HomeMy WebLinkAboutCUP 2018-0023; BUENA VISTA RESERVIOR SITE; UPDATE GEOTECHNICAL LETTER AND RESPONSE TO CITY REVIEW COMMENTS; 2019-02-04GEOCON
INCORPORATED
GEOTECHNICAL U ENVIRONMENTAL U MATERIALS
Project No. G2225-52-01
February 4, 2019
Schmidt Design Group
1310 Rosecrans Street, Suite G
San Diego, California 92106
Attention: Mr. Mark Moss
Subject: UPDATE GEOTECHNICAL LETTER AND
RESPONSE TO CITY OF CARLSBAD REVIEW COMMENTS
BUENA VISTA RESERVOIR SITE
CARLSBAD, CALIFORNIA
References: 1. Limited Geotechnical Investigation, Buena Vista Reservoir Site, Carlsbad, California,
prepared by Geocon Incorporated, revised January 11, 2019 (Project No. G2225-
52-01).
2. Pt Review for CUP 2018-0023 (DEV2018-0222) - Buena Vista Reservoir Site,
Carlsbad, California, prepared by the City of Carlsbad Planning Division, dated
January 28, 2019.
Dear Mr. Moss:
We prepared this letter to update our referenced geotechnical report and address the referenced review
comments provided by the City of Carlsbad regarding the Buena Vista Reservoir Site. The reviewer's
comments are listed herein with our response immediately following.
Comment 5: Correct the Geocon soils report dated 2/9/18 as follows:
Comment 5a: Recommendation 7.1.8 is a double negative. If you are stating that the site
development will not impact adjacent sites or create settlement of improvements,
then remove 'not' after 'will' and 'will not' later in the sentence. The statement is
effectively stating. If you are stating it will impact, then indicate what mitigation
is needed to not impact the adjacent property and improvements.
Response: Our updated recommendation for Section 7.1.8 of our referenced report should
read: We do not expect site development will impact adjacent sites and create
settlement to public improvements.
Comment Sb: Under recommendation 7.12.1, clarify that the minimum street section for a local
street per City of Carlsbad Engineering Standards is 4 inch AC/4 inch AB.
Response: We calculated the flexible pavement sections in general conformance with the
Caltrans Method of Flexible Pavement Design (Highway Design Manual, Section
608.4) using an estimated Traffic Index (Ti) of 5.0, 5.5, 6.0, and 7.0 for parking
6960 Flanders Drive 0 San Diego, California 92121.2974 I Telephone 858.558.6900 0 Fax 858.558.6159
Of. - ,
Shawn Foy Weedon
GE 2714
stalls, driveways, medium truck traffic areas, and heavy truck traffic areas,
respectively. We updated Table 7.12.1 to provide the presents the preliminary
flexible pavement sections incorporating the City of Carlsbad minimum pavement
design thickness for a "local street."
TABLE 7.12.1
PRELIMINARY FLEXIBLE PAVEMENT SECTION
Assumed Assumed Asphalt Class 2
Location Traffic Subgrade Concrete* Aggregate
Index R-Value (inches) Base (inches)
Parking stalls for automobiles 5.0 20 4 5 and light-duty vehicles
Driveways for automobiles 5 53 4 4
20 4 7 and light-duty vehicles
53 4 4 Medium truck traffic areas 6.0 20 4 9
Driveways for 7.0 53 4 4
20 1 4 1 12 heavy truck traffic
*The minimum street section for the City of Carlsbad.
Comment Sc:
Response:
On sheet C-lU of Worksheet C.4-1 (page 3 of 4), the last sentence to the answer
under criteria 5 should read 'Infiltration in the northwest corner...', not
northeast. Infiltration testing was not performed at the northeast corner.
The correction has been made to the last sentence of the answer under criteria 5
on page 3 of 4 of the enclosed Worksheet C.4-1.
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
Keei A. James 0 FESS,c
RCE 79438 A.
KAJ:SFW:dmc (co ( No. 79438
Enclosure: Worksheet C.4-1&,
(e-mail) Addressee
(e-mail) Latitude 33
Attention: Nick Psyhogios
Project No. G2225-52-01 -2- February 4, 2019
1(Categorization KnIT Infiltration T1FeasibilityZIMYitfksji Worksheet C.4-1
Part I - Full Infi1tftion Feasibility Screening Critena
Would infiltrationof the full design volumdbc feaáSible from a physical perspetive without any undesirable
consequences that cannot bercasonably mitigated?
