10.1
Purpose and Scope
Part 3 of the report presents a general geologic
and geotechnical evaluation of the original three specific
study areas previously mentioned in the Preface of this report.
The emphasis is on evaluating factors that influence soil
stability, and presenting general remedial measures for the
types of slope instability found in the West Seattle, Magnolia/Queen
Anne, and Madrona study areas.
The purpose for our studies and recommendations
regarding stability improvements in these study areas is to
provide the City of Seattle (City) with information that can
be used to prioritize remedial efforts and to develop order-of-magnitude
budgets based on the cost data given in Part 2, Section
8.0 of this report. The remedial measures presented are
intended to be preliminary, with final scopes of work and
corresponding cost estimates based on additional engineering
studies and subsurface explorations.
The purpose described above has been accomplished
in accordance with the following scope of services:
We
field checked the location of the reported landslides in
the original three study areas. During this effort and
an additional field visit, we evaluated the alternatives
for stability improvements in the areas based upon the conditions
observed (slide type, groundwater and surface water conditions,
soil stratigraphy, etc.).
For
each study area, we prepared a description of the topography,
geologic and groundwater conditions, slide types, timing,
and slide locations.
We
divided each study area into smaller Stability Improvement
Areas where landslide activity has been prevalent. For
each smaller area, we evaluated the conditions contributing
to current instability and/or potential future instability.
Based
on the above, we formulated stability improvements for consideration
in the Stability Improvement Areas. The types of improvements
recommended are described in Part 2, which also presents
unit costs relative to the various types of improvements.
The
above scope of work is presented in this part of the report
and is summarized in Table
3-1. The table provides preliminary estimates of quantities
(length, square footage, etc.) related to improvements in
the various areas.
In general, two site visits were made to each
Stability Improvement Area, as indicated above. The first
site visit, actually made prior to formulating the improvement
areas, was primarily to field check the database locations
and make appropriate changes in the database. The second
site visit was for the purpose of formulating general types
of measures that could be considered by the City and/or private
property owners to improve stability and reduce landslide
risk. Specific sites were not evaluated. The stability improvements
listed on Table 3-1
include homeowner education; existing storm drainage facilities
maintenance; storm drainage facilities improvement, as may
be indicated by future observations or studies; subdrainage
systems; fill stabilization; and retaining wall construction.
The number, length, square footage, etc., listed on the table
are rough estimates presented only to formulate order-of-magnitude
budgets. Upon further studies needed to prioritize improvements,
such studies may conclude that the extent or type of recommended
improvements may or may not be needed, or that changes and/or
additions may be advisable.
It should be mentioned here that some landslides
have occurred outside the designated Stability Improvement
Areas. These are usually isolated cases and the improvement
areas were selected for locations where instability was prevalent.
For landslides outside the designated areas, the stability
improvement methods described in Part 2 of this report would
apply, including homeowner education and drainage control.
The stability measures recommended do not consider
the location of property lines and relate to improvements
made on City property, private properties, or both. Since
landslides and areas of potential instability do not obey
property boundaries, improvements are sometimes necessary
on both public and private land to suitably improve stability
in an area. Therefore, the improvements recommended in Part
3 are those that could be made by the City to protect utilities,
drainage features, streets, and other City facilities; and
also those measures or actions to be taken by the City and/or
adjacent property owners to improve stability of an unstable
slope. In the latter case, the City and private property
owners should coordinate efforts to improve stability and/or
provide protection (such as catchment walls) should instability
take place. It is anticipated that some improvements will
be made by the City, while other improvements or protection
will be the responsibility of private property owners.
It should be noted here that there are always
risks of damage to property and structures involving landslides,
for property located on or adjacent to a slope. Property
owners need to accept those risks. Although the recommended
improvements and homeowner education can lead to immediate
or eventual improved slope stability conditions, private property
owners should also obtain professional geotechnical advice
to reduce current risks for their properties.
The analyses and recommendations presented in
Part 3 of this report must be considered only in conjunction
with the Limitations Section
1.5 presented in the Preface of this report.
10.2
Actions by City
In the succeeding sections of Part 3, various
improvement measures and other actions are presented that
we recommend be considered by the City. These actions include:
Providing homeowner education materials regarding
actions private property owners can take to reduce instability.
Maintaining and/or improving storm drainage facilities.
Conducting further detailed engineering studies in
areas of prevalent landslides, including subsurface explorations.
Implementing stability improvements.
Coordinating stability improvements with private
property owners.
Homeowner education is important so that the
public is made aware of the factors that cause landslides
and the steps homeowners should take to improve stability.
Information should be provided to homeowners relative to prudent
construction practices and obtaining professional advice for
improving stability for existing homes, additions, or new
construction. It is particularly important that homeowners
learn that filling on a slope (especially at the top of a
slope), or cutting into a slope (especially at the toe), can
lead to instability and should only be undertaken with proper
advice and consultation with competent geotechnical engineers
or engineering geologists. Even the placement of yard waste
on a slope decreases stability and, therefore, should be properly
composted on flat ground or taken off-site. Homeowners should
also be required to properly maintain and control their on-site
drainage systems and to discharge drainage in accordance with
applicable regulations, since improperly channeled water decreases
slope stability, particularly when concentrated.
In addition to the above, we recommend that
the City continue to conduct neighborhood informational meetings
to facilitate two-way discussion regarding stability matters.
Valid concerns of homeowners should be taken into account
in planning and implementing improvements. We also recommend
that the general public be made aware of a telephone "hot
line" that can be readily reached to report locations
of poor drainage, landslides, or potential instability.
In areas of potential landsliding, it is important
that existing storm drainage facilities be maintained. In
addition, storm drainage improvements could be considered
when indicated by subsequent observations and studies. In
this regard, the City has retained a consulting engineering
firm (Black & Veatch) to evaluate surface drainage systems
throughout the city. The scope of this "Needs Assessment"
included visual observation of the roadway runoff where it
had potential to impact landslide-prone slopes. Their studies
are to be coordinated with the landslide studies presented
herein, with the goal of improving stability conditions.
In the succeeding sections of this report, recommendations
regarding maintaining and/or improving storm drainage facilitates
are subject to the evaluations and recommendations to be made
by Black & Veatch. Therefore, prioritizing and budgeting
relative to surface drainage improvements are beyond this
current landslide study.
As stated previously, the stability improvements
presented in Part 3 are preliminary and for the purpose of
providing the City with information they can use to prioritize
remedial efforts and develop "ballpark" budgets.
Further detailed studies, including subsurface explorations,
should be undertaken by the City to determine final scopes
and design of remedial measures, and more accurate cost estimates.
Geotechnical and other consultants should be used as appropriate.
Implementing stability improvements by the City would consist
of preparing plans and specifications using the data presented
in Part 2 of this report, and observing actual construction
to verify suitable conformance with project requirements.
