CANOD and geotech report.pdfCity of Edmonds
Critical Area Notice of Decision
Applicant:
Property Owner:
Critical Area File
Permit Number:
7 "s
Site Location:
Parcel Number:
1, 0 600
Project Description:
Conditional Waiver, No critical area report is required for the project described above.
1. There will be no alteration of a -Critical Area or its required buffer,
2. The proposal is an allowed activity pursuant to ECDC 23.40.220, 23.50.220, and/or
23.80,040,
3. The proposal is exempt pursuant to ECDC 23.40.230.
F -I Erosion Hazard. Project is within erosion hazard area. Applicant must prepare an erosion and
sediment control plan in compliance with ECDC 18,30,
O�Critical Area Report Required. The proposed project is within a critical area and/or a critical area
buffer and a critical area report is required. A critical area report has been submitted and evaluated
for compliance with the following criteria pursuant to ECDC 23.40.160:
1. The proposal minimizes the impact on critical areas in accordance with ECDC 23.40.120,
Mitigation sequencing;
2. The proposal does not pose an unreasonable threat to the public health, safety, or welfare
on or off the development proposal site;
3. The proposal is consistent with the general purposes of this title and the public interest;
4. Any alterations permitted to the critical area are mitigated in accordance with ECDC
23.40.110, Mitigation requirements.
5. The proposal protects the critical area functions and values consistent with the best
available science and results in no net loss of critical functions and values; and
6. The proposal is consistent with other applicable regulations and standards.
❑ Unfavorable Critical Area Decision. The proposed project is not exempt or does not adequately
mitigate its impacts on critical areas and/or does not comply with the criteria in ECDC 23.40,160 and
the provisions of the City of Edmonds critical area regulations. See attached findings of
noncompliance,
Favorable Critical Area Decision. The proposed project as described above and as shown on the
attached site plan meets or is exempt from the criteria in ECDC 23.40.160, Review Criteria, and
complies with the applicable provisions of the City of Edmonds critical area regulations. Any
subsequent changes to the proposal shall void this decision pending re -review of the proposal.
❑ Conditions. Critical Area specific condition(s) have been applied to the pen -nit number referenced
above. See referenced permit number for specific condition(s).
i4l-41< e C' /&/"' '14
-
Reviewer J
Signature-'
Date
Appeals: Any decision to approve, condition, or deny a development proposal or other activity based on the
requirements of critical area regulations may be appealed according to, and as part of, the appeal procedure, if any,
for the permit or approval involved.
Revised 12/16/2010
Peter Chopelas, PE
Engineering & Design Services
307 N. Olympic Ave, Suite 208
Arlington, WA 98223
(360) 653-4615; Chopelasandassociates@gmail.com
eo'tecnical ep®rt
March 23, 2012
Revised: May 15, 2012
Eric Thuesen
509-9"~ Avenue N.
Edmonds WA 98020
Ericthuesen@,frontier.com
1-425-772-9401
Subject: geotechnical investigation for plan ck # 2011-0775 accessory structure (cabana) at the
above address
SUNINaRY
A geotechnical investigation of the above property was performed to address slope stability,
erosion control and drainage issues. This sloping site has been extensively altered due to
previous construction activities including extensive cut and fill of the slope. A small accessory
structure (cabana) and a retaining wall is proposed to be built below the residence at the top of
slope below the home. The investigation consisted of a site inspection to examine soils and site
conditions and to take measurements on the sites. A review of published material on local
geology from the USDA Soil Survey of Snohomish County, and a review of the geotechnical
report done by Dennis Bruce PE, dated Jan 13th, 2005 for this site.
The investigation found that the site can be made satisfactory with deep foundations for the
proposed accessory building, and a retaining wall with footings deep enough to get below the fill,
and the use of surface stabilization measures. The building site for the cabana is on shallow old
fill placed at the top of the slope to make the back yard terrace larger. There is an existing site
drainage system, including yard drains, and a conveyance system that transports all roof and
driveway run-off to the base of the hill. The fill, though stable for landscaping features, is not
suitable for permanent buildings. Therefore the proposed building and retaining wall should be
built on foundations deep enough to bare on the original undisturbed subsoil. This will allow the
proposed development with a generous safety factor of 1.5 or greater.
