HomeMy WebLinkAboutSDP 2021-0016; CARLSBAD OAKS NORTH LOT 3; INFILTRATION FEASABILITY CONDITION LETTER; 2023-05-22INFILTRATION FEASIBILITY
CONDITION LETTER
CARLSBAD OAKS NORTH
BUSINESS PARK – LOT 3 CARLSBAD, CALIFORNIA
PREPARED FOR
SAN DIEGO, CALIFORNIA
MAY 22, 2023PROJECT NO. 06442-32-34
THE TECHBILT COMPANIES
Project No. 06442-32-34
May 22, 2023
Tech Contractors
3575 Kenyon Street
San Diego, California 92110
Attention: Mr. Raul Guzman
Subject: INFILTRATION FEASIBILITY CONDITION LETTER CARLSBAD OAKS NORTH BUSINESS PARK – LOT 3
CARLSBAD, CALIFORNIA
References: 1. Update to Geotechnical Report, Carlsbad Oaks North Business Park – Lot 3,
Carlsbad, California, dated May 5, 2023, prepared by Geocon Incorporated (Geocon
Project No. 06442-32-34).
2. Update Geotechnical Report, Carlsbad Oaks North Business Park – Lot 3, Carlsbad,
California, dated May 26, 2021, prepared by Geocon Incorporated (Geocon Project
No. 06442-32-34).
3. Grading Plans for: Lot 3, Whiptail Loop, Carlsbad, California, prepared by Spear &
Associates, Inc., undated.
4. Post Development DMA Plan for: Lot 3, Whiptail Loop, Carlsbad, prepared by prepared
by Spear & Associates, Inc. undated.
Dear Mr. Guzman:
In accordance with the request of Mr. Josh Zeigler with Spear & Associates, Inc., we prepared this
Infiltration Feasibility Condition Letter for the subject project located in Carlsbad, California (see
Vicinity Map).
GEOCON
INCORPORATED
G E OT E CHN I CAL ■E NV I RONMENTA L ■ MA T ER I A L S
6960 Flanders Drive ■ Son Diego, California 92121-297 4 ■ Telephone 858.558.6900 ■ Fax 858.558.6159
Geocon Project No. 06442-32-34 - 2 - May 22, 2023
Vicinity Map
SITE DESCRIPTION
The lot is bounded by Whiptail Loop to the east, open space to the west and south, and Pacifica Vista
Commerce Center (formerly Lot 23) to the north. The property mostly consists of compacted fill at
existing grade. Granitic rock is located along the northeast portion of the lot. Ascending and descending
2:1 (horizontal:vertical) fill and cut slopes are located along the perimeter of the lot with a maximum
height of approximately 50 feet. Sparse low lying grass/weeds are spread across the property. A
temporary paved drive isle and sales trailer is located along south margin of the sheet-graded pad.
The sheet-graded pad portion of the lot slopes from the northwest to southeast with elevations varying
from approximately 328 feet above Mean Sea Level (MSL) to approximately 316 feet MSL. A
temporary detention basin is located in the southeast margin of the pad. Existing improvements within
the lot consist of a storm drain system that was constructed as part of the temporary detention basin. The
slopes are landscaped with shrubs and trees with an active irrigation system.
Geocon Project No. 06442-32-34 - 3 - May 22, 2023
PROSOSED DEVELOPMENT
We understand that the proposed development includes grading the existing sheet-graded pad to support
three, single-story buildings. We anticipate that the buildings will consist of concrete tilt-up construction
supported by conventional continuous and isolated spread footings with slab-on-grade construction.
Additional improvements consist of underground utilities, surface parking/driveways, retaining walls,
hardscape and bio-retention systems for storm water.
STORM WATER MANAGEMENT DISCUSSION
We understand storm water management devices are being proposed in accordance with City of
Carlsbad BMP Design Manual (January 2023 Update). If not properly constructed, there is a potential
for distress to improvements and properties located hydrologically down gradient or adjacent to these
devices. Factors such as the amount of water to be detained, its residence time, and soil permeability
have an important effect on seepage transmission and the potential adverse impacts that may occur if
the storm water management features are not properly designed and constructed. We have not performed
a hydrogeological study at the site. If infiltration of storm water runoff occurs, downstream properties
may be subjected to seeps, springs, slope instability, raised groundwater, movement of foundations and
slabs, or other undesirable impacts as a result of water infiltration.
