HomeMy WebLinkAboutMS 16-04; VIASAT BRESSI RANCH CAMPUS; REPSPONSE TO CITY COMMENTS; 2016-11-11. . GEOCON
INCORPORATED
GEOTECHNICAL • ENVIRONMENTAL. MATERIALS
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Project No. G1928-52-01
November 11, 2016
ViaSat
6155 El Camino Real
Carlsbad, California 92009
Attention: Mr. Ryan Hatch
Subject: RESPONSE TO CITY COMMENTS
VIASAT - BRESSI RANCH
CARLSBAD, CALIFORNIA
References: 1. Geotechnical Investigation, ViaSat, Bressi Ranch, Carlsbad, California, prepared by
Geocon Incorporated, dated May 23, 2016, revised July 5, 2016 (Project No. G1928-
52-01).
2. Plancheck #2, Viasat Bressi Ranch Campus, prepared by the City of Carlsbad - Land
Development Engineering, dated November 9, 2016 (Project No. MS 16-04).
Dear Mr. Hatch:
We prepared this letter to address geotechnical review comments provided by the City of Carlsbad
regarding the subject project. The following provides the redline city comments (based on report section
number) and our response.
Section 7.6: Permanent soil nail walls and tie back walls are provided in this project. Revise
section title, or add another section. Provide recommendations for permanent
wall design and construction.
Response: We consider the recommendations provided in our referenced report to be
applicable for the design of both permanent and temporary soil nail and tieback
anchor walls. If permanent walls are proposed, an increased design factor of
safety for the wall and corrosion protection for the soil nails/tiebacks will need
to be determined by the wall designer. We will provide a review letter of the
wall plans to check that the design is acceptable from a geotechnical standpoint.
Section 7.6.14: Laboratory test results indicate several tested internalfriction angles are lower
than recommended values. Additionally, it is concluded that the site contains
expansive soils, which may have an internal friction angle lower than the
recommended value. Clarify.
Response: Results from our direct shear and triaxial shear testing indicate that the fill
materials (Qcf and Qpcf) possess a friction angle ranging from 26 and 37
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degrees and a cohesion ranging from 500 and 3,200 psf (Tables B-Ill and
B-IV). We evaluated the recommended friction angle of 29 degrees and
cohesion of 400 psf for the fill materials using a lower bound evaluation of the
data. Due to the varying nature of the fill materials, we are not able to feasibly
provide layer specific design parameters; however, we consider the strength
parameters that we provided to be an accurate representation of the prevailing
conditions of the fill.
Section 7.6.15: Provide design bond stress for permanent tieback anchor walls.
Response: The design bond strength for tieback anchors is depends on several factors
including soil conditions, anchor sizing, drilling method, grout mix and grout
placement. As a result, the design bond strength should be evaluated by the wall
structural engineer. We will provide a review letter of the wall plans to check
the plans are adequate from a geotechnical standpoint.
Section 7.7.7: Provide design bond strength for soil nails.
Response: Refer to our response for Section 7.6.15 herein.
Section 7.13.1: Section 7.2.1 concludes that the site fill soil has "medium to high expansive"
soils. Please confirm the recommended equivalent fluid density of 40 pcffor
site conditions.
Response: The recommended equivalent fluid density of 40 pcf is based on the assumption
that retaining wall compacted backfill material will have an expansion index of
90 or less. In areas where the soldier pile walls are retaining soil with a "high"
expansion potential (expansion index of 91 to 130), an equivalent fluid density
of 40 pcf can be used for unrestrained walls with a level backfill.
Section 7.13.4: The recommended additional seismic lateral earth pressure of 21H appears
high for the given mapped PGAm of 0.44g. Although no explicit formula for the
additional lateral earth pressures is defined in the CBC, IBC, or ASCE 7-10.
The current Caltrans practice uses 1/3 to 1/2 PGAm to quantify the seismic
lateral earth pressures. More flexible retaining walls have lower design seismic
horizontal acceleration coefficients. Clarify.
Response: We used methodology from several resources (including Caltrans, Army Corps
of Engineers, NAVFAC) and based on Mononobe-Okabe equations to calculate
the recommended seismic lateral earth pressure provided. Although there is not
a consensus within the engineering community as to what method is most
accurate, our calculations using several published methods indicate that a
seismic lateral earth pressure of 21H is applicable for the soil and seismic
conditions at the site.
Section 7.13.4: Provide recommendation for a seismic horizontal acceleration coefficient for
the permanent soil nail wall design and global stability analysis for other
permanent retaining wall design.
Response: There are several methods the soil nail wall engineers have used to evaluate the
seismic slope stability. Therefore, we provide the design site acceleration and
the soil nail structural engineer can evaluate the applicable horizontal seismic
coefficient to use in their analyses.
Project No. G1928-52-01 -2- November 11, 2016
If you have any questions regarding this response, or if we may be of further service, please contact the
undersigned at your convenience.
Very truly yours,
GEOCON INCORPORATED
Shawn Foy eedon
GE 2714
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No. 2714 Matthe-,6. oeve
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SFW:MRL:dmc
(e-mail) Addressee
Project No. G1928-52-01 -3- November 11, 2016