These measures will reduce risk of landslides, increase slope stability and will not increase
surface water discharge, sedimentation or erosion on downstream sites. This development will
not increase surface water discharge, and it will not decrease slope stability but rather increase
stability and it will not adversely impact;other:pr',tical areas.
Peter Chopelas PE
ray[%'
Project Description
The building site is a residential building lot of approximately 0.2 acres served by a municipal
sewer and a public water supply, and with an existing residence and garage built on a terrace on a
sloping site. There is a proposed accessory building of 320 SF, with a 360 SF attached deck, a
retaining wall and back fill to enlarge the useful backyard, placed at the top of a steep slope
below the existing residence. The building site is about 160 feet from the toe of the slope, and
about 50 feet in elevation above it. The foundation will be deep enough to reach below the old
fill to bear on original undisturbed soil. These are Allowed Activities in a critical area per
23.80.090. B (1).
23.40.120 Mitigation sequencing.
1. The impact cannot be avoided since it is the property owner's desire to increase the
safety factor of the slope and enlarge the useful yard area by adding an engineered
retaining wall and building the cabana with an engineered foundation on the edge of a
small level terrace.
2. The impacts are minimized by building a modest building and enlarging the useful area
of the backyard by about 300 SF. Site disturbances on the slope should be kept to a
minimum during construction, it should be done during the dry season, and all exposed
soil will be covered during the construction phase. Both temporary and permanent soil
stabilizing measures are recommended.
3. There are no impacts to wetland nor to any habitat area since none are disturbed. The
site will be more stable and surface vegetation restored after the project is completed.
4. Both the foundation and retaining structure are engineered structures and will result in
a safer, more stable slope condition than the existing site conditions.
5. The recommendations for the proposed project will result in long term slope stability.
Recommendations include installing stabilizing engineered structures, protecting and
maintaining the vegetation on the slope, and to minimize unnecessary site disturbances.
6. No compensation for impact to wetlands, frequently flooded areas, and fish and
wildlife habitat conservation areas are required since none are disturbed.
7. Monitoring the hazard will be done during the construction since the soil conditions
have to be inspected prior to placing the footings, and during the construction to verify
design conditions are met. Design adjustments may be made to minimize any hazards.
Site Conditions
The terrain slopes steeply to the west at about 32 percent average grade from a near level terrace
on the east half where the existing residence is located. The terrace has existing lawn and
landscaping and there is mix of grasses and landscaping shrubs and small to medium trees on the
slope. There is no significant surface drainage on the site, and no seepage on the site. There is
an existing drainage and conveyance system to transport surface discharge from the roof tops and
driveway to the base of the slope to the west. The proposed construction site is at about 50
percent grade, which was probably created by placement of fill on the slope from the spoils of
the foundation excavations when the house was built since deep fill was found near the top of the
slope.
Local Geology
The subject site lies along a slope forming the edge of the highland plateau of the Edmonds area.
The general stratigraphy (soil layering) of the area is characterized by fluvial and glacio=fluvial
sediments from sea level to around 200 feet, with glacial till and sand deposit plains that make up
the surficial sediments on most of the plateau. The USDA Soil Survey of Snohomish County
maps the soil as Everett gravelly sandy loam and Alderwood gravelly sandy loam. The Everett
soil is deep excessively drained soil on terraces and outwash plains. It is formed in glacial out
wash. The sub -soil is dark brown gravelly sandy loam down to about 60 inches, with increasing
gravel at the deeper depths. The Alderwood gravelly sandy loam is moderately well drained soil
on till plans, moderately deep over a hardpan, and formed in glacial till. Both of these soils are
very stable and suitable for building site development.
