HomeMy WebLinkAboutPD 2023-0021; THE BLUFFS SLOPE REPAIR; RESPONSE TO CITY REVIEW SHEET SURFICIAL WASHOUT EROSION ON NORTHERN SLOPE; 2023-08-24GeoSite Engineering, Inc. 27068 La Paz Road, #571, Aliso Viejo, CA 92656 949-940-6828
August 24, 2023 Triumph Property Management Project No: 23004
9454 Wilshire Blvd., Suite 800 Report No: 23-003 Beverly Hills, California 90212 Attention: Mr. David M. Page
Subject: Response to City Review Sheet Surficial Washout Erosion on Northern Slope 2701 Avenida de Anita Along Marron Road Carlsbad, California
References: See Appendix A Dear Mr. Page: Geosite Engineering Inc. is presenting herein responses to the City Review of our Reference 1
report. A copy of the review sheet is included in Appendix D. Our responses to the review sheet are provided below in numerical order to match the review sheet. The location of illustrations and appendices are given below. Figure 1, Geologic Map
Figure 2, Geotechnical Map Figure 3, Cross Section A-A’ Figure 4, Cross Section B-B’ Appendix A, List of References Appendix B, Exploration Logs
Appendix C, Laboratory Testing Results Appendix D, Slope Stability Analyses Appendix E, City Geotechnical Report Review Sheet RESPONSE TO GEOTECHNICAL REPORT REVIEW
Comment No. 1. There are no test pit logs, etc., for the exploration reportedly performed at the washout areas. Please provide the test pit logs and show the location on the Geotechnical Map, or further describe the subsurface exploration performed to clarify how the depth of failure debris and subsurface soil conditions at the failed areas of the slope were determined.
Response to Comment No. 1 The requested exploration logs are provided in Appendix B and the approximate boring locations are shown on the Geologic Map, Figure 1. Our field exploration was performed on April 7, 2023 and consisted of mapping the surficial slope failure areas and the excavation and sampling of five exploratory borings within the slope washout areas. These borings
August 24, 2023 Project No: 23004 Report No: 23-003 Page No: 2
were drilled to evaluate overall soil conditions at the site and to collect representative samples for laboratory testing.
In both slope areas, failure was caused by water overflowing the midslope v-ditch and eroding the slope face. As a results, there was very little failure debris on the slope as it had been scoured and washed down the slope onto the sidewalk and street. The thickness of the failure debris was generally less than 6 inches. All loose slough is to be removed and recompacted as engineered fill soil as a part of the slope reconstruction process.
Comment No. 2. There are no laboratory test results provided in the report; please provide as available. Please provide laboratory testing as necessary (strength testing, etc.) to support the recommendations for the proposed repair and conclusion with respect to the surficial slope stability of the repair (see comment #5 below).
Response to Comment No. 2 Laboratory testing was performed on selected samples and the results of
this testing is provided in Appendix C. Laboratory testing was performed on selected samples of soil considered representative of those encountered. Laboratory tests included the determination of moisture content, dry density, Expansion Index, Atterberg Limits, and direct shear strength.
Comment No. 3. Please clarify the thickness of loose debris current existing within the washout areas and mantling competent existing fill soils from the mass grading of the slope.
Response to Comment No. 3 Please refer to Response to Comment No. 1.
Comment No. 4. Please confirm the inclination of the fill slope where of the two washouts occurred, as the contours shown on the Topographic Survey and Grading Plans appear to indicate an inclination of 1.5:1 (H:V).
Response to comment No. 4 The slope ratio for both failure areas has been checked and is estimated to be 1.5 to 1 (horizontal to vertical)
Comment No. 5. As the apparent 1.5:1 (H:V) inclination of the existing slope (and thus the proposed slope repair) is steeper that the 2:1 (H:V) inclination for unreinforced fill slopes allowed in the City of Carlsbad Ordinance, please provide the results of surficial slope stability analysis that demonstrates the post-repair factor of safety of 1.5 for the wash-out areas without the use of a soil reinforcement (geogrid). Please provide the justification for the soil parameters used in the surficial stability analysis (see comment #2 above).