Presented below is a discussion for items addressing Appendix D.3 of the BMP Design Manual.
Public health and safety requirements that affect infiltration locations.
Please see attached Table D.1.1 for planned BMP basins A and B. Where applicable, we
provided response to items presented on the table. Based on our responses, the site is restricted.
The phase of the project in which the geotechnical engineer first analyzed the site for
infiltration feasibility.
We analyzed for infiltration feasibility in the preliminary/planning phase.
Results of previous geotechnical analyses conducted in the project area, if any.
We prepared geotechnical report dated May 26, 2021 (Reference No. 2) as part of the planning
phase of development. The lot consists of compacted fill and dense granitic rock. Planned
grading will result with these units at grade. As discussed in the report, the compacted fill
consists of silty sands, and mixtures of angular gravel and boulders generated from blasting
operations in granitic rock. Soils consisting of sandy clays were placed in deeper fill areas.
Infiltrating into compacted fill generally results in settlement and distress to improvements
placed over the compacted fill; as well as slope instability. It is our opinion the compacted fill
is unsuitable for infiltration of storm water runoff due to the potential for adverse settlement and
slope instability. The granitic bedrock is also sufficiently dense that infiltration water would be
expected to perch on this unit.
Geocon Project No. 06442-32-34 - 4 - May 22, 2023
The development status of the site prior to the project application (i.e., new development with
raw ungraded land, or redevelopment with existing graded conditions.)
The project is redevelopment with existing graded conditions.
The history of designs discussions for the project footprint, resulting in the final design
determination.
There were no locations on the property where the storm water basins could be located that
would be outside of graded areas. Final design determination was based on estimated ultimate
as-graded conditions (compacted fills, granitic rock), planned improvements (i.e., buildings,
underground utilities, surface improvements, retaining wall structures) and bordering
developments.
Full/partial infiltration BMP standard setbacks to underground utilities, structures, retaining
walls, fill slopes, and natural slopes applicable to the DMA that prevent full/partial
infiltration.
The project is and will be underlain by compacted fill and granitic rock after planned grading is
completed. Based on our grading recommendations, we assume that the upper 5 feet of fill and
cut areas may consist of very low to low expansive soils. Ascending and descending 2:1
(horizontal:vertical) fill and cut slopes are located along the perimeter of the lot with a maximum
height of approximately 50 feet. Infiltration into compacted fill could result in soil movement
resulting in either heave or settlement and slope instability. Existing improvements border the
east margin of the project. Full or partial infiltration near planned and existing improvements
(i.e., building foundations, retaining walls, underground utilities, surface improvements) is not
recommended. Infiltration near existing and planned improvements will likely result in lateral
water migration and soil movement (heave and/or settlement) which could result in structural
distress.
The physical impairments (i.e., fire road egress, public safety considerations, etc.) that
prevent full/partial infiltration.
The property consists of ascending and descending 2:1 (horizontal:vertical) fill and cut slopes
located along the perimeter of the lot with a maximum height of approximately 50 feet.
Additional physical impairments are the planned buildings and existing surface and
underground improvements bordering the property.
The consideration of site design alternatives to achieve partial/full infiltration within the
DMA.
A site design alternative to include full or partial infiltration would be limited to a lined, deep
dry well system founded in the underlying granitic rock. This is considered practically
infeasible.
The extent site design BMP’s requirements were included in the overall design.
This question is best answered by the project Civil engineer, if needed.
Geocon Project No. 06442-32-34 - 5 - May 22, 2023
Conclusion or recommendation from the geotechnical engineer regarding the DMA’s
infiltration condition.
Based on the responses provided above, it is our professional opinion that entire site (planned
DMA’s) is not feasible for partial or full infiltration and the property should be considered to
possess a No Infiltration condition in accordance with City of Carlsbad BMP Design Manual
(January 2023). Infiltration would create an un-mitigatable risk of impact to improvements,
slope instability, and lateral seepage migration that could adversely impact public and private
improvements.