This soil has good strength and is not prone to slumps or landslides. It is subject to surface
erosion when exposed to the weather and measures are required to prevent erosion. This soil is
not prone to liquefaction.
The site shows no evidence of landslides or deep soil instability. There were some minor tension
cracks in the soil near the top of the slope, but this was old fill that has been settling and causing
surface irregularities. This does not pose a hazard to this site or the adjoining site, and the slope
will be reinforced with the addition of the proposed structures.
There is no flooding risk on this property. There are no wetlands or surface water on this
property, however several hundred feet down slope there is a wetland at the toe of the slope.
Geotechnics
Surface soils have been inspected over most of the site and on the slopes by visual inspections.
The underlying undisturbed soil on the subject site is deep silty sand, this is old alluvial outwash.
The soil on the proposed building site below the existing home is undifferentiated fill 1 to 4 feet
deep, likely the spoils from the house site excavations. It is a mix of sand and gravel and has
organic material and other debris mixed into it. This was placed at the top of the slope, over
steepening the upper 10 feet or so of the slope.
Soil log 1
Soil log 2
Soil log 3 Soil log 4
0-12" mixed fill
0-36" mixed fill
0-48"+ mixed fill 0-18" fill
12"+ sandy loam
36"+ sandy loam
18"+ sandy loam
Soil log table
Slope stability: Analysis of the natural grade of the steeper parts of the site using the Singh
diagrams indicates that the slope shows a Factor of Safety of 1.8. This reduces to about 1.6 for
dynamic seismic conditions. This indicates a stable slope with a large safety factor over the 1.5
and 1.2 minimum requirements respectively. The upper filled slope is also currently stable, but
with a lower safety factor of about 1.2. This is acceptable for landscaping features, but deeper
footings must be used to get into the underlying original'subsoil. The soil properties of the
undisturbed Everett soil is as follows: density (gamma)= 130 PCF; Cohesion (C)= 100 PSF,
height of the slope is 50 feet.
Erosion: All of the existing site is well protected from erosion with established vegetation All
of the slopes over 30 percent on this site is subject to severe erosion if the soil is exposed.
Minimize disturbing any vegetation on the slope. Any exposed soil on the site must have deep
rooting ground cover installed as soon as practical, avoid concentrated surface flows on the
slopes, and to transport by tight -line roof and driveway runoff to the base of the slope through the
existing drainage system.
23.80.060 and 23.80.070 Development standards.
The recommendations in the following section will achieve the following:
1. It will not increase the threat of the geological hazard to adjacent properties beyond
predevelopment conditions; and will reduce them since it will improve the safety factor.
2. It will not adversely impact other any other critical areas.
3. These recommendations are designed so that the hazard to the project is eliminated or
mitigated to a level equal to or less than predevelopment conditions; and
4. All are certified as safe as designed, and under anticipated conditions, by a qualified engineer
licensed in the state of Washington.
Note that construction buffers for the proposed development is zero. The proposed changes will
stabilize the slope to bring the safety factor well above the 1.5 minimum, so no buffers are
required for this project. Therefore section 23.80.070 (1) does not apply. However, it is
recommended to keep heavy construction equipment back from the top of the slope at least 10
feet during construction.
This proposed development is certified to:
a. Reduce, and not increase surface water discharge or sedimentation to adjacent properties
beyond predevelopment conditions;
b. It will increase, and not decrease slope stability on adjacent properties; and
c. These alterations will not adversely impact other critical areas;
3. Design Standards. Development within an erosion or landslide hazard area and/or buffer shall
be designed to meet the following:
a. The proposed development has a design salty factor of 1.5 or above static, and 1.2 and above
dynamic, after the recommendations are accomplished. There is no reduction in the safety factor
for the site, or to any adjacent properties.