Since landslides and potential instability cut
across property boundaries, a cooperative effort between property
owners is advisable in obtaining the greatest benefits of
stability improvements. In addition to homeowner education,
previously discussed, the City should facilitate the processing
of permits submitted by private property owners so remedial
work can take place expeditiously to improve stability. Variances
to code requirements should be allowed where needed to improve
stability for private and/or public properties. Temporary
and/or permanent easements on or across City property could
be granted, where allowed by ordinance, such as when needed
to construct protective structures or to allow gravity flow,
in lieu of pumped drainage, for suitably designed drainage
facilities on private properties. Coordination between the
City and private property owners may also include shared costs,
such as by Challenge Grants or Local Improvement Districts
(LIDs).
10.3
Actions by Private Property Owners
Improvement of stability involves actions not
only by the City, but actions by private property owners.
Such actions by private property owners should include accepting
existing conditions and the risks of slope instability. Measures
should accordingly be implemented on private properties as
may be needed to protect and improve stability for existing
property, structures, additions, or new construction. Those
measures to be taken by private property owners are the same
types of improvements presented in Part 2 of this report,
and professional advice should be obtained from geotechnical
and other appropriate consultants regarding the improvements.
Such advice should also be obtained by prospective buyers
of property in slide potential areas.
Stability improvements would include proper
drainage of surface water, including suitable discharge of
roof gutter downspouts. Surface water should not be improperly
channeled to or concentrated on slopes and particularly not
onto adjacent property. Other remedial measures would consist
of properly designed subdrains, site grading, soil retention
systems (walls, soil reinforcement, tieback anchors, etc.),
drilled drains, or other measures as conditions may dictate.
Of particular concern are structures located
above or at the bottom of a potentially unstable slope. Private
property owners should seek professional advice regarding
such measures as underpinning walls and/or tieback anchors
near the top, or catchment/retaining walls at the bottom of
a slope.
Private property owners should take advantage
of the homeowner education materials prepared by the City
or other entities. Cooperation with the City and with adjacent
property owners is also important so that remedial measures
can be coordinated to achieve the greatest benefits of stability
improvement. Private property owners should also notify the
City regarding areas observed with poor drainage, landsliding,
or potentially unstable ground, so that drainage and stability
improvements can be coordinated between City and private property
owners as appropriate.
10.4
Additional Considerations
The contributing factors to instability, as
described for the Stability Improvements section of this report,
include terms such as surface drainage, runoff, storm water
runoff, surface water runoff, etc. Such drainage or runoff
includes that from pavement areas as well as from soil or
vegetated areas. The more pervious the soil, such as sand
and/or gravel, the more that rainfall will infiltrate the
ground, which reduces the amount of runoff. Conversely, for
more impervious soils like silt or clay, runoff will be greater.
Runoff also takes place from vegetated slopes, being greater
for areas of sparse vegetation than for slopes with heavy
vegetation.
Cuts at or near the toe of a slope, or fills
on or near the top, are also contributing factors to instability.
Such factors, particularly where cuts or fills took place
years ago, may still have some influence on the stability
of an area; however, such a factor may or may not be the predominant
cause of recent or future instability. For example, a road
cut area may remain stable for years, yet experience instability
as the direct result of such things as a leaking or broken
pipe, improper drainage from adjacent property, new filling
or excavation on a slope, or other unwitting actions by owners
or adjacent property owners. Each occurrence of instability
requires evaluation to assess the predominant factor or factors
leading to slope failure.
In describing some of the Stability Improvement
Areas, we noted remedial measures of landslides that had recently
been completed or were taking place. However, there are probably
other remedial measures being planned, in progress, or completed
by the City or private property owners that are not mentioned.
Furthermore, we have not mentioned specific locations where
surface drainage improvements have recently been undertaken
or are being planned in conjunction with the "Needs Assessment"
portion of the surface drainage studies by Black & Veatch.
11.0 WEST
SEATTLE
The West Seattle area contains the most documented
landslide events of the three study areas and of the whole
city, as well as one of the two specific areas with the highest
density of landslides, i.e., the Alki Avenue S.W. area. (The
other area with the highest density of landslides is the Perkins
Lane W. area in Magnolia.) In the early part of this century,
West Seattle consisted primarily of summer homes that Seattle
residents used only seasonally. Initially, Alki Avenue Southwest
was constructed on piles around the Duwamish Head to provide
access to the summer beach houses at the base of the Duwamish
Head bluff. Later, Alki Avenue Southwest was filled to create
a permanent roadway, which eliminated shoreline erosion at
the base of Duwamish Head. The City of Seattle annexed the
Arroyo Heights and Seola Beach areas, south of Lincoln Park,
in the 1950s; therefore, instability south of Lincoln Park
prior to the 1950s is not recorded in the City files.
11.1
Site Description
West Seattle is comprised of two linear ridges
separated by Longfellow Creek (refer to Figure
B-1). These north-south ridges and parallel depressions
were shaped by the last glacial ice to occupy this area.
West Seattle is bounded on the east by the Duwamish Waterway
and on the west by Puget Sound. The eastern longitudinal
ridge (Puget Ridge) is bounded on the east by West Marginal
Way aligned between the base of the slope and the Duwamish
Waterway. West of Puget Ridge is Longfellow Creek, which
is one of Seattle s longest and lowest gradient streams.
Pigeon Point represents the northern-most extension of this
lineal ridge. West of Longfellow Creek the ground surface
rises to a maximum elevation of 425 feet (High Point) atop
a broad plateau representing the second longitudinal ridge.
The margins of this broad ridge are steep and drained by several
short and steep streams including Fairmount Gulch, Schmitz
Creek, Fauntleroy Creek, and Seola Creek. This west ridge
is bounded on the west by Puget Sound. Both longitudinal
ridges extend farther south, beyond the city limit.
11.2
Soil Stratigraphy
Soils deposited during the most recent glaciation
of the central Puget Lowland dominate the surface geologic
conditions in West Seattle. Because West Seattle is south
of the Seattle Fault (an east-west-trending reverse fault,
dipping to the south), Tertiary bedrock is shallower in depth
south of the fault, relative to those areas north of the fault.
Tertiary bedrock outcrops sporadically near Alki Point along
the beach and just east of the Alki Point lighthouse. For
the most part, the bedrock is not landslide prone. One shallow
colluvial landslide occurred on the west slope of one of these
topographic bedrock highs.
The primary geologic units in West Seattle are
the Vashon glacial deposits, although older, glacially deposited
and nonglacial soils are present in stream cuts and at lower
elevations. The glacially transported soils consist of all
ranges in particle size, from clay to boulders. They can
be divided into six broad categories based on the environment
in which they were deposited: pre-Vashon glacial deposits,
pre-Vashon nonglacial deposits, glaciolacustrine deposits
(Lawton Clay), advance outwash (Esperance Sand), lodgement
till (Vashon Till), and recessional outwash.
