HomeMy WebLinkAboutCDP 2020-0003; CROWLEY RESIDENCE; INFILTRATION FEASIBILITY DISCUSSION; 2020-04-01Geotechnical • Geologic • Coastal • Environmental
5741 Palmer Way • Carlsbad, California 92010 • (760) 438-3155 • FAX (760) 931-0915 • www.geoso1ls1nc.com
DATE:
TO:
Attn:
From:
MEMORANDUM
April 1, 2020
Don Looney Architect, AIA
2947 Harding Street, Suite 211
Carlsbad, California 92008
Mr. Don Looney
Robert G. Crisman, CEG 1934
David W. Skelly, RCE 47857
W.O. 7759-A-SC
SUBJECT: Discussion of Infiltration Feaslbllity 4367 Highland Drive, Carlsbad,
San Diego County, California 92008
References: 1. "Carlsbad Storm Water Design Manual {BMP Design Manual) for Permanent Site Design
with Respect to Storm Water Treatment and Hydromodification Management, dated
February 16, 2016, by the City of Carlsbad.
2. "United States Department of Agriculture, National Resources Conservation Service, 2020,
Custom Soils Report for San Diego County Area, 4367 Highland Drive, Carlsbad, California,
dated March 24, 2020, by the United States Department of Agriculture.
3. "Soil Survey, San Diego Area, California, Part I and Part II, dated 1973, by the United
States Department of Agriculture.
4. "Preliminary Grading Plan, Coastal Development Permit, 4367 Highland Drive, Carlsbad,
92008," J.N. 3028, Plot date February 4, 2020, by Pasco Laret Suiter & Associates.
In accordance with your request and authorization, and per the City's BMP design manual
(Reference No. 1), GeoSoils, Inc. (GSI) has prepared the following discussion of soil
infiltration rates and storm water BMP design for the subject site. This discussion is based
on a "desk top" evaluation of infiltration rates, as evaluated by the United States
Department of Agriculture (see Reference Nos. 2 and 3), analysis, and a review of the site
grading plan (Reference No. 4).
USDA Study
A review of the United States Department of Agriculture database (Reference Nos. 3 & 4),
indicates that the upper, roughly 80 percent ofthe site is underlain with soils belonging to
the Marina Loamy Sand, with the lowermost 20 percent of the site underlain with the
Carlsbad gravelly loamy sand. Per the USDA, Marina soils were evaluated with infiltration
rates ranging from 0.57 to 1.98 inches per hour, while Carlsbad soils were evaluated with
infiltration rates ranging from 1.98 to 5.98 inches per hour. Both soils series belong to
Hydrologic Soil Group (HSG) "B." For this site, the Marina Loamy Sand, with an infiltration
rate of 0.57 inches per hour, is considered the controlling soil type, and should be
considered in BMP design.
Infiltration Feaslblllty
In general accordance with the City BMP design manual (Reference No. 1), the infiltration
feasibility for this site was evaluated. An evaluation of the soils infiltration characteristics
and potential impact on site development was performed for this evaluation, using a "desk
top" analysis. Based on our review, including; adjacent slopes, existing (or proposed)
utility backfill, and/or existing moisture-sensitive improvements, such as pavements, and
utility trench backfill, foundations, retaining walls, and below grade building walls, would
likely be adversely affected by soil infiltration, including offsite improvements, causing
settlement and distress.
Storm water BMP's can adversely affect the performance of the onsite and offsite
structures foundation systems by: 1) Increasing soil moisture transmission rates through
concrete flooring; 2) reducing the stability of slopes and; and 3) increase the potential for
a loss in bearing strength of soil. Current plans for this project (Reference No. 4) indicate
BMP basins located below the building pads, and on a natural slope with an approximate
gradient of about 3:1 (h:v). Each BMP appears to be constructed using retaining walls.
As indicated in the attached City Form 1-8, "Categorization of Infiltration Feasibility
Condition," a full infiltration BMP does not appear feasible due to the potential for distress
to adjacent retaining walls and an increase in the potential for slope instability. A partial
infiltration BMP may be considered in conjunction with mitigation, including, but not limited
to: deepened foundations, reduced soil design parameters (i.e., bearing value, lateral
pressures), and the consideration of hydrostatic pressure in wall design.