Response to Comment No. 5 The slope ratio of the re-built slope will be steeper than 2 to 1 (horizontal to vertical). However, the existing slope has shown good performance over time with no visible surficial failures over the last 25 to 30 years. Also, the slope failures were not caused by typical surficial slumping, but were caused by erosion (scour) from water flowing down the
August 24, 2023 Project No: 23004 Report No: 23-003 Page No: 3
slope. Therefore, it is recommended that the slope be re-constructed to its original slope ratio provided that the fill soil materials exhibit strength properties equal to or better than the original slope. Based on the surficial stability calculations provided in Appendix D, the compacted fill soil should have a minimum cohesion of 300 psf and friction angle of 25 degrees to achieve a
surficial factor of safety of 1.5.
Comment No. 6. Please provide recommendations (allowed height of vertical cuts, inclination for backcuts, etc.) for temporary slopes associated with the proposed slope repair.
Response to Comment No. 6 The allowable heights for temporary slopes associated with the proposed slope repair should be as follows: maximum 4 feet vertical with slopes above this height trimmed back to a 1 to 1 slope ratio for a total height of 10 feet.
Comment No. 7. Please show the locations of the proposed fill key and limits of grading for the two areas of slope repair on the Geotechnical Map (Figure 3).
Response to Comment No. 7 The location of the proposed fill keys and limits of grading are shown on the Geotechnical Map, Figure 2 included herein.
Comment No. 8. Please provide an updated "Grading Detail - Figure 6" that addresses and illustrates the actual location and topographic condition of the proposed repair for the washout areas within the existing 1.5:1 (H:V) slope. Please show a) the existing and pre failure condition/inclination of the slope, b) the existing washout and any associated loose
debris, and c) the recommended fill key and benching for the proposed repair.
Response to Comment No. 8 Previous Cross Sections A-A’ and B-B’ (Figures 3 and 4) have been updated to show the requested information.
Comment No. 9. Please provide a summary list of all geotechnical observation and testing services that should be performed as part of this slope repair project.
Response to Comment No. 9 The geotechnical observation and testing services required during remedial grading are as follows:
A.All fill keys and slope benches are to be observed and approved by the geotechnicalconsultant prior to placement of any fill soils. Field memorandum will be provided to thegrading contractor to show the limits of approved keys and benches and to indicate that
placement of fill soil has been approved by the Geotechnical Consultant.
B.Imported fill soils are to be tested and approved by the Geotechnical Consultant prior toplacement as engineered fill soil. Samples will be collected and tested for compaction
August 24, 2023 Project No: 23004 Report No: 23-003 Page No: 4
characteristics and shear strength. Fill soil is to have a minimum cohesion of 300 psf and friction angle of 25 degrees.
C.Removal bottoms should be scarified and recompacted to a firm and moist condition prior to
fill soil placement. Engineered fill soil is to be placed in 4 to 6-inch loose lifts and be
compacted to at least 90 percent relative compaction at a moisture content of 110 percent ofoptimum moisture content per ASTM D 1557.
D.All remedial grading work is to be performed under the part-time observation and testing of
the soil engineer. Fill soil placement will be tested for relative compaction and moisture
conditioning by the Geotechnical Consultant at not more than two-foot vertical intervals.
E.Subgrades for the repaired sections of the v-ditch should be observed and probed by theGeotechnical consultant prior to placement of reinforcing and concrete. A memorandum is
to be provided for each subgrade area to indicate observation and approval by the
Geotechnical Consultant to proceed with construction.
LIMITATIONS
Our work was performed in accordance with generally accepted practice in the engineering geologic and soils engineering field. No further warranty is offered or implied. Conclusions presented are based on visual observation, review of investigations by others, and information
collected from others.
The opportunity to be of service is appreciated. If there are any questions, please call.
Respectfully submitted,
GEOSITE ENGINEERING, INC.