Where applicable, liners and subdrains are recommended in the design and construction of bio-
filtration systems. The liners should be impermeable (e.g. High-density polyethylene, HDPE,
with a thickness of about 30 mil or equivalent Polyvinyl Chloride, PVC). The subdrains should
be perforated within the liner area, installed at the base and above the liner, be at least 4 inches
in diameter. The subdrains outside of the liner should consist of solid pipe. The penetration of
the liners at the subdrains should be properly waterproofed. The subdrains should be connected
to the project storm-drain system. The devices should also be installed in accordance with the
manufacturer’s recommendations.
An Exhibit for all applicable DMA’s that clearly labels:
Figure 1 (geologic map) and Figure 2 (DMA exhibit) are attached. The geologic map shows
existing geologic conditions. Figure 2 shows planned DMA’s. The project grading plan
(Reference No. 3; not included) shows existing and select planned underground improvements.
As previously discussed, there are no locations on the property where structural BMPs can be
located where full/partial infiltration can be proposed due to existing sloping terrain, geologic
condition and existing improvements bordering the project. Infiltration would create an un-
mitigatable risk impacting planned structures, slope instability and lateral seepage migration
that could adversely affect existing and planned improvements (adjacent underground and
surface improvements).
If you have any questions regarding this letter, or if we may be of further service, please contact the
undersigned at your convenience.
Very truly yours,
GEOCON INCORPORATED
Emilio Alvarado
RCE 66915
EA:arm
(e-mail) Addressee
(e-mail) Spears & Associates, Inc.
Attention: Mr. Josh Zeigler
3
2
0
33
0
34
0
35
0
36
0
37
0
3
1
0
3
2
0
33
0
34
0
35
0
360
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5
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5
31
5
32
5
3
3
5
34
5
355
365
36
5
35
5
34
5
33
5
BUILDING "A"
BUILDING "B"
BUILDING "C"
2
2
2
2
22
2
2
22
3
1
11
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3
3
3
3
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3
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1 1 1
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3
3
3
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4
4
4 44
4
4
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5
5
66
6
6
6
7 7
6
6
7
6
3
4
T-8
Kgr
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T-2
T-5
T-3
T-4
T-6
295
273
350
343
324
304
317
304
312
299
307
296
297
284
276
269
260
270
284
298
309
312
296
298
299
284
271 269
258
290
308
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Qcf/
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Kgr
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C
B'
BA
A'
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PROPERTY BOUNDARY
6960 FLANDERS DRIVE - SAN DIEGO, CALIFORNIA 92121 - 2974
PHONE 858 558-6900 - FAX 858 558-6159 SHEET OF
PROJECT NO.
SCALE DATE
FIGURE
Plotted:05/19/2023 2:04PM | By:JONATHAN WILKINS | File Location:Y:\PROJECTS\06442-32-34 (Con Lot-3)\SHEETS\06442-32-34 GeologicMAP.dwg
GEOTECHNICAL ENVIRONMENTAL MATERIALS
1" =
GEOLOGIC 1AP
CARLSBAD OAKS NORTH BUSINESS PARK - LOT 3
CARLSBAD, CALIFORNIA
30' 05 - 22 - 2023
06442 - 32 - 34
1 1 1
252
257
T-9
GEOCON LEGEND
........COMPACTED FILLQcf
........APPROX. LOCATION OF EXPLORATORY TRENCH EXCAVATIONS
........APPROX. LOCATION OF GEOLOGIC CONTACT
........APPROX. LOCATION OF GEOLOGIC CROSS-SECTIONC C'
........GRANITIC ROCK (Dotted Where Buried)Kgr
........APPROX. BOTTOM ELEVATION OF COMPACTED FILL
........APPROX. ELEVATION OF CANYON SUBDRAIN
........APPROX. LOCATION OF EXISTING CANYON SUBDRAIN
........APPROX. LIMITS OF BUILDING PAD UNDERCUT
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■
6960 FLANDERS DRIVE - SAN DIEGO, CALIFORNIA 92121 - 2974
PHONE 858 558-6900 - FAX 858 558-6159 SHEET OF
PROJECT NO.