A seventh geologic unit in West Seattle is colluvium,
which is a by-product of the weathering, erosion, and movement
of the previously deposited soils. Colluvium is an accumulation
of eroded soils and landslide debris on moderate or steep
slopes. At some locations, it exists as a thin rind of soft
or loose soil on very steep slopes such as the Duwamish Head
bluff area. When direct precipitation and/or groundwater
seepage saturate colluvium (generally soft or loose), it can
lose strength and fail. Resulting failures occur typically
as either a shallow or deep-seated colluvial slide. Colluvium
mudflows (debris flows) commonly travel for a significant
distance (greater than 50 feet) beyond the toe of the steep
slope and are common throughout the landslide history of West
Seattle. Colluvial landslides also occur where colluvium
on a bench becomes unstable due to water pressures and moves
over the top and down the face of steep bluffs.
11.3
Groundwater
Groundwater plays an important role in slope
instability in West Seattle. There are three general types
of groundwater present in this study area:
Groundwater
perched atop the lodgement till after percolating down through
the relatively permeable recessional outwash near the highest
elevations of West Seattle. (This source of groundwater
has not contributed to instability in West Seattle to the
same extent as the other two types of groundwater identified
below.)
Groundwater
perched atop glaciolacustrine deposits after percolating
through "windows" or cracks in the overlying lodgement
till, and through the relatively permeable advance outwash
sand.
Groundwater
perched on slopes at the contact between the overlying loose
or soft colluvial soils and the glacially overridden soils.
As mentioned earlier, a key stratigraphic marker
for landslide location is the contact between the advance
outwash sand (Esperance Sand) and the underlying glaciolacustrine
silt and clay (Lawton Clay), i.e., "The Contact"
(Tubbs, 1974). The contact
includes interbedded layers of silt, clay, and sand, which
transition between the two geologic units. West Seattle has
the longest trace of this sand-clay contract of any neighborhood
within the City of Seattle (refer to Figure
B-2). Although this contact is pronounced and well exposed
in the Alki area, it extends continuously southward along
the west-facing slope to the Arroyo Heights area. This contact
is also present on the east-facing slope west of West Marginal
Way and on the slope west of Longfellow Creek. This hydrologic
discontinuity produces springs on the flanks of all of the
West Seattle hills.
11.4
Landslide Types
11.4.1
High Bluff Peeloff Landslides
Please refer to Section
4.1.1 for a detailed description of high bluff peeloff
type landslides.
There are no documented high
bluff peeloff type landslides in West Seattle. The main
reason for the absence of high bluff peeloff landslides
in this area is the presence of Harbor Avenue Southwest,
Alki Avenue Southwest, and Beach Drive Southwest along the
shoreline of Puget Sound. These roadways protect the base
of the steep slopes against shoreline erosion along West
Seattle, thereby eliminating undercutting of the bluffs.
South of Lincoln Park, no high
bluff peeloff landslides are documented in this area; however,
the absence of documented landslides may be a reflection
of the relatively recent (1950s) annexation of this area
by the City of Seattle.
11.4.2
Groundwater Blowout Landslides
As previously described, a key
stratigraphic marker for landslide location is the contact
between advance outwash sand and an underlying glaciolacustrine
silt and clay. Groundwater blowout landslides occur at
this contact or other locations where pervious soil zones
with high groundwater pressure influence the ground displacement.
Therefore, the initiation point of earth movement, also
referred to as the headscarp, generally lies on or near
the contact between the pervious soil and the underlying
less permeable soil. Because colluvium is usually involved
in groundwater blowout landslides, it is common to classify
them merely as shallow colluvial landslides. For this reason,
there is an anomalously low incidence of reported groundwater
blowout landslides throughout West Seattle. Figure
B-2 illustrates the locations of documented groundwater
blowout type landslides in West Seattle. For reference,
the sand-clay contact (Tubbs,
1974) is also shown on the map. Nearly all of the groundwater
blowout landslides were initiated at the sand-clay contact.
A high percentage of shallow colluvial landslides were also
initiated at or near this contact, and some may be improperly
classified in the historical records.
11.4.3
Deep-Seated Landslides
Deep-seated landslides were
identified in the database as ground displacement deeper
than about 6 to 10 feet. The plane of movement may be arcuate
or relatively planer and may involve glacially overridden
soils as well as the surficial colluvial soils.
A map illustrating the distribution
of deep-seated landslides in West Seattle is presented on
Figure B-3.
The highest densities of deep-seated landslides in West
Seattle occur along Alki Avenue S.W., Delridge Way S.W.
(23rd Avenue S.W.), S.W. Jacobsen Road, 5900-block of Beach
Drive S.W., S.W. Admiral Way, and the intersection of Chilberg
Avenue S.W. and Boyd Place S.W. With the exception of Delridge
Way, all of the densest concentrations of deep-seated landslides
occur at or near the sand-clay contact, similar to the groundwater
blowout and shallow colluvial landslides. Grading of roadways
by either cutting material from the toe of a slope or placing
fill at the top of a slope may be one of the influences
of the deep-seated failures.
The deep-seated landslides shown
on Figure B-3
in the Alki Avenue area occurred on a topographic bench
formed at the contact between the Esperance Sand and the
underlying Lawton Clay; refer to Figure
3-1. The bench was formed by the erosion, sliding,
and gradual regression of the upper portion of the bluff
composed of Esperance Sand. The mechanism for this type
of landslide is as follows:
-
Landslides from the upper slope deposit
thick colluvium on the bench.
-
As colluvium accumulates on the bench,
it becomes unstable due to groundwater pressure at
the contact between the colluvium and the clay/silt
(Lawton Clay), decreasing the frictional resistance
to sliding.
-
The thick wedge of unstable material
then translates along the lower portion of the bench,
depositing debris (trees, vegetation, and colluvium)
over the top of the bluff and onto the lower slope.
Deep-seated, rotational sliding predominates on the
bench with the slide planes reaching depths as much
as 50 to 60 feet into the thick wedge of colluvium
and slide debris on the bench.
-
The added material on the lower slope
becomes unstable because of several factors, including:
abundant groundwater emerging along the sand-clay
contact, the steep slope angle, and the relative lack
of vegetation on the lower slope. Shallow colluvial
landslides predominate along the steep, lower slope,
and trees from the bench move downslope with the colluvium.
11.4.4
Shallow Colluvial Landslides
Shallow colluvial landslides
occur when loose, mostly heterogeneous soil on a moderate
to steep slope becomes saturated. Commonly, these landslides
result in rapidly moving saturated soil acting as a viscous
fluid that can travel significant distances. In cases where
the travel distance of the slide mass is greater than 50
feet, it was termed a debris flow for purposes of this study
(refer to Section 4.1.4).
A map showing the distribution
of historical shallow colluvial landslides in West Seattle
is presented in Figure
B-4. Shallow colluvial landslides make up 74 percent
of the total reviewed landslides in West Seattle. Although
they typically result from short duration heavy precipitation,
regional groundwater also can be a factor. This is illustrated
by the frequent occurrence of shallow colluvial landslides
in West Seattle close to the sand-clay contact.
