Ordinance 2445WSS:jt
7/3/84
ORDINANCE NO. 2445
AN ORDINANCE OF THE CITY OF EDMONDS, WASHING -
TON, AMENDING CHAPTER 19 OF THE EDMONDS
COMMUNITY DEVELOPMENT CODE BY THE ADDITION OF
A NEW SECTION 19.00.020 RELATING TO THE
ISSUANCE OF BUILDING PERMITS IN THE LANDSLIDE
HAZARD AREA, AMENDING CHAPTER 20 OF THE
EDMONDS COMMUNITY DEVELOPMENT CODE BY THE
ADDITION OF A NEW SECTION 21.55.060 RELATING
TO THE DEFINITION OF LANDSLIDE HAZARD, LIFTING
A MORATORIUM IMPOSED BY RESOLUTIONS 428, 431,
450, 587 AND 590 AND FIXING A TIME WHEN THE
SAME SHALL BE EFFECTIVE.
WHEREAS,'the City Council by Resolutions 428, 431,
450, 587 and 590 has imposed a moratorium on building and
building permits within an area commonly known as Meadowdale;
WHEREAS, the City Council imposed said moratorium in
the interest of public health, safety and welfare, in order to
protect against earth subsidence and landslide hazard as
identified in a final report commissioned by the City from
Roger Lowe Associates, Inc.;
WHEREAS, the hazards identified in said report have
been addressed by the City of Edmonds to the extent possible
through public works designed to reduce ground water levels by
the improvement of public storm and sanitary sewerage; and
WHEREAS, the projects and hearing complete and
represent the last of the steps available to the City to
address the problem in light of current scientific,
engineering and architectural standards; and
WHEREAS, the City has a legal obligation to warn
property owners and other persons within the Meadowdale area
of the hazards identified by the Roger Lowe Associates, Inc.
report;
WHEREAS, the City desires to permit resumption of
development in the area to the extent consistent with public
safety and under reasonable assurances which protect both
persons in the area and all of the citizens of Edmonds.
NOW, THEREFORE, THE CITY COUNCIL OF THE CITY OF
EDMONDS, DO ORDAIN AS FOLLOWS:
Section 1. Chapter 19.00 of the Edmonds Community
Development Code is hereby amended by the addition of a new
Section 19.00.020 relating to the issuance of building permits
in the earth subsidence and landslide hazard area to read as
follows:
1'9'.00.020 EARTH SUBSIDENCE AND LANDSLIDE HAZARD
AREA, BUILDING PERMITS
1. Notwithstanding any contrary provision of these
ordinances or of the Uniform Building Code, all
applications for building permits received for any
site, any portion of which lies within an area
designated by the Final Report, Landslide Hazards
Investigation prepared by Roger Lowe Associates,
Inc. as on file with the City Clerk as having an
ascertainable hazard, shall be governed by the
provisions of this section. In addition to all
other requirements of these ordinances, the
following restrictions and provisions shall apply
to all building permits and applications processed
after the lifting of a moratorium imposed by
Resolution 428, as amended by Resolutions 431, 450
587 and 590.
A. All such applications for building permit
shall disclose on their face whether or not
they lie within the earth subsidence and
landslide hazard area. All such applicants
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shall be provided with a copy of the Landslide
Hazard Map, which is a part of the Roger Lowe
Associates, Inc. report.
B. All such applications shall be accompanied by
a foundation design prepared by an engineer
licensed by the state of Washington and
schooled in the design of foundations, based
upon a soils report by a geotechnical engineer
licensed by the State of Washington, which
design addresses those hazards identified as
applicable to the site by the Landslide Hazard
Report. Prior to issuance of any occupancy
permit, the building official shall be
supplied with a statement from the designing
engineer that he/she has conducted a site
inspection and that the foundation conforms to
the engineer's design. Nothing in this
subsection shall be deemed to limit the
discretion vested in the building official by
the Uniform Building Code to impose greater or
more restrictive requirements of the applicant
or to require additional design, recommenda-
tion or review by professionals hired by the
applicant.
C. The building permit application shall require
acknowledgement by the applicant that he/she
has been provided with or has waived receipt
of a copy of the Landslide Hazard Map and
agrees to release and indemnify the City from
any and all claims, damage or loss resulting
to any party resulting from:
1. The construction and/or design of the
building or structure by the applicant or
applicant's employees or contractors;
and/or
2. The provision of false information by the
applicant or applicant's agents or
employees in the permit application;
and/or
3. Any risk or hazard of which the applicant
could reasonably have had notice of from
the Roger Lowe Associate, Inc. report and
the Landslide Hazard Map.
Nothing herein shall be deemed to require the
applicant to release or indemnify the city, its
- 3 -
officers or employees from loss or damage arising
from the City's sole negligence.
D. All applications for building permits, subdivision,
planned residential development and development
applications of any kind for sites within the earth
subsidence and landslide hazard area shall provide
the appropriate reviewing body or official with a
covenant, executed and acknowledged by the owners of
the site in a form that can be recorded in the
Snohomish County Auditor's office. The covenant
shall provide:
1. A statement by the owner(s) that he/she will
inform his/her successors and assigns that the
site, lot or development area is within an area
identified by the Roger Lowe Associates Inc.
report; of the risks associated with development
thereon; and of any conditions, prohibitions or
restrictions on development;
2. A legal description of the property affected;
3. The official number and date of the permit or
other action for which the covenant was required;
and
4. A statement waiving the right of the owner,
his/her heirs, successors and assigns to assert
any claim against the City by reason of or
arising out of issuance of the permit or other
development approval by the city for the subject
property.
2. All reviews conducted under the provisions of the Edmonds
Community Development Code for building permit,
subdivision or planned residential development with
reference to any site located in the landslide hazard
area shall include consideration of the removal of any
tree having a diameter of 8 inches or greater. Such
trees shall be shown on any map, plat or site development
plan required by the Code. Consideration of such
application shall include the requirement that such trees
not be damaged or removed; provided, however, that an
applicant may be permitted to remove such trees if
adequate replacement of such trees, as determined by the
appropriate City Board or City official, is provided for
by the applicant. Bonding may be required. Subdivision
and planned residential development shall contain
covenants which prohibit the damaging or removal of trees
eight inches or greater without adequate replacement.
- 4 -
3. The report of Roger Lowe Associates and the Landslide
Hazard Map are hereby incorporated by reference and made
a part of this ordinance as fully as if herein set forth.
Copies of the report and map shall be maintained in the
office of the City Clerk, Planning Department and with
the building official and shall be available for
inspection during all normal working hours. Individual
copies of the report and map may be obtained by the
public upon the payment of the cost of reproduction.
Section 2. Chapter 21.55 of the Edmonds Community
Code is hereby amended by the addition of a new section
21.55.060 relating to the definition of earth subsidence and
landslide hazard area:
21.55.060 EARTH SUBSIDENCE AND LANDSLIDE HAZARD
AREA.
Those areas identified in the Roger Lowe Asso-
ciates, Inc. report as on file with the City Clerk
and the accompanying landslide hazard map as
having a greater than 0% probability of landslide
or subsidence hazard.
Section 3. The moratorium imposed by Resolution 428
and modified by Resolutions 431, 450, 587 and 590 is hereby
declared to be at an end. Any and all applications for build-
ing permit, subdivision or planned residential development
received under the terms of the moratorium shall be processed
in accordance with the provisions of this ordinance and no
right shall vest by virtue of the application until the
effective date of this ordinance lifting the moratorium and
imposing the described restrictions on permit issuance.
Section 4. This ordinance shall be in full force
and effect five (5) days after passage and publication by
posting as provided by law.
APPROVED:
MAYOR, LARRY S. NAUGH EN
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ATTEST/AUTHENTICATED:
IT CLERK, JACQUELINE G. PARRETT
APPROVED AS TO FORM:
OFFICE THE ITY ATTORNEY:
BY I A Lafl,
FILED WITH THE C Y CLERK: July 11, 1984
PASSED BY THE CITY COUNCIL: July 17, 1984
POSTED: July 20, 1984
EFFECTIVE DATE: July 25, 1984
ORDINANCE NO. 2445
AFFIDAVIT OF POSTING ORDINANCE
STATE OF WASHINGTON
ss:
COUNTY OF SNOHOMISH
JACQUELINE G. PARRETT
, being first duly sworn
on oath deposes and says that the is over the age of eighteen (18)
years and is competent to testify as to the matter stated herein.
There is no official newspaper or other newspaper printed and
published within the City. In accordance with RCW 35A.12.160, on
the 20th day of July , 1984, affiant posted true and
correct copies of the attached Ordinance No. 2445 , passed by the
City Council on the 17 day of July , 19 84, at the
official posting places for City notices which are the public
bulletin boards at the following locations:
Edmonds Civic Center
250 Fifth Avenue North
Edmonds, Washington 98020
Edmonds Public Library
Civic Center, 250 Fifth Avenue North
Edmonds, Washington 98020
Edmonds Branch of United States Post Office
201 Main Street
Edmonds, Washington 98020
DATED this 20 day of July
, 1984.