SCALE DATE
FIGURE
Plotted:05/19/2023 2:58PM | By:JONATHAN WILKINS | File Location:Y:\PROJECTS\06442-32-34 (Con Lot-3)\SHEETS\06442-32-34 DMA MAP.dwg
GEOTECHNICAL ENVIRONMENTAL MATERIALS
1" =
D1A 1AP
CARLSBAD OAKS NORTH BUSINESS PARK - LOT 3
CARLSBAD, CALIFORNIA
30' 05 - 22 - 2023
06442 - 32 - 34
1 1 2
-----
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L • .•.•.•.•.• ~ . . . . .
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tr MINIMUM CURB DEPTH IS 12 INCHES; CURBS SHOULD BE
VERTICAL ELBOW DOWELED TWO FEET INTO ADJACENT PCC
PAVEMENT WITHIN 10 FEET OF TH£ 8/0FILTRATION BASIN.
CATCH BASIN RISER EMERGENCY OVERFLOW
STORM DRAIN STENCIL SD-13
3.00' MIN BENCH
w
.. ---.... . --.
. . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
J" MIN. HARDWOOD MULCH
MAINTENANCE CLEAN-OUT {TO
REMAIN CLOSED AT AU TIMES)
6" MIN. FROM F.S. OF BASIN
6" P.C.C. CURB
PER SDRSD G-1
WHEN NEXT TO A SLOPE ------BASIN CREST (MIN.)
AMENDED SOIL (K=5 =-------..\\.••(i
IN/HR) IJ40% POROSITY
1 • 1 VERTICAL SLOPE
J" WASHED SAND (ASTl,f NO. 33} --...
OVERLYING J" OF CHOKING STONE
(ASTl,f NO. 8) LAYER IJ 40% POROSITY
IMPERVIOUS MEMBRANE (PER SOILS ---
£NG/N£ER} MIN JO-MIL POLYETHYLENE
LINER SUCH AS LAYFIELD £NV/RO LINER
GRA/fl TOP
i§l
....:
3.,0-■1~5=::::1'0---~30====61"0---~90 1000 EUOJO (OR EQUNALENT) W/ALL r SEAMS HEAT WELDED. LINE ENTIRE BASIN.~
~L TO STORm ,
Gli4/fl BOTTOM
ASTl,f NO. 57 STONE
LAYER IJ 40% POROSITY
SCA LE: 1 " 30' DRAIN sYST£M
INSTALL TRASH CAPTURE SCREEN OVER
OUTLET PIPE, BRAND• 8/0CLEAN, MODEL•
CPS U SHAPED L= J.7; H= 1.9'.
6" DIA. PERFORATED PVC UNDERDRAIN
(J" OFFSET IN ORIFICE PLATE)
ORIFICE DRIUED THROUGH MAINTENANCE PLATE
IJCONNECTION (INTERIOR SIDE OF CATCH BASIN}
BIOFILTRA TION BASIN
TYPICAL DETAIL & OUTLET CONNECTION
NOT-TO-SCALE
-· .............. . . . . . . . . . . . . -• .. ,., .•-.. , ·, .. ~ ~---
COREDRILL KEYHOLE TYPE OPENING FOR
MAINTENANCE PLATE REMOVAL WfTHOUT
REMOVING THE BOLTS
12"x12" PLATE
½ "x4" STAINLESS ST££L
WEDGE ANCHOR BOLT
¾ • THICK GAL VANIZ£0
MAINTENANCE PLATE
EPOXY SEAL TO CATCH
/ ~ -" BASIN INTERIOR WAU
I , Iii'\ __ -1--6" DIA. PERFORATED PVC
I "' t;: / UND£RDRAIN CONNECTION
8 \, :/ 8 (EXTERIOR)
ORIFICE DRIUED THROUGH MAINTENANCE
PLATE IJ UNDERDRAIN CONNECTION.
A=1.68'. 8=1.59"
MAINTENANCE PLATE DE TAIL
ATTACH TO INTERIOR INLET WALL OF
36"x36" CATCH BASIN
. t
t
t
. . .
