Response to Landau-10.04.2019.pdfN
Main Office
17311-135'" Ave NE, A-500
Woodinville, WA 98072
(425) 486-1669 - FAX (425) 481-2510
October 2, 2019
Carl and Sharon Larsen
9832 Marine View Drive
Mukilteo, Washington 98275
NELSON GEOTECHNICAL
ASSOCIATES, INC.
GEOTECHNICAL ENGINEERS & GEOLOGISTS
Updated Geotechnical Peer Review Comment Response Letter
Larsen Property Short Plat
15729 — 75" Place West
Edmonds, Washington
NGA File No. 999017
Dear Mr. and Mrs. Larsen:
Engineering -Geology Branch
5526 Industry Lane, #2
East Wenatchee, WA 98802
(509) 665-7696 - FAX (509) 665-7692
This letter presents the results of our updated geotechnical peer review comment response for your
planned short plat project located at 15729 — 75t' Place West in Edmonds, Washington.
INTRODUCTION
We previously prepared a report for this property that included recommendations for the proposed short
plat and underpinning support of the existing residence dated February 28, 2018. We also prepared plan
review letters for the proposed improvements to the existing residence dated March 20, 2018, July 25,
2018, and August 23, 2018. We prepared preliminary plan review letters for the proposed short plat
development dated August 9, 2018, May 6, 2019, and August 13, 2019. Project plans consist of
subdividing the existing property into three separate properties. A new single-family residence is
planned for the new upper eastern lot.
The site falls within "Zones B through D" of the North Edmonds Earth Subsidence and Landslide
Hazard Area Report prepared by Landau Associates for the City of Edmonds. This designation requires
that certain features be included (or excluded) in the design. Such features include the restriction of cuts
and fills, the need for tightlining runoff into an approved system, the need to design foundations and
retaining walls to withstand high lateral earth pressures and potential loss of soil beneath parts of the
foundation, the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the
elimination of any and all irrigation systems.
Updated Geotechnical Peer Review Comment Response Letter NGA File No. 999017
Larsen Property Short Plat October 2, 2019
Edmonds, Washington Page 2
We have addressed all of these requirements in our previous report and previous supplemental letters.
We recommend that our previous report and letters be thoroughly reviewed. The conclusions and
recommendations provided in this letter should be considered supplemental to the conclusions and
recommendations provided in our previous report and letters. For our use in preparing this letter, we
were provided with a set of plans titled "Larsen 3-Lot Short Plat," dated July 31, 2019, prepared by CG
Engineering.
LANDAU PEER REVIEW COMMENTS AND OUR RESPONSES
As a part of the City of Edmonds review of this project, Landau Associates performed an updated peer
review of the proposed project and our previous geotechnical reports and letters in accordance with
Edmonds Community Development Code Chapter 19.10. Landau Associates updated comments and
opinions were presented in a technical memorandum dated September 6, 2019. In the following section,
we summarize the relevant geotechnical concerns raised by Landau Associates, followed by our
response.
Comment 1:
The geotechnical letter, dated August 12, 2019 by Nelson Geotechnical Associates provides results of
slope stability analysis conducted based on the proposed development' however, the resulting Factor of
Safety (FS) under seismic conditions is 1.1. The FS under seismic conditions should be 1.2 or greater, as
required in ECDC 23.80.070 (A.4.a).
Response:
Our previous slope stability analyses were updated and reanalyzed using updated soil parameters that
reasonably reflect their engineering characteristics of the site soils. These updated soil parameters were
based on our understanding of the site soil conditions along with engineering properties as described in
"Geotechnical Properties for Landslide -Prone Seattle — Area Glacial Deposits," W.Z. Savage, M.M.
Morrissey, and R.L. Baum, USGS, 2000. The soil parameters used in our updated analyses are presented
in Figures 1 and 2. The critical slip surfaces for these analyses achieved factors of safety greater than or
equal to 1.7 and 1.2 for the static and seismic cases, respectively indicating relatively stable conditions in
accordance with ECDC 23.80.070 (A.4.a).
NELSON GEOTECHNICAL ASSOCIATES, INC.