SUBSCRIBED AND SWORN to before me this _20 day of
July 19 84•
otary Public in and for the
State of Washington, residing
at r m VlhoA
THIS[ MAC tih1oU11? 0-0 DI- USID iN ,ONJUNC TION WIIH 111E
FINAL REPORT �38"?_
LANDSLIDE HAZARDS INVESTIGATION
MEADOWDALE AREA
EDMONDS, WASHINGTON f y -3405 --
A FULL DESCRIPTION OF THE LANDSLIDE -HAZARD CLA,7SIFICATION IDS i ) /f�����' / it
11 1 SOS '
AND A SUMMARY OF THE PROCEDURES USED 10 CONSTRUCT THIS �[ �/ - - f, \�, /
MAP ARE INCLUDED IN THAT REPORT
KEY � ,r= �' 15�"• .r aw
HAZARD
f NONE IDENTIFIED
2, HAZARDS FROM ENCROACHING LANDSLIDE IVATERIALe,
3 HAZARDS FROM GROUND FAILURE IN MATEHIAI. THAT
HAS NOT PREVIOUSLY FAILED`�,
4 HA7APCS FROM GROUND FAILUREy
IN PnE.n1I-IY J �/4� O 4 1\ , .1. �� I �r_�. .�•. /'I`
FAILED MATERIAL Y{ •� 3 02 r" ''~' -+-� -T ` -
PROCESS j - ( _�. Q_'• a' f..Y-" - ' -
f �• i
A SLUMPS y. 35
Q I�
a DEBRIS SLIDES
C DEBRIS AVALANCHES jj 1 r v• �� "�If1-�� ����_ ,�_ - - '�
D DEBRIS FLOWS
II
PROBABILITY ` 4A25 ` �M ^f 1� ' f•
PROBABILITY OF OCCURRENCE DURING A 2, VFAR 2Cp
PERIOD. IN PERCENT (^- / ' �. L -
Icy-
2A9 a d 43 - _ W. p c �
1 4 0 •� 41 � II Db � J
EXAMPLE: 4A90 r II I ON, , -
LOCATIONS WITHIN THIS MAP UNIT ARE GENERALLY SUBJECT 2 ?i\
TO GROUND -FAILURE HAZARD FROM AN E1lIS71NG SLUMP, 11MP, ( 3 2A/
WITH A 90 PERCENT PROBABILITY OF OCCURRENCE -
DURING A 25 LEAP PERIOD ~� r. 1 2D-
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f
October 16, 1979
City of Edmonds
Public Works Department
200 Dayton Street
Edmonds, Washington 98020
Attn: Mr. Fred Herzberg, P.E.
Director of Public Works
Gentlemen:
Transmitted to you with this letter is our "Final Report, Landslide
Hazards Investigation, Meadowdale Area, Edmonds, Washington". This report
is submitted in partial fulfillment of our agreement dated May 22, 1979. The
scope of our work is described in that agreement.
We have previously submitted to you an Interim Report dated August 10, 1979,
and a Preliminary Draft Final Report dated September 17, 1979. In accordance
with our agreement, we are submitting to you three copies of our final report.
We will supply additional copies of the final report at our cost upon request
by the City.
We appreciate the opportunity of working with you on this project. If
you have any questions, or if we can be of further assistance, please call.
DWT/JWK/cg
3 copies submitted
Yours very truly,
ROGER LOWE ASSOCIATES INC.
G.i
i
Donald W. Tubbs, Geologist
7Jon'W. Koloski, Associate
TABLE OF CONTENTS
INTRODUCTION
1
GEOLOGY AND SITE CONDITIONS
2
Geologic History
2
Site Geology
6
Landslide History
8
Site Hydrology
11
LANDSLIDE -HAZARD MAPPING
14
Landslide -Hazard Classification
14
Description of the Landslide -
Hazard Categories
17
ALTERNATIVE LAND-USE/RISK-REDUCTION
20
MEASURES
General
20
No Special Measures
20
Non -Construction Measures
21
Construction Measures
21
Engineering Analysis
26
CONCLUSION
31
USE OF THIS REPORT AND WARRANTY
34
APPENDIX - BORING LOGS
A-1
e
FINAL REPORT
LANDSLIDE HAZARDS INVESTIGATION
MEADOWDALE AREA
EDMONDS, WASHINGTON
INTRODUCTION
This report presents the findings of our investigation of landslide
hazards in the Meadowdale area. The Meadowdale study area is located in the
northern part of the City of Edmonds, in Section 5 and adjacent portions of
Sections 7 and 8 of Township 27N, Range 4E, Willamette Meridian. This area
includes an ancient, and still active, landslide approximately 40 acres in extent
and portions of an adjacent upland to the north, east and south.
The purpose of our investigation is to obtain information on the geologic
and hydrologic conditions in and adjacent to the Meadowdale landslide complex as
a basis for developing recommendations regarding development in the Meadowdale area.
Our work has been conducted in two phases; we summarized the results of Phase 1
in an interim report dated August 10, 1979 and this report is submitted as part
of Phase 2. The specific scope of work for this phase includes: subsurface ex-
ploration, engineering analysis, and preparation and submission of a
final report presenting the findings of the study. The contents of this report
includes:
1. A description of the site geology.
2. A geologic map of the site.
3. A description of the landslide -hazard classification.
4. A landslide -hazard map.
5. A discussion of alternative land-use/risk-reduction measures.
6. The results of engineering analysis of those measures.
7. A discussion of the assumptions and suggested rational that may
be used by the City in making decisions regarding land-use/risk-
reduction measures in the Meadowdale study area.
` City of Edmonds October 16, 1979 Page 2
GEOLOGY AND SITE CONDITIONS
Geologic History
The Quaternary history of the central Puget Sound lowland has been domin-
ated by several glacial episodes, during each of which a lobe of Cordilleran ice
advanced into the area from the north. The glacial advances were separated by inter-
glacial periods during which climatic conditions were in many respects similar to
climate conditions prevailing today. The existence of glacial drift (i.e. sediments
deposited by, or in association with, glacial ice) predating the most recent
glaciation of the Puget Sound lowland has been recognized since about the turn of
the century, but most of the current concepts of local Quaternary stratigraphy have
been developed during the past 25 years (Figure 1).
The oldest glaciation thus far recognized in the Puget Sound lowland is
the Orting Glaciation, which was immediately succeded by the Alderton Interglaciation.
The materials deposited during these time intervals, the Orting Drift and the Alder -
ton formation respectively, are recognized in the Puyallup and Green Valleys, but
correlative sediments have not been found elsewhere in the Puget Sound lowland.
The next glaciation recognized in the Puget Sound lowland is the Stuck
Glaciation. Glacial drift that may be correlative to the Stuck Drift has been
recognized in the southern Puget Sound lowland, in Island County and also within the
Meadowdale study area. These sediments are discussed in more detail below. Following
the retreat of the Stuck glacier, the interglacial Puyallup formation was deposited
throughout much of the Puget Sound lowland.
The Salmon Springs Glaciation followed the Puyallup Interglaciation.
The Salmon Springs deposits at the type section near Sumner include two drift
sheets seperated by a few feet of nonglacial sediments, including peat and volcanic
ash. This section was originally interpreted as representing two glacial advances
during a single glaciation (Figure 1a), but more recently workers in the northern
Puget Sound lowland have advanced an alternative view: that the Salmon Springs
deposits represent two major glaciations between which glacial ice completely
evacuated the Puget Sound lowland (Figure lb). Outcrops near the southern end of
Whidbey Island show a pre -Fraser succession of two tills and intervening nonglacial
sediments. The upper pre -Fraser till there has been designated the Possission
Drift, the lower till the Double Bluff Drift, and the intervening nonglacial
sediments the Whidbey formation. Mapping by the Washington State Department of
Natural Resources has shok-rn that both the Double Bluff Drift and the Whidbey
a
City of Edmonds
October 16, 1979
Paae 3
FIGURE 1
GEOLOGIC - CLIMATE UNITS IN THE CENTRAL PUGET SOUND LOWLAND
la. Generally accepted chronology
Fraser Glaciation (youngest)
Olympia Interglaciation
Salmon Springs Glaciation
Puyallup Interglaciation
Stuck Glaciation
Alderton Interglaciation
Orting Glaciation (oldest)
lb. Alternative interpretation
Fraser Glaciation (youngest)
Olympia Interglaciation
Possession Glaciation
Whidbey Interglaciation
Double Bluff Glaciation
Puyallup Interglaciation
Stuck Glaciation
Alderton Interglaciation
Orting Glaciation (oldest)
City of Edmonds October 16, 1979 Paae 4
formation extend into the IMeadowdale study area.
The Olympia Interglaciation immediately preceeded the most recent
glaciation. During the Olympia Interglaciation, and apparently also during the
previous interglaciations, the Puget Sound lowland probably looked much like it
does today, except for differences in the existence and position of the marine
inlets that presently comprise Puget Sound. Hills that were several hundred feet
high, with steep slopes and relatively flat tops, existed in many of the same
positions as the present hills; they were surrounded by flood plains and shallow
lakes in which layers of clay, silt and sand were deposited. Fossil pollen evi-
dence indicates that during the later part of the Olympic Interglaciation the
climate of the Puget Sound lowland was cooler and wetter than at present. Alpine
glaciers began to form in the mountains of western Washington and an ice sheet was
developing in the mountains of western British Columbia. The alpine glaciers in
western Washington soon retreated, but the ice sheet in the mountains of western
British Columbia continued to expand into the lowlands of southwestern British
Columbia and northwestern Washington. Thus began the main phase of the Fraser Glacia-
tion, called the Vashon Stade.
Approximately 15,000 years ago a lobe of Cordilleran ice, the Puget Lobe,
pushed south into the Puget Sound lowland far enough to block the northward -flowing
drainage to the Strait of Juan de Fuca. This resulted in a large proglacial lake
which drained southward into Grays Harbor via the lower Chehalis Valley. Water and
sediment entered the lake from the glacier, which constituted the northern
boundary of the lake, and alsofrom the highlands on both sides. The coarser sedi-
ment carried by the water was dropped as the streams entered the lake, while the
silt- and clay- sized particles settled to the bottom in the quieter water at some
distance from the ice margin. A widespread deposit of silt and clay was thus
created which constitutes the Lawton Clay Member of the Vashon Drift. The Lawton
Clay is found throughout much of the central portion of the Puget Sound lowland,
but does not appear to be present within the Meadowdale study area.
As the Puget Lobe advanced farther south a thick unit of proglacial sand
was deposited. This unit, the Esperance Sand Member of the Vashon Drift, spread
over not only the Lawton Clay, but also the hills of older material that protruded
through the Lawton Clay. Most of the eastern portion of the Meadowdale study area,
above an altitude of approximately 230 feet, is underlain by Esperance Sand.
City of Edmonds October 16, 1979. Paae 5
The Esperance Sand locally becomes coarser and more pebbly near its top,
grading into the more poorly sorted Vashon advance outwash - sand and gravel
deposited directly in front of the advancing ice by glacial meltwater streams.
The Puget Lobe of the Vashon glacier overrode the study area and advanced southward
to a position about 15 miles south of Olympia. At its maximum the ice thickness
in the vicinity of the Meadowdale study area was approximately 4,000 feet. In
moving over the unconsolidated sands and clays, the ice scoured out the newly
deposited material more readily than it did the older deposits. It thus eroded
troughs where previously there had been valleys aligned parallel to the direction
of ice movement, and left behind hills with cores of older sediments.
