HomeMy WebLinkAboutPD 2020-0004; WILSON RESIDENCE ADDITION; RESPONSE TO REVIEW COMMENTS SUBMITTED BY HETHERINGTON ENGINEERING; 2020-05-04H RECEIVED
JUN 08 2020
LAND DEVELOPMENT
May 4, 2020 ENGINEERING
Mr. Drew Wilson Project No. 19-2110
7953 Sitio Vaquero
Carlsbad, California 92009
Subject: Response to Review Comments submitted by Hetherington Engineering, Inc. dated
April 9, 2020, Project No. 9102.1, Log No. 20947
References: Limited Geotechnica1 Investigation, Two Story Detached Home Expansion, 7953 Sitio
Vaquero, Carlsbad, California, prepared by Eternia, Inc., dated June 17, 2019, Project
No. 19-2110.
Foundation Plans and Details, Wilson Residence, 7953 Sitio Vaquero, Carlsbad,
California, prepared by Quails Engineering, dated November 19, 2019 (Sheets S1.0,
S1.2, S2.0, SD3)
"Grading Plans for: Wilson Residence Addition", by Raam Development, Inc., undated
(Sheets 1 through 2)
Dear Mr. Wilson:
At your request and authorization, Eternia, Inc. is providing our response to the subject review
comment letter provided by Hetherington Engineering, Inc., consultant to the City of Carlsbad (see
enclosures). Our responses to the review comments are provided as follows:
1. Grading and foundation recommendations provided in the referenced report remain applicable to
the project Based on the geologic setting and anticipated earthwork for construction of the proposed
Page No. 2
Job No. 19-2110
project, the soils underlying the site are classified as Site Class D - default, according to the 2019
California Building Code (CBC). The seismic parameters according to the 2019 CBC are summarized
in the following table. The seismic parameters provided assume Equivalent Lateral Force (ELF) design
is permitted using the exceptions noted in Section 11.4.8 of ASCE 7-16. A site-specific seismic hazard
analysis was not performed for this investigation.
2019 CBC - Seismic Parameters
Mapped Spectral Acceleration Parameters S = 0.957 and S1 = 0.349
Site Coefficients Fa = 1.2 and F = 1.95
Adjusted Maximum Considered Earthquake
Spectral Response Parameters
S= 1.148 and SM1 =O.681
Design Spectral Acceleration Parameters S Ss = 0.765 and SD1 = 0.454
Peak Ground Acceleration (PGAM) 0.501
All other recommendations provided in the referenced report remain applicable to the project
Acknowledged.
The referenced foundation and grading plans were reviewed and were found in general
conformance with the project soils report. No additional recommendations are deemed necessary at
this time.
Attached is a current grading plan, Grading Plans for: Wilson Residence Addition, undated, by
RAAM Development This plan contains existing topography, b) existing and proposed
structure/improvements, c) proposed finished grade, d) locations of subsurface explorations, e)
geologic contacts, and remedial grading limits. The grading plan has been modified as requested.
H Eternia, Inc.
Page No. 3
Job No. 19-2110
Geologic units underlying the subject site include artificial fill, which is composed of sandy clay (CL),
which is dark olive brown in color, wet, and soft and displays medium expansive characteristics. This
material was observed over the surface of the project area and encountered within both exploratory
borings. Santiago formation is mapped underlying the fill soils. This unit was not encountered during
the field investigation within either exploratory excavation or on the surface.
The subject property lies less than 15 km from the nearest active fault, the Newport-Inglewood-
Rose Canyon fault, which lies offshore. Since this fault is capable of producing an earthquake greater
than M7, it is considered to be a 'near-fault site', in accordance with ASCE 7-16, 11.4.1.
The updated seismic design table is provided under the response to comment no. 1. The Risk
Category for the project is Risk Category H.
No known active faults cross or trend toward the subject properly. The site does not lie within
an Aiquist-Priolo (AP) Special Studies zone. There is no AP earthquake fault zones map for the Rancho
Santa Fe Quadrangle.
As with most of southern California, the subject site is situated in an area of active and potentially
active faults. Active faults present several potential risks to structures, the most common of which are
strong ground shaking, dynamic densification, liquefaction, mass wasting, and surface rupture at the
fault plane. The following four factors are the principal determinants of seismic risk at a given location:
Distance to seismogenically capable faults.
The maximum or "characteristic" magnitude earthquake for a capable fault.
Seismic recurrence interval, in turn related to tectonic slip rates.
Nature of earth materials underlying the site.
H Eternia, Inc.
Page No.4
Job No. 19-2110
Based upon proximity to regionally significant, active faults, ground shaking is considered to be the
primary hazard most likely to affect the site. Characteristics of the major active fault zones selected
for inclusion in analysis of strong ground shaking are listed in the following table. Numerous
significant fault zones are located at distances exceeding 40 kilometers from the site, but greater
distances, lower slip rates, and/or lesser maximum magnitudes indicate much lower risk to the site from
the latter fault zones than those listed below.
Distance Fault Reference
Slip Rate Fault
Fault Zone' from Site Length Earthquake
(mm/yr)' Type1
(kin) (J)l M(i)'
Newport-
Inglewood/Rose 10 70±7 1.5±0.5 7.2 B Canyon
Coronado Bank
(rl-ss)
31 185±19 3.0±1.0 7.6 B
Elsinore, Temecula
section 38 43±4 5.0±2.0 6.8 A
San Andreas
(Coachella Segment) 111 96±10 25.0±5.0 7.2 A
(rl-ss)
California Department of Conservation, Geological Survey, 2007 (Appendix A), California Fault Parameters for
the National Seismic Hazard Maps and Working Group on California Earthquake Probabilities, CGS Special Report
203A, USGS Open File Report 2007-1437A.