Russell C. Lamb, G.E. 2207 Chief Geotechnical Engineer Date Signed: 8/24/2023
RCL/rcl
Distribution: Addressee (email) Aldrich Consulting Engineers (2 hard copies)
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GEOLOGIC MAP -FIGURE 1
LEGEND
•
Af
Tsa
BORINGB-5
APPROX. LIMITS OF WASH OUT
ARTIFICIAL FILL SOIL
SANTIAGO FORMATION BEDROCK
60'
I
GEOTECHNICAL MAP -FIGURE 2
LEGEND
APPROXIMATE LIMITS OF GRADING
0 KEYWAY lS'WIDExS 'DEEP MINIMUM
O'
I
30'
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60'
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,........_ ....i 110 µJ > µJ ....l -< µJ V) z < µJ ::E
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E--< µJ µJ c.i.. -A WEST AREA A' V-DITCHPRE-EROSION GRADE � ....l -< ORIGINAL GRADE � µJ Cl -r./) Af FILL ------1---:-------1 . ---
� ------- . J---.J , ,----1· ____ , ....----·r---r --� -------POSSIBLE BENCHING Tsa BEDROCK SCALE 1 INCH = 20 FEET HO RIZO NT AL AND VERTICAL CROSS SECTION A-A' - FIGURE 3 110 80 50 BENCHING TYPICALz 0 ..... !-< <C
Gi 50 ,-..l µ.l
-
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so B EAST AREA V-DITCHPRE-EROSION GRADE ORIGINAL GRADE � I
i:.i.l Cl ..... r.r; Af FILL '(•1 ,-. ,-----' r-------------·,---... , •-- - .... POSSIBLE BENCHING Tsa BEDROCK SCALE 1 INCH = 20 FEET HORIZONTAL AND VERTICAL CROSS SECTION B-B' - FIGURE 4B' 110
80
so BENCHINGTYPICAL
APPENDIX A
LIST OF REFERENCES
1.GeoSite Engineering, Inc., 2023, “Preliminary Investigation Report, Surficial Washout
Erosion on Northern Slope, 2701 Avenida de Anita Along Marron Road, Carlsbad,California,” Project No: 23004, Report No: 23-002, dated May 21.
2.City of Carlsbad, 2023, “Geotechnical Report Review, The Bluffs Slope Repair, Marron
Road,” Project ID: PD2023-0021, Grading Permit No.: GR2023-0032, dated July 31.
APPENDIX B
EXPLORATION LOGS
Q) ----'O ~'R -a. ~ -Q) Q) BORING B-1 Q)
~ .D E Q) -
o_ GRAPHIC LOG (l) .... .._ -::::, Q) ro .... -Q) >, -.c iii 0. en ::, c:: (.) -(SCALE: 1"=1') .c -Q) ro ·--i5 E en --en Description -a. .!I! --c:: a. c:: a.
Q) c:: ro '3 0 0 I Q) Q)
0 => en al ~o c:: 0 0
-o -·o. ~ 0-1': FILL: Dark brown clayey SAND, silty-fine grained,
• 1 -[X slightly moist, stiff, few rootlets . . 1 -
-2 ·--2 -~ @1'-2 1: Dark brown clayey SAND, silty-fine grained,
-3 -moist, stiff, fewer amount of rootlets. • 3 -
:
-4 -@2 1-3.51: Brown/orangish-gray clayey SAND, firm, ·4 -
moist. No roots.
-5 --·5.
I
TD: 3.5'
i -6 --No groundwater encountered. • 6 -
-"'O Q) ~~t;: -Q) C. ~
0 t) ~ Q) .0 E 0 C. BORING B-2 Q)
~ ... Q)-Q)-;:,,. GRAPHIC LOG ~ -:J Q) (0 '-:Jc:u--.c in a en .... Q) (ti ·-(SCALE: 1"=1') z +-' ~ E -'1e. (/)--"' Description a. e-·-C: C. C: c:: co -::, 0 0 I Q) a 0 => en m :!!!t)C:()
• O • ·o.
0-1': FILL: Dark brown clayey SAND, silty-fine grained,
-1 -slightly moist, stiff, few rootlets. • 1 • ~ -2 .. ·2· ~ @1'-2': Dark brown clayey SAND, silty-fine grained,
-3 . -moist, stiff, fewer amount of rootlets. . 3. ~ -4 • @2'-3': Brown/gray clayey SAND, firm, moist. No ·4.
roots.
-5 --·5.
. 6 .• @3.5': Brown/orange clayey SILT, moist, slightly firm, ·5 -no rootlets.
-7 -• 7 -
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GRAPHIC LOG
(SCALE: 1"=1')
BORING 8-3
Description
0-3': FILL: Dark brown/gray clayey SAND, silty-fine
grained, slightly moist, stiff, few large roots.
@3': Brown/orange clayey SILT, sightly moist, slightly
firm, few rootlets.
