010 CBC and the USGS Ground Motion Parameter Calculator (Version
5.1.0), should be used in the design of the proposed improvements.
CBC (2010) Seismic Code - Parameters for the Site
Description Values CBC Reference
Site Class D Table 1613.5.2
Short Period Spectral Acceleration S8 1.100 Figure 1613.5(3)
1-Second Period Spectral Acceleration S1 0.417 Figure 1613.5(4)
Short Period Site Coefficient F8 1.06 Table
1613.5.3(1)
1-Second Period Site Coefficient F 1.583 Table
1613.5.3(2)
Adjusted Short Period Spectral SMS 1.166 Equation 16-36 Acceleration
Adjusted 1-Second Period Acceleration SMI 0.659 Equation 16-37
Design Short Period Spectral Response SDS 0.777 Equation 16-38 Parameter
Design 1-Second Period Spectral SDI 0.440 Equation 16-39 Response Parameter
Based on our review, the footings for the proposed buildings will be founded in
competent fill material. Footings founded in compacted fill may be designed for a
maximum allowable bearing pressure of 2,500 psf and should extend a minimum
of 24 inches beneath lowest adjacent finish grade.
To account for potential redistribution of forces during a seismic event, basement
walls should also be checked considering an additional uniform seismic pressure
distribution equal to 8 H psf, where H equals the overall retained height in feet.
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971009-068
Leighton should observe and test all fill placement during grading and observe
footing excavations prior to concrete placement to confirm that the soil conditions
are as anticipated.
The conclusions and recommendations provided in this update letter are based on as-
graded subsurface conditions observed during the previous site grading. The
interpolated subsurface conditions should be checked in the field during construction.
Construction observation of all onsite excavations and field density testing of all
compacted fill should be performed by a representative of this office. All footing
excavations should be reviewed by this office prior to placing steel or concrete.
If you have any questions regarding this letter, please contact this office. We appreciate
this opportunity to be of service.
Respectfully submitted,
LEIGHTON AND ASSOCIATES, INC.
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ENGINEERING
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Mike D. Jensen, CEG 2457 William D. Olson, RCE 45283
Project Geologist Associate Engineer
Attachments: Appendix A - References
Figure 1 - Site Location Map
Figure 2— Cross-Section A-A' and B-B'
Plate I - Geotechnical Map
Distribution: (2) Addressee
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Project: 971009-068 Eng/Geol: WDO/MDJ SITE LOCATION MAP Figure 1
Scale: 1 = 2,000' Date: May, 2013 Pacific Ridge School Expansion
Base Map; ESRI Resources 2013 Bressi Ranch PA-13
Thematic Into: Leighton
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Appendix A
References
California Building Standards Commission (CBSC), 2010, California Building Code (CBC)
Hoffman Planning, 2013, Preliminary Grading Plans, Sheets C-4 and C-5, Pacific Ridge
School, Carlsbad, California
Leighton and Associates, Inc., 1997, Preliminary Geotechnical Investigation, Bressi Ranch,
Carlsbad, California, Project No. 4971009-002, dated July 29, 1997.
-, 2001, Supplemental Geotechnical Investigation for Mass Grading, Bressi Ranch,
Carlsbad, California, Project No. 971009-0015, dated March 14, 2001.
2004a, Deep Fill Area Settlement Monitoring Recommendations, Bressi Ranch,
Carlsbad, California, Project No. 971009-014, dated February 13, 2004, revised
February 16, 2004.
-, 2004b, As-Graded Report of Mass Grading, Planning Areas PA-13 and PA-14,
and a Portion of PA-15, Carlsbad Tract No. 00-06, Bressi Ranch, Carlsbad,
California, Project No. 971009-014, dated September 17, 2004.
-, 2006b, Preliminary Geotechnical Conclusions and Recommendations
Concerning the Proposed Right-Turn Lane on the West Side of El Fuerte Street,
Bressi Ranch Planning Area PA-13, Carlsbad, California, Project No. 971009-044,
dated September 27, 2006.
-, 2006c, Geotechnical Response to City of Carlsbad Review Comments and
Addendum Recommendations for the Proposed Development of the Pacific Ridge
Private School, Parcels 2 through 4 of Bressi Ranch Planning Area PA-13, Carlsbad,
California, Project No. 971009-044, dated November 3, 2006.
-, 2007, Final As-Graded Report of Rough, Fine, and Post Grading, Parcels 2
through 4,Planning Areas PA-13, Bressi Ranch, Carlsbad, California, Project No.
971009-049, dated October 30, 2007.
-, 2008a, Geotechnical Update Study, Permanent Campus Facilities and Campus
Expansion, Carlsbad, California, Project No. 971009-053, dated July 10, 2008.
-, 2008b, Geotechnical Evaluation of Water Intrusion and Soil Moisture Beneath
Temporary Building, Pacific Ridge School, Carlsbad, California, Project No. 971009-
056, dated November 8, 2008.
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