U.S. Department of the Interior, Geological Survey, 2008, website: 2008 National Seismic Hazards Maps - Source
Parameters, httusd!earthguake.uur pov/cfusionlhazf.ultn 2008 search/auery result&cfoi.
Southern California Earthquake Data Center, website: Significant Earthquakes and Faults,
httid/scedc.caltech.edu/significantlfault-indax.htnil
Fault Geometry: (ss) strike slip; (r) reverse; (n) normal; (ri) right lateral; (II) left lateral; (0) oblique; (45 N) direction.
H Eternia, Inc.
Page No. 5
Job No. 19-2110
Based on our field investigation and laboratory testing results, it is our opinion that the proposed
development will be safe against hazards from landslide, settlement or slippage and the proposed
construction will have no adverse effect on the geologic stability of the adjacent properties or future
developments provided the recommendations presented in the referenced report are followed.
The foundation and slab recommendations provided in the report are consistent with the
requirements of Section 1808.6 of the 2019 California Building Code. Recommendations applicable
to the medium expansive potential of the site include:
. Moisture conditioning the onsite soils to 2 to 4 percent above optimum moisture content
for all compacted fill.
Providing a minimum of 2 feet of moisture conditioned compacted fill underneath
footings.
Embedding exterior footings a minimum of 24 inches below adjacent grade
Embedding interior footings a minimum of 18 inches below lowest adjacent grade
Recommending minimum reinforcement for footings and slabs
Providing a minimum of 18 inches of compacted fill underneath slabs.
10. For unrestrained retaining wall conditions, an active earth pressure developed at a rate of 50
psf per foot of depth should be utilized for level backfill.
For restrained retaining wall conditions, an at-rest earth pressure of 70 psf per foot of depth should be
utilized for level backfill.
The "at-rest" condition applies toward braced walls that are not free to tilt. The "active" condition
applies toward unrestrained cantilevered walls where wall movement is anticipated. The structural
designer should use judgment in determining the wall fixity and may utilize values interpolated
between the at-rest and active conditions where appropriate. A triangular distribution of static earth
pressures should be used in the design.
H Eternia, Inc.
Page No. 6
Job No. 19-2110
For walls 6 feet in height or greater, the seismic earth pressure should be considered in addition to the
static earth pressure.
The lateral active seismic earth pressure (NOT including static active earth pressure) developed at a
rate of 15 psf per foot of depth should be utilized for level backfill. An inverted triangular distribution
of seismic active earth pressure should be used in the design.
The lateral at-rest seismic earth pressure (NOT including static at-rest earth pressure) developed at a
rate of 10 psf per foot of depth should be utilized for level backfill. A rectangular (uniform) distribution
of seismic at-rest earth pressure should be used in the design.
These values are applicable only to properly drained backfill with maximum slope inclination as
indicated with no additional surcharge loadings and do not include a factor of safety other than
conservative modeling of the soil strength parameters.
For walls with uniform surcharge loading, (located within a IV: 1H plane from the bottom of the wall),
the increase in active pressure can be calculated as the product of 0.41 (Ka) and the surcharge load, q,
(i.e., 0.41q) for level backfill. The increase in at-rest pressure can be calculated as the product of 0.58
(Ko) and the surcharge load, q for level conditions. The resulting additional surcharge pressure should
be applied to the wall as a rectangular distribution, from top to bottom.
Backfill behind retaining walls (within H12 of the back of the wall, where H is the height of the wall)
should consist of a soil of sufficient granularity that the backfill will properly drain. The granular soil
should be classified per the USCS as GW, GP, SW, SP, SW-SM or SP-SM. Surface drainage should
be provided to prevent ponding of water behind walls. A drainage system consisting of either of the
following should be installed behind all retaining walls:
H Eternia, Inc.
Page No. 7
Job No. 19-2110
A 4-inch-diameter perforated PVC (Schedule 40) pipe or equivalent at the base of the stem
encased in 2 cubic feet of granular drain material per linear foot of pipe or
Synthetic drains such as Enkadrain, Miradrain, Hydraway 300 or equivalent
Perforations in the PVC pipe should be 3/8 inch in diameter and facing down. Granular drain material
should be wrapped with filter cloth such as Mirafi 140 or equivalent to prevent clogging of the drains
with fines. Walls should be waterproofed to prevent nuisance seepage. Water should outlet to an
approved drain.
11. Flatwork should be a minimum of 4 inches in thickness supported by a minimum of 12 inches
of properly moisture conditioned compacted fill. Flatwork should consist of at least No.3 reinforcing
bars spaced at 24 inches on center each way. Use of maximum control joint spacing of no more than
8.0 feet in each direction and a construction joint spacing of 10 to 12 feet should be used in the design
of flatwork. Construction joints that abut foundations or slabs should include a felt strip, or approved
equivalent, that extends the full depth of the exterior slab. It is recommended that exterior slabs be
isolated from adjacent foundations.
If the subgrade earth materials are allowed to become saturated, there is a risk of vertical differential
movement of the exterior concrete hardscape, sidewalks, curbs / gutters, etc. Therefore, proper
drainage should be established away from such improvements and minimal precipitation or irrigation
water allowed to percolate into the earth materials adjacent to and/or under the exterior concrete
flatwork or hardscape, curbs / gutters, etc.
ETERNI4
Eternia, Inc.
Page No.8
Job No. 19-2110
We appreciate this opportunity to provide geotechnical services for this project. If you have questions
or comments concerning this report, please contact us at your convenience.
Respectfully submitted, FE
Eternia, Inc. 00
CO
fu No. GE3085 n" cc
Principal Engineer
Distribution: Mr. Drew Wilson (PDF)
Enclosures: Review Comment Letter
Eternia, Inc.
Review Comment Letter
H Eternia, Inc.
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