TD: 3.5'
No groundwater encountered.
·,I
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0-3': Dark brown/gray silty CLAY, slightly moist, soft-1 1 . -1 . firm, large roots .
]-2-
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~ @3-3.51: Brown/orange clayey SILT/silty CLAY, sightly . 3 • • 3 • ~ moist, slightly firm, few rootlets.
. 4 . ·4 •
TD: 3.5' . 5 .. No groundwater encountered. ·5 •
-"O Q) ;e ~~ --Q) a. BORING 8-5 -Q) Q)
~ .0 E e...o_ GRAPHIC LOG ~ ~ Q) -Q) > .._.. ::, Q) ro L.:::, C (.) +-'
.._..
.c ciia Cl) -a> ro ·-(SCALE: 1"=1') .c -:S E ~ rn --rn Description -a. --C O. C a.
Q) c: ro ::i a a , a> Q)
0 :::::, Cl) al :; o E o 0
-o -FILL: ·o • --
l,X
0-2.5': Brown/gray clayey SILT, slightly moist, firm,
-1 -few rootlets. • 1 -
-2 .• • 2. :
-3 .•
.........._____
@2.5'-3.5': Brown/gray clayey SAND, fine-grained, . 3 •.
moist, firm to stiff, no rootlets.
-4 -"4·
TD: 3.5' -5 -• No groundwater encountered. ·s ..
APPENDIX C
LABORATORY TESTING
Sample Sample Water Dry Saturation Expansion Depth Content Borehole (ft) Length Type Soil Unit Classification Density (%) Index (in) (%) (pcf)
B-1 1.00 20.3 68
B-1 2.00 21.5
B-2 1.00 20.6
B-2 2.00 20.9 59
B-2 3.00 SANDY SIL T(ML) 24.8
B-3 3.00 22.2
B-4 1.50 14.1
B-4 2.50 17.8
B-4 3.00 12.7 47
B-5 2.50 LEAN CLAY with SAND(CL) 24.5
Figure No. C-1
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0 0 20 40 60 80 100
LIQUID LIMIT
BOREHOLE DEPTH LL PL Pl Fines Classification• B-2 3.00 39 26 13 66 SANDY SIL T(ML)
0 B-5 2.50 47 24 23 74 LEAN CLAY with SAND(CL)
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GRAIN SIZE IN MILLIMETERS
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BOREHOLE DEPTH Classification l LL I PL Pl Cc Cu •I B-5 2.5 LEAN CLAY with SAND(CL) l 47 l 24 23
BOREHOLE DEPTH I D100 I D60 I D30 I D10 %Gravel %Sand %Silt I %Clay•I B-5 2.5 I 0.85 I 0.056 I 0.008 I 23.4 50.2 24.1
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SAMPLE:B-1 @ 1'-2'PEAK ULTIMATE
φ'35 o 32 o
C'1,100 PSF 275 PSF
IN-SITU AS-TESTED
STRAIN RATE:0.0030 IN/MIN γd 119.7 PCF 119.7 PCF
(Sample was consolidated and drained)wc 12.1 %15.1 %
DIRECT SHEAR TEST RESULTS FIGURE C-5
Description:
Dark Brown Clayey SAND (SC)
0
2000
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NORMAL STRESS [PSF]
Peak Strength Test Results
35 Degrees, 1100 PSF Cohesion
Ultimate Strength Test Results
32 Degrees, 275 PSF Cohesion
a
..________.__I I...__________.
..________.__I I...__________.
SAMPLE:B-5 @ 1.5'-2.5''PEAK ULTIMATE
φ'35 o 32 o
C'600 PSF 350 PSF
IN-SITU AS-TESTED
STRAIN RATE:0.0030 IN/MIN γd 105.3 PCF 105.3 PCF
(Sample was consolidated and drained)wc 19.7 %22.2 %
DIRECT SHEAR TEST RESULTS FIGURE C-6
Description:
Brown-Gray Clayey SAND (SC)
0
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NORMAL STRESS [PSF]
Peak Strength Test Results
35 Degrees, 600 PSF Cohesion
Ultimate Strength Test Results
32 Degrees, 350 PSF Cohesion
..________.__I I...__________.
..________.__I I...__________.
