Lanninggeotech.pdfChopelas & Associates, Inc.
Engineering & Design Services
3611 168'x' St. NE, Suite D R E C E I V E
Arlington, WA 98223
(360) 653-4615; FAX: (360) 651-2085 NOV 0`7 2006
DFVFI QPMFNT SFRVICPS CTR.
September 28, 2006 CITY of EDMONDS
Geotechnical Summary Report
Mike and Chrystal Lanning
18604 Sound View Place
Edmonds, WA 98020 C VPV
Summary
A geotechnical review of property at the above address was performed to address slope stability,
erosion control and drainage issues. The investigation consisted of a site inspection and a review
of published materials by the USES and the USDA
It was found that the slope is stable and safe from earthquakes and landslides. The sandy
soil is highly prone to severe erosion when exposed if protective measures are not immediately
taken. Installing lawn and landscaping, as well as preserving the existing native ground cover,
and taking reasonable protective measures, the proposed grading and landscaping on this site can
be done with reasonable assurance of long term slope and site safety.
WARNING: Immediate action must be taken to provide cover for the exposed soil. The
building department should allow immediate authorization to proceed with ground cover
per the recommendations of this report even before the permit review is complete because
of the potential for early heavy rainfall.
I. Project Description
Grading and extensive landscaping has been started and are proposed near a coastal bluff for an
existing single family residence and garage, a driveway, on a community sewer system and
public water supply are proposed for this residential site.
H. Site Conditions
The building site is a residential building lot of approximately one third acre with a public sewer
system and water supply. The proposed grading and landscaping are on a sloping terrace 10 feet
back from the top of a Puget Sound coastal bluff approximately 30 feet high. Most of the site is
currently stripped of the original lawn leaving exposed soil. The bluff is vegetated with thin,
invasive shallow rooting shrubs such as berries, morning glories, peas, etc. Most of the site
slopes to the west toward the top of the bluff at from 10 to 20 percent grade. The grade on the
bluff ranges from about 60 to 100 percent.
There is no evidence of past surface erosion or earth movement anywhere on or near the
bluff. The trees on the adjoining properties near the bluff are true and strait with little or no
signs of correction. There is no sign of seepage of moisture .found anywhere. along the slope. The
lower part of the property is approximately 100 feet from the Puget Sound shoreline.
M. Subsurface Conditions
The subject site lies on the western edge of the Lynnwood plateau. The general stratigraphy (soil
layering) of the area is characterized by fluvial and glacio-fluvial sediments from sea level to
around 400 feet, with glacial till plains making up the surficial sediments of most of the plateau.
The soils at the site are mostly sandy with silty sand and clayey silt cropping out at the toe of the
slope. These sediments belong to the transitional beds and Vashon Till plains (Minard 1482).
The classification of the soils on the site is Alderwood gravelly sandy loam. There is from 3 to 6
feet deep gravelly sand over about a 20 to 30 foot thick layer of Vashon till. This till is described
as a "concrete -like mixture of clay, silt sand and gravel ... very dense and tough,... and forms near
vertical natural and cut slopes for long periods of time .... it [is] Iargely immune to instability
problems associated with saturation by ground and surface water. Vashon till has good seismic
stability as well." This soil was observed on the site by visual inspection of the excavations and
on the bluff.
The very top of the bluff appears slightly over -steepened by shallow fill from past old
grading activities. This was probably done when the site was cleared for building the home 30
plus years ago, or perhaps when the sewer line was installed. It has naturally revegetated by
invasive plants and landscaping clippings. Despite the loose fill and sparse surface vegetation
there was no significant surface slumping or landslides observed along the bluff. The rest of the
bluff is at or below its natural angle of repose.
IV. Slope Stability
Landslide hazard:
The overall slope on the site is stable from deep seated slumps or Iandslides. The sandy soils are
at or below their natural angle of repose, and there is little surface moisture. There is no
evidence of surface sliding or slumps, all the mature trees and vegetation on the adjoining sites
along the bluff indicate long term stable growth. Analysis indicates that "at rest' there is a slope
stability factor of 1.2 or higher despite the over -steepening. Before the oversteeping of the upper
10 feet or so of the bluff the "at rest" safety factor is 1.5. Only the upper shallow filled portion
of the slope is at risk of instability since the compact and dense material below the till has a
safety factor of 2.0 or more, and is not susceptible to moisture. Since the upper sandy material
does not retain moisture it too will not be affected by a surface run-off. The location of the
proposed grading for the patio has an "at rest" safety factor of 2.0 or more.
