20160915090337.pdfOctober 9, 2012
HWA Project No. 2012-000-21 Task 24`M,y,,
Jeremy Anderson
.. 9 °)rl+°s �R
10425 Little John Court
0
Edmonds, Washington 98020 UIN)
Subject: GEOLOGICALLY HAZARDOUS AREAS ASSESSMENT,
Garage Pad & Rockery ,.
10425 Little John Court
Edmonds, Washington 98020
Dear Mr. Anderson;
This assessment addresses the applicable requirements of the Edmonds City Development Code
(ECDC) with regard to the construction of a rockery and garage pad (to accommodate a future
detached garage) along the eastern side of your property located at 10425 Little John Court (See
Figure 1). We understand that initial communications with the City of Edmonds indicate that the
proposed pad and rockery will be constructed within the an area potentially considered to be a
critical area (Geologically Hazardous Area) that most likely consists of a potential Erosion
Hazard Area. The purpose of this assessment is to determine whether the proposed rockery and
pad construction will impact local slope stability and the potential for site erosion consistent with
the requirements of the ECDC. Our work included a site reconnaissance, limited subsurface
exploration, ECDC code review, and writing this assessment report.
PROJECT DESCRIPTION
We currently understand that the owner desires to provide additional parking for vehicles along
the east side of the existing residential structure. To accomplish this, minor site grading will be
required to provide means of access and a level parking area for vehicles. Consideration is being
given to installation of a rockery wall to protect the low cuts anticipated along the north and east
side of the parking pad. The pad will likely be 30 feet wide and 30 feet long and extend parallel
to the east side of the existing residential structure along its toe and into the adjacent existing
hillslope, as shown on Figure 2. We understand that a detached garage structure approximately
25 feet by 25 feet in area may be constructed within the garage pad, as part of a future project.
Currently, the area is being landscaped and invasive vegetation is being removed.
2131.2 :30th Drava; H..,
Suite 110
Bothell, WA,,. 980211-701.0
Tel: 425 74.01.06
Fax: 425,774.2714
October 9, 2012
HWA Project No. 2012-000-21 Task 24
SITE DESCRIPTION AND OBSERVATIONS
The subject lot is located within the north central portion of the cul-de-sac comprising Little John
Court (see Figure 2). Residential lots border all but the south side of property where it abuts
Little John Court. The central and western portions of the lot consist of a relatively planar
terrace that very gently (<2%) declines toward the west. The eastern portion of the property
consists of a slightly terraced hillslope that is gently to moderate inclined to the northeast at an
average gradient of 22 percent. There are currently no visible signs of previous slope instability,
or evidence of slope creep that might be inferred from trees with bent trunks. Slope vegetation
consists mainly of ivy with several large coniferous trees. No slope vegetation that is typically
indicative of seepage or very moist conditions was noted.
SUBSURFACE EXPLORATIONS
The soil conditions within the approximate location of the future garage pad and rockery location
were explored by means of two hand auger borings, one 20 feet east of the southeastern house
corner and one near the northeastern corner of the proposed rockery by our licensed geologist. It
should be noted that the soil and ground water conditions described are only for the specific
dates and locations reported and, therefore, are not necessarily representative of other locations
and times. It is anticipated that water conditions will vary depending on seasonal precipitation,
local subsurface conditions and other factors.
SUBSURFACE CONDITIONS
According to the geologic map by Mackey Smith (1975), "Preliminary Surficial Map of the
Edmonds East and Edmonds West Quadrangles, Snohomish and King Counties, Washington
the slope consists of Esperance Sand comprising thinly bedded, mostly clean, fine to medium
sand with pebbles and scattered lenses of coarse gravel. This unit is usually medium dense to
dense when undisturbed. Slopes comprised of Esperance Sand typically slough to the angle of
repose of sand of around 30'to 33' (about 65 percent grade). When slope instability occurs in
Esperance Sand, it is mainly confined to loose surficial layers, but deep-seated slides can occur
near the contact with less permeable glacial silts and clay near the base of the deposit. Soil
conditions are shown on Figures 4 and 5 along with a legend to terms used on the hand borings
on Figure 3.
SOILS
According the NRCS soil map for the project area, the site is underlain by Alderwood-Everett
gravelly sandy loam 25 to 75 Percent Slopes Series. The Everett component of the series is
comprised of soils that formed on glacial outwash such as the Esperance Sand. These soils are
considered to excessively drained and only pose a moderate potential for erosion. However
these soils are considered by the City of Edmonds as being a potential erosion hazard.
Geologically Hazardous Areas Assessment 2 HWA GeoSciences Inc.