The highest concentrations of
shallow colluvial landslides occur along 47th Avenue S.W.,
Atlas Place S.W., S.W. Jacobsen Road, and along the Alki
area of West Seattle. The conspicuous double row of landslides
on the northwest-facing slope of the Alki Avenue area represents
shallow colluvial landslides occurring on two distinct topographic
levels. The southeastern-most row of landslides is located
along the upper bluff, which is composed primarily of overridden
Esperance Sand (advance outwash) (refer to Figure
3-1). The lower or northwesternmost row of shallow
colluvial landslides is located along the lower slope below
the bench. Between the two distinct slopes is the bench
created by ongoing deep-seated landsliding in the thick
accumulations of reworked Esperance Sand (colluvium). Few
of these landslides are reported because they are on forested,
undeveloped property.
11.5
Landslides with Debris Flows
Debris flows are shallow colluvial landslides
and generally consist of rapid movements of saturated soils
that act as a fluid and travel considerable distances. As
mentioned previously, landslides that have runout distances
of greater than 50 feet are considered debris flows in this
study.
Figure
B-5 presents a map showing the distribution of debris
flows in West Seattle. The Alki Avenue area of West Seattle
has the highest density of debris flows in the City of Seattle.
A debris flow typically begins as either a groundwater blowout
or shallow colluvial landslide on a steep slope. These slides
may also initiate as an earth fall of saturated colluvial
debris from a bench onto the lower slope. Debris flows include
not only mud but wood debris and other objects that can act
as projectiles that may cause structural damage to structures
in their path. Structures situated at the toe of the slope
along Alki Avenue are susceptible to this type of landslide
because of their close proximity to the steep slope.
In the vicinity of the 1300 block of Alki Avenue
S.W., an area with several debris flow landslides (1956, 1983,
1997), colluvial landslides on the upper slope near Sunset
Avenue S.W. flowed into a confined, short, steep gully and
down to Alki Avenue S.W. (refer to Figure
B-5). Once in the gully, the slide debris mixed with
additional water in the intermittent stream channel, decreasing
the viscosity and increasing the volume and runout distance
of the debris flow.
11.6
Timing of Landslides
A map showing the historical distribution of
landslides by decade is presented in Figure
B-6. Areas where the landslides are chronologically dispersed
through time in West Seattle include Beach Drive S.W., Alki
Avenue S.W., and Delridge Way (23rd Avenue S.W.). More recent
(post-1960) landslides dominate the 47th Avenue S.W. and Seola
Beach Drive S.W. areas. As previously discussed, these two
areas were not significantly developed until after 1960, so
it is likely that older landslides occurred in these two areas
but were not reported.
11.7
Severe Storm-Related Landslides
A map illustrating the distribution of landslides
in West Seattle during the four most notable landslide winters
(1933/34, 1971/72, 1985/86, 1996/97) is presented in Figure
B-7. The most notable trend in the quantity and distribution
of the severe storm-related landslides in this area is the
high proportion of 1996/97 landslides in West Seattle. The
scarcity of 1933/34 and 1971/72 reported landslides may be
a function of lesser urban development during those time frames
rather than the relative magnitude of the earlier severe storms.
11.8
Potential Slide Areas
The City of Seattle presently regulates development
in steep slope and potential slide areas. Historical landslide
locations and the location of the sand-clay contact were used
by the Department of Design Construction and Land Use (DCLU)
to define the Potential Slide Areas, as described in Section
20.0 of this report. Figure
B-8 illustrates the location of the potential slide areas
with all of the landslides in the database for West Seattle.
About 63 percent of the reviewed landslides in West Seattle
fall within the existing Potential Slide Areas. The obvious
areas where landslides occur outside of the potential slide
areas are the 47th Avenue S.W. area, Seola Beach Drive S.W.
area, and the upper slope along Alki Avenue S.W.
11.9
Stability Improvements
This section presents possible stability improvements
that could be made by the City to protect utilities, drainage
features, streets, and other City facilities. It also presents
measures that could be made by the City and adjacent property
owners to improve the stability of an unstable slope. We
present further comments regarding educating private property
owners on steps they may take to improve stability.
The West Seattle area has been divided into
ten smaller Stability Improvement Areas, where landslide activity
has been prevalent. As shown on Figure
B-9 (Appendix B, Map Folio), the ten areas are as follows:
-
-
-
-
-
-
-
-
-
-
For each area, we summarize the general subsurface
conditions, landslide types and causes, and present actions
that could be considered for improving slope stability. Table
3-1, located following the text in Part 3 of this report,
presents a summary of this information.
11.9.1
23rd Avenue S.W.
In the 23rd Avenue S.W. Stability
Improvement Area, as designated on Figure
B-9, 24 landslides were recorded. Both deep-seated
and shallow colluviual landslides occurred, and a number
of landslides were not identified as to the type. The landslides
in this area have taken place along the west-facing slope
generally between 21st Avenue S.W. and Delridge Way S.W.
at the toe of the slope. Instability in this area was reported
as early as 1914. The most recent landsliding took place
in January 1997, which damaged 23rd Avenue S.W. (one block
east and uphill of Delridge Way) at S.W. Dakota Street.
As a result of the January 1997 landslide, several properties
on the downhill (west) side of 23rd Avenue were also damaged
by the earth movement.
The landslides that occurred
in this area prior to 1960 reportedly were related to grading
of 22nd and 23rd Avenues, presumably caused by cutting into
the slope on the east side of these streets. The instability
that took place following 1960 was generally related to
filling on private properties on the west side of 23rd Avenue,
or cutting into the slope on private properties east of
Delridge Way.
The subsurface conditions in
this area consist of a silt-clay colluvium that is up to
25 feet thick, located over stiff to hard clay. Groundwater
levels are typically high because this area is at and near
the toe of a slope. The sand-clay contact (Tubbs,
1974) has not been mapped in this area. The contributing
factors to instability are the soil conditions on this slope
(colluvium over stiff to hard clay), undercutting or filling
on the slope, and high groundwater levels/seepage. The
landslides were triggered by heavy rainfall that resulted
in surface runoff and infiltration into the slope soils.
To improve stability for 23rd
Avenue S.W. at the Dakota Street right-of-way (not a through
street), a buried, drained, secant-type soldier pile wall
was constructed along the west edge of the street. The
wall length was about 110 feet. Repaving the street east
of the new wall included provisions to control surface drainage.
With wall construction, stability was improved for 23rd
Avenue; however, instability could still occur downhill
from the wall, particularly on private properties where
owners should obtain professional advice for improving stability
on their sites.
Recommended actions in this
area would include homeowner education and storm drainage
systems maintenance and/or improvement, including the improvement
of storm drainage from private properties uphill from 23rd
Avenue. Finger drains could also be considered to improve
stability for the toe of the hillside upslope of 23rd Avenue.
11.9.2
Admiral Way
The Admiral Way Stability Improvement
Area is the east-facing slope situated as shown on Figure
B-9. In this area, a total of 26 shallow colluvial
and deep-seated landslides have been recorded for this area,
beginning in 1917. Some of the landslides occurred on the
steep slope uphill from S.W. Admiral Way, and others took
place on the steep slope downhill. The most recent instability
occurred uphill from Admiral Way in early 1998, which resulted
in the City constructing remedial measures including a rock
buttress near the top of the slope, and a 120‑foot-long,
drained soldier pile and concrete lagging wall (6 to 8 feet
high) along the toe of the slope on the west side of Admiral
Way.