Liquefaction is another risk during seismic events. Liquefaction is not a hazard at this
location because well drained gravely sands do not liquefy, nor does the underlying dense till
Since there is no significant new impervious area there is virtually no change to the hydraulic
loading of the underlying material. With the existing 70 -foot building setback from the top of the
slope, the house site has a calculated safety factor of about 2.5.
Erosion hazard:
Where the slope is not subject to surface flows and where it has deep rooting vegetation and
ground cover, no erosion occurs. As long as concentrated flows of surface water are avoided,
and the vegetation is maintained, the site is safe from erosion hazard.
However, the deep sandy soils when exposed to concentrated surface flow are highly
subject to surface erosion and mass wasting. Therefore disbursing all downspout discharge in a
sheet to lawn or Iandscaping, and avoiding any concentrated flow, is essential. Because of the
large risk of fouling and failure of subsurface absorption beds, subsurface beds are not as
desirable as surface discharge since it is more easily monitored and corrected if necessary.
V. Recommendations
The proposed grading and landscaping adjacent to the bluff can be safely done without
increasing the risk to the coastal bluff. Long term slope stability and safety to the bluff and
building will be achieved if the following recommendations are followed:
1. The existing and any proposed conventional footing designs are safe with a soil bearing
capacity of 2000 PSF. All new footings must rest on clean undisturbed subsoil. Building
setbacks from the top of bluff should be 30 feet. Footings do not need footing drains.
2. All roof down spouts should be directed to splash blocks and crushed rock beds away from
the foundation. It should be disbursed in a sheet to lawn and landscaping to facilitate surface
infiltration. Under no circumstances should concentrated surface flows be allowed to be
discharged in the direction of the top of the bluff.
3. Soil removed during construction should not be stored or disposed of on the slope. Soil
disturbances should be kept 10 feet back from the top of bluff. Heavy construction vehicles
should be kept back 20 feet from the top of the slope. Under no circumstances should any
surface soil or vegetation be disturbed within 10 feet of the top of the slope.
4. Cuts in native undisturbed soils should be no steeper than 2 horizontal to 1 vertical. Filled
slopes should be no steeper than 3 h: I v and compacted. All landscaping fill should be
compacted to 92 percent or more of max density, structural fill under slabs or walkways
should be compacted to a minimum of 95 percent of max density (modified proctor method).
The on-site sandy soil free of organic material and debris is suitable for fill.
5. As soon as possible after the'final grading, all exposed soil needs to be covered with topsoil
and planted with lawn and landscaping or other vegetation that will retard erosion and
stabilize upper soil layers from surface erosion. Mulch, jute mat, straw or plastic sheet may
be used to temporarily cover exposed soil.
6. Not required for this project, but it is recommended to prevent possible shallow surface
slumps, that the shallow rooting invasive plants on the bluff be eventually removed by hand
clearing (no heavy equipment), any loose soil compacted, and then replanted with deep
rooting native shrubs suitable for erosion control. Suitable trees are vine maple, oak -leaf
mountain ash, pink Idaho locust, or dogwood, and native shrubs are salal, Oregon grape,
snowberry, kinnikinnick, white rock rose, and currant. Other native species may be planted
as recommended by qualified landscape professionals knowledgeable in soil stabilization
If you have any questions or if you are in need of further assistance, please do not hesitate to call.
1 EXPIRES 12-14-"GGCro ' !
Peter Chopelas PE
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Applicant:
;Mike and Chrystal Lanning .
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18604 Sound View Place
i Edmonds, WA 98020
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{426} 744-1893
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Tax Account Number:
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Engineering & Design Services
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Arlington, WA 48223
(360) 6534615; FAX: (360) 651-2085
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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]
Soil name and ;Depth; USDA_ texture
map symbol ; ;
Everett --------- 1
Classification
;
; Unified
!
,
; AASHTO
;
, rag-
;ments ;
1 > 3 1
;inches; 4
Percentage
sieve
,
; 10
passing
number--
, ,
; 40 1 200
, ,
;Liquid ;
; limit 1
; ;
Plas-
ticity
index
n ,
,
,
,pet--!-,,
t
1, 2, 3----------1
0-7 ;Gravelly sandy
1GM
;A-2,
A-4,;
0-5
155-70
150-65
135-55
120-40
; 20-30 ;
NP -5
Alderwood ;
; loam.
;
; A-1
;
;
;
;
1
1 ;
7-351Very gravelly
;GM
1A-1,
A-2
;
0-10
;35-55
;35-50
;25-45
;t0-30
; 20-30 ;
NP -5
loam, very
1 gravelly sandy
1 loam.