Some of the material eroded by the glacier was redeposited farther to
the south as advance outwash. Much of it however, was incorporated into the Vashon
till - a nonsorted, nonstratified sediment deposited directly by the glacial ice.
Most of the Vachon till is very compact because it was plastered onto the ground
surface under the weight of several thousand feet of ice. However, as the glacier
began to melt, a.less compact layer of till was left at the surface as residual
debris from the thawing dirty ice. The Vashon till caps portions of the upland
surfaces in the eastern half of the Meadowdale study area.
The recession of the Puget Lobe was quite rapid. By approximately 13,500
years ago the ice had melted back to approximately the latitude of the Meadowdale
study area, and by 11,000 years ago the ice front had retreated up the Fraser
Valley. As it retreated, the ice uncovered a glacially- sculpted landscape
of uplands and intervening valleys. South of the melting ice, and along the
edges of residual ice lobes within the valleys, meltwater streams locally deposited
Vashon recessional outwash. Some of these deposits have been mapped within the south-
eastern portion of the Meadowdale study area.
As the glaciers in various parts of the world melted, sea level rose
rapidly and marine waters invaded Puget Sound. Sea level appears to have attained
approximately its present position about 7,000 years ago. Since then, currents
and wave action have cut away at the base of the glacially -formed slopes, occasionally
oversteepening them and causing landslides to occur.
UI.y of Edmonds October 16, 1979 Page 6
t.e Geology
The events discussed in the previous section resulted in a complex
assemblage of glacial and non -glacial sediments and landforms. A geologic map
ur the Meadowdale study area is presented as Figure 2 (in pocket) and a cross
section of the study area is shown in Figure 3. These figures are based partially
Of, previous studies of the Meadowdale area and partially on our own surface and
%obsurface investigation. Our field mapping has concentrated on identifying the
Character and distribution of the pre-Vashon sediments, because of their importance
for slope stability.
The oldest materials found within the Meadowdale study area belong
to the Double Bluff Drift. These materials are locally exposed in the northwest
Portion of the study area up to approximately 150 feet above sea level (mllw),
and also were encountered in Borings 2 and 5. Within the study area the Double
Bluff Drift includes silt, sand, gravel, and pebbly silt and clay. These materials
are highly consolidated due to compaction by glacial ice, and some of the gravel
i! cemented to form a poorly -indurated conglomerate. The upper surface of
the Double Bluff Drift slopes irregularly downward toward the west, and is apparently
the east wall of a roughly north -south trending valley that was filled with
`e diments during the Puyallup Interglaciation.
These interglacial sediments comprise the Whidbey formation. Within
the Meadowdale study area the Whidbey formation consists of sand, silt, and
Clay, and extends from at least 25 feet below to approximately 230 feet above
Wa level (mllw). The lowest 50 feet of the Whidbey formation encountered in
the borings is predominantly sand and the upper 200 feet is predominantly silt
and clay, but contains a few sand beds. The silt and clay beds in the upper
Part of the Whidbey formation have an important influence on the movement of
!Jroundwater, as will be described below.
Overlying the Whidbey formation within the Meadowdale study area is the
E:perance Sand Member of the Vashon Drift. The Esperance Sand consists of
l'I'latively clean, fine to coarse sand which was spread over the top of the older
""its during the advance of the Vashon glacier. The upper portion of the slopes
"Id most of the upland surface within and adjacent to the eastern half of the
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City of Edmonds October 16, 1979 Page 8
Meadowdale study area is underlain by the Esperance Sand. This unit is a significant
aquifer in the vicinity of the Meadowdale study area. Groundwater that infiltrates
beyond the depth of plant rooting in areas underlain by the Esperance Sand percolates
downward until it is stopped by the silt and clay beds within the upper part
of the Whidbey formation. The water then moves laterally to where it emerges
at the seeps and springs that are present near the trace of the top of the Whidbey
formation.
Thick deposits of Vashon advance outwash do not appear to be present
within the Meadowdale study area. In the eastern half of the study area the
Esperance Sand is locally capped by Vashon till. The Vashon till is a nonsorted,
nonstratified deposit of silt, sand and gravel, and may locally constitute an
impediment to the downward percolation of groundwater. In the southeastern part
of the Meadowdale study area, some Vashon recessional outwash has been mapped,
but extensive deposits of recessional outwash deposits do not appear to be present.
Landslide History
The only major post -glacial deposit within the Meadowdale study area is
the material which constitutes the mass of the Meadowdale landslide complex.
This landslide complex consists of blocks of sandy and silty material derived
primarily from the Whidbey formation, the Esperance Sand and, near the north
end of the landslide, the Double Bluff Drift. The landslide mass has a north -
south extend of about 3,200 feet and extends eastward as much as 650 feet from
the Burlington Northern Railroad right-of-way. The landslide ranges in elevation
from near sea level to approximately 150 feet above mean lower low water. The
thickness of the landslide mass is generally between 20 and 50 feet. Portions
i
of the landslide mass near the eastern boundary of the landslide complex
elevation of about 125 feet, appear to consist of relatively large, coherent
landslide blocks. Most of the remainder of the landslide, especially the portion
west of 75th Place West, is composed of more deformed material which moves much
more frequently than the larger, more coherent blocks.
We believe that the initial movement of the Meadowdale landslide
complex occurred sometime within the past 7000 years, and probably within the
past two or three thousand years. Some previous workers have suggested that
landsliding in the Meadowdale area began during the retreat of the Puget Lobe
City of Edmonds October 16, 1979 Page 9
of the Vashon glacier. However, the toe of the Meadowdale landslide complex is
located at the eastern edge of a 600-foot wide wave -cut bench which has
largely formed since sea level attained its present level approximately
7,000 years ago. The overall form of the Meadowdale landslide complex, and the
position of its toe near the present shoreline, indicates that the initial
failure of the landslide complex occurred subsequent to the formation of the
wave cut bench.
Active landslidinq_ in the Meadowdale area has been occurring primarily
within a zone 400 feet wide located parallel to and immediately east of the
Burlington Northern Railroad tracks. The earliest documented movement occurred
during the winter of 1946-47. This movement involved an area extending from
approximately 220 feet south to approximately 1,050 feet south of the Laebugten
Wharf. The head scarp was located approximately 410 feet east of the Burlington
Northern trackage. The slide down -dropped a sizeable portion of 76th Avenue West,
rendering it impassible to vehicular traffic.
Landslide activity in the Meadowdale area durinq 1953-54 is mentioned
in newspaper accounts but the exact location could not be ascertained from available
data.
The most thoroughly documented and extensive landsliding occurred in
the winter of 1955-56. According to maps supplied to us, movement occurred in
two areas: the northern area was centered approximately 1550 feet north of the
Laebugten Wharf, with the head scarp located 120 feet east of 75th Place West, and
the southern area Was centered about 850 feet north of the wharf, with the head
scarp situated 75 feet east of 75th Place West. However, on the basis of our field
observations and discussions with long-time local residents we believe that movement
during the winter of 1955-56 affected a much larger area. Geomorphic evidence
suggests that the landslide extended from about 1,920 feet north of Laebugten Ulharf
to approximately 220 feet south of the intersection of North Meadowdale Road and
75th Place West. The head scarp was located approximately 570 feet east of the
Burlington Northern Railroad trackage.
City of Edmonds October 16, 1g74 Page 10
Numerous smaller landslides occurred between 1948 and 1956. Information
derived from maps supplied to us indicates that as many as 9 smaller landslides
occurred in the area of the foot of the northern third of the 1955-56 landslide.
Two smaller landslides are shown as having occurred in the area of the foot of
the southern third of that landslide. A fairly substantial landslide also
occurred in the 1960's. The area of that movement was roughly the same as that
of the 1946-47 slide, but was slightly greater in extent. The exact date of
these landslides could not be determined from information available to us.
Maps provided to us indicate that a landslide occurred during the
winter of 1970-71, centered approximately 220 feet northeast of the Laebugten
Wharf access road. The area of movement was approximately 350 feet long and
extended 50 feet east of 75th Place West. The landslide reportedly also moved
3 inches in the 5-year period between 1971 and 1976.
Two widely separated landslides occurred in 1973-74. The southern area
of movement was approximately 425 feet long and was centered approximately
350 feet north of the Laebugten Wharf and 65 feet east of the Burlington Northern
.. Railroad tracks. The head scarp for this landslide was located about 80 feet
I east of 75th Avenue West. The northern area of movement was approximately 220 feet
long and was centered 1200 feet north of the wharf and 200 feet east of the
railroad tracks. The head scarp for this landslide extended to the west edge
of 75th Place West.
In summary, it appears that historic landsliding has occurred within
a zone approximately 400 feet wide adjacent to and immediately east of the
Burlington Northern Railroad tracks. Except for one reported incidence of
approximately 4 inches of lateral movement of the railroad tracks, the Burlington
_. Northern Railroad tracks do not appear to be within the area subject to ground
movement. It appears that nearly all the active failure surfaces within the
Meadowdale landslide complex emerge at the surface east of the Burlington Northern
Railroad tracks and that the primary failure surface probably emerged at
approximately the position of the shoreline prior to the construction of the
railroad.
City of Edmonds October 16, 1979 Page 11
We believe that the initial failure of the Meadowdale landslide complex
was related to shoreline erosion, and that the landslide probably originated
sometime within the past few thousand years. The landslide complex has
continued to be active due to erosion of its toe by shoreline processes. The
construction of the Burlington Northern Railroad, with associated shoreline
erosion prevention measures, has been a stabilizing influence on the landslide.
Site Hydrology
Some of the episodes of movement described above reportedly
occurred during periods of unusually heavy.precipitation. The relatively small
number of such events, and uncertainties regarding the dates of initial or
accelerated • movement, preclude the construction of climatic correlations
based on those reports. However, our previous studies of the relationships
between landsliding and climatic factors in the central Puget Sound lowland
have demonstrated that landsliding in this area is closely related to precipitation.