October 9, 2012
HWA Project No. 2012-000-21 Task 24
CONCLUSIONS
Geologically Hazardous Areas
Based on our assessment the site does not meet the City of Edmonds criteria for a Landslide
Hazard Area. However construction of the pad and rockery are considered an alteration within a
potential erosion hazard area by the ECDC. In accordance with ECDC Section 23.80.060,
alterations of geologically hazardous areas or associated buffers may only occur for activities
that.
1. Will not increase the threat of the geological hazard to adjacent properties beyond
predevelopment conditions;
2. Will not adversely impact other critical areas;
3. 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. Are certified as safe as designed and under anticipated conditions by a qualified engineer
or geologist, licensed in the state of Washington.
In addition, in accordance with ECDC Section 23.80.070.A.2, alterations of an erosion hazard
area and/or buffer may only occur for activities for which a hazards analysis is submitted and
certifies that:
a. The development will not increase surface water discharge or sedimentation to
adjacent properties beyond predevelopment conditions;
b. The development will not decrease slope stability on adjacent properties; and
c. Such alterations will not adversely impact other critical areas.
The geologic map and our explorations indicate that the project area is underlain by Esperance
Sand. These soils consist of medium dense to dense, granular, well drained, sands that are
typically not subject to global or deep rotational failure. In addition, the present site
development plans do not include any mass grading beyond leveling the pad area with only
slight modification along the slope toe to accommodate a low rockery (< 4 feet in height) and
will not impair local stability to any significant degree. Based on our understanding of the
proposed work, as stated in the previous project understanding section, it is our opinion that
creating a level parking area extending out from the existing house laterally 30 feet and
constructing a rockery to protect the slope cuts will not have any adverse effects on the existing
slope on the east side of your property as long as the following conditions are met during
construction:
Geologically Hazardous Areas Assessment 3 HWA GeoSciences Inc.
October 9, 2012
HWA Project No. 2012-000-21 Task 24
• Leveling of the garage pad footprint should be accomplished by conducting a balanced
cut and fill thereby minimizing the height of slope cuts and soil exports (if any). Site
soils that are free of organic material may be reused as fill.
• The garage pad should be sized such that it extends 5 feet beyond the exterior walls of the
future footprint of the detached garage structure (i.e. the pad should be 30 feet by 30 feet
for a 25-foot by 25-foot garage structure).
• The rockery should be constructed in accordance with the guidelines contained in the
Association of Rockery Contractors (ARC Manual). The base rocks should be embedded
at least one foot below the adjacent pad grade on properly compacted crushed rock placed
over unweathered native soil. The actual depth to unweathered native outwash may vary
and rockery base depth should be adjusted accordingly.
• The rockery should be battered a minimum of 6V:1H.
• No rocks less than 1-man size should be used in order to minimize threat of pull down by
climbing children.
• Positive drainage in the form of a gravel encapsulated perforated drainpipe should be
installed behind the wall base.
In our opinion the most significant risk posed by this project in this portion of the site is likely a
temporary increased potential for erosion during construction. This potential hazard will need to
be mitigated by post construction covering of the pad area with compacted crushed rock, and
replacement of the disturbed slope with suitable native planting and landscaping sufficient to
prevent soil erosion. We believe that the construction of the pad and rockery will not adversely
affect the adjacent slope or any adjacent critical areas assuming the following conditions are met
during construction.
Best Management Practices (BMP' S) including limiting activity during wet weather, and
proper disposal or recompaction of surface soils materials that has been disturbed during
construction must be followed.
If these steps are followed then we conclude that the proposed improvements will not impair
local stability to any significant degree and should be permitted as allowed under the provisions
specified in ECDC Sections 23.80.060 and 23.80.070.A.2
CONDITIONS AND LIMITATIONS
We have prepared this letter report for use by Jeremy Anderson and his designated agents for use
in construction of a garage pad and rockery along the eastern side of his existing single family
residence. Experience has shown that soil and ground water conditions can vary significantly
over small distances. Inconsistent conditions can occur between explorations and may not be
detected by a geotechnical study of this nature. If, during future site operations, subsurface
Geologically Hazardous Areas Assessment 4 HWA GeoSciences Inc.
October 9, 2012
HWA Project No. 2012-000-21 Task 24
conditions are encountered which vary appreciably from those described herein, HWA should be
notified for review of the recommendations of this letter report, and revision of such if necessary.
The scope of work did not include environmental assessments or evaluations regarding the
presence or absence of wetlands or hazardous substances in the soil, surface water, or ground
water at this site. HWA does not practice or consult in the field of safety engineering, and
construction safety considerations are the responsibility of the Contractor or property owner.
We appreciate the opportunity to be of service. If you have any questions or concerns, please do
not hesitate to contact us.
Sincerely,
HWA GEOSCIENCES INC.
Steven E. Greene, L.G., L.E.G.