The subsurface conditions consist
of colluvium on the steep slopes overlying glacially overridden
native soils. In some areas, fill may be present, such
as for backyards. The original construction of Admiral
Way likely included some fills along the east side and cutting
along the west. A 6- to 8-foot-high rail (trolley) and
concrete lagging toe wall is present along much of the west
side of Admiral Way. A portion of this wall failed at the
time of the 1998 landslide, and other portions of the wall
are bulging or have been overtopped by slope erosion debris
or previous landslides. The slopes both west and east of
Admiral Way exhibit active signs of creep. Based on available
subsurface information, the colluvium on the slopes is 10
or more feet deep. The sand-clay contact (Tubbs,
1974) extends through this area.
The factors that contribute
to instability in this area are steep topography, relatively
deep colluvium on the slope, high groundwater levels/seepage,
storm water runoff, and cutting and filling. The triggering
mechanism is generally heavy rainfall with surface water
runoff and infiltration.
Stability improvements that
could be considered are trench subdrain installations, wall
construction, storm drainage systems maintenance and/or
improvement, and homeowner education. Subsurface drainage
is probably the most cost-effective method for improving
slope stability in the area. Interceptor trench subdrains
parallel to contours uphill from Admiral Way, or trench
subdrains at intervals perpendicular to contours (finger
drains) could be effective. Such subdrains should extend
through the colluvium and into the glacially overridden
soils. Stronger, higher walls for toe support and increased
catchment area for slide debris could also be considered
to protect Admiral Way. Refer to Table
3-1, for estimated lengths of subdrains and wall that
could be considered for budgeting purposes in this area.
A comprehensive study and improvement to storm drainage
is recommended for east of 35th Avenue S.W. and north of
S.W. Spokane Street. The instability downhill of Admiral
Way occurred mostly on private properties where homeowner
education and prudent development practices should be followed.
11.9.3
Fairmount Gulch
The Fairmount Gulch Stability
Improvement Area consists of a large, steep-sided ravine
that extends from Harbor Avenue to the southwest where Admiral
Way crosses the ravine on a high bridge; refer to Figure
B-9. Eleven landslides have been recorded for this
area, mostly on the east-facing slope of the ravine. Ten
landslides were listed as shallow colluvial with three of
them debris flows, and one was not identified as to type,
although it was likely also a shallow colluvial landslide
based on the database comment. The earliest recorded landslide
occurred in 1937, and instability has been reported through
the years. Only one event involving instability (tension
cracks in backyard) was reported due to the 1996/1997 storm.
The subsurface soils in this
area, based on geology mapping and our experience in this
area (no explorations reviewed), consist of colluvium overlying
glacially overridden soils. The overridden soils consist
of sand over clay, and the sand-clay contact (Tubbs,
1974) is present at lower elevations in the ravine.
The landslides reported in this area are primarily failures
in colluvium and/or yard fills placed by private property
owners. One landslide was reported in 1995 to be related
to road fill placed for Belvidere Avenue S.W. This street
is located in the ravine near the sand-clay contact. The
factors contributing to instability are steep topography,
loose fill and/or colluvium on the slope, high groundwater
levels with associated seepage particularly near the sand-clay
contact, and heavy rainfall (triggering cause) that saturates
the loose soil.
It is recommended that work
by the City to improve stability include maintaining existing
storm drainage facilities and improving them when indicated
by future observations in this area. Springhead drains
installed at known seepage points could reduce infiltration
and saturation of colluvial soils by groundwater springs
and seeps. Homeowner education is recommended to include
providing information regarding prudent construction and
site drainage practices, and obtaining professional advice
for improving stability for existing property, additions,
or new construction.
11.9.4
Harbor Avenue
Sixty-one landslides have been
recorded for the Harbor Avenue Stability Improvement Area.
Most of the landslides were reportedly of the shallow colluvial
type (48), while a few were listed as deep-seated (8) and
groundwater blowouts (5). These landslides have generally
occurred on the easterly- and northerly-facing steep slopes
in this area; refer to Figure
B-9. The earliest recorded landslide occurred in 1916
and instability has occurred continually through the years,
including 1998. A number of landslides occurred in this
area during the 1996/97-winter storm (13) including a large
deep-seated landslide at California Way S.W. and Ferry Ave
S.W., which closed California Way S.W. for several months.
There are three general areas
of instability in this area: the east-facing slope between
Victoria Avenue S.W. and Harbor Avenue S.W., the east-facing
slope between Palm Avenue S.W. and California Way S.W.,
and the north to northwest-facing slope between California
Lane S.W./California Way S.W. and Alki Avenue S.W./Harbor
Avenue S.W. Shallow colluvial landslides and debris-flows
dating back to about 1933 have impacted structures at the
toe of the slope, east of Victoria Avenue. The slope below
Palm Avenue exhibits abundant groundwater seepage near the
sand-clay contact and is the location for two relatively
large deep-seated landslides that occurred in early 1996
and in early 1997: the 1300-block of California Way, and
California Way/Ferry Avenue, respectively. The pavement
along the east side of Palm Avenue was cracked and had settled
at the time of our visit in 1998. The City improved the
stability of the slope in the 1300-block of California Way
by constructing a drainage blanket retained by a 45-degree
earth slope reinforced with geogrids. This repair was the
first use of reinforced slopes with geosynthetics in Seattle.
The City improved the stability of the California Way/Ferry
Avenue landslide using subsurface drainage, crib walls,
grading, and buttressing. A buried, soldier pile wall approximately
110 feet long, was also constructed along the east side
of California Way to improve roadway stability. The structures
at the base of the northerly-facing slope below California
Lane and California Way have been impacted by gradual bluff
regression and sloughing since 1955. Structures on the
bench, in the vicinity of California Lane, have been impacted
by at least two deep-seated landslides.
The subsurface soils in this
area, based on geologic mapping and our experiences in this
area, consist of colluvium overlying glacially overridden
soils. The glacially overridden soils consist of slightly
silty sand (outwash) over silty clay (glaciolacustrine).
The sand-clay contact (Tubbs,
1974) is present at approximately elevation 100 feet
(±20 feet) throughout
the Harbor Avenue Stability Improvement Area. A bench of
variable width exists at the top of the clay unit with roughly
10 to 60 feet thickness of colluvium accumulated from up-slope
sources. The factors that contribute to instability in
the Harbor Avenue area include steep topography, loose colluvium
over glaciolacustrine clay, high groundwater levels/seepage,
and cutting at the toe of the slopes. The landslides reported
in this area typically initiate at or near the bench with
debris traveling down the lower clay slope.