We do not believe that freeze/thaw events have a significant influence on landslide
activity in the central Puget Sound lowland.
As will be shown in the results of our stability analyses, the
stability of the Meadowdale landslide complex is extremely sensitive to
groundwater levels within the landslide. Water enters the area of the Meadowdale
landslide complex via three routes: as precipitation directly into the area
of the landslide, as water imported for domestic use, and as inflow from
adjacent areas.
Water that enters the area of the Meadowdale landslide complex as
precipitation can follow several paths: Some of it does not infiltrate
into the soil, but instead immediately exits as direct surface runoff. Most of
the precipitation that does enter the soil is retained within the zone of
plant roots and is later evapotranspi rated, but a portion of it percolates
downward to the water table and thus contributes to the groundwater within the
landslide.
City of Edmonds October 16, 1979 Page 12
Some of the water imported for domestic use enters the landslide as
leakage from the distribution system, and most of the remainder is ultimately
discharged into septic systems. Much of this water percolates downward
to the water table; in places where the water table is very near the surface,
some of the septic system effluent re-emerges nearby.
A large quantity of water enters the area of the Meadowdale landslide
complex from the east. Some of this water is direct surface water runoff
from the adjacent hillslopes and upland areas, and some of this water is ground-
water which has emerged from the seeps and springs along the contact between
the Esperance Sand and the Whidbey formation. Water from both of these
sources enters the area of the Meadowdale landslide complex as surface water.
Our investigation has disclosed no significant quantity of groundwater discharging
directly into the landslide.
Much of the surface water inflow seeps into the upper portion of
the landslide complex and contributes to the groundwater within the landslide.
The portion of such inflow that becomes groundwater is largely related to
the level of the water table within the landslide complex. When the water
table is low, a larger portion of the inflow enters the groundwater system
than when the water table is high. In the absence of quantitative information
on the amount of such groundwater recharge, we have estimated that on an annual
basis approximately half of the inflow becomes groundwater.
Table 1 summarizes the annual groundwater recharge of the Meadowdale
landslide complex for each of three conditions. The first column presents
our estimates of the amount of recharge under natural (i.e. pre -development)
conditions. The second column presents our estimates under the present
- conditions, and the third column represents a condition of complete residential
development. For the estimates pertaining to complete residential development
we assumed approximately twice the present housing density, both within the
area of the Meadowdale landslide complex and in the adjacent areas east of the
landslide, and no additional storm or sanitary sewers.
City of Edmonds October 16, 1979 Page 13
Recharge Source
Precipitation
Imported water
Inflow
TOTAL
TABLE 1
ANNUAL GROUNDWATER RECHARGE
MEADOWDALE LANDSLIDE COMPLEX
(units: millions of cubic feet per year)
Natural
Present
Fully Developed
0.9
0.7
0.6
0.0
0.5
1.0
2.8
4.0
5.3
3.7
5.2
M
City of Edmonds October 16, 1979 Page 14
LANDSLIDE -HAZARD MAPPING
Landslide -Hazard Classification
E Regional slope -stability mapping has been conducted in the Puget
Sound lowland since 1972. In 1973, the U. S. Geological Survey published a
map showing relative slope stability in part of west central King County.
That map used both slope inclination and the nature of the subsurface material
as criteria for evaluating slope stability. Excepting areas extensively
modified by human activity, the map showed areas sloping less than 150 as being
relatively stable, areas sloping greater than 15% and underlain by stratigraphic
units containing tight silt or clay as being relatively unstable, and areas
sloping greater than 15°S and underlain by other materials as being of
intermediate stability. A companion report demonstrated that there was a good
correspondence between locations of landslides known to have occurred during
early 1972 and the mapped slope -stability categories.
Later publications also established a strong correlation between
landslide locations and the position of the contact between the Esperance Sand
and either the Lawton Clay or pre-Vashon sediments. This is primarily an
effect of the contrasting permeabilities of these stratigraphic units. Where
the Esperance Sand overlies the Lawton Clay or pre-Vashon sediments such as
the Whidbey formation, the movement of groundwater within the Esperance Sand is
largely controlled by the uppermost laterally -extensive silt or clay bed within
the underlying material. The trace of the lower contact of the Esperance Sand
is often the location of considerable seepage, which decreases slope stability
in the vicinity of that contact.
Subsequent mapping of relative slope stability in the Puget Sound
lowland has generally been based on the principles established in the early
publications. The Washington State Department of Natural Resources has
produced several published and unpublished relative slope stability maps, one
of which includes the Meadowdale study area.
There are, however, at least three general limitations that are inherent
in these slope -stability maps. One such limitation results from the
City of Edmonds October 16, 1979 Paae 15
scale of mapping. Most slope -stability mapping in the Puget Sound area has been
conducted at a scale of 1:24,000. At this scale, a line which is 1/20th of an
inch wide on a map represents 100 feet on the ground, and errors resulting from
drafting of the map can result in the boundaries between various slope -stability
areas being misplaced by several hundreds of feet.
A second limitation of most existing slope -stability maps is related
to the sources of information upon which they are based. Most such maps are
constructed from existing topographic and geologic maps, and most of those maps
were originally produced at scales of 1:24,000 of 1:62,500. Even if the
slope -stability map is drafted at a larger scale, it can be no more accurate
than the sources of information upon which it is based.
A third limitation of relative slope -stability maps is that they
depict inferred "slope stability" rather than "landslide hazard". Landslides
originating within a relatively unstable area shown on such maps can affect both
upslope areas (e.g. - by the retrogressive fa.ilure of slumps) and downslope areas
(e.g. - by the veneration of debris avalanches). In using slope -stability maps
to made decisions about landslide hazards, it is necessary to consider the
possible effects of landslides originating within a relatively unstable area upon
adjacent, more stable areas.
The map which we have compiled as part of this study avoids all of
these difficulties. First, at the selected scale of 1 inch = 200 feet it is
possible to show sufficient detail for planning decisions affecting individual
sites. Second, the availability of topographic maps at that scale having a contour
interval of 5 feet, together with the surficial reconnaissance and subsurface
exploration, provides a sufficient information base for the construction of a map
at the selected scale. Third, the map that we have designed depicts "landslide
hazard" rather than "slope stability".
The landslide -hazard classification that we have utilized is shown in
Figure 4. Each landslide -hazard category is represented by an identifier composed
of up to three fields: the first field indicates the nature of the hazard, the
second field indicates the type of landslide process that presents the hazard, and
the third field indicates the probability of occurrence within a 25-year period.
City of Edmonds October 16, 1979 Page 16
FIGURE 4
LANDSLIDE -HAZARD CLASSIFICATION
MEADOWDALE STUDY AREA, EDMONDS, WASHINGTON
General
This landslide -hazard classification is based on (1) the nature of
the hazard, (2) the type of landslide process that creates the hazard, and (3) the
probability of occurrence. The general representation of the various categories
is of the form:
X Y Z
where X is a number indicating the nature of the hazard,
Y is a letter indicating the type of process, and
Z is a number indicating the probability of occurrence, in percent,
during a 25-year period.
Class 4A80, for instance, represents an area of high ground -failure hazard
due to reactivation of an existing slump.
Key
Hazard
1 No identifiable landslide hazard (fields Y and Z are
null fields)
2 Hazard from encroaching landslide material
3 Ground -failure hazard from movement of material that has
not previously failed.
4 Ground -failure hazard from renewed movement of previously -
failed material
Process
A
Slumps
B
Debris slides
C
Debris avalanches
D
Debris flows
City of Edmonds October 16, 1979 Page 17
This landslide -hazard classification has been developed to best express the
types and degrees of hazards within the Meadowdale study area. The classification
includes elements from previous classification schemes and elements developed
during this study, and has evolved in response to comments from City of Edmonds
personnel.
Each of the three fields used to represent the various landslide -hazard
_. categories serves a purpose. The first field provides for the desiqnation of
hazardous areas both upslope and downslope from unstable zones. The second field
designates the type of landslide process that presents the hazard, and is useful
for identifying the types of structures which might be susceptible to damage
and the types of damage that might occur. The degree of hazard is expressed in
the third field, which indicates the probability of occurrence within a 25-year
period. This time interval was chosen because it allows the expression of a wide
range of probabilities in a compact, two -digit format, and also because it roughly
corresponds to established time periods used in decisions relative to home
mortgages and the amortization of commercial structures.
Description of the Landslide Hazard Categories
The landslide -hazard map of the Meadowdale study area is presented as
Figure 5 (in pocket). This map is our interpretation of landslide hazards within
the Meadowdale study area based on the geomorphic and geologic information
available to us. This information was obtained from topographic maps, aerial
photographs, surface reconnaissance, and subsurface exploration. We believe
that the accuracy of the map is appropriate to its present scale of 1 inch = 200 feet;
the map should not be enlarged beyond its present scale and the boundaries -
between map -unit areas should not be presumed more accurate than to within a few
tens of feet. As additional information becomes available, it may be appropriate
to refine the map -unit identifiers or the boundaries between some of the map units.
Also, certain land-use/risk-reduction measures that are described in the next
section of this report will result in changes in the degree of hazard for some
of the map units.
City of Edmonds October 16, 1979 Page 18
Class 1 areas are areas of no identifiable landslide hazard using
the landslide -hazard mapping procedure adopted for this study. This does not
necessarily mean that there are no landslide hazards in these areas. Local
topographic and geologic variations, and also extensive modification of areas
by human activities, can result in landslide hazards not identified in this report.
Class 2 areas are areas subject to hazards from encroaching landslide
material. Such areas include areas downslope from active landslides, relatively
flat areas at the base of landslide -prone slopes, and areas near the mouths of
gulleys subject to debris flows. Within the Meadowdale study area, the primary
areas subject to encroaching landslide material are located along the Burlington
Northern right-of-way, along the foot of the steep scarp which defines the eastern
edge of the Meadowdale landslide complex, and along the swales downslope from
some of the gulleys that enter the landslide complex from the east. Within the
Burlington Northern right-of-way the encroachment hazard is from slumps and debris
avalanches, and near the base of the east scarp of the landslide complex the
hazard is from debris avalanches. In the swales downslope from some of the gulleys
entering the eastern portion of the landslide complex the encroachment hazard
is from debris flows. The areas subject to debris -flow hazards are very broad
and poorly defined, due to the gentle coutours of the Swale bottoms and possible
channeling or diversion by cultural features.