b. The Structures and improvements are placed to to avoid geologically hazardous areas and to
improve the slope safety factors, and will have no affect on other critical areas;
c. The proposed structures and improvements minimize alterations to the natural contour of the
slope, and foundations are tiered where possible to conform to existing topography;
d. Structures and improvements are located to preserve the most critical portion of the site and its
natural landforms and vegetation;
e. The proposed development do not result in greater risk or a need for increased buffers on
neighboring properties;
f. Retaining walls are used to allow the maintenance of existing natural slope rather than graded
artificial slopes;
g. The development minimizes impervious lot coverage, the proposed building is modest in size;
4. Vegetation Retention. All vegetation on the slopes shall be left undisturbed outside of the
construction area;
5. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year;
provided, that the director may extend or shorten the dry season on a case-by-case basis
depending on actual weather conditions;
6. Point Discharges. Point discharges from surface water facilities and roof drains onto or
upstream from erosion or landslide hazard area are prohibited. All roof and surface
discharge shall be collected and tight -lined to the existing site drainage system where it is
conveyed to a an approved discharge point where there is no erosion hazard.
7. Prohibited Development. No On-site sewage disposal systems are proposed for the site
Recommendations
The proposed building location can be used with a reasonable assurance of long term slope
stability and safety to the building if the following recommendations are followed:
1. Conventional footings may be used as long at it is placed a minimum of 12" into the original
undisturbed sub -soil below the old fill. Bearing capacity .is 2000 PSF. Equivalent fluid
pressure of 35 PCF for retaining walls for drained on-site back fill. Coefficient of friction
for the soil is 0.55 and passive soil (lateral bearing) pressure of 350 PCF. All bearing soil
should be inspected by the engineer before concrete is poured.
2. The up hill footings of the accessory structure can be supported on two 24" square x 10"
thick concrete footings, one near each corner of the building. These should be placed below
the fill 12" into original undisturbed subsoil. The footings should be reinforced with (3) #4
rebars in each directions (3" clearance from soil). A 12" diameter Sonotube formed concrete
column can be installed on top of these footings up to grade, this should be reinforced with
(4) vertical #4 bars with a #3 horizontal hoops every 6". The excavated soil must be back
filled and compacted over and around these footings up to grade.
Roof run-off should be directed to a catch basin in the existing conveyance system and tight -
lined to the base of the slope. No drains or concentrated flows should discharge over the
ground or on the slope.
4. A silt fence should be placed on the downhill side of the construction site to attenuate silt -
laden run-off if necessary. Silt fences should be placed around catch basin inlets. Soil that is
to be removed and reused around the site should be kept away from the top of the slope.
Soils not to be reused, on the site should be hauled off site to an approved location.
5. The vegetation on the steep slopes should be replaced as soon as practical after construction.
Any disturbed soil in the area of the slope should be protected with jute matte or straw and
replanted with native vegetation. After final grading any disturbed or exposed soil needs to
have topsoil spread and compacted and planted with' vegetation that will retard erosion and
stabilize the upper soil layers.
Grass seed mixture based on soil type, used for erosion control, should be as follows:
Common Name
Perennial ryegrass
Annual ryegrass
Red Fescue
Percent by Weight
40
20
40
This should be applied at a hydro -seed rate of 1 LB per 1000 sq. ft. (or 60 LB. per acre); or a
hand seed rate for 2 LB. per 1000 sq. ft. (or 87 LB. per acre). Fertilizer shall be 10-20-20,
applied at the rare of 10 LB. per 1,000 sq. ft. (or 400 LB. per acre).
The following native vegetation are acceptable for permanent plantings on disturbed slopes
and buffers:
Trees*( 3-4' size; plant 6' o.c.)
Shrubs*(to be 1-3' size; plant 2 foot
O.C.
Vine Maple
Kinnick-kinnick
Big leaf maple
Salal
Nobles fir
Snowberry
Grand fir
Vinca Minor
Oregon Grape
Red current
Other native species as recommended by qualified landscape professional knowledgeable in soil
stabilization may also be used.
Local topography (from Snohomish County GIS data base)
Project site
I
\
f 3
II
1
I
t
+ �l
s
t
� `
!