Recommended improvements that
could be considered by the City and private landowners in
this area consist of storm drainage systems maintenance
and/or improvement, road fill replacement, springhead drain
installation at identified seepage points, retaining/catchment
wall installation, trench subdrain installation, and homeowner
education. Surface drainage in the vicinity of California
Lane could be improved in order to reduce infiltration into
the thick colluvial soils along the bench. We recommend
that existing and new drainage facilities installed in the
area by the City or private landowners be checked and maintained
on a regular basis for proper functioning. Interceptor
trench subdrains may be appropriate along the bench area
in the vicinity of California Lane and downslope of Palm
and Victoria Avenues. Consideration could also be given
to removing the fill portion of Palm Avenue and replacing
it with compacted material to reduce settlement and pavement
cracking and resulting surface water infiltration into downslope
soils. Retaining/catchment walls would be effective along
the toe of the lower slope below Victoria Avenue and at
the northernmost tip of the Stability Improvement Area.
Furthermore, installation of springhead drains could be
considered in discrete areas of acute groundwater seepage
along the steep slopes to prevent saturation of the colluvial
soils by spring water. We accordingly recommend that property
owners in this area obtain geotechnical advice regarding
precautions to reduce the risk to properties, including
catchment walls at the base of the slope and surface drainage
at the top of the slope.
11.9.5
Alki Avenue
The Alki Avenue Stability Improvement
Area is a northwesterly-facing slope situated as shown on
Figure B-9.
In this area, a total of 106 landslides (deep-seated, groundwater
blowout, and shallow colluvial) have been recorded in this
area since 1916. Approximately one-third of the landslides
occurred along the upper bluff, just west of Sunset Avenue
S.W. The others occurred along the lower bluff behind the
properties along Alki Avenue S.W. While approximately 33
slides were reported in this area due to the 1996/97 winter
storm, the most notorious landslide occurred in the spring
of 1974 where a large-scale deep-seated event threatened
properties along the 1400-1700 blocks of Alki Avenue S.W.
The subsurface conditions consist
of thick accumulations of colluvium (up to 50 to 60 feet
thick) on a midslope bench and thin rinds blanketing the
steep slopes, as shown on Figure 3-1. Underlying the colluvium
is an upper unit of glacially overridden outwash sand (Esperance
Sand) with glaciolacustrine clay (Lawton Clay) and older,
pre-Vashon silt/clay and sand below. Some fills placed
for roads and residential construction may be present along
the upper sand bluff just west of Sunset Avenue and in the
vicinity of California Lane and Bonair Drive, where the
construction of these streets likely included fills along
the west margins of the roads. Near the 1300-block of Sunset
Avenue, the City installed a buttress fill and a drained
soldier pile and lagging wall to mitigate landslides that
occurred on the upper steep slope just below Sunset Avenue.
Several other remedial measures in this area included crib
walls and soldier pile and lagging walls to protect structures
along the upper bluff, and catchment walls and surface drainage
behind structures at the toe of the lower slope.
The contributing factors to
instability in this area are steep topography, colluvium
on the slope, high groundwater levels/seepage, cutting and
filling, and heavy rainfall and associated infiltration
(triggering mechanism).
With respect to the instability
during the spring of 1974, Shannon & Wilson, Inc. (Shannon
& Wilson) was retained by the City to conduct a geotechnical
study of the area. Based on geologic reconnaissance and
subsurface borings, a report dated July 1975 recommended
two conceptual preliminary design alternatives. One alternative
was to design a large earth buttress (including subdrains)
on the bench. The other alternative was to construct a
large, tied-back cylinder pile wall on the bench in conjunction
with trench subdrains. Because of the great depth of colluvium
on the bench, such measures to improve stability would be
extensive and expensive. Upon further exploration and evaluation
in 1999, a scheme of horizontal drains and deep trench drains
was chosen to increase stability of this slope and particularly
the bench area. To help fund this work, the City applied
for and received a Federal Emergency Management Agency (FEMA)
grant. Although some improvement in stability conditions
is anticipated above and below the bench area, some risks
of instability would still be present. Property owners
above and below the bench area would still need to seek
geotechnical advice and take precautions to reduce the risk
to their properties.
Other improvements that the
City could consider consist of storm drainage systems maintenance
and/or improvement, subdrain and springhead drain installations
along the bench area outside the project area described
in the preceding paragraph, and homeowner education. Homeowner
education is probably the most cost-effective method for
improving slope stability in this area. Property owners
in the Alki Avenue Stability Improvement Area should avoid
making improper cuts and fills, maintain existing drainage
systems, seek geotechnical advice, and take precautions
to reduce risk to their properties.
It is recommended that the
City consider evaluating, repairing, and maintaining existing
City-owned drainage pipes that have been installed over
the years in this area (a drainage map is available in City
files). Furthermore, it is recommended that the City coordinate
efforts (expeditious processing of permits or other cooperative
effort as described in Sections
1.5 and 10.2 in this report) with
private property owners along Alki Avenue, relative to building
catchment walls along the toe of the slope for protection
of the structures.
11.9.6
Boyd Place/Chilberg Place
The Boyd Place/Chilberg Place
Stability Improvement Area consists of a west-facing steep
slope, as indicated on Figure
B-9. Seven landslides are recorded for this area, mostly
in the vicinity of the Boyd Place S.W. and Chilberg Place
S.W. intersection. Three landslides were listed as shallow
colluvial and four, at the Chilberg/Boyd intersection, were
listed as deep-seated. The earliest recorded landslide
occurred in 1964 and consisted of a setdown along the Boyd
Place right-of-way. Other landslides in the vicinity of
this intersection, along Boyd Place, have occurred in 1971,
1974, and 1997. During the 1997 earth movement, and probably
as a result of this instability, a water main ruptured,
exacerbating the situation. Remedial measures included
an 85-foot-long wall installed along the west side of Boyd
Place to retain the road fill and a 55-foot-long wall was
installed along the east side of Boyd Place to retain the
cut slope. These two walls consisted of soldier piles with
concrete lagging. An 83-foot-long reinforced concrete retaining
wall was also constructed along the downhill side of Chilberg
Place to provide support for that roadway. The City also
installed catch basins and other drainage improvements in
the vicinity.
The subsurface conditions in
this stability improvement area generally consist of colluvium
overlying glacially overridden native soils. In some areas,
existing fill is present, such as for backyards and roads.
The glacially overridden soils consist of outwash sand overlying
glaciolacustrine silt and clay. The sand-clay contact is
located in the vicinity of the intersection of Chilberg
and Boyd Place. Associated groundwater seeps and springs
exist in this area.
The factors that contribute
to instability in this area are steep topography, abundant
groundwater seeps and springs associated with high groundwater
levels, storm water runoff, and cutting and filling. The
triggering mechanism is generally heavy rainfall with surface
water runoff and infiltration into downslope soils.
It is recommended that actions
by the City in this area consist of maintaining and/or improving
storm drainage systems, particularly in areas outside of
the 1997 Chilberg/Boyd Place project area. Homeowner education
is also recommended.
11.9.7
Jacobsen Road
The Jacobsen Road Stability
Improvement Area is a west-facing slope situated as shown
on Figure B-9.