Form 1-8, "Categorization of Infiltration Feasibility Condition," and Form 1-9, "Factor of
Safety and Design Infiltration Rate Worksheet," are attached.
Onsite Filtration/Infiltration-Runoff Retention Systems
General design criteria regarding the use of onsite filtration-infiltration-runoff retention
systems (OIRRS) are presented below.
Should onsite infiltration-runoff retention systems (OIRRS) be required for Best
Management Practices (BMPs) or Low Impact Development (LID) principles for the project,
some guidelines should/must be followed in the planning, design, and construction of such
systems. Such facilities, if improperly designed or implemented without consideration of
the geotechnical aspects of site conditions, can contribute to flooding, saturation of
Don Looney Architect, AIA
4367 Highland Drive, Carlsbad
File:e :\wp 1217700\7759adoi.mem GeoSoils, Inca
W.O. 7759-A-SC
April 1, 2020
Page 2
bearing materials beneath site improvements, slope instability, and possible concentration
and contribution of pollutants into the groundwater or storm drain and/or utility trench
systems.
Some of the methods which are utilized for onsite infiltration include percolation basins,
dry wells, bio-swale/bio-retention, permeable pavers/pavement, infiltration trenches, filter
boxes and subsurface infiltration galleries/chambers. Some of these systems are
constructed using native and import soils, perforated piping, and filter fabrics while others
employ structural components such as storm water infiltration chambers and
filters/separators. Every site will have characteristics which should lend themselves to one
or more of these methods, but not every site is suitable for OIRRS. In practice, OIRRS are
usually initially designed by the project design civil engineer. Selection of methods should
include (but should not be limited to) review by licensed professionals including the
geotechnical engineer, hydrogeologist, engineering geologist, project civil engineer,
landscape architect, environmental professional, and industrial hygienist. Applicable
governing agency requirements should be reviewed and included in design
considerations.
Based on our evaluation, the following issues should be addressed when considering any
storm water BMP design:
•
•
•
•
The probability of limited space and proximity of settlement-sensitive improvements
to potential treatment area BMPs.
The presence of a thin layer of engineered fill overlying formation (as-built condition)
and the potential for developing a shallow, perched water table beneath
foundations.
Potential for adverse performance of planned improvements such as floor slabs,
below grade walls, and foundations, due to potential settlementfrom saturation, or
other distress due to water vapor transmission.
The potential for the migration of subsurface water offsite, beneath adjacent
residential properties, or streets, and/or into utility line trenches.
The following geotechnical guidelines should be considered when designing onsite
infiltration-runoff retention systems:
• It is not good engineering practice to allow water to saturate soils, especially near
slopes or improvements; however, the controlling agency/authority may now
require this.
• Areas adjacent to, or within, the OIRRS that are subject to inundation should be
properly protected against scouring, undermining, and erosion, in accordance with
the recommendations of the design engineer.
Don Looney Architect, AIA
4367 Highland Drive, Carlsbad
File:e :\wp 12\7700\7759adoi.mem GeoSoils, Inc.
W .0. 7759-A-SC
April 1 , 2020
Page 3
• Should they be required, where infiltration systems are located near slopes or
improvements, impermeable liners and subdrains should be used along the bottom
of bioretention swales/basins located within the influence of such slopes and
structures. Impermeable liners used in conjunction with bioretention basins should
consist of a 30-mil polyvinyl chloride (PVC) membrane that is covered by a
minimum of 12 inches of clean soil, free from rocks and debris, with a
maximum 4:1 (h:v) slope inclination, or flatter, and meets the following minimum
specifications:
Specific Gravity (ASTM D792): 1.2 (g/cc, min.); Tensile (ASTM D882):
73 (lb/in-width, min); Elongation at Break (ASTM D882): 380 (%, min);
Modulus (ASTM D882): 32 (lb/in-width, min.); and Tear Strength
(ASTM D1004): 8 (lb/in, min); Seam Shear Strength (ASTM D882)
58.4 (lb/in, min); Seam Peel Strength (ASTM D882) 15 (lb/in, min).
• Subdrains for basins should consist of at least 4-inch diameter Schedule 40 or
SDR 35 drain pipe with perforations oriented down. The drain pipe should be
sleeved with a filter sock.
• Utility backfill within OIRRS should consist of a two-sack mix of slurry.
Final project plans (infiltration, grading, precise grading, foundation, retaining wall,
landscaping, etc.), should be reviewed by this office prior to construction, so that
construction is in accordance with the conclusions and recommendations of this report.