�i
\O
+
i
I
IN
TOPOI map printed on 03/23/12 from "Untitled.tpo"
122.400000 W 122.383330 W 122.366670 W 122.350000 W WGS64122,333330 W
,4ulyy^$.. t y+ x t
-t--#
a
kY
-47 iR3,_
ENRONYt
k-
Z
oy� xr as
M , �`— t'�zt"r �.:. 2. - { D s hY,�} .;
M�"r'-tr#` ?�:y.'2_'# -i-. Z`'. i r...r"t
M— 4k ik
'l"t P g y tar
M [� - r. r "3-'x: t.;,,:,z; ♦L ,ar3"� e. .tt. � c - s qty -�
TAM
O7 3r'a-`w E, ``"t x -r a "��, t*.' S4'r-�.:. s� k.. F- i. "' S a -.-'s z e •tr'� '�-!l. r > f,= yL ' t k 'n
ON
i c p
O� G � x 1 -a' �'"-4i"'r. :Yx k4:
�3
10-4
x d
l4 r r +
'� :�
{
c� C f LI a
t0
k
Sc`
..;/ C
st✓,�`C��rT
ry `,✓r—'-'�`i r t .1� .r _ �. s a i r to a yr" ,r-.33 -'" r
t a �-�^ � -� fit#- � � "�� tt v .1 s� fat � r ��^ �c - � � -t# ! x • ' 6 f,�t.'_ .�
t .,y if ;T i z;,?1'
Z 5 -test
v-�y i g -"`K � ���:. c� �'t �t�iE
o �, "ti'Y a` `� v. t — 5 -:ct`uF s y t-
tt�r i ane �. Y
tp • �, x`ry 3,-�.,�.��w--`r't a,,,,, r- �x t. f 5 s � �Lv 17 � k� "' ) rr ' 1 ., -r �., � i
.-t � �'}t�-�`-�„ . s� �r � yam' f5'+3csa st v. � fi {.. ,r t ,� ; � r. , . + ^. f� } � i t z L• 4- !� h,-
__ �3% • y t �` Protect site ,tt K `
N
�-� �, ,.cxr-�. 'P'+ � z--• S t+. if � r � � '� piteg,C Ind.
}
C `.�'"'�s' x. .���ry ��3 � ��l�r S � k -•�. '+� o� "` Q, C�,4' > ""_h �. G .r .! �: � �r {- �' t y3 �a � 1 `- `r
k� % r �.� �''�/ -. r � � � `• Mt } aan ie. � '� � 7 ;� y `'* e'} sl,. rc� N` .s:- « r s t.=
�a •'�
! s sr �j rn �4tsC1r ch
i^� € r .� :! .. �r78� r
zr �i-'i} : \ t tic / r, .r V° E
u .�R -�`r
�# -: �-�x-v,� i�...t..�, ,ic�f 'fl � :-. i � `?�U t ..� 3'" �.� - t � r r � `�`� s 4 � � _ � � X Y �� st• B� 4' �'�" QC �. ��,
G a t. .; `fi"t e '3-•. .,.> r k A t. a - - .t7 II`' --±' i 1I
Ami
> t _
i
NL
.., x q � t -a >•=x-�'4 trS� � � e .�. > > � -„ �� t a 4� 1 t tra,x �'`
� .. - ° itcc
2 t,� ii �..,
+<.'i : ,y �.� i�
o std {'i ,jt t z . } f
M p is i -r 4'`� `} L a �' Y i 4 P` s !."r , ' t '"="
CO�-:r-. -s` ?.r.. rCT `�k? . F t {. r + gait z< Cem. '� f" f5�r�`: :� f y. y.