·,-'. ,._. :
-,---~---._,
DRAINAGE MANAGEMENT AREA BREAKDOWN
DRAINAGE AREA SURFACE SYMBOL DRAINAGE AREA ~' DRAINAGE AREA 'B' SELF MITIGATING
SOIL TYPE
BUILDING ROOFTOP
PARKING/DRIVE AISLE
IMPERVIOUS AREA
HARSCAP/NG/SIDEWALK
LANDSCAPING
OMA SUB-AREA:
TOTAL SITE AREA:
"/)"
34,696 SQ FT
40,935 SQ FT
6,294 SQ FT
36,449 SQ FT
118,374 SQ FT
20,052 SQ FT 0 SQ FT
42,495 SQ FT 0 SQ FT
7,878 SQ FT 0 SQ FT
35,473 SQ FT 1,504 SQ FT
105,898 SQ FT 1,504 SQ FT
225,776 SQ FT
LEGEND OF SYMBOLS
--------
------
m-----
GEOCON
lNC O.H.P O RAT E D
•
PROPERTY LIN£
STR££T C£NT£RLIN£
EXISTING CDNTDURS
DIR£CTIDN DF DRAINAGE
DRAINAGE AREA
DRAINAGE MANAGEMENT AREA D£SIGNA TIDN
LONGEST FLDW PATH
PRDPDS£D STORM DRAIN
& CATCH BASIN (PVT>
•
TABLE D.1-1 (APPENDIX D) CONSIDERATIONS FOR GEOTECHNICAL ANALYSIS OF INFILTRATION RESTRICTIONS - BMP BASIN A
Restriction Element
Is Element
Applicable?
(Yes/No)
Mandatory
Considerations
BMP is within 100’ of Contaminated Soils
BMP is within 100’ of Industrial Activities Lacking Source Control
BMP is within 100’ of Well/Groundwater Basin
BMP is within 50’ of Septic Tanks/Leach Fields No
BMP is within 10’ of Structures/Tanks/Walls No
BMP is within 10’ of Sewer Utilities Yes
BMP is within 10’ of Groundwater Table No
BMP is within Hydric Soils
BMP is within Highly Liquefiable Soils and has Connectivity to Structures No
BMP is within 1.5 Times the Height of Adjacent Steep Slopes (≥25%) Yes
County Staff has Assigned “Restricted” Infiltration Category
Optional
Considerations
BMP is within Predominantly Type D Soil Yes
BMP is within 10’ of Property Line Yes
BMP is within Fill Depths of ≥5’ (Existing or Proposed) Yes
BMP is within 10’ of Underground Utilities Yes
BMP is within 250’ of Ephemeral Stream
Other (Provide detailed geotechnical support)
Result
Based on examination of the best available information,
I have not identified any restrictions above.☐
Unrestricted
Based on examination of the best available information,
I have identified one or more restrictions above.☒
Restricted
TABLE D.1-1 (APPENDIX D) CONSIDERATIONS FOR GEOTECHNICAL ANALYSIS OF INFILTRATION RESTRICTIONS - BMP BASIN B
Restriction Element
Is Element
Applicable?
(Yes/No)
Mandatory
Considerations
BMP is within 100’ of Contaminated Soils
BMP is within 100’ of Industrial Activities Lacking Source Control
BMP is within 100’ of Well/Groundwater Basin
BMP is within 50’ of Septic Tanks/Leach Fields No
BMP is within 10’ of Structures/Tanks/Walls No
BMP is within 10’ of Sewer Utilities Yes
BMP is within 10’ of Groundwater Table No
BMP is within Hydric Soils
BMP is within Highly Liquefiable Soils and has Connectivity to Structures No
BMP is within 1.5 Times the Height of Adjacent Steep Slopes (≥25%) Yes
County Staff has Assigned “Restricted” Infiltration Category
Optional
Considerations
BMP is within Predominantly Type D Soil Yes
BMP is within 10’ of Property Line Yes
BMP is within Fill Depths of ≥5’ (Existing or Proposed) Yes
BMP is within 10’ of Underground Utilities Yes
BMP is within 250’ of Ephemeral Stream
Other (Provide detailed geotechnical support)
Result
Based on examination of the best available information,
I have not identified any restrictions above.☐
Unrestricted
Based on examination of the best available information,
I have identified one or more restrictions above.☒
Restricted