'
35 `Cemented---------'
+
'
4•:
Alderwood-------;
0-7 ;Gravelly sandy
1GM
;A-2,
A-4,;
0-5
155-70
50-65
;35-55
120-40
; 20-30 1
NP -5
loam.
;
; A-1
7-351ve-. gravelly
;GM
1A-1,
A-2
;
0-10
;35-55
135-50
125-45
110-30
20-30 ;
NP -5
loam, very
gravelly sandy.
loam.
1
35 ;Cemented ---------1 -__
; -__
;
--
---
Everett---------1 0-6 ;Gravelly sandy ISM ;A-1, A-2 1 0-10 165-85 150-75 4130-50 115-30 1 15=25 ; NP -5
11 e
1 1 loam. ; , ; ; 11;
1 6-181Very gravelly :GP -GM, GM ;A-1 ; 5-10 ;30-60 ;20-50 ;10-30 1 5-25 1 15-25 ; NP -5
; ; sandy loam. 1 ; ; ; 1 ; ; 1 ;
;18-601Very gravelly ;GP ;A-1 ; 5-20 125-50 ;15-45 1 5-20 1 0-5 1 --- ; NP
; coarse sand,
1 ; very gravelly ; ; 1 ;
loamy sand,
extremely
gravelly sand.
5*:
Al*,derwood-------1 0-7 ;Gravelly sandy :GM ;A-2, A-4,; 0-5 155-70 150--65 135-55 120-40 ; 20-30 ; NP -5
loam. ; ; A-1 ; 1 ; ; ; ; 1
7-351Very gravelly 1GM 1A-1, A-2 1 0-10 135-55 ;35-50 ;25-45 110-30 ; 20-30 1 NP -5
1loam, very 1
gravelly sandy ; ; 1
+ , loam.
1 35 ;Cemented-------- __- -- ; --- 1 --- --- { --- --- ; --- ; ---
r +
i k 1 I { 1 1 1 1 ! 1
Urban land.
7----------------; 0-9 ;Silty clay loam ;ML, OL 'A-4 e 0 ; 100 ; 100 ;95-100180-95 ; 25-40 ; NP -10
Bellingham ; 9-601Siity clay, clay,;CL, CH 1A-7 ; 0 1 100 195-100;95-100185-100; 45-65 1 20-40 .
; silty clay loam.;
1
1
8----------------; 0-6 :Mucky silty clay 1ML, CL 1A-6, A-7 1 0 ; 100 1 100 ;85--100180--1001 35-45 ; 1A-20
Bellingham 1 1 loam. ' ' ; 1 ' '
, 1 ! , 1 1
Variant ; 6-42;Silty clay loam, 1CL 1A-7 1 0 1 100 ; 100 185-100;80-100; 40-50 1 20-25
1 1 silty clay.
;42-601Stratified loamy ;CL -ML, CL,;A-2, A-4,1 0 1 100 1 100 140-80 120-60 ; 20-35 ; 5-15
; fine sand to ; SM -SC, SC; A-6 1 1 ; ; ;11;
; 1 silty clay loam.;
I
9, lo, 11--------1 0-8 ;Loam -------------:ML ;A-4, A-5 ; 0 190-95 185-95 170-80 160-70 1 35-45 1 5-10
Cathcart ; 8-35;Loam, sandy loam,lML, SM ;A-4, A-5 1 0 180-95 180-90 170-85 145-65 1 35-45 ; 5-10
silt loam.
135-601Loam, clay loam :ML, SM ;A-4 ; 0 1$0-90 175-85 165-75 140-60 1 30-40 1 5-10
l 1 1 { 1 k 1 1 ; 1
12*.
k + 1 1 1 [ f 1 1 I
-Cryohemi.stS
13---------------1 0-9 ;Fine sandy loam ;SM ;A-2, A-4 ; 0 1 100 190-1001$0-90 130-45 1 25-30 1 NP -5
Custer ; 9-35;Loamy fine sand, :SP -SM, SM 1A-2, A-3 i 0-20 195-100;90-100170-80 1 5-20 ; --- ; NP
1 sand.
;35-601Fine sand, sand, ;SP -SM ;A-3 ; 4-20 ;95-100160-.$0 ;60-70 ; 5-10 --- 1 NP
gravelly coarse 1
1 ; sand. ; ; ; 1 ; ; i
1 f 1
, 1 ,
See footnote at end, of table_
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