Based on our review, supplemental recommendations and/or further geotechnical studies
may be warranted. It should be noted that structural and landscape plans were not
available for review at this time.
Closure
Inasmuch as our study is based upon our review and engineering analyses, the
conclusions and recommendations presented are professional opinions. These opinions
have been derived in accordance with current standards of practice, and no warranty is
express or implied. Standards of practice are subject to change with time. GSI assumes
no responsibility or liability for work or testing performed by others, or their inaction, or
work performed when GSI is not requested to be onsite, to evaluate if our
recommendations have been properly implemented. Use of this report constitutes an
agreement and consent by the user to all the limitations outlined above, notwithstanding
any other agreements that may be in place. In addition, this report may be subject to
review by the controlling authorities
Attachments: Form 1-8 -"Categorization of Infiltration Feasibility Condition"
Form 1-9 -"Factor of Safety and Design Infiltration Rate Worksheet"
Don Looney Architect, AIA
4367 Highland Drive, Carlsbad
Fite:e:\wp12\7700\7759a.doi.mem GeoSoils, Inc-
W.O. 7759-A-SC
April 1 , 2020
Page 4
Appendix I: Forms and Checklists
Categorization of Infiltration Fcasibi}jty Form I-8
Condition
Part 1-full Infiltration Feaaibilit;y Screening Criteria
Would infilttadon of the full design volume be feasible from a physical perspective without any undesirable
consequences that cannot be reasonably mitigated ?
Criteria Screening Question
Is the estimated reliable infiltration rate below proposed
facility locations greater than 0.5 inches per hour? The response
to this Screening Question shall be based on a comprehensive
evaluation of the factors presented in Appendix C.2 and Appendix
D.
Provide basis:
Yes No
Per the USDA, the Marina Loamy Coarse Sand occupies the uppermost 80 percent of the site, while
the Carlsbad Loamy Sand occupies the lowermost 20 percent of the site. Infiltration rates vary from
0.57-1.98 inches per hour (IPH) for the Marina soil, and 1.98-5.98 (IPH) for the Carlsbad soil. Both
soils are categorized as Hydrologic Soil Group B. As the Marina Loamy Coarse Sand appears to be
the dominant soil type, an infiltration rate in the range of 0.57 -1.98 (IHP) should govern BMP design.
Please note that a minimum factor of safety= 2.0 (per Form 1-9) has not been applied.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/ data source applicability.
2
Can infiltration greater than 0.5 inches per hour be allowed
without increasing risk of geotechnical hazards (slope stability,
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:
Planned BMP's are intimately associated with site retaining walls, and the overall site slopes
westward at a gradient of about 3:1 (h:v). Full infiltration will adversely affect wall performance and
slope stability.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/ data source applicability.
1-3 February 2016
Appendix I: Forms and Checklists
Criteri
a
3
Form 1-8 'Page 2 of 4
Screening Question
Can infiltration greater than 0.5 inches per bout be allowed
without increasing risk of groundwater contamination (shallow
water table, storm water pollutants 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.3.
Provide basis:
Yes No
The site is not located near a drainage course. Based on our geotechnical site exploration,
groundwater is greater than 20 feet below existing site grades, and likely at much lower elevations
(>50 feet) near sea level.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of study/ data source applicability.
4
Can infiltration greater than 0.5 inches per hour be allowed
without causing potential water balance issues such as chan.ge
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:
t/
The site is not located near a drainage course. Based on our geotechnical site exploration,
groundwater is greater than 20 feet below existing site grades, and likely at much lower elevations
(>50 feet) near sea level.
Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative
discussion of srudy / data source applicability.
Part 1
Result
*
If all answers to rows 1 -4 are "Yes" a full infiltration design is potentially feasible.
The feasibility screening category is Full Infiltration
lf any answer from row 1-4 is "No", infiltration may be possible to some extent but
would not generally be feasible or desirable to achieve a "full infiltration" design.
Proceed to Part 2
NO
>tTo be completed usmg gathered site mfonnatlon and best profess1onal 1udgment cons1denng the definition of MEP ID
the MS4 Pennit. Additional testing and/or studies may be required by the City to substantiate findings.