M is r a. �k -c fl t" f �t4 hE r 37`f .,.._ ,(,.. +r U 1 y �..y 3��tt` i
� ' � � � AN 1
i sirg
swell 4 llts«: f tt 1 a •. } r ��� > t i i z' Y.~ r .. s f' r t Fl 1
R 771
122.40000° W 122.383330 W 122.36667° W 122.35000° W WGS84122.333330 W
TN* JMN ,. 6 S I MILE
17!' UJd FEET o sots IR?t) MEfEBS
Map created with TOPOIO @2003 National Geographic(www.nationalgeogtaphic.comftopo)
C
M
Z
7
h
h
h
D
f
I
I
- SHEET NUMBER 56
r -{.,..,1'7.41 D � c
1 2 MILES
2 KILOMETERS
yz()') Q�-r SJr--
-'= Snohomish County Area, Washington
TABLE 13. --ENGINEERING INDEX PROPERTIES
[The symbol < means less than; > means more than. Absence of an entry indicates that data were not estimated)
1 Classification
Soil name and !Depth! USDA texture ! rag- ' ercentage passing
,ments { sieve number-- !Liquid ` Plas-
map symbol { Unified ! AASHTO 1> 3 1 ;
!inches!
, I 200 , limit , ticlty
n ' I ,inches, 4 ; 10 ; 40 ' ; ; index
See footnote at end of table.
i
1 Pot-,
rpl2,3--d 1 0-7 !Gravelly sandy iGM
1 loam.
!A-2, A-4,;
0-5
;55-70 150-65 135-55 120-40
1 20-30
1 NP -5
1 I
! 7-351Very gravelly !GM
1 loam, very i
1 A-1 '
!A-1 A-2 1
'
' , , i
0-10 1
' 35-55 35-50 25-45 ,10-30
,
1
, 20-30
i
, NP -5
! gravelly sandy
1
I I
1
i ! loam.
35 !Cemented---------'
Alderwood-------1 0-7 !Gravelly sandy !GM
A-2,. A-4,,
0-5
155-70 ,50-65 ,35-55 120-40
, 20-30
1 NP -5
! ! loam. !
1 A-1 '
' ,
,
1 7-351Very gravelly !GM
!A-1, A-2 1
i
0-10 ;35-55 135-50 125-45 1_10-30
1 20-30
,
1 NP -5
loam, very 1
! !
! ! !
SIT -C ! ! gravelly sandy- !
j ! ! loam.
!
!
35 'Cemented -,--------I ---
! --- !
---
i --- ! --- ! --- ! ---
! ---
Everett ---------i 0-6 !Gravelly sandy !SM
!A-1, A-2 1
0-10
165-85 150-75 1'30-50 1115-30
! 15-25
i NP -5
i 1 loam.
1 6-18!Very gravelly !GP -GM,
GM 1A-1 1
5-10
130-60 120-50 110-30 1 5-25 1
15-25 1
NP -5
! sandy loam.
1118-60!Very gravelly !GP
!A-1 !!
5-20
,25-50 115-45 , 5-20 1 ;
0-5
--- !
NP
coarse sand,
,
! ! very gravelly
I
i
! ! loamy sand, !
,
! ! extremely
1 I gravelly sand.
1 !
!
6:*:
iderwood------- 1 0-7 !Gravelly sandy iGM
, 1
' 11
iA-2, A-4,,�
0-5
i 1 !
,55-70 ,50-65 135-55 ,20-40 ,
i
20-30 !
NP -5
loam. 1
1 A-1 '
' I 1,
,
! 7-35!Very gravelly !GM
!
!A-1 A-2 '
0-10135-551
1 1 1
35-50 25-45 10-30
20-30 1
NP -5
11 loam, very !
!
! ! gravelly sandy
! ! loam. !
1 35 !Cemented ---------'•
1 I
rban land.
1 1
,
'--------------1 0-9 !Silty clay loam. lML, OL
=llingham 1 9-60iSilty clay, clay,lCL, CH
1A-4
!A-7
! 100 195-100!80-95 !
25-40 1
NP -10
silty clay loam.!
!
1 1
0
1 100 95-100195-100185-100!
! ! ! ! !
45-65 1
!
20-40
--------------! 0-6 !Mucky silt clay
1 y y ,ML, CL
Allingham , , loam. !
