HomeMy WebLinkAboutAppendix E.3 - Nakano_Addendum to Geotechnical Report and Response to City of San Diego Review Comments_020922Project No. 07516-42-02
February 9, 2022
Tri Pointe Homes
13400 Sabre Springs Parkway, Suite 200
San Diego, California 92128
Attention: Ms. April Tornillo
Subject: ADDENDUM GEOTECHNICAL REPORT AND
RESPONSE TO CITY OF SAN DIEGO REVIEW COMMENTS
NAKANO
SAN DIEGO, CALIFORNIA
References: 1. City of San Diego Geology Review Comments prepared by Patrick Thomas dated
November 17, 2021.
2. Update Geotechnical Investigation, Nakano Property, Chula Vista, California prepared
by Geocon Incorporated dated September 18, 2020 (Project No. 07516-42-02).
3. Nakano – Site Plan, City of Chula Vista, California, prepared by Civil Sense, Inc.,
dated February 3, 2022.
Dear Ms. Tornillo:
We prepared this addendum to provide an updated geologic map and to respond to review comments
from the City of San Diego, LDR-Geology department (Reference 1). The updated geologic map and
the log from a recent boring and test pit are appended. The review comments and our responses are
presented below.
Issue 3:Submit an addendum geotechnical report that specifically addresses the following:
Response: The recommendations presented in our Update Geotechnical Investigation (Reference 2)
remain applicable. This response serves as the requested addendum report.
Issue 4:Clarify if the southeastern corner of the subject property is underlain by a landslide
(geologic hazard zone 22) with a rupture surface deeper than the extent explored in the
test pits.
Response: The southeast corner of the property is not underlain by a landslide. A second large
diameter boring (LD-2) was drilled to a depth of 70 feet at the east end of the proposed
cut slope (Figure 1 and appended boring log). The boring bottom elevation is
approximately 16 feet below proposed pad grade at toe of slope. Observations made
during downhole logging show similar geologic conditions to those observed in LD-1,
i.e., a massive to poorly bedded very fine-grained silty sandstone dipping approximately
14 degrees due west. There are no slide planes, bedding plane shears, or other geologic
features indicative of landsliding present in LD-1 or LD-2.
Geocon Project No. 07516-42-02 - 2 - February 9, 2022
Additionally, we excavated an additional test pit (TP-24) downslope of TP-2 to confirm
the geologic conditions present in the smaller cut slope at the southeast corner of the
project. The geologic unit and structure observed in TP-24 are consistent with the
conditions observed in LD-1 and LD-2. Boring and test pit logs for LD-2 and TP-24 are
included with this report.
Issue 5:Per the Sate’s Guidelines for Evaluating Seismic Hazards in California (2008), the
commonly accepted factor of safety for slopes is >1.1 for dynamic loads. The projects’
geotechnical consultant should demonstrate that the site will comply with this standard
following project completion.
Response: We performed a seismic slope stability analysis for cross sections C-C’ and D-D’ in
accordance with Recommended Procedures for Implementation of DMG Special
Publication 117: Guidelines for Analyzing and Mitigating Landslide Hazards in
California, prepared by the Southern California Earthquake Center (SCEC), dated June
2002.
The seismic slope stability analysis was performed using an unweighted acceleration of
0.16g, corresponding to a 10 percent probability of exceedance in 50 years. In addition,
a deaggregation analysis was performed on the 0.16g value for the site. A modal
magnitude and modal distance of 6.1 and 11.1 kilometers, respectively, were used for
the analysis.
Using the parameters discussed herein, an equivalent site acceleration, kEQ, of
approximately 0.1g was calculated (see Figure 4). Using this kEQ, we get a factor of
safety of 1.86 for Section C-C’ (see Figure 5) and 2.2 for Section D-D’ (see Figure 6). A
slope is considered acceptable by the screening analysis if the calculated factor of safety
is greater than 1.0 using kEQ; therefore, the slopes pass the screening analysis for seismic
slope stability.
Issue 6:Provide the Slope stability calculations.
Response: Results of our stability analyses are appended.
Issue 7:Clarify if the site will have a factor of safety of 1.5 or greater with respect to surficial
slope stability following completion of the project based on utilizing a depth of
saturation of 5 feet. Per the City’s “Guidelines for Geotechnical Reports,” if the depth
of saturation is used in the analysis is less than 5 feet, the shallower depth must be
justified.