In this area, a total of 18 landslides (deep-seated, shallow
colluvial, and groundwater blowout) are recorded in the
database since 1933. Some of the landslides occurred on
the steep slope on the east side of S.W. Jacobsen Road,
and others, including several deep-seated landslides, have
occurred on the steep to moderate slope along the west side
of Jacobsen Road. The most recent instability occurred
downhill from Jacobsen Road in early 1997, which resulted
in severe structural damage to two residences west of Jacobsen
Road. Remedial measures have been planned by private property
owners to improve slope stability and repair the damaged
structures. The City placed an asphalt curb to prevent
surface water from infiltrating the slope soils west of
Jacobsen Road.
The subsurface conditions consist
of colluvium on the steep slopes overlying glacially overridden
native soils. In some areas, existing fills may be present,
such as for residences along the west side of Jacobsen Road.
The original construction of Jacobsen Road likely included
some fills along the west side and cutting along the east.
The sand-clay contact with its associated groundwater seepage
exists along the downslope side of the southern portion
of Jacobsen Road, and crosses to the uphill side of Jacobsen
Road to the north. The slopes on both sides of Jacobsen
Road exhibit signs of soil creep. Based on available subsurface
information, we estimate that the colluvium on the slopes
is 10 or more feet deep.
The factors that contribute
to instability in this area are the steep topography, relatively
deep colluvium on the slope, high groundwater levels/seepage,
and cutting and filling. The triggering mechanism is generally
heavy rainfall with surface water runoff and infiltration.
Stability improvements that
the City could consider consist of surface drainage maintenance
and/or improvement, interceptor trench subdrain installation,
and homeowner education. Surface drainage is probably the
most cost effective method for improving slope stability
along the west side of Jacobsen Road. An interceptor trench
subdrain along the east (upslope) side of Jacobsen Road
may be appropriate, unless a suitably functioning subdrain
is already in place. Installation of curbs and gutters
to prevent surface water from Jacobsen Road from flowing
onto and infiltrating the downslope areas west of the roadway
could also be considered. Information should be provided
to property owners regarding proper cutting and filling,
and controlling their on-site drainage systems.
11.9.8
Beach Drive/Atlas Place
The Beach Drive/Atlas Place
Stability Improvement Area, as shown on Figure
B-9, consists of the following: 1) an upper, west-facing
steep slope between 49th and 50th Avenue S.W. (east of Atlas
Place) and Atlas Place S.W.; 2) a bench approximately 300
feet wide (upon which Atlas Place is constructed); and 3)
a lower, west-facing moderate slope west of Atlas Place
that extends down to Beach Drive S.W. Twenty-five landslides
have been recorded for this area. Six landslides were listed
as deep-seated and the others were the shallow colluvial
type. The earliest recorded landslide occurred in 1927,
and instability has been reported through the years. Four
landslides occurred during the winter storm of 1996/97,
including a deep-seated event in the 5900-block of Atlas
Place.
The subsurface soils in this
area, based on geologic mapping and our experience in this
area (no explorations reviewed), consist of colluvium overlying
glacially overridden native soils. The overridden soils
consist of sand overlying clay, and the sand-clay contact
(Tubbs, 1974) is present
at roughly the same elevation as the bench. The slope instability
reported in this area is located along the steep slope west
of Atlas Place, along the steep slope east of the 6500-block
of Beach Drive, and along the west shoulder of Beach Drive.
The landslides that have been
reported in this area occurred primarily in colluvium and/or
road cuts and fills for both Beach Drive and Atlas Place.
Instability along the west margin of Beach Drive appears
to result from fills placed during the construction of the
roadway. Ponding water and road-settlement were observed
along Beach Drive during our field reconnaissance. Shallow
colluvial landslides along the east (uphill) side of Atlas
Place appear to be the result of cutting into the slope
without any slope retention measures. Surface water is
also contributing to instability between Beach Drive and
Atlas Place. The City placed an asphalt curb along the
west side of Atlas Place to prevent surface water from infiltrating
the downslope areas. In summary, the factors contributing
to instability are the steep topography, cutting and filling,
surface water, and high groundwater levels/seepage.
Actions the City could consider
consist of improvement of the Atlas Place street grade with
curbs, gutters, and storm drainage facilities; removal and
replacement of existing loose soils along the west side
of Beach Drive, and education of property owners in this
area. Improvement of the Atlas Place street grade would
include the retention of the cut-slopes, a possible interceptor
trench subdrain along the centerline of the roadway, and
provisions for surface drainage along the full length of
the roadway. Springhead drains would be effective in capturing
groundwater seeps and springs along the cut slope east of
Atlas Place. Improving stability of Beach Drive could include
removal of the existing fill soils and replacement with
lightweight, structural fill material. It is recommended
that homeowner education include proper methods for controlling
on-site drainage systems and discharging drainage in accordance
with City regulations.
11.9.9
47th Avenue S.W.
The 47th Avenue S.W. Stability
Improvement Area is a steep, west-southwest-facing slope
situated as shown on Figure
B-9. In this area, one deep-seated, one groundwater
blowout, and 19 shallow colluvial landslides have been recorded
since 1955. Some of the landslides occurred on the steep
slope uphill from 47th Avenue and others took place on the
steep slope downhill of 47th Avenue. Others occurred in
the vicinity of Maplewood Place S.W. (private road) located
near the southern edge of this stability improvement area
The most recent instability took place at the intersection
of 47th Avenue and Maplewood Place during the 1996/97 winter
storms. This resulted in the City constructing remedial
measures, including a gabion wall on the east side of 47th
Avenue, a soldier pile and lagging wall along the west side
of Maplewood Place, and drainage improvements.
The subsurface conditions consist
of colluvium on the very steep slopes overlying glacially
overridden native soils. Fills are present in some areas
such as for residential backyards, based on the landslide
descriptions. The overridden native soils consist of limited
occurrences of glacial till overlying outwash sand with
glaciolacustrine clay below. The sand-clay contact (Tubbs,
1974) is present east of 47th Avenue at elevation 170
feet (±30 feet). The landslides that have been reported
in this area are primarily failures in colluvium resulting
from surface water runoff and groundwater seepage near and
downslope from the contact. Numerous groundwater seeps
and hydrophitic vegetation exist along the east (uphill)
side of 47th Avenue.
The factors that contribute
to instability in this area are steep topography, improper
cutting and filling, high groundwater levels with associated
seepage particularly near and downslope from the sand-clay
contact, and improperly directed surface water. For example,
improper cutting into the toe of the slope on both private
and public properties, or private utility failures (water
and sewer lines) reportedly influenced approximately five
of the recorded slides in this Stability Improvement Area.
Stability improvements that
we recommend the City and private property owners consider
are surface drainage systems maintenance and/or improvement
and homeowner education. The City could consider placing
a curb/gutter along the west side of 47th Avenue S.W. to
prevent infiltration of surface water into downslope areas,
particularly upslope of Maplewood Place, a private road.