Criteria Screening Question Yes No
Is the estimated reliable infiltration rate below proposed
facility locations greater than 0.5 inches per hour? The response
1 to this Screening Question shall be based on a comprehensive X
evaluation of the factors presented in Appendix C.2 and Appendix
D.
Provide basis:
We performed 4 Aardvark Permeameter tests within the northwest corner and western side of the site within the Old
Paralic Deposits. The following presents the results of our field infiltration tests:
Northwest corner of site:
P-1 within Qop: 0.049 inches per hour with a FOS=2.
P-2 within Qop: 0.043 inches/hour with a FOS=2.
Western side of site:
P-3 at 4.3 feet deep within Qop: 0.468 inches per hour with a FOS=2.
P-4 at 5.7 feet deep within Qop: 0.295 inches/hour with a FOS=2.
The northwest corner of the site has an average infiltration rate of 0.046 inches/hour with a factor of safety of 2 and
the western side of the site has an average infiltration rate of 0.382 inches/hour with a factor of safety of 2, which is
less than the required 0.5 inches/hour rate. Therefore, full infiltration in either area should be considered infeasible.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/data source applicability.
Can infiltration greater than 0.5 inches per hour be allowed
without increasing risk of geotechnical hazards (slope stability,
2 groundwater mounding, utilities, or other factors) that cannot
be mitigated to an acceptable level? The response to this
Screening Question shall be based on a comprehensive evaluation of
the factors presented in Appendix C.2.
Provide basis:
Slopes to the neighboring properties below exist at the eastern and western property lines. Infiltrating in this area could
cause buildup of hydrostatic pressures, which could destabilize the existing slopes or walls. Seepage could occur from
he slope that flows into the adjacent properties. Mitigation measures could include deepening the bottom of the BMP
facility and lining the sides down to an elevation at least 1 foot below the toe of the adjacent slopes or retaining walls.
However, if we were to extend the basin deeper, we expect the infiltration rates would be reduced to about the rates we
obtained from the northwestern portion of the property due to similar elevations. Therefore, the infiltration would b
considered infeasible. If the devices are setback at least 50 feet from the top of slopes, the elevations would be too high
o practically install infiltration devices.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/data source applicability.
Project No. G2225-52-01 February 4, 2019
Worksheet C.4-1 Page 2 of 4
Criteria I
- .• Screening Question . .
I
Yes No
Can infiltration greater than 0.5 inches per hour be allowed
without increasing risk of groundwater contamination (shallow
water table, storm water pollutants or other factors) that cannot x be mitigated to an acceptable level? The response to this
Screening Question shall be based on a comprehensive evaluation of
the factors presented in Appendix C.3.
Provide basis:
Appendix C Section C.3.2 of the BMP Design Manual states that the depth to seasonably high groundwater tables
beneath the base of any infiltration BMP must be greater than 10 feet for any infiltration BMP to be allowed. We
expect that groundwater exists at depths of greater than 100 feet below the site. Therefore, we do not expect that
infiltration greater than 0.5 inches per hour would increase the risk of groundwater contamination.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/data source applicability.
Can infiltration greater than 0.5 inches per hour be allowed
without causing potential water balance issues such as change
of seasonality of ephemeral streams or increased discharge of
contaminated groundwater to surface waters? The response to
this Screening Question shall be based on a comprehensive
evaluation of the factors presented in Appendix C.3.
Provide basis:
We expect that groundwater exists at depths of greater than 100 feet below the site. Therefore, we do not expect that
infiltration greater than 0.5 inches per hour would cause water balance issues.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/data source applicability.
If all answers to rows 1 - 4 are "Yes" a full infiltration design is potentially feasible.
Part 1 The feasibility screening category is Full Infiltration
Not Full
If any answer from row 1-4 is "No", infiltration may be possible to some extent but Infiltration
would not generally be feasible or desirable to achieve a "full infiltration" design.