Class 3 areas are subject to ground -failure hazards in material that
has not previously failed. Processes responsible for these hazards include debris
slides and slumps that can occur on the steep slopes east of the Meadowdale
landslide complex. Debris slides can be expected in areas sloping more than
15 percent that are underlain by the Whidbey formation or the Double Bluff Drift.
Slumps can be expected in areas immediately upslope from the contact between
the Esperance Sand and the Whidbey formation where those slopes are steeper than
approximately 60 percent. In such areas the horizontal width of the hazardous
area has been estimated by projecting a plane sloping 60 percent upwards into
the slope to the line where it emerges higher on the slope.
City of Edmonds October 16, 1979 Page 19
Class 4 areas are subject to ground -failure hazards due to renewed
movement of previously -failed material. The only identified areas of Class 4
hazards are within the area of the Meadowdale landslide complex. Within that
landslide, two styles of failure have been identified. The portions of the
upper, eastern part of the landslide complex are subject to relatively infrequent
movements of large, relatively coherent blocks of material. This material includes
blocks of the Whidbey formation, the Esperance Sand and, near the north end of the
landslide complex, the Double Bluff Drift. The lower, western portion of the
landslide complex is subject to relatively more frequent movements of smaller
masses of more deformed material. The style of movement of the western portion
of the landslide complex includes aspects of both slumps and earthflows; for purposes
of mapping the process has been designated as slumping.
The landslide hazards described above differ in the amount of risk they
pose for various types of structures. Encroachment hazards can have serious
effects on surface structures but may cause relatively little damage to buried or
overhead utilities. Ground -failure hazards can cause more severe damage to
surface structures and seriously damage subsurface utilities, but unless the
amount of movement is large, may result in only minor damage to overhead utilities.
Ground -failure hazards cause damage to most structures not by movement
but by relative movement. Movement of one part of a structure relative to
another part results in distortion. Typical wood -frame, single-family houses can
tolerate a limited amount of distortion, beyond which damage results. As noted
above, the Meadowdale landslide complex includes some areas that move as relatively
coherent blocks and some areas that involve more internal deformation. The latter
areas are likely to experience greater damage.
City of Edmonds October 16, 1979 Page 20
ALTERNATIVE LAND-USE/RISK-REDUCTION MEASURES
General
In our opinion, the future risk to private and City property due to
landsliding can be significantly reduced by the implementation of land -use controls
and/or certain construction measures. The degree of such risk reduction is described
in a subsequent section of this report. In the list below, the risk -reduction
measures are briefly described and are compared to a "do nothing" alternative.
The discussion of each of these measures addresses their implementation both
within the Meadowdale landslide complex and in the drainage basin adjacent to the
landslide. All of the construction measures involve new construction and could be
applied to all'or part of the landslide at any given time. New construction within
the adjacent drainage basin is also discussed. The engineering qualities of
each measure, including approximate relative construction costs of the various
alternatives, are discussed under the subheading entitled "Engineering Analyses."
No Special Measures
- 'This is essentially a "do nothing" alternative. The procedure
would be to terminate the existing moratorium and allow continued development of
the land within and adjacent to the landslide area according to the "conventional"
permit process. The type of damage experienced in the past would be expected
to continue. An increase in the density of building units would be expected
to result in increased groundwater levels within the active landslide if septic
systems are used for sanitary waste treatment and if no storm sewer system is
constructed. Property -damage within the active landslide area would increase
since areas which are currently undeveloped would be occupied by buildings
or other property improvements. Indeed, we expect that property damage within
the active landslide area would increase even if no additional development
takes place within the area of the active landslide if development in the
adjacent drainage basin continues in the present format.
City of Edmonds October 16, 1979 Page 21
Non -Construction Measures
Non -construction measures include land -use controls to prohibit or
restrict development of land on, or around, the slide area. Examples of this
type of land -use control include the present moratorium prohibiting all building
within the landslide and adjacent areas and application of certain restrictions
to building development proposals. Such restrictions might include restrictions
on the use of dry wells and/or imposition of development density ratios. In
addition, short plat applications or individual building permit applications
could require a site -specific geotechnical evaluation of the effect of the
proposed development on the overall stability of the landslide.
This scheme assumes that land development will continue in and around
the landslide area, and an ultimate increase in building density therefore would
be expected. Even if very conservative (i.e. highly restrictive) land -use measures
are adopted, property damage within the active landslide area probably would not
be reduced. At best, this type of risk -reduction measure could be expected to
slow the rate of increase of property damage.
Construction Measures
Construction measures which will result in landslide -hazard risk -reduction
can be subdivided into four basic groups. These include: 1) retaining facilities,
2) regrading, 3) lowering the water table, and 4) increasing the strength of the
soils within the landslide. The various construction measures considered in our
analyses and the applicability of these measures to improvements within the
active landslide area, or in the adjacent drainage basin, are indicated in Table 2.
An engineering assessment of the various construction measures and an approximate
cost comparison of these measures are presented in a subsequent section of this report.
City of Edmonds October 16, 1979 Page 22
TABLE 2
CONSTRUCTION MEASURES FOR LANDSLIDE-HAZARD/RISK-REDUCTION
Construction Measure Possible Applicability in Meadowdale Study Area li
Within Area of Active Within Other Parts of
Landsliding Drainage Basin
Cantilevered retaining walls or X
crib walls
Pile or caisson walls X
Earth Buttress- X
Regrading X
Sanitary sewers
X X
Storm sewers and surface water controls X X
Subsurface drains X
Soil strength enhancement by grouting X
or compaction
Gravity Retaining Walls: This alternative includes the construction of
conventional gravity retaining walls along the toe of the existing slope adjacent
to the east side of the Burlington Northern Railroad. Gravity walls include
conventional footing -supported cantilevered retaining walls founded within or
beneath the landslide, and also crib walls or bin walls.
City of Edmonds October 16, 1979 Page 23
Gravity walls are appropriate to prevent undercutting of the toe or for
resisting relatively small landslides, but they are not generally suitable for
large unstable masses due to the enormous lateral pressure exerted on the walls.
In our opinion, the circumstances in the Meadowdale study area preclude the use
of conventional gravity walls for general landslide -hazard risk -reduction.
.. Pile or Caisson Walls: Pile or caisson walls consist of large diameter
reinforced concrete piles which are supported in undisturbed native materials
well below the unstable mass to be retained. A common ratio of embedment is
2 feet within the supporting material for every 1 foot above the supporting material.
This type of retaining wall could be located along the toe of the landslide or
at other locations within the area of the landslide where retaining walls are
needed to provide effective slope retention.
Caisson or pile walls are extraordinarily expensive to construct and,
at best, provide retention for soils to only a limited distance upslope from
each retaining wall. If only one retaining wall is installed parallel and
adjacent to the railroad tracks, we believe it would be decades before landslide
movement would be significantly reduced near the head of the landslide area.
Multiple retaining walls would, of course, have the advantage of providing slope
retention at a number of locations within the slide area but would be proportionately
more expensive. We do not recommend the use of pile or caisson walls for landslide -
hazard risk reduction in the Meadowdale study area.
Earth Buttress: Construction of a massive earth buttress adjacent to
the railroad tracks would provide slope retention roughly equal to that
provided by a retaining wall at the same location. The buttress would be difficult
_. to construct from available soils within the area of the landslide due to the high
clay and moisture content of those materials. Like retaining walls, the buttress
would provide retention for only a limited distance upslope from the structure.
The massive dimensions necessary for the buttress would impinge on private
properties that border the existing shoreline. To be effective, the buttress must
be founded in undisturbed native materials below the slide zone and must include
a volume of fill on the order of 1/4 to 1/2 the volume of the total slide mass. In
City of Edmonds
Octobe►- 16, 1979 Page 24
our opinion, the utility and dimensions of such an earth buttress render that
alternative unsuitable for application in the Headowdale study area.
Regrading: The objectives of regrading include elimination of the
disturbed and easily mobilized landslide material, reduction of the driving
force within the landslide mass, and reconfiguring the landslide for the purpose
Of treating localized areas or for control of surface runoff. The form taken
by regrading includes 1) removal of all or part of the landslide material to
create a generally flatter slope configuration, or 2) excavation to create benches
or local regrading to mitigate problems of a smaller scale.
The most significant disadvantage of regrading in the Meadowdale landslide
area is the problem of dealing with existing houses and streets, together with the
problem of what. to do with excavated soil. As noted elsewhere in this report,
the overall slide mass ranges from about 20 to more than 50 feet thick. Removal
of all of the slide material would require excavation of an enormous amount of soil.
Selective removal of material from the landslide would tend to reduce restraint
on the landslide material further upslope and would increase the potential for
renewed movement or upslope retrogression of the landslide. We do not believe that
regrading is applicable as a general landslide-hazard/risk-reduction measure in
the Meadowdale study area.
Sanitary Sewers: Construction of sanitary sewers in and adjacent to
the landslide area will tend to reduce the amount of groundwater within the
landslide, thereby tending to improve the stability of the landslide. As described
elsewhere in this report, a substantial amount of the groundwater within the
landslide is derived from existing septic tank sewage treatment systems within
and near the landslide area.
The disadvantage of installing sanitary sewers within the existing
landslide include the anticipated maintenance required by continuing readjustments
within the landslide mass. In addition, some breakage of the sewer pipes will
probably occur where the pipes cross failure zones, and the leakage of water
into such areas would be a significant negative influence on stability of the
landslide. It is our opinion that installation of sanitary sewers within and
adjacent to the landslide area will have the advantageous effect of lowering
the water table within the landslide and that this effect more than offsets
any potential disadvantages induced by leakage from the sewer pipes.
City of Edmonds October 16, 1979 Page 25
Storm Sevier and Surface Water Controls: The purpose of storm sewers
is the collection of runoff from impervious areas such as roofs, streets, etc.