!A-6, A-7 !
!
0
! 100 ! 100 1185-100180-1001
35-45 !
10-20
variant 6-421Silty clay loam, !CL
! silty clay. !
!
.1A-7 1
! i
0
! ! ! ! !
1 100 1, 100 185-100!80-100!
,
!
40-50 !
20-25
42-601Stratified loamy 1CL-ML,
! fine
CL,IA-2, A-4,1
0
1 100 1 100 140-80 !20-60 ;
20-35 1
5-15
sand to 1 SM -SC,
SC! A-6 !
! ! silty clay loam.!
! !
10, 11--------1 0-8 !Loam-------------'
thcart AML
8-351Loam,
!
!A-4, A-5 ,
0
190-95 1185-95 1170-80 160-70 !
35-45 !
5-10
1 sandy loam,1ML, SM
1 , silt loam.
!A-4, A-5 1
0
180-95 180-90 1170-85 145-65 1
,
35-45 1
5-10
35-60 Loam, clay loam IML, SM
4-4 i
0
180-90 175-85 !65=75 140760 1
,
30-40 1
5-10
yohemists
,
I
-------------=,! 0-9 ,Fine sandy loam 11SM !A-2 A-4 1 0
ster 19-351:Loamy fine sand, !SP -SM, SM !A-2, A-3 1 0-20
! _ 1
' ! ! 130-45
1
25_30 1
S
NNP
1 1 sand. i
i ,
.i
!9501000!90-100!70-80 15 20
i
1
f35-60 Fine sand, sand, jSP-SM
!A-3 1 0-20°1195-100160-80
160-70 1 5-10 1
,
--- !
NP
! ! gravelly coarse
1 sand.
I ! I
I i•
� [ 1 I i
,
See footnote at end of table.
Peter Chopelas, PE
Engineering & Design Services
307 N. Olympic Ave, Suite 208
Arlington, WA 98223
(360) 653-4615;
May 17, 2012
Eric Thuesen
509-9'h Avenue N.
Edmonds WA 98020
Eriethuesen@fi-ontier.com
Chopelasandassociates@gmail.com
Mitigation Plan
per section 23.40.130
Subject: Slope stabilization measures for plan ck # 2011-0775 accessory structure (cabana) at
the above address
Project Description
The building site is a residential building lot of approximately 0.2 acres served by a municipal
sewer and a public water supply, and with an existing residence and garage built on a terrace on a
sloping site. There is a proposed accessory building of 320 SF, with a 360 SF attached deck, a
retaining wall and back fill to enlarge the useful backyard, placed at the top of a steep slope
below the existing residence. The building site is about 160 feet from the toe of the slope, and
about 50 feet in elevation above it.
Goals and Objectives
The project will reduce potential landslides, stabilize the existing slope, enhance erosion
control on the steep slopes, and will enhance and add to native vegetation on the hillside.
1. Potential impacts are uncontrolled run-off, erosion and destabilizing the slope during _
construction phase. There is new impervious area proposed and the additional loads on
the slopes created by the proposed structures. These will be mitigated by using an
engineered design, construction timing, limiting disturbing the slope to between May 1
and October 1St, use of a silt fence on the down hill side of the site disturbance, cover any
exposed soils on the slops within 48 hours. Permanent measures will be to direct roof
run-off to the existing stormwater conveyance system, and replanting disturbed soil with
native vegetation suitable for soil stabilizing surface soils.
There are no wetlands, habitat, or shorelines affected by this project, so no habitat
mitigation measures are necessary. However, replanting of the hillside with native
vegetation will enhance the natural environment and provide for improved habitat.
2. Slope stability measurements were done using traditional Singh diagrams. This has
been in use for many decades and has proven to result in safe and stable slopes. This
methodology results is a more stable slopes as indicated by more modern analysis
methods.
3. If all recommendations are followed, the likelihood of success are estimated to me 98
percent or greater. The methods proposed have long histories of successful outcomes.