Senior Engineering Geologist
Attachments:
Figure 1
Site Vicinity Map
Figure 2
LIDAR and Exploration Plan
Figure 3
Legend of Terms and Symbols Used on Exploration Logs
Figure 4
Log of Handhole HH-1
Figure 5
Log of Handhole HH-2
Geologically Hazardous Areas Assessment 5 HWA GeoSciences Inc.
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GEOLOGICALLY HAZARDOUS AREAS ASSESSMENT
HWAGEOSCIENCES INC. ANDERSON GARAGE PAD
10425 LITTLE JOHN COURT
FIGURE No,
NROJECT NO
2012-000 T24
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RELATIVE DENSITY OR CONSISTENCY VERSUS SPT N-VALUE
TEST SYMBOLS
COHESIONLESS SOILS
COHESIVE SOILS
%F
Percent Fines
Approximate
AL
Atterberg Limits: PL = Plastic Limit
DensityN
Consistency Approximate
blows/ft c N blows/ft Undralned Shea r
( ) y ( )
LL =Liquid Limit
Relative /)
y(°
"""'
Strength (psf)
CBR California Bearing Ratio
Very Loose
0 to 4 0 - 15 Very Soft
0 10 2 4250
CN
Consolidation
Loose
4 to 10 15 - 35 Soft
2 to 4 250 - 500
DID
Dry Density (pcf)
Medium Dense
10 to 30 35 - 65 Medium Stiff
4 to 6 500 - 1000
IDS
Direct Shear
Dense
30 to 50 65 - 85 Stiff
8 to 15 1000 - 2000
GS
Grain Size Distribution
Very Dense
over 50 85 - 100 Very Stiff
15 to 30 2000 - 4000
K
Permeability
Hard
over 30 >4000
MD
Moisture/Density Relationship (Proctor)
.................
_..-............... .....
SCS SOIL CLASSIFICATION
SYSTEM
MR
Resilient Modulus
PID
PhotoionizationDevice Reading
MAJOR DIVISIONS
GROUP DESCRIPTIONS
PP
Pocket Penetrometer
Approx. Compressive Strength (tsf)
Gravel and
GW Well -graded GRAVEL
SG
Specific Gravity
Coarse
Clean Gravel
TC
Triaxial Compression
Grained
Gravelly Soils
(little or no fines)
— � '°'�
° GP poorly -graded GRAVEL
TV
Torvane
Approx. Shear Strength
Soils
More than ...�...._._. ...
Gravel
50% of CoarseGM
Silly GRAVEL VE L
UC
(tsf)
Unconfined Compression
Fraction Retained Fines (appreciable
onNo.4Sieve amount of fines)
GC Clayey GRAVEL
SAMPLE TYPE SYMBOLS
Sand and Clean Sand
SW Well -graded SAND
®
2.0" OD Split Spoon (SPT)
Sandy Soils the 0r n0 fines
( )
��--
(1401b. hammer with 30 in. drop)
More than
Poorly -graded SAND
SP Poo grad
Shelby Tube
50% Retained••••••
50% or More
on No.
Sand with
of Coarse
SM Silty SAND
3-1/4" OD Split Spoon with Brass Rings
200 Sieve
Fines (appreciable
Size
Fraction Passing
amount of fines)
No 4 Sieve
SC Clayey SAND
d
0
Small Bag Sample
r
Large Bag (Bulk) Sample
Fine
Silt
Liquid Limit
m ..
Grained
and
Less than 50%
✓
CL Lea n CLAY
Core Run
Soils
Clay
�.... ., 6 ....--- ...................
— OL Organic SILT/Organic CLAY
Non-standard Penetration Test
... ........_....-. � , a _ _
(10" OD split spoon)
MH Elastic SILT
50% or Moreand
Passing
Silk
Liquid Limit
More
_ .........,,,__
CH � Fat CLAY
GROUNDWATER SYMBOLS
No. 200 Sieve
Clay50% or
Groundwater Level (measured at
Size
A� OH Organic SILTIOrganic CLAY
time of drilling)
mm. .-._---......
................. ....._..........
Highly Organic Soils
. . ......
� PT PEAT
Groundwater Level (measured in well or
,-........ .
open hole after water level stabilized)
COMPONENT DEFINITIONS
COMPONENT PROPORTIONS
COMPONENT
SIZE RANGE
PROPORTION RANGE
DESCRIPTIVE TERMS
Boulders
Larger than 12 in
< 5%
Clean
Cobbles
3 in to 12 in
Gravel
3 in to No 4 (4,5mm)
5 - 12%
Slightly (Clayey, Silty, Sandy)
Coarse gravel
3 in to 3/4 in
Fine gravel
3/4 in to No 4 (4.5mm)
12 - 30%
Clayey, Silty, Sandy, Gravelly
Sand
No. 4 (4.5 mm) to No. 200 (0,074 mm)
Coarse sand
No. 4 (4.5 mm) to No. 10 (2.0 mm)
Medium sand
No. 10 (2 0 mm) to No. 40 (0.42 mm)
30-50%
Very (Clayey, Silty, Sandy, Gravelly)
Fine sand
No. 40 (0.42 mm) to No, 200 (0,074 mm)
•. ...... �
�—•••••• •••
Silt and Clay
Smaller than No. 200 (0.074mm)
Components are arranged in order of increasing quantities.