The efficient control of surface water runoff to minimize infiltration and
surface water inflow from adjacent areas will have the effect of generally
lowering the water table within the landslide. The effect of this water table
reduction would be in addition to the reduction accomplished by construction
of sanitary sewers. As noted above, the effect of lowering the water table within
_. the landslide will be to increase the stability of the landslide and thus will tend
to reduce the magnitude and frequency of landslide movements. Buried storm sewers
are subject to cracking or parting at locations where the pipelines cross failure
zones, as described above for sanitary sewers. We believe that construction
of storm sewers and/or other surface water control facilities in the drainage
basin adjacent to the Meadowdale landslide complex, and to a lesser extent within
the landslide,• would have the advantageous effect of lowering the water table
within the landslide and would thus tend to improve the stability of the landslide.
Subsurface Drains: Subsurface drains include perforated pipelines installed
in excavated trenches and relatively small diameter perforated pipes installed by
drilling at a nearly horizontal attitude to intersect water bearing layers. The
i
purpose of both types of drains is to intercept and divert groundater to
accomplish a lowering of the water table. The effects of this type of drainage
control are significantly more widespread than limited structural or regrading
measures; groundwater intercepted at the head of a landslide usually results
in lowering of the water table throughout the downslope area, thus reducing the
potential risk of movement for the entire landslide area.
Horizontal (drilled) drains.are much less expensive to.construct than
excavated trench drains. In addition, horizontal drains can usually be installed
over a broad area from a single construction location. To function, these drains
must intercept a water bearing layer. Therefore, it is common to experience
a very low ratio of productivity per lineal foot of drain installed as compared
with trench drains. In addition, the efficiency of horizontal drains is considerably
less than trench drains and experience has shown that with time, most horizontal
drains cease to function and need to be replaced.
In the Meadowdale landslide area, it is our opinion that installing
drains at a few locations of known, or suspected, groundwater concentration will
result in a substantial reduction in the groundwater input to the landslide area.
We estimate that construction of interceptor drains in known seepage areas, such
City of Edmonds October 16, 1979 Page 27
TABLE 3
RELATIVE COST COMPARISON, CONSTRUCTION MEASURES FOR LANDSLIDE -HAZARD RISK -REDUCTION
Construction Measure
Estimated Project Cost
Comments
Cantilevered retaining
$2,000,000
Not Applicable
walls (1)
Crib walls (1)
$1,700,000
Not Applicable
Pile or caisson walls (1)
$7,100,000
Applicable - but not
recommended
Earth buttress'(1)
$2,100,000
Not Applicable
Regrading (2)
$2,000,000
Not Applicable
Sanitary sewers (3)
$1,200,000
Effective
Storm Sewers (3,4)
$1,000,000
Effective
Subsurface drains
Trench drains
$ 150,000 (5)
Effective
Drilled drains
$ 120,000 (6)
Effective, but with
functional limitatior
Notes:
(1) Assume wall or buttress length of 3100 feet along railroad tracks with an
average height of 20' above existing railroad grade.
(2) Assumes removal of approximately 250,000 cubic yards of material from that
portion of the landslide complex upslope from 75th Place West. Does not
include costs of relocating streets and houses.
(3) Storm and sanitary sewer project costs include 40 acres within the active
landslide zone, 40 acres within "steep slope" areas, and 120 acres of upland
areas.
(4) The storm sewerage project cost does not include detention facilities or an
offshore outfalI.
City of Edmonds October 16, 1979 Page 28
(5) The estimated cost of trench drains includes 1670 feet of collector drain
and 4030 feet of discharge drain. The estimate does not include easements
or regulatory discharge permits.
(6) The estimated cost of drilled drains includes 4860 feet of collector drain
plus 4030 feet of discharge drain. The estimate does not include easements
or regulatory discharge permits.
Our analyses were performed on a representative cross-section of the
Meadowdale landslide complex located approximately along the section A -A' shown
in Figures 2 and 3. The stability of the landslide varies with time, mainly
due to changes in groundwater conditions, and also with space. At any given
time, the factor of safety for different parts of the landslide complex may vary
by 109 or more, even within individual map units shown on the landslide -hazard
map. As groundwater levels rise, an increasing portion of the area of the
landslide approaches the critical state and some local areas may experience failure
before the cross-section which we analyzed reaches a factor of safety of 1.0
r (i.e.- assumed failure conditions).
The stability analyses were performed using Spencer's Method, which
can be utilized to evaluate any shape of shear surface. The procedure used to
calculate the factor of safety is a limit equilibrium method of slices which
satisfies all conditions of static equilibrium for each slice. The factor of
safety is taken as the ratio of the shear strength of the soil available on the
shear surface to the shear strength mobilized in order to maintain equilibrium.
The specific assumptions made for the analysis of the existing soil profile
and modified profiles are summarized below. The results of the analyses are
shown in chart form as Table 4.
To quantify the existing stability conditions, the existing soil
profile was analyzed recognizing that slope failure occurs when groundwater
levels in the landslide rise to or near the ground surface. A specific non -
circular shear surface location was chosen based on our field explorations and
geologic reconnaissance. Soil strength parameters were adjusted until a factor
of safety approximating 1.0 was attained when the water table was located near
the ground surface. The groundwater level was then relocated to the current
depths indicated by our piezometer readings to determine the approximate
existing factor of safety.
City of Edmonds October 16, 1979 Page 29
TABLE 4: SUMMARY OF RESULTS OF STABILITY ANALYSES
Soil Profile
Existing Conditions(1) At or near
ground surface
Existing Conditions(1) At current
level (2)
Modified: At or near
Retaining Facilities ground surface
Modified: At or near
Regrading ground surface
Modified: 5 feet below
Lowered Water Table current level
Modified: 10 feet below
Lowered Water Table current level
Approximate
Percent Increase in
Factor of Safety
Factor of Safety
1.00
-
1.17 ,:
17
1.09
9
1.04
4
1.33'
33
1.47
47
Notes:
(1) Topographic conditions as shown on maps dated March 1965 provided by the
City. Soil conditions as disclosed by our surface and subsurface investigation,
July - September, 1979.
(2) Groundwater levels as measured on September 10, 1979:'
City of Edmonds October 16, 1979 Page 30
To analyze the effect of a caisson wall or earth buttress located at
the toe of the landslide, it was assumed that the retaining facility would
be extremely rigid and that the shear surface would be redirected to intercept the
ground surface upslope of the retaining facility.
For the regrading alternative it was assumed that the entire surface of
the landslide upslope of 75th Place West was regraded and flattened to a uniform
_. inclination of roughly 15 percent. The shear surface location used for this
alternative was the same as that used for the existing conditions.
To evaluate the effect of lowering the water table, the groundwater level
was uniformly lowered across the landslide by 5 feet and 10 feet below its current
level. The same shear -surface location was used as was utilized for the
existing conditions.
City of Edmonds October 16, 1979 Page 31
CONCLUSIONS
We believe that the original failure of the Meadowdale landslide
complex occurred several thousand years ago as a result of shoreline erosion and
that it continued to be active due to erosion of its toe by shoreline processes.
The construction of the Burlington Northern Railroad and associated shoreline
protection measures was a stabilizing influence on the slide, but without
further stabilizing measures continued movements can be expected to occur as
the landslide adjusts itself toward a more stable internal distribution of stresses.
The past movements have occurred, and in the absence of further stabilizing measures
can be expected to continue to occur, primarily during periods of high groundwater
levels.
Groundwater within the Meadowdale landslide complex is derived from
several sources. Some of the water is natural groundwater recharge from
precipitation falling within the area of the landslide. Additional water is
derived from the septic systems of the houses within the landslide area. However,
the majority of the groundwater within the landslide is derived from groundwater
and surface water sources in the drainage basin east of the landslide complex.
This water enters the area of the landslide as surface water inflow. We estimate
that the amount of groundwater recharge in the Meadowdale landslide complex
is approximately 40% greater than under natural (i.e. pre -development) conditions.
If development within the area of the landslide complex and in the drainage basin
to the east continues in its present format, we estimate that an additional
40% increase in recharge would occur.
Our stability analyses have shown that the overall stability of the
Meadowdale landslide complex is extremely sensitive to the amount of groundwater
within the landslide. As inflow into the area of the landslide increases due
to development in the drainage basin east of the landslide, critical groundwater
levels will be attained more often, and both the magnitude and frequency of
landsliding will increase. As shown in Table 4, a greater increase in
_. stability can be attained by lowering the water table within the landslide than
by any other measures analyzed.
City of Edmonds October 16, 1979 Page 32
Lowering of the groundwater level within the landslide can be achieved
by a system of collector drains near the head of the landslide or by some
combination of sanitary sewers and storm sewers, either within the area of the
landslide complex or within the drainage basin to the east. The relative effects
of each of these measures and their relative costs are summarized in Table 5. The
_-- effects of storm and sanitary sewers within and adjacent to the landslide complex
are additive. Thus, a combination of storm and sanitary sewers within the drainage
basin adjacent to the landslide would result in approximately a 13o increase in
the factor of safety. We believe that an increase in the factor of safety on the
order of 13-15 percent would result in approximately a tenfold decrease in landslide
activity..
If the City chooses to reduce the risk of landslide hazard by one of
- these measures, it should be assumed that settlement and minor readjustments of
the landslide material will continue to occur for a period of decades, albeit
at a reduced rate. These movements will continue to cause damage to existing
and future public utilities and private structures. The cost of maintaining
these utilities as well as the original capital cost of the installation should
I
be considered in any decisions.
Any construction measure for landslide -hazard risk -reduction in the Meadowda7
study area will require additional investigations for the design of the
facilities. Whichever alternative the City chooses for dealing with landslide
hazards in the Meadowdale study area, we recommend that a monitoring program
be implemented to correlate episodes of movement within the landslide complex to
groundwater levels within the landslide and to monitor the effectiveness of the
chosen alternative. The monitoring system should include observations of a
network of survey points together with observations of the water levels in the
piezometers installed during this investigation.
