HomeMy WebLinkAboutCUP 2018-0014; FIRE STATION NO. 2; UPDATE ADDENDUM TO GEOTECHNICAL EVALUATION REPORT; 2021-01-14
5710 Ruffin Road | San Diego, California 92123 | p. 858.576.1000 | www.ninyoandmoore.com
January 14, 2021
Project No. 108715014
Mr. Steven Stewart City of Carlsbad – Public Works
1635 Faraday Avenue Carlsbad, California 92008
Subject: Update Addendum to Geotechnical Evaluation Report Fire Station No. 2 1906 Arenal Road Carlsbad, California
Dear Mr. Stewart:
In accordance with your request, we have prepared this update addendum letter to our geotechnical
evaluation report for the project. Specifically, this letter serves to update the recommended site
seismicity and seismic design parameters provided in the referenced geotechnical report (Ninyo &
Moore, 2017) for the proposed new Fire Station No. 2 to be constructed at 1906 Arenal Road in
Carlsbad, California. The purpose of this letter is to provide updated geotechnical conclusions and
recommendations, as appropriate, to meet the current jurisdictional guidelines and requirements since
the report was originally issued, including updated seismic design parameters for the site based on the
2019 California Building Code (CBC). Conditions, findings, and recommendations presented in our
report (Ninyo & Moore, 2017) remain valid and applicable to the project, unless specifically
superseded herein.
PROJECT INFORMATION
The project will replace the fire station building with a new two-story building with living spaced and
dormitories situated above the apparatus bay (Delawie, 2020). The planned structure will be founded
on conventional spread and perimeter foundations with a slab-on-grade floor. Other site
improvements will include site walls, a trash enclosure, an emergency generator pad, pavements,
and underground utilities.
Geotechnical & Environmental Sciences Consultants
Ninyo & Moore | 1906 Arenal Road, Carlsbad, California | 108715014 | January 14, 2021 2
BACKGROUND
Our subsurface exploration for the project was conducted on August 30 and 31, 2017 and consisted
of drilling, logging, and sampling four exploratory borings to depths up to approximately 20 feet. Our
borings encountered a layer of fill material that was in turn underlain by old alluvium and materials
of the Santiago Formation. Depth to materials of the Santiago Formation ranged from approximately
3½ to 10 feet. Groundwater was not encountered.
As discussed in our geotechnical evaluation report (Ninyo & Moore, 2017), the site is located in a
seismically active area and the potential for strong motion in the project area is considered significant
during the design life of the proposed improvements. Updated information regarding strong ground
motion and seismic design parameters for the site are presented below.
STRONG GROUND MOTION
Considering the proximity of the site to active faults capable of producing a maximum moment
magnitude of 6.0 or more, the project area has a high potential for experiencing strong ground
motion. The 2019 CBC specifies that the Risk-Targeted, Maximum Considered Earthquake (MCER)
ground motion response accelerations be used to evaluate seismic loads for design of buildings and
other structures. The MCER ground motion response accelerations are based on the spectral
response accelerations for 5 percent damping in the direction of maximum horizontal response and
incorporate a target risk for structural collapse equivalent to 1 percent in 50 years with deterministic
limits for near-source effects. The horizontal peak ground acceleration (PGA) that corresponds to
the MCER for the segments was calculated as 0.51g using a web-based seismic design tool
(SEAOC/OSHPD, 2021).
The 2019 CBC specifies that the potential for liquefaction and soil strength loss be evaluated, where
applicable, for the Maximum Considered Earthquake Geometric Mean (MCEG) peak ground
acceleration with adjustment for site class effects in accordance with the ASCE 7-16 Standard. The
MCEG peak ground acceleration is based on the geometric mean peak ground acceleration with a
2 percent probability of exceedance in 50 years. The MCEG peak ground acceleration with
adjustment for site class effects (PGAM) was calculated as 0.53g using a web-based seismic design
tool that yielded a mapped MCEG peak ground acceleration of 0.44g for the site and a site
coefficient (FPGA) of 1.2 for a Site Class C.
Ninyo & Moore | 1906 Arenal Road, Carlsbad, California | 108715014 | January 14, 2021 3
Seismic Design Parameters
Design of the proposed improvements should be performed in accordance with the requirements of
governing jurisdictions and applicable building codes. Table 1 presents the seismic design
parameters for the site in accordance with the CBC (2019) guidelines and adjusted MCE spectral
response acceleration parameters (SEAOC/OSHPD, 2021).