Response: For cut slopes in Tertiary age formational units, a saturation depth of 5 feet is
unreasonable due to the impermeable nature of cemented geologic units. Therefore, the
calculation presented in the referenced report which uses a saturation depth of 4 feet
remains applicable. However, we have performed additional surficial slope stability
analyses for fill slopes using a saturation depth of 5 feet (see Figure 7).
Issue 8:Based on the slope stability analyses, the geotechnical consultant must provide a
professional opinion whether or not slopes within and adjacent to the proposed
development will have a factor of safety of 1.5 or greater with respect to gross and
surficial stability following completion of the project.
Response: Based on the results of our stability analyses, included herein, the slopes in and adjacent
to the proposed project have a factor of safety of 1.5 or greater for gross and surficial
stability following completion of the project, provided the grading recommendations in
Reference 2 are followed.
Geocon Project No. 07516-42-02 - 3 - February 9, 2022
Issue 9:The referenced plans indicate a cut slope at a gradient of 1.8 horizontal feet to 1
vertical foot. Revise the gradient to 2 horizontal feet to 1 vertical foot per the City of
San Diego Municipal Code Section 142.0133.
Response: The Project Civil Engineer will adjust slope gradients to meet City of San Diego
requirements.
If there are any questions regarding this correspondence, or if we may be of further service, please
contact the undersigned at your convenience.
Very truly yours,
GEOCON INCORPORATED
Rodney C. Mikesell
GE 2533
Rupert S. Adams
CEG 2561
RCM:RSA:arm
(e-mail) Addressee
(e-mail) Civil Sense, Inc.
Attention: Mr. Inh Ling
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Seismic Slope Stability Evaluation
Input Data in Shaded Areas
Project Nakano Computed By RCM
Project Number 07516-42-02
Date 02/08/22
Filename Case 1_Proposed Slope_Seismic
Peak Ground Acceleration (Firm Rock), MHAr,g 0.16 10% in 50 years
Modal Magnitude, M 6.1
Modal Distance, r, km 11.1
Site Condition, S (0 for rock, 1 for soil)1
Yield Acceleration, ky/g NA <-- Enter Value or NA for Screening Analysis
Shear Wave Velocity, Vs (ft/sec)NA <--
Max Vertical Distance, H (Feet)NA <--
Is Slide X-Area > 25,000ft2 (Y/N)N <-- Use "N" for Buttress Fills
Correction for horizontal incoherence 1.0
Duration, D5-95|med, sec 6.670
Coefficient, C1 0.5190
Coefficient, C2 0.0837
Coefficient, C3 0.0019
Standard Error, HT 0.437
Mean Square Period, T m, sec 0.550
Initial Screening with MHEA = MHA = k maxg Approximation of Seismic Demand
ky/MHA NA Period of Sliding Mass, Ts = 4H/Vs, sec NA
fEQ(u=5cm) = (NRF/3.477)*(1.87-log(u/((MHA r/g)*NRF*D5-95)))0.4730 Ts/Tm NA
kEQ = feq(MHAr)/g 0.076 MHEA/(MHA*NRF)NA
Factor of Safety in Slope Analysis Using kEQ 2.20 NRF = 0.6225+0.9196EXP(-2.25*MHA r/g)1.26
Passes Initial Screening Analysis MHEA/g NA
ky/MHEA = ky/kmax NA
Normalized Displacement, Normu NA
Estimated Displacement, u (cm) NA
FIGURE 4
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Slope Height, H (feet)
Vertical Depth of Stauration, Z (feet) 5
Slope Inclination 2.00
Slope Inclination, I (degrees) 26.6
Unit Weight of Water, JW (pcf)62.4
Total Unit Weight of Soil, JT (pcf)125
Friction Angle, I (degrees)27
Cohesion, C (psf) 300
Factor of Safety = (C+(JT-JW)Z cos2i tanI)/(JTZ sin i cos i)1.71
References:
Surficial Slope Stability Evaluation - Fill Slopes
(1) Haefeli, R. The Stability of Slopes Acted Upon by Parallel Seepage , Proc. Second International Conference,
SMFE, Rotterdam, 1948, 1, 57-62.
(2) Skempton, A. W., and F. A. Delory, Stability of Natural Slopes in London Clay , Proc. Fourth International
Conference, SMFE, London, 1957, 2, 378-81.
5&0/5
SLOPE STABILITY ANALYSIS
NAKANO
CHULA VISTA, CALIFORNIA
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