City of Edmonds October 16, 1979 Page 33
TABLE 5
RELATIVE
EFFECTS OF MEASURES TO LOWER WATER
TABLE
MEADOWDALE
LANDSLIDE COMPLEX
Recharge
Water Table
Factor of Safety
Decrease
_Decline (1)
Increase (1)
Cost
(106 ft3/yr)
(ft)
M
(5)
Interceptor drains
2.0
5-7
15
150,000
Storm sewers
Within landslide
0.1
2
1
150,000
complex
Within adjacent
0.9
32
9
850,000
drainage basin
Sanitary sewers
Within landslide
0.4
12
4
200,000
complex
Within adjacent
0.4
12
4
1,000,000
drainage basin
(1) Compared to current, existing conditions as noted on Table 4
City of Edmonds October 16, 1979 Page 34
USE OF THIS REPORT AND WARRANTY
We have prepared this report for use by the City of Edmonds, for
planning purposes. The data and report should be provided to the public, at the
discretion of the City, for informational purposes but not as a warranty of
site -specific surface or subsurface conditions. There are possible variations
in surface and subsurface conditions between the explorations and also with time.
Within the limitations of the schedule and budget for our work, and within
the limits of accuracy of data provided to us by others, we warrant that our work
has been done in accordance with generally accepted practice in this area. No
other warranty, express or implied, is made.
The scope of our work did not include services related to construction
safety precautions and is not intended to recommend or direct construction means,
methods, techniques, sequences or procedures, except as specifically described,
and then only for consideration in design, not for construction guidance.
We appreciate the opportunity of working with you on this very
interesting project. We are available to answer any questions concerning this
report or to provide additional services is necessary.
Very truly yours,
ROGER LOWE ASSOCIATES INC.
Donald W. Tubbs, Geologist
Jon W. Koloski, Associate
JWK/DWT/kc
3 copies submitted
APPENDIX
LANDSLIDE HAZARD INVESTIGATION
MEADOWDALE AREA
EDMONDS, WASHINGTON
Subsurface conditions were explored by means of 7 test borings at
the locations shown on the geologic map, Figure 2. The borings ranged from
41.5 to 150 feet in depth. They were drilled by a truck -mounted drill rig
using a 4-inch hollow stem auger, except for the lower 45 feet of Boring 2
which was drilled by the rotary method.
The explorations were located by our representative who continuously
logged the exploration, examined and classified materials recovered, and
selected locations for obtaining representative soil samples. Soils were
classified in accordance with the Unified Soil Classification System which
is described on Plate A-1. The field logs, modified to reflect the results
of laboratory examination and testing of the samples are presented on Plates
A-2 through A-18.
Relatively undisturbed soil samples were obtained using a 3-inch
0 D heavy duty sampler with 2.5 inch brass liner rings. The heavy duty sampler
consists of a barrel which can be split and disassembled for the removal of
samples. The sampler was generally driven with a 140 lb. hammer falling a
distance of 30 inches, except in holes Nos. 1 and 3 and at depths greater than
50 feet and 35 feet in holes No. 2 and 5 respectively, where a 320 lb. hammer
was used. The sample conditions, recovery and resistance to driving are noted
on the exploration logs.
All samples were sealed in containers to prevent moisture loss, labeled
and taken to our laboratory. Selected samples were tested to determine the field
moisture and density, and for soil strength characteristics. The moisture and
density data are presented on the logs of the exploration.
UNIFIED SOIL CLASSIFICATION SYSTEM
MAJOR DIVISIONS LETTER DESCRIPTIONS
SYMBOL
WELL-r?ACC aU',f LS, 'PA'•f ;-WO
GRAVEL CLEAN GRAVELS GW -II -PE S. LITTLE :A 'a rI'ES
AND
COARSE
GRAVELLY (LI -.E C4'c FeES) GP PoCPLY-7?ACED rPA••f LS. CPA.fL-
GRAINED SOILS WC-ImPES, LITTLE rp 'O Fl•ES
SOILS T SOY GRAVELS GM SILTY W"ILS, C.UYEL-S-O-SILT
CF C"a� FRAC- WITH FINES-IXr,PEs
C14' °.4 .L r CLAYEY GPAVELS, GRAVEL•SAND-Slit
Ci+ •C. a SIEVE 1OF 1' IS) A+CLN'
OF %CES� GC H1ATI:RES
SAND WELL GPLASUCS, GRAVELLY SVCS.
a N
AND CLEAN SANDS SW CED LITTLE C FI'Es
SANDY SOILS ILI^.E CR .vD FINES) Sp pCCpLY--,RA0E3 SL"CS, :AAVELLY S'CS,
,K TM4 5i , LITTLE DI •c Fees
OF -ATERIAL I5
Lxvf4 Tww •c. WPE THAT+ 50%
zoo SIEVE SI:E OF :CARSE FAAC- SANDS SM SILTY SVCS. SAIC-SILT -t XTLRES
T104 its` WITH FINES
`C. a SIEVE
W-ECIASLE AACIHT Sc CLAYEY SACS. SUO-CLAY MIMPES
OF
IWPCMIC SILTS. ANo VET FINE SV05,
ML ROCK FLCA. SILr OR CLAYEY FIrE
FINE SILTS AMS itT:LAYEY SILTS WITH SLIG+YT
GRAINED AND LIQUID LIMIT
LESS
trcPLA% tc curs CF ELCVLL n
SOILS CLAYS THAN 50
GL PLASTICITY, GRAVY CLAYS.AYS. SVOY
CLAYS. SILTY CLAYS, LEAN CLAYS
QL LRGNC V41C SILTS AC.W."4IC SILTY
CLAYS OF LCYI PLASTICITY
MH MRGAN1C SILTS. "MACECUS CH OIATO-
-AGE :IS FirE SANG CA SILTY SOILS
ACRE
SILTS
�, 5C% LIQUID LIMIT
OF - --RTAL I5 AND GREATER CI { IrcpG%vic curs cr etc.. RLAS'tcrr
"ER T'A" �- THAN 50 FAT CLAYS
zoo SIEVE SI:E CLAYS
ClGAMC CLAYS OF "E3fUr TO HIGH
OH PLASTICITY. CRGANIC SILTS
PT PEAT. K"S. SWAW SOILS Wlrr' HIGH
HIGHLY ORGANIC SOILS ORC.41C CarrEWS
NOT_, DUAL SY" S tNC 1CA'E SCPCERLINE SOIL CLLSSIFICATICN
KEY TO SAM PLE DATA
25.2 I 91.7 I 35 BLOWS REQUIRED TO DRIVE
SAMPLER 12 INCHES.
DRY DENSITY, (PCF)
STROKE 30 INCHES.
MOISTLRE CONTENT, (% OF DRY DENSITY) (P INDICATES HES1
DEPTH OR ELEVATION (FEET)
SAMPLE RECOVERY SAMPLER TYPE GRAPHIC LOG
I I SM LETTER SYMBOL FOR
UNDISTURBED HD — 3 INCH SPLIT SOIL TYPE
TUBE SAMPLER
DISTURBED TW — 3 INCH THIN WALL DISTINCT CONTACT
SAMPLER BETWEEN SOIL STRATA
NO RECOVERY SPT — 2 INCH SPLIT GRADUAL CHANGE
TUBE SAMPLER gTEyEEN SOIL STRATA
C — ROCK CORE
B — BLOC DISTURBED BOTTOM OF SCRING
SAMPLE
UNIFIED SOIL CLASSIFICATION SYSTEM
ROGER LOWS RSSOCIATES INC. AND KEY TO SAMPLE DATA
City of Edmonds October 16, 1979 Plate A-2
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER
FEET CONTENT (PCF) FOOT
0
WEST SIDE OF 70TH AVE. W.
BORING ONE 160 FEET NORTH OF 160TH ST. SW
SURFACE ELEVATION 350' (DATUM MLLW)
GRAPHIC
LOG SOIL DESCRIPTION
5
10
5 Jill
14
SW
BROWN SILTY SAND WITH SOME PEBBLES
(LOOSE, DAMP)
FILL
LIGHT BROWN WELL GRADED hEDIL,M TO COAF
SW
SAND WITH SOME PEBBLES (DENSE TO VERY
39
DENSE, DAMP )
15
ESPERANCE SAND
51 Jill
20
39 Jill
25
56 Jill
30
SP
LIGHT BROWN TO GRAYISH LIGHT BROWN POI
53
GRADED MEDIUM SAND WITH OCCASIONAL LEI
35
OF WELL GRADED SAND AND OCCASIONAL LEI
WITH SOME GRAVEL
ESPERANCE SAND
39
-+0
.1o, —
)RLY
JSES
JSES
ROGER LOWE RSSOCIRTES INC.
LOG OF EXPLORATION
City of Edmonds October 16, 1979 Plate A-3
I In7n
-1 _ A A
t Edmonds uc Lo:.er — - , r t a kc n---
BORING ONE CONTINUED
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG
80
c
1 1<
1 11
I 11
12
SOIL DESCRIPTION
SP
LIGHT BROWN TO GRAYISH LIGHT BROWN POO
GRADED MEDIUM SAND WITH OCCASIONAL LEN
OF WELL GRADED SAND AND OCCASIONAL LEN
WITH SONS GRAVEL
44 ����
ESPERANCE SAND
IS
61 In�
10
5.2
102.0
40 ini
5
46 Jill
0
SP
GRADES TO FINE SAND
7.0
97.8
57 Jill
5
80 Jill
0
61
5
37 �I�I
0
RLY
3E S
SE S
ROGER LOWE RSSOCIRTES INC.
LOG OF EXPLORATION
City of Edmonds October 16, 1979 Plate A-5
BOR MG ONE CONTINUED
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG SOIL DESCRIPTION
SILT
;AND
ROGER LOWE ASSOCIATES INC.