NOTES: Soil classifications presented on exploration logs are based on visual and laboratory observation.
Soil descriptions are presented in the following general order: MOISTURE CONTENT
Density/consistency, color, modifier (if any) GROUP NAME, additions to group name (if any), moisture DRY Absence of moisture, dusty,
content Proportion, gradation, and angularity of constituents, addilior al comments. dry to the touch.
(GEOLOGIC INTERPRETATION) MOIST Damp but no visible water.
Please refer to the discussion in the report text as well as the exploration logs for a more WET Visible free water, usually
soil is below water table.
complete description of subsurface conditions.
Geologically hazardous Areas Assessment LEGEND OF TERMS AND
8`0 Garage Pad & Rockery SYMBOLS USED ON
HWAGEOSCIENCESINC 10425 Little John Court EXPLORATION LOGS
Edmonds, eshlntert
PROJECTNO,: �1 -000 T24 r
LEGEND 2012-000 T024.GPJ 10/9112
DRILLING COMPANY: HWA GeoSciences Inc.
DATE STARTED: 10/5/2012
DRILLING METHOD: Hand Tools
DATE COMPLETED: 10/5/2012
SAMPLING METHOD: Grab
LOGGED BY: D. Coltrane
LOCATION: See Figure 2.
Lug
w
Z
r
U
a 2
N U
Standard Penetration Test
J
w C
H
(140 lb. weight, 30" drop)
Z
0
0
m
w
I= U
EL U
n. a
Z o
2
> y
W
o (0 D DESCRIPTION
co
a
0
0
w
0 .,... _ _ .
.._
Topsoil.
GRA��.......
� mmmaded GRAVEL with
Loosemediumbown, poorly graded
31
cobbles,dens dry.
)('
-p
Medium dense, light brown, poorly graded SAND with gravel,
° moist.
SP Dense, yellowish brown, poorly graded SAND with gravel,
SM moist.
i
Hand Hole terminated at an approximate depth of 3 feet below
the existing ground surface due to difficult digging. No
groundwater seepage was observed while conducting this
hand hole.
0
20 40 60 80
100
Water Content (%)
Plastic Limit 1 Liquid Limit
NOTE: This log of subsurface conditions applies only at the specified location and on the date indicated
Natural Water Content
and therefore may not necessarily be indicative of other times and/or locations.
Geologically Hazardous Areas Assessment HAND HOLE:
r Garage Need & Rockery HH-1
AGuE + IE CES INC. 10425 Little John Court PAGE: 1 of 1
Edmonds, Washington
PROJECT NO.: 2012-000 T024 FIGURE: 4
HANDHOL-DSM 2012-000T024.GPJ 10/9/12
DRILLING COMPANY: HWA GeoSciences Inc.
DRILLING METHOD: Hand Tools
SAMPLING METHOD: Grab
LOCATION: See Figure 2.
co
g
U
J
J O
a� Co v
o4d)
D ...�W,rr...
5-
Topsoil.
DATE STARTED: 10/5/2012
DATE COMPLETED: 10/5/2012
LOGGED BY: D. Coltrane
w
U
m
Z
D
N
W
¢
Standard Penetration Test Z
Z
u,
H
o
(140 lb. weight, 30" drop) O
w
a
w
a
wm
N
w
Z
z o
x
O
w �,
DESCRIPTION
a Z
O
c�7
w
Loose to medium dense, brown, poorly graded GRAVEL with
sand, silt, and cobbles, dry.
....... _- ... Y.,,
b Medium dense, light brown, graded SAND with gravel,
moist.
SP Medium dense to dense, yellowish brown, poorly graded
I' SM SAND with silt and +gravel', moist.
Hand boring terminated at an approxiamte depth of 2.5 feet
below the existing ground surface due to hard digging. No
Groundwater seepage was observed while conducting this
hand boring.
NOTE: This log of subsurface conditions applies only at the specified location and on the date indicated
and therefore may not necessarily be indicative of other times and/or locations.
g 20 40 60 80
Water Content (%)
Plastic Limit F-.*""......1 Liquid Limit
Natural Water Content
Geologically Hazardous Areas Assessment HAND HOLE:
Garage Pad & Rockery HH-2
HWAGEOSCIENCES INC. 10425 Little John Court PAGE: 1 of 1
Edmonds, Washington
PROJECT NO.: 2012-000 T024 FIGURE: 5
HANDHOL-DSM 2012-000T024.GPJ 10/9/12