I-4 February 2016
Appendix I: Forms and Checklists
Form 1-8 Page 3 of 4
Part 2 -Partial Infiltration vs. No Iofilttatioo Feasibility Screr:oimr Criteria
Would infiltration of wate1' in any appreciable amount be physically feasible without any negative
consequences that cannot be reasonably mitigated?
Criteria
5
Screening Question
Do soil and geologic conditions allow for infiltration in any
appreciable rate or volume? The response to this Screening
Question shall be based on a comprehensive evaluation of the
factors presented in Appendix C.2 and Appendix D.
Provide basis:
Yes No
Per the USDA, the Marina Loamy Coarse Sand occupies the uppermost 80 percent of the site, while
the Carlsbad Loamy Sand occupies the lowermost 20 percent of the site. Infiltration rates vary from
0.57-1.98 inches per hour (IPH) for the Marina soil, and 1.98-5.98 (IPH) for the Carlsbad soil. Both
soils are categorized as Hydrologic Soil Group B. As the Marina Loamy Coarse Sand appears to be
the dominant soil type, an infiltration rate in the range of 0.57 -1.98 (IHP) should govern BMP
design. Please note that a minimum factor of safety = 2.0 (per Form 1-9) has not been applied.
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.
6
Can Infiltration in any appreciable quantity be allowed
without increasing risk of geotechnical hazuds (slope
stability, 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 Appendi.'C C.2.
Provide basis:
Mitigation including, but not limited to: deepened foundations, and reduced soil design parameters
(bearing value, lateral pressures, etc.) would be necessary for retaining wall improvements in the
vicinity of any BMP.
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.
1-5 February 2016
Appendix I: Forms and Checklists
Criteria
7
Form 1-8 Page 4 of 4
Screening Question
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:
Yes No
Based on our geotechnical site exploration, groundwater is greater than 20 feet below existing site
grades, and likely at much lower elevations (>50 feet) near sea level.
Summarize findings of studies; provide reference to studies, calculations, maps, data soucces, etc. Provide narrative
discussion of study/ data source applicability and why it was not feasible to mitigate low infiltration rates.
8
Can infiltration be allowed without violating downstream
water rights? The response to this Screening Question shall be
based on a comprehensive evaluation of the factors presented in
Appendix C.3.
Provide basis:
Down stream water rights are considered a legal matter and typically do not fall within the purview
of geotechnical engineering. However, GSI is not aware of any significant downstream water rights
issues of concern on the adjoining properties. Given the relatively low infiltration rate evaluated
onsite, infiltration should not significantly affect down stream water rights, from a geotechnical
perspective.
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.
Part2
Result*
Tf all answers from row 5-8 are yes then partial infiltration design is potentially feasible.
The feasibility screening category is Partial Infiltration.
If any answer from row 5-8 is no, then infiltration of any volume is considered to be
infeasible within the drainage area. The feasibility screening category is No Infiltration.
partial Infiltration
*To be completed using gathered site information and best professional judgment cons1denng the definition of MEP in
the MS4 Permit. Additional testing and/or studies may be required by the City to substantiate findings.
1-6 February 2016
Appendix I: Forms and Checklists
Form 1-9 : Factor of Safety and Design Infiltration Rate Worksheet
Factor of Safety I ntiltration Rate Form 1-9
\Vorksheet
Factor Criteria Factor Description Assigned Factor Product(p)
Weight(w) Value(v) p =wxv
Soil assessment methods 0.25 3 0.7S
Predominant soil texture 0.25 I 0.25
A Suitability Site soil variability 0.25 I 0.25 Assessment
Depth to groundwater/impervious layer 0.25 I 0.25
Suitability Assessment Safety Factor, SA = I:p +:58 Use 2.0 min.
Level ofpretreabnentlexpected sediment loads 0.5
Redundancy/resiliency 0.25
B Design
Compaction during construction 0.25
Design Safety Factor, S8 = :Ep
Combined Safety Factor, S..,..1= SA x S8
Observed Infiltration Rate, incb/hr, K,, ..... oo1 0.57 ( corrected for test-speci fie bias)
Design Infiltration Rate, in/hr, ¾esla• = K,,., ... od I S,0.,, 0.285
Supporting Data
Briefly describe infiltration test and provide reference to test forms:
Infiltration rate per USDA
*Design Criteria has been left blank due to the fact that design plans for an infiltration basin have not
been created yet.
1-7 February 26, 2016