Updated Geotechnical Peer Review Comment Response Letter NGA File No. 999017
Larsen Property Short Plat October 2, 2019
Edmonds, Washington Page 3
Comment 2:
The slope stability analysis under seismic conditions provided in the August 13, 2019 geotechnical letter
shows a critical slip surface that extends approximately 75 ft from the top of the slope; however, the
preliminary Geotechnical report, dated February 28, 2019 by Nelson Geotechnical Associates
recommends a setback of only 60 ft from the top of the slope. Based on the analysis provided, a 60 ft
setback could result in a structure being constructed too close to the crest of the slope, especially when
compared to the yet to be determined distance that would be required to achieve a FS of 1.2 under
seismic conditions. The design team should clarify this inconsistency.
Response:
As a result of our updated slope stability analysis, the critical slip surface under seismic conditions
extended approximately 60 feet back from the top of the slope and achieved a minimum factor of safety
of 1.2. In our opinion, the previously recommended structure setback of 60 feet from the top of the steep
slope should be adequate and should provide adequate protection to the proposed structure as a result of a
shallow and deep-seated landsliding from a large seismic event.
Comment 3:
The slope stability analysis provided in the August 13, 2019 geotechnical letter includes a 1,000 pcf
surcharge for the proposed residence. We presume that Nelson Geotechnical Associates intended to say
1,000 psf. Nevertheless, the Preliminary Geotechnical Report recommends an allowable bearing
pressure of 2,000 psf for the proposed residence. While we recognize that the entire building footprint
will not be subjected to a surcharge pressure of 2,000 psf, it is our opinion that the geotechnical
engineer should analyze the stability of the slope using a higher surcharge pressure (i.e., 2,000 psi
where shallow foundation will be located and possibly a lower surcharge pressure in areas of the
building footprint that will be subjected to lighter loads (e.g., beneath concrete slabs -on -grade).
Response:
The computer program Slope/W utilized for the slope stability analysis models surcharge as an overall
density in pounds per cubic foot (pcf). The surcharge utilized in our analysis was modeled as a material
with an overall density of 1,000 pcf on one foot thick resulting in a uniform surcharge of 1,000 psf. In
our updated slope stability analyses, we have increased the surcharge to represent a uniform surcharge of
2,000 psf which is conservative for the actual loading representative of the proposed structure. The
critical slip surfaces for these analyses achieved factors of safety greater than or equal to 1.7 and 1.2 for
the static and seismic cases, respectively indicating relatively stable conditions The results of our
updated analyses are presented in Figures 1 and 2.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Updated Geotechnical Peer Review Comment Response Letter NGA File No. 999017
Larsen Property Short Plat October 2, 2019
Edmonds, Washington Page 4
Comment 6:
Neither the Civil Plans dated July 31, 2019 by CG Engineering, nor the Preliminary Geotechnical
Report, Dated February 28, 2019 by Nelson Geotechnical Associates acknowledge the seasonal
restrictions for clearing described in ECDE 23.080.070 (A.6). A note should be added to the Civil Plans
with the purpose of identifying the seasonal restrictions.
Response:
In accordance with ECDE 23.080.070 (A.6), we recommend that clearing and grading activities be
performed between May V and October I" of the year. This seasonal grading restriction may be
extended and/or shortened by the City of Edmonds on a case -by -case basis depending on actual weather
conditions. We recommend that be indicated in the civil plans. Provided our recommendations provided
in our previous report regarding clearing and grading activities are followed and NGA is retained to
observe grading activities on the site during construction, it is our opinion that site clearing and grading
should not adversely impact existing slope stability conditions within the property.
MINIMUM RISK STATEMENT
Provided that the recommendations in this lcttcr, the geotechnical report dated February 28, 2018, the
letters dated March 12, 2018 and August 13, 2019, and this letter are followed during construction, the
areas disturbed by construction should remain stable. Therefore, the risk of damage to the proposed
development or to adjacent properties form soil instability should be minimal, and the proposed grading
and development should not increase the potential for soil movement.
CLOSURE
We recommend that NGA be retained to provide monitoring and consultation services during
construction to confirm that the conditions encountered are consistent with those indicated by the
explorations, to provide recommendations for design changes should the conditions revealed during the
work differ from those anticipated, and to evaluate whether or not earthwork activities comply with
contract plans and specifications.
r•s
NELSON GEOTECHNICAL ASSOCIATES, INC.
Updated Geotechnical Peer Review Comment Response Letter
Larsen Property Short Plat
Edmonds, Washington
NGA File No. 999017
October 2, 2019
Page 5
We appreciate the opportunity to provide service to you on this project. Please contact us if you have
any questions regarding this letter or require further information.