4. Only steep slope areas are affected by this proposal, all mitigation is oriented towards
steep slope development. All off site impacts will be prevented by following the
recommendations in the Geotech report.
B. Performance Standards. Regular inspections of the site must be made to insure that off
site run-off avoided by containing all surface run-off on-site during construction. And
that those construction recommendations are followed.
C. Detailed Construction Plans: These are detailed in the Recommendations section of the
Geotech report. These include:
1. Conventional grading and footings recommendations. And all bearing soil should be
inspected by the engineer before concrete is poured.
2. Roof run-off should be directed to a catch basin in the existing conveyance system and
tight -lined to the base of the slope. No drains or concentrated flows should discharge
over the ground or on the slope.
3. A silt fence should be placed on the downhill side of the construction site to attenuate
silt -laden run-off if necessary. Silt fences should be placed around catch basin inlets.
Soil that is to be removed and reused around the site should be kept away from the top of
the slope. Soils not to be reused on the site should be hauled off site to an approved
location. Clearing shall be allowed only from May 1st to October 1st of each year;
provided, that the director may extend or shorten the dry season on a case-by-case basis
depending on actual weather conditions.
4. The vegetation on the steep slopes should be replaced as soon as practical after
construction. Any disturbed soil in the area of the slope should be protected with jute
matte or straw and replanted with native vegetation. After final grading any disturbed or
exposed soil needs to have topsoil spread and compacted and planted with vegetation
that will retard erosion and stabilize the upper soil layers.
Grass seed mixture based on soil type, used for erosion control, should be as follows:
Common Name Percent by Weight
Perennial ryegrass 40
Annual ryegrass 20
Red Fescue 40
This should be applied at a hydro -seed rate of 1 LB per 1000 sq. ft. (or 60 LB. per acre);
or a hand seed rate for 2 LB. per 1000 sq. ft. (or 87 LB. per acre). Fertilizer shall be 10-
20-20, applied at the rare of 10 LB. per 1,000 sq. ft. (or 400 LB. per acre).
The following native vegetation are acceptable for permanent plantings on disturbed
slopes and buffers:
Trees*( 3-4' size; plant 6' o.c.)
Shrubs*(to be 1-3' size; plant 2 foot
O.C.
Vine Maple
Kinnick-kinnick
Big leaf maple
Salal
Nobles fir
Snowberry
Grand fir
Vinca Minor
Oregon Grape
Red current
Other native species as recommended by qualified landscape professional knowledgeable in soil
stabilization may also be used.
.D. Monitoring Program. Inspections should be done during the construction at each
phase, and upon completion of the replanting of the exposed soil. It should not be
necessary to monitor the health of the vegetation for more than one year after completion.
If the plants are growing, healthy, and providing ground cover, no further action should
be necessary.
E. Contingency Plan. Project modifications may have to be done if unforeseen soil
conditions occur during the soil and excavation inspections, or if unusually wet weather
or soil conditions are encountered.
F. Financial Guarantees. Applicant will post bonds, or a pledge of financial responsibility
(at the director's option) to ensure mitigation, maintenance, and monitoring. [Ord. 3527
§ 2, 2004]. The size of this project is not sufficient to require extensive bonds.
N
a
W
C
y
G
•fir, ,�,
,�,�_...,,r:"..',,r" �
cC
G
F. p
N •�
N
501
�
U
�
F m o
U N G G
rn RS
.ti cO '
3
q O
u" o ro:e
kr
5❑ aUi
a o o
0 F4 cv
o
Po+ • 3 O
ro�
ai
b
O
Ei41
a
p
4j
U b
+✓ .�
U�
,D G�
U O N cd
..O O O
N
O
O
Eb rn
y U
F'�•r ' O
Q? d' ro C
.N. ro N
('�
E^ O N
7
s�
1S
0
u
E�
•fir, ,�,
,�,�_...,,r:"..',,r" �
44O
F m o
7
s�
1S
0
u
E