Proceed to Part 2
*To be completed using gathered site information and best professional judgment considering the definition of MEP in the
MS4 Permit. Additional testing and/or studies may be required by the City to substantiate findings.
Project No. G2225-52-01 February 4, 2019
Worksheet TIUPage TTt
Part 2— Partial Infiltration vs. No Infiltration Feasibility ScreeningCriteria
Would infiltration of water in any appreciable amount he physically feasible without any negativ e
consequences that cannot be.reasonablymitigated?
Criteria Screening Question Yes No
Do soil and geologic conditions allow for infiltration in any
5 appreciable rate or volume? The response to this Screening x Question shall be based on a comprehensive evaluation of the
factors presented in Appendix C.2 and Appendix D.
Provide basis:
We performed 4 Aardvark Permeameter tests within the northwest corner and western side of the site within the Old
Paralic Deposits. The following presents the results of our field infiltration tests:
Northwest corner of site:
P-I within Qop: 0.049 inches per hour with a FOS=2.
P-2 within Qop: 0.043 inches/hour with a FOS=2.
Western side of site:
P-3 at 4.3 feet deep within Qop: 0.468 inches per hour with a FOS=2.
P-4 at 5.7 feet deep within Qop: 0.295 inches/hour with a FOS=2.
The northwest corner of the site has an average infiltration rate of 0.046 inches/hour with a factor of safety of 2 and
the western side of the site has an average infiltration rate of 0.382 inches/hour with a factor of safety of 2, which is
less than the required 0.5 inches/hour rate. Therefore, based solely on infiltration rate, partial infiltration in the western
side of the site could be feasible. Infiltration in the northwest corner of the site should be considered infeasible.
Can Infiltration in any appreciable quantity be allowed
without increasing risk of geotechnical hazards (slope
6 stability, groundwater mounding, utilities, or other factors) x that cannot be mitigated to an acceptable level? The response
to this Screening Question shall be based on a comprehensive
evaluation of the factors presented in Appendix C.2.
Provide basis:
Slopes to the neighboring properties below exist at the eastern and western property lines. Infiltrating in this area could
cause buildup of hydrostatic pressures, which could destabilize the existing slopes or walls. Seepage could occur from
he slope that flows into the adjacent properties. Mitigation measures could include deepening the bottom of the BMP
acuity and lining the sides down to an elevation at least 1 foot below the toe of the adjacent slopes or retaining walls.
However, if we were to extend the basin deeper, we expect the infiltration rates would be reduced to about the rates we
obtained from the northwestern portion of the property due to similar elevations. Therefore, the infiltration would be
considered infeasible. If the devices are setback at least 50 feet from the top of slopes, the elevations would be too high
o practically install infiltration devices.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/data source applicability and why it was not feasible to mitigate low infiltration rates.
Project No. G2225-52-01 February 4, 2019
Worksheet C.4-1 Page 4 of 4
Criteria Screening Question Yes No
Can Infiltration in any appreciable quantity be allowed
without posing significant risk for groundwater related
concerns (shallow water table, storm water pollutants or other
factors)? The response to this Screening Question shall be based
on a comprehensive evaluation of the factors presented in
Appendix C.3.
Provide basis:
We expect that groundwater exists at depths of greater than 100 feet below the site. Therefore, we do not expect that
infiltration greater than 0.5 inches per hour would increase the risk of groundwater contamination.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/data source applicability and why it was not feasible to mitigate low infiltration rates.
Can infiltration be allowed without violating downstream
8 water rights? The response to this Screening Question shall be x based on a comprehensive evaluation of the factors presented in
Appendix C.3.
Provide basis:
We understand this is not a common issue within the County of San Diego and we are unaware of downstream water
rights.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/data source applicability and why it was not feasible to mitigate low infiltration rates.
If all answers from row .1-4 are yes then partial infiltration design is potentially feasible.
Part 2 The feasibility screening category is Partial Infiltration.
Result* i i If any answer from row 5-8 s no, then infiltration of any volume s considered to be No Infiltration
infeasible within the drainage area. The feasibility screening category is No Infiltration.
"To be completed using gathered site information and best professional judgment considering the definition of MEP in the
MS4 Permit. Additional testing and/or studies may be required by the City to substantiate findings.
i
Project No. G2225-52-01 February 4, 2019