LOG OF EXPLORATION
t ^ 1070 D1 , 4.- A c
V I LV U 1 CU111Unub
VC LUGCr
WEST SIDE OF 72ND AVE. W
18 FEET NORTH OF N. ME ADCWDALE RD.
BORING TWO SURFACE ELEVATION 200 FEET
(DATUM MLLW)
DEPTH DRY
BLOWS
IN MOISTURE DENSITY
PER
GRAPHIC
FEET CONTENT (PCF)
FOOT
LOG
SOIL DESCRIPTION
0
SW
BROWN WELL GRADED SAND WITH SOME ORGANIC
MATTER AND SOME GRAVEL; INCLUDES BEDS OF
POORLY GRADED SAND (LOOSE, DAMP)
FILL
5
SW
BROWN TO GRAY WELL GRADED COARSE SAND WITH
SOME GF'AVEL AND OCCASIONAL LENSES OF SILT
(DENSE, WET)
92
ALLUVIUM
10
85 iin
1 15
42/11" lilt
1 20
24 ini
1 25
ML DARK. CRAY SILT WITH SOME CLAY (STIFF TO
VERY STIFF, DAMP TO WET)
62 ini WHICEFY FORMATION
1 30
76 1111
1 35
40 I ( 52
ROGER LOWE ASSOCIATES INC. LOG OF EXPLORATION
City of Edmonds Octcber 16, 1979 Plate A-7
BOR I "IG TWO COPIT I NUED
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG SOIL DESCRIPTION
40
TH
VEL
SILTY
80
ROGER LOWE ASSOCIATES INC.
LOG OF EXPLORATION
CL
DARK GRAY SILTY CLAY (VERY STIFF. DAMP
WITH LENSES OF FINE SAND (MEDIUM DENSE
85 I�l�
MOIST)
45
WHIDBEY FORMATION
44
50
'
SP
DARK GF:AY POORLY GRADED MEDI Ufv1 SAND WI
LENSES OF SILTY SANG AND LENSES OF GRA
(VERY C ENSE , DAMP)
55
64 Jill'NHIDf.'EY
FORMATION
60
89
65
57
ML
70
101 nu
SM
DARK GRAY, GENERALLY PEBBLY SILT.
CLAY AND SILTY SAND (VERY DENSE, DAMP
[`-OLHLE BLUFF DRIFT
75
132 I���
City of Edmonds October 16, 1979 Plate A-8
BORING T1,40 CONTINUED
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG SOIL DESCRIPTION
80
100/3" 1111
ML
DARK GRAY, GENERALLY PEBBLY SILT, SILTY
SM
CLAY AND SILTY SAND (VERY DENSE, DAMP)
DOUBLE BLUFF DRIFT
SILTY GRAVEL, 83 TO 88 FEET
85
"
103/5 Jill
90
95
100
96 Jill
BORING TERMINATED AT 96.5 FEET
ON 8/17/79
ROGER LOWE RSSOCIATES INC.
LOG OF EXPLORATION
City of Edmonds October16, 1979 Plate A-9
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER
FEET CONTENT (PCF) FOOT
0
BOR I PSG THREE
GRAPHIC
LOG
74TH PLACE W.
36' NORTH OF N. MEADOWDALE RD.
SURFACE ELEVATION 100 FEET
(DATUM MLLW)
SOIL DESCRIPTION
S
DARK BROWN SILTY SAND WITH ORGANIC
MATTER (LOOSE, DAMP)
TOPSOIL
SM
DARK BROWN TO GRAY SILTY SAND WITH S(
3
ORGANIC MATTER (LOOSE, I'10IST)
5
LANDSLIDE hiATERIAL
Sp
DARK CRAY SAND AND SILTY SAND (VERY 1
4/15"
TO MEDIUM DENSE, WET) WITH BLOCKS OF
10
DARK GRAY, SomETIMES PEBBLY SILT ANO
CLAY (SOFT TO HARD, DAMP TO MOIST)
LANDSLIDE MATERIAL
4 Jill
15
99.5
3 Jill
2027.9
14 In)
25
8 ini
GRAVELLY SAND, 28 TO 33 FEET
30
GW
DARK GRAY SANDY GRAVEL (VERY LOOSE,
35
35/6" I���
LANDSLIDE MATERIAL
DARK GRAY CLAY (VERY STIFF TO HARD,
CL
G !1
48
WHI06EY FORMATION
lip
WET)
DAMP)
ROGER LOWE RSSOCIRTES INC.
LOG OF EXPLORATION
City of Edmonds October 16, 1979 Plate A-10'
BORING THREE CONTINUED
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG
40
I
i
SOIL DESCRIPTION
1APP )
ROGER LOWE RSSOCIRTES INC.
LOG OF EXPLORATION
City of Edmonds October 16, 1979 Plate A-11
WEST SIDE OF 75TH PLACE W.
BORING FOUR 69 FEET NORTH OF N. MEADOWDALE RC'
SURFACE ELEVATION 67 FEET
(DATUM MLLW)
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FOEET CONTENT (PCF) FOOT LOG
I
3
4
SOIL DESCRIPTION
SM
DARK BROWN SILTY S
MATTER (LOOSE, DA
TOPSOIL
SM
R DARK BROWN TO GRAY
5
ORGANIC MATTER AND
11 �11�
MOIST)
LANDSLIDE MATERIAL
ML
DARK GRAY LAIIINATEI
STIFF TO STIFF, DAI
0
12
LANDSLIDE MATERIAL
5
18 llil
p
33.8
88.4
4
5
35.9
86.9
17
CL
DARK GRAY CLAY CST:
LANDSLIDE MATERIAL
p
DARK GRAY CLAY (HAF
CL
24.9
100.5
68 ����
WHIDBEY FORMATION
5
74 Jill
AND WITH ORGANIC
MP )
SILTY SAND WITH SOME
TRACE OF CLAY (LOOSE,
) CLAYEY SILT (.MEDIUM
V TO MOIST)
FF, DAMP)
0, DAMP)
ROGER LOWE ASSOCIATES INC. I LOG OF EXPLORATION
C : tY Of Edmonds
Octo'--er 16, 1979 Plate A-V:
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER
4EET CONTENT (PCF) FOOT
85
45
BORING FOUR CONTINUED
GRAPHIC
LOG
m
SOIL DESCRIPTION
DARK GRAY CLAY (HARD, DAMP)
WHIDBEY FORMATION
BORING TERMINATED AT 46.5 FEET
ON 8/25/79
ROGER LOWE ASSOCIATES INC. I LOG OF EXPLORAT110%
City of Edmonds October 16, 1979 Plate A-13
75TH AVENUE W.
BORING FIVE 100 FEET SO. OF N. MEADOWDALE
SURFACE ELEVATION 47 FEET
(DATUM MLLW)
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG SOIL DESCRIPTION
0
SW DARK GRAY SAND WITH SOME GRAVEL (LOOSE
TO DENSE, DAMP)
FILL
5
78 Jill
10 CL DARK GRAY CLAY (VERY STIFF TO HARD,
30 DAMP TO MOIST)
LANDSLIDE MATERIAL
15
70 11111 I LENSES OF LIGHT GRAY SAND, 13 TO 18 F CET i
SP LIGHT BROWN FINE TO VERY FIND SAND
20 (VERY DENSE, DRY TO MOIST)
117 WHIOSEY FORMATION
25 1 5.4 I 104.4 I 112 1111
30
t-R
90/6 " 1111
75 1111
40 J I I Lit J
ROGER LOWE ASSOCIATES INC. LOG OF EXPLORATION
UILY UI CUI.IUIIUZ)
BORING FIVE CO1ITMUED
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG SOIL DESCRIPTION
4 69
SP DARK GRAY FINE TO MEDIUM SAND (VEgY DENSE,
MOIST TO WET)
WHIDBEY FORMATION
1 45
65/6" 1111
1 50
70 1111
1 55
87 Jill
1 60
132 11111 I SOME GRAVEL, 57,5 TO 60.5 FEET
I 65
50/1" Jill
70
75 1 DARK GFAY SILTY COARSE TO MEDIUM SAND WITH'
50/6„ SOME GRAVEL (VERY DENSE, DAMP)
DOUBLE BLUFF DRIFT
BORING TERMINATED AT 97.0 FEET
80 ON 8/26/79
F EXPLORATION
ROGER LOWE ASSOCIRTES INC. LOG 0 E L
A . 1F 1979 Plate A-15
li I Ly V 1UI11V IIUZ.
WEST SIDE OF 75TH PLACE W.
125 FEET NORTH OF 158TH AVE. W.
BORIIG SIX SURFACE ELEVATION 113 FEET
(DATUM MLLW)
DEPTH
DRY BLOWS
IN MOISTURE
DENSITY PER
GRAPHIC
FEET CONTENT
(PCF) FOOT
LOG
SOIL DESCRIPTION
0
$W
BROWN GRAVELLY FILE TO MEDIUM SAND (LOOSE,
(LOOSE, DRY)
FILL
ML
LIGHT BROWN CLAYEY SILT WITH LENSES OF
5
FINE SAND (STIFF, DRY)
28
I'�'
LANDSLIDE MATERIAL
10 Sp
BROWN FINE TO MEDIUM SAND (MEDIUM DENSE TO
25 1111 VERY DENSE, DRY TO DAMP)
LANDSLIDE MATERIAL
I 15
I 20
I 25
67 ini
47 �II'� I LENSES OF CLAY AND SILTY CLAY, 18 TO 28
FEET
81/9" "'I
30 50/3 1, Jill GRAVEL, 28 TO 33 FEET
BROWN GRAVELLY SAND (DENSE, DAMP)
35 SW LANDSLIDE MATERIAL
110 (1I'
40 1 �►
ROGER LOWE RSSOCIRTES INC. LOG OF EXPLORATION
-. th_ 1979 Plate A-16
City of Edmonds October 16, 1979 Plate A-17
DEPTH DRY
IN MOISTURE DENSITY
FEET CONTENT (PCF)
0
WEST SIDE OF 75TH PLACE W.
BORI"IG SEVEN APPROXIMATELY 16400 BLOCK
SURFACE ELEVATION 83 FEET
(DATUM MLLW)
BLOWS
PER GRAPHIC
FOOT LOG SOIL DESCRIPTION
DIST
)RY TO
ROGER LOWE RSSOCIRTES INC. I LOG OF EXPLORATION
City of Edmonds October 16, 1979 Plate A-12
BORING SEVEN CONTINUED
DEPTH DRY BLOWS
IN MOISTURE DENSITY PER GRAPHIC
FEET CONTENT (PCF) FOOT LOG
4
59 11111 CL
45
SOIL DESCRIPTION
DARK GRAY CLAY (HARD TO VERY HARD. DRY TO
DAMP)
WHIDBEY FORMATION
BORING TERMINATED AT 41.5 FEET
ON 8/28/79
ROGER LOWE RSSOCIFITES INC. I LOG OF EXPLORATION