Table 1 – 2019 California Building Code Seismic Design Parameters
Seismic Design Factors Value
Risk Category IV
Seismic Design Category D
Site Class C
Site Coefficient, Fa 1.2
Site Coefficient, Fv 1.5
Mapped Spectral Acceleration at 0.2-second Period, Ss 1.007g
Mapped Spectral Acceleration at 1.0-second Period, S1 0.365g
Spectral Acceleration at 0.2-second Period Adjusted for Site Class, SMS 1.208g
Spectral Acceleration at 1.0-second Period Adjusted for Site Class, SM1 0.548g
Design Spectral Response Acceleration at 0.2-second Period, SDS 0.805g
Design Spectral Response Acceleration at 1.0-second Period, SD1 0.365g
Soil Corrosivity
As part of our previous evaluation of the site (Ninyo & Moore, 2017), laboratory testing was
performed on a representative sample of near-surface soil to evaluate soil pH, electrical resistivity,
water-soluble chloride content, and water-soluble sulfate content. The soil pH and electrical
resistivity tests were performed in general accordance with California Test Method (CT) 643. The
chloride content test was performed in general accordance with CT 422. Sulfate testing was
performed in general accordance with CT 417.
The results of the corrosivity testing indicated an electrical resistivity of 650 ohm-centimeters (ohm-cm),
a soil pH of 7.4, a chloride content of 190 parts per million (ppm), and a sulfate content of 0.026 percent
(i.e., 260 ppm). Based on a comparison with the California amended (Caltrans, 2019) AASHTO (2017)
corrosion criteria, the onsite soils would be classified as corrosive. Corrosive soils are defined as soil with
an electrical resistivity of 1,100 ohm-cm or less, a chloride content of 500 ppm or greater, a sulfate content
of 0.15 percent (1,500 ppm) or greater, and/or a pH equal to or less than 5.5.
Ninyo & Moore | 1906 Arenal Road, Carlsbad, California | 108715014 | January 14, 2021 4
Concrete
Concrete in contact with soil or water that contains high concentrations of water-soluble sulfates that
can be subject to premature chemical and/or physical deterioration. As noted, the soil sample tested
in our previous evaluation (Ninyo & Moore, 2017) indicated a water-soluble sulfate content of
0.026 percent by weight (i.e., 260 ppm). Based on the American Concrete Institute (ACI) 318 criteria,
the site soils would correspond to exposure class S0. For this exposure class, ACI 318 recommends
that normal weight concrete in contact with soil possess a compressive strength of 2,500 pounds per
square inch (psi) or more.
LIMITATIONS
The geotechnical services described in this report have been conducted in general accordance with
current practice and the standard of care exercised by geotechnical consultants performing similar
tasks in the project area. No warranty, expressed or implied, is made regarding the conclusions,
recommendations, and opinions presented in this report. Our conclusions and recommendations are
based on an analysis of the observed site conditions and the references listed. If actual conditions
differ from those described in this report, our office should be notified and additional recommendations,
if warranted, can be provided upon request. Please note that our scope of services for this project did
not include an evaluation of any potential environmental hazards at the site.
We appreciate the opportunity to be of service on this project.
Respectfully submitted, NINYO & MOORE
Nissa M. Morton, PG, CEG Senior Project Geologist Jeffrey T. Kent, PE, GE Principal Engineer
NMM/JTK/gg
Attachment: References
Ninyo & Moore | 1906 Arenal Road, Carlsbad, California | 108715014 | January 14, 2021
REFERENCES
American Association of State Highway and Transportation Officials (AASHTO), 2017, AASHTO LRFD Bridge Design Specifications, 8th Edition: dated September.
American Concrete Institute (ACI), 2019, ACI 318 Building Code Requirements for Structural Concrete and Commentary.
American Society of Civil Engineers (ASCE), 2017, Minimum Design Loads and Associated Criteria for Building and Other Structures, ASCE Standard 7-16.
California Building Standards Commission, 2019, California Building Code: California Code of
Regulations, Title 24, Part 2, Volumes 1 and 2, based on the 2018 International Building Code.
California Department of Transportation (Caltrans), 2019, California Amendments to the AASHTO LRFD Bridge Design Specifications (2017 Eighth Edition): dated April.
Hart, E.W., and Bryant, W.A., 2007, Fault-Rupture Hazard Zones in California, Alquist-Priolo Earthquake Fault Zoning Act with Index to Earthquake Fault Zones Maps: California Department of Conservation, California Geological Survey, Special Publication 42, with Supplement 1 added in 2012, Supplement 2 added in 2014, Supplement 3 added in 2015, and Supplement 4 added in 2016.
Delawie, 2020, 75% Design Development Plan Set, Carlsbad Fire Station #2, 1906 Arenal Road, Carlsbad, CA 92009: dated December 12.
Ninyo & Moore, 2017, Geotechnical Evaluation, Fire Station No. 2, 1906 Arenal Road, Carlsbad, California, Project No. 108438001: dated October 18.
Ninyo & Moore, 2020, Proposal for Update Addendum to Geotechnical Evaluation, Fire Station No. 2, 1906 Arenal Road, Carlsbad, California: dated December 4.
Structural Engineering Association of California (SEAOC), Office of Statewide Health Planning and Development (OSHPD), 2021, U.S. Seismic Design Maps website, https://seismicmaps.org/: Accessed in January.
United States Geological Survey (USGS), 2021, 2008 National Seismic Hazard Maps – Fault Parameters Database, World Wide Web, https://earthquake.usgs.gov/cfusion/hazfaults_2008_search/query_main.cfm: accessed in
January.