APPENDIX D
SLOPE STABILITY ANALYSES
SURFICIAL SLOPE STABILITY ANALYSIS
ADAPTED FROM: U.S. Geological Survey
Prof. Paper 851
Campbell, 1975
SOIL TYPE: EXISTING AND IMPORTED FILL SOIL
1.5 TO 1 (HORIZONTAL TO VERTICAL) SLOPE
DATA: C = Cohesion = 300 PSF
4> = Angle of Interior Friction= 25 degrees
y = Total Density= 120 PCF
i = Slope Angle = 34 degrees
Ds = Depth of Saturation = 4 feet
Ya = Buoyant Density (y -62.4) = 57.6 PCF
STATIC SAFETY
FACTOR =
C + (rs) * Ds * COS2 i * TAN cj>
y * D5 * SIN i * COS i
STATIC FACTOR OF SAFETY: 1.68
SURFICIAL SLOPE STABILITY ANALYSIS
JOB NO: 23004 AUGUST2023 !FIGURE: 0-1
APPENDIX E
CITY GEOTECHNICAL REPORT REVIEW SHEET
7 of9
RE: Bluffs Grading Permit Processing
GEOTECHNICAL REPORT REVIEW
DATE: July 31 , 2023
TO: City of Carlsbad
Land Development Engineering
1635 Faraday Avenue
Carlsbad, CA 92008
Attention: David Rick
PROJECT ID: PD2023-0021
GRADING PERMIT NO.: GR2023-0032
SUBJECT: The Bluffs Slope Repair, Marron Road (1 st review)
Items Submitted by ADDlicant Items Being Returned to Aoolicant
• "Preliminary Geotechnical Investigation, Surficial • Written report review comments.
Washout Erosion on Northern Slope, 2701
Avenida de Anita along Marron Road, Carlsbad ,
California,' by GeoSite Engineering, Inc., dated
May 21 , 2023. i
Based on our review of the submitted geotechnical report, we are providing the following
comments that should be addressed prior to the next submittal. Please provide complete and
thorough written responses to all comments.
GEOTECHNICAL COMMENTS:
-Image (49).jpg
1. There are no test pit logs, etc., for the exploration reportedly performed at the washout
areas. Please provide the test pit logs and show the location on the Geotechnical Map, or
further describe the subsurface exploration performed to clarify how the depth of failure
debris and subsurface soil conditions at the failed areas of the slope were determined.
2. There are no laboratory test results provided in the report; please provide as available.
Please provide laboratory testing as necessary (strength testing, etc.) to support the
recommendations for the proposed repair and conclusion with respect to the surficial slope
stability of the repair (see comment #5 below).
3. Please clarify the thickness of loose debris current existing within the washout areas and
mantling competent existing fill soils from the mass grading of the slope.
8/18/2023, 10:55 AM
8 of9
GR2023-0032
July 31 , 2023
Page 2 of 2
RE: Bluffs Grading Permit Processing
4. Please confirm the inclination of the fill slope where of the two washouts occurred, as the
contours shown on the Topographic Survey and Grading Plans appear to indicate an
inclination of 1.5: 1 (H:V).
5. As the apparent 1.5: 1 (H:V) inclination of the existing slope (and thus the proposed slope
repair) is steeper that the 2: 1 (H:V) inclination for unreinforced fill slopes allowed in the
City of Carlsbad Ordinance, please provide the results of surficial slope stability analysis
that demonstrates the post-repair factor of safety of 1.5 for the wash-out areas without the
use of a soil reinforcement (geogrid). Please provide the justification for the soil
parameters used in the surficial stability analysis (see comment #2 above).
6. Please provide recommendations (allowed height of vertical cuts, inclination for backcuts,
etc.) for temporary slopes associated with the proposed slope repair.
7. Please show the locations of the proposed fill key and limits of grading for the two areas
of slope repair on the Geotechnical Map (Figure 3).
8. Please provide an updated "Grading Detail -Figure 6" that addresses and illustrates the
actual location and topographic condition of the proposed repair for the washout areas
within the existing 1.5:1 (H:V) slope. Please show a) the existing and pre failure
condition/inclination of the slope, b) the existing washout and any associated loose debris,
and c) the recommended fill key and benching for the proposed repair.
9. Please provide a summary list of all geotechnical observation and testing services that
should be performed as part of this slope repair project.
8/18/2023, 10:55 AM