Sincerely,
NELSON GEOTECHNICAL ASSOCIATES, INC.
Lee S. Bcllah, LG
Project Geologist
WA S_ `
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35215 w
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Khalcd M. Shawish, PE
Principal
LSB:KMS:dy
Two Figures Attached
NELSON GEOTECHNICAL ASSOCIATES, INC.
Slope
Stability Analysis- Cross Section A -A' Proposed Static Conditions
Morgenstern -Price most critical surface with minimum FOS = 1.7
Co
-4 3
N
Soldier Pile Retaining Wall and Roadway Fill
Internal Friction Angle: 34°
Moist Unit Weight: 130pcf
O
c r
Cohesion: 0 psf
Cf) U v
o m m
m
o �
Proposed Residence (2,000psf Surcharge)
U) 7
iv
o
6 D 0
1.7
60-foot Setback
> o to
v
0
240
Glacial Till/Advance Outwash
240
N
m
230
Internal Friction Angle: 36°
230
Fn
a
220
Moist Unit Weight: 130pcf
220
(n '*
210
Cohesion: 100 psf
210
sv
200
200
n
190
180
190
180
170
170
New
Z
160
75th Ave SW
160
mm
a
^
G)
0,
150
10
a
0,
1ao
Existing
140
"'m Z
>
v
130
Residence
130
mD
120
Whidbey Formation
120
o >
Z
0
0
110
Internal Friction Angle: 34°
110
to
m
100
Moist Unit Weight: 120 pcf
100
z
> r
d
90
Cohesion: 1100 psf
90
2
z
(n N
N 0
>
(V
80
70
Landslide Deposits
80
70
m
z
A
W
60
Internal Friction Angle: 28°
60
m
> m
50
Moist Unit Weight: 110 pcf
50
° o
F 2,
O
40
Cohesion: 0 psf Critical Slip Surface
3
�N
to 0
(n m
30
20
30
20
d r0
--
Z =
10
10
8�
n Z
0
0
3 --4
D
-10
-20 0 10 20 30 40 50 60 70 80 90 110 130 150 170 190 210 230 250 270 290 310 330 350 370 390 410 430 450 470 490 510 530 550 570 590 610
-10
630
w� N
r
Distance (feet)
z
0
J
0)
tp
N
(�
N
O
7
NOTES: Not to scale
Elevations are Approximate
Cn
co
W
N
0
Slope
Stability Analysis- Cross Section A -A' Proposed Seismic Conditions
„
(0(
Morgenstern -Price most critical surface with minimum FOS = 1.2
CCD
-
with a seismic coefficient of ground acceleration = 0.27g
N
�
N
Soldier Pile Retaining Wall and Roadway Fill
Internal Friction Angle: 34°
n
p
Moist Unit Weight: 130pcf
o'
U r
Cohesion: 0 psf
m v
oo('
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Proposed Residence (2,000psf Surcharge)
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6
D
1.2
=
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D
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0
240
Glacial Till/Advance Outwash
240
k
Q
230
Internal Friction Angle: 36°
230
Cn
Ct) w
220
Moist Unit Weight: 130pcf
220
_m -�
210
Cohesion: 100 psf
210
U)
3
200
200
n'
190
180
190
180
170
170
New m
Z
160
75th Ave SW
160
a m
^
150
10
a
14o
Existing
14o
"'m Z
D
v 130
Residence
130
m
120
Whidbey Formation
120
o rY
Z
O 110
Internal Friction Angle: 34°
110
to
1A
+' 100
Moist Unit Weight: 120 pcf
100
Z
D r
d 90
Cohesion: 1100 psf
90
N O
N 80
80
Z
m
O z
LLJ 70
Landslide Deposits
70
A
60
Internal Friction Angle: 28°
60
N
D m
50
Moist Unit Weight: 110 pcf
50
a
F
Cohesion: 0 sf Critical Slip Surface
40
3
�N
N�� m
to
N m
30
20
30
20
d r0
=
10
10
0
n z
o
F
0
3 �m i
0
-10
-20 0 10 2030405060708090 110 130 150 170 190 210 230 250 270 290 310 330 350 370 390 410 430 450 470 490 510 530 550 570
-10
590 610 630
w� N
r
Distance (feet)
z
0
m
�
m
o
�
v
m
<_
N
O
7
NOTES: Not to scale
Elevations are Approximate
W
N
2016 NGA Proiect
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