Furthermore, it is recommended that the City facilitate
the processing of permits regarding design, access, and
construction efforts with private property owners along
Maplewood Place, with respect to catchment wall construction
along the toe of the cut slope for protection of the structures.
A soldier pile retaining wall could also be considered along
the west margin of 47th Avenue upslope of the Maplewood
Place dead-end to improve stability for the street. It
is recommended that homeowner education emphasize the need
to obtain professional advice before cutting and/or filling
along any slopes. Private property owners in this area
should control their on-site drainage systems and discharge
drainage in accordance with regulations, since improperly
channeled water decreases slope stability.
11.9.10
Seola Beach
The Seola Beach Stability Improvement
Area consists of a moderately steep to steep-sided ravine
that extends from Puget Sound to the north-northeast for
approximately one mile; refer to Figure
B-9. All of the landslides recorded in the database
for this area are on the west side of the ravine. The east
side of the ravine is outside of the Seattle City Limits.
Along the west side, a total of six landslides (shallow
colluvial and deep-seated) have been recorded, beginning
in the spring storm of 1986.
The subsurface conditions based
on geologic mapping and our experience in the area (no explorations
reviewed) consist of colluvium overlying glacially overridden
outwash sand and gravel. There is no lacustrine clay exposed
in this area below the outwash sand and gravel. Therefore,
the sand-clay contact is not mapped in this area. The landslides
that have been reported in this area primarily occurred
in colluvium and/or yard fills placed by private property
owners. One landslide/debris flow was reported in 1986
to be related to the rupture of a sewer main on the upper
plateau, north of the south end of Seola Beach Drive S.W.
The runout of debris reached Puget Sound.
The factors contributing to
instability are moderately steep to steep topography, private
backyard fills and/or colluvium on the slope, and heavy
rainfall (triggering cause) that saturates the loose soil
and causes failure.
In the long-term, there do
not appear to be practical remedial measures that the City
could take to prevent the natural occurrence of landslides
in this area other than homeowner education.
12.0 Magnolia/Queen
Anne
While Magnolia and Queen Anne are two distinct
topographic highs, they share similar geology conditions and,
therefore, are treated as a single study area. Perkins Lane
West, located along the southwestern margin of Magnolia, is
similar to Alki Avenue in West Seattle in that it contains
a very high density of historical reported landslide events.
12.1
Site Description
Magnolia and Queen Anne are two distinct topographic
highs separated by Interbay, a north trending linear depression
(refer to Figure
B-10). Magnolia, offset north with respect to Queen Anne,
reaches a maximum elevation of 375 feet. Queen Anne, similar
in size to Magnolia, reaches a maximum elevation of 400 feet.
The area is bordered by Puget Sound and Elliot Bay to the
west and southwest, the Lake Washington Ship Canal to the
north, Lake Union to the east, and downtown Seattle to the
south. Steep slopes surround both Magnolia and Queen Anne.
The bluff along the west side of Magnolia, extending from
Smith Cove to the Lake Washington Ship Canal, is locally armored
against wave action and is the steepest slope in Magnolia.
Kinnear Park and the slope west of Aurora Avenue are among
the steepest slopes on Queen Anne.
12.2
Soil Stratigraphy
Soils deposited during the most recent glaciation
of the central Puget Lowland dominate the surface and subsurface
geologic conditions in the Magnolia and Queen Anne study area.
Because Magnolia and Queen Anne lie north of the Seattle Fault,
Tertiary bedrock is buried below roughly 3,000 feet of glacial
and non-glacial sediments.
The primary geologic units involved with landsliding
in Magnolia and Queen Anne are the Vashon glacial deposits.
The glacial soils consist of all ranges in particle size from
clay to boulders and may be divided into four broad categories:
glaciolacustrine deposits (Lawton Clay), advance outwash (Esperance
Sand), lodgement till (Vashon Till), and recessional outwash.
Colluvium is also present along the lower portions
of the hillsides in the Magnolia and Queen Anne study area.
Particularly thick accumulations of colluvium occur along
the Perkins Lane West area of Magnolia. Colluvium also forms
a thin rind on steep slopes all around Magnolia and Queen
Anne.
12.3
Groundwater
Groundwater plays a key role in slope instability
in Magnolia and Queen Anne. The contact between advance outwash
sand and underlying glaciolacustrine silt and clay is exposed
in slopes around both Magnolia and Queen Anne. Prominent
springs associated with this contact occur throughout these
areas including Perkins Lane W., Kinnear Park, 15th Avenue
W., and Westlake Avenue N. Figures B-11 through B-19 illustrate
the location of the sand-clay contact.
12.4
Landslide Types
12.4.1
High Bluff Peeloff
High bluff peeloff-type landslides
occur in only a few discrete areas in the City of Seattle.
A map showing the distribution of high bluff peeloff landslides
in the Magnolia and Queen Anne study area is presented in
Figure B-11.
Areas where the slopes are near vertical resulting from
either wave action at the base of the slope or the presence
of resistant lodgement till, or both, are present in Kinnear
Park, Lawtonwood, and along the southwestern shoreline of
Magnolia. With the exception of Kinnear Park and portions
of Perkins Lane W., there is little or no armoring along
the toe of the slope below the high, steep bluffs. The
high bluff peeloff landslide located at the northern tip
of Magnolia likely occurred as a result of undercutting
by wave action at the base of the bluff. In 1997, a high
bluff peeloff landslide occurred along a short section of
steep bluff east of the northern portion of Perkins Lane.
The high bluff peeloff landslides along the southwest margin
of Magnolia occurred on the steep bluff above (east of)
Perkins Lane W. The very steep, bare bluff south of the
southern end of Perkins Lane West has a long history of
high bluff peel-off type landslides, but the City files
do not have information on these events because they generally
have little effect on structures or transportation routes.
12.4.2
Groundwater Blowout Landslides
A map showing the distribution of groundwater
blowout landslides in the Magnolia and Queen Anne area is
presented in Figure
B-12. The contact between the advance outwash (Esperance)
sand and the glaciolacustrine silt and clay (Lawton Clay)
is also shown. As stated previously, without accurate reporting
and analysis of the landslide event, it is difficult to
distinguish between a shallow colluvial landslide and a
groundwater blowout landslide. Several landslides described
in the historical records as shallow colluvial landslides
may, in fact, be groundwater blowout landslides. In Magnolia,
the Works Progress Administration (WPA) completed several
projects designed to capture and redirect groundwater for
slope stabilization purposes. The WPA projects are marked
on Figure B-10
with a pick and shovel symbol.
12.4.3
Deep-Seated Landslides
A map illustrating the locations of deep-seated
landslides in the Magnolia and Queen Anne areas is presented
on Figure B-13.
The highest density of deep-seated landslides is located
along the west side of Queen Anne and Perkins Lane W. Along
the west side of Queen Anne, several deep-seated landslides
were reported, including a very large area of instability
that was active from 1951 to 1956 in the vicinity of 12th
Avenue W. and W. Blaine Street. The Perkins Lane W. landslides
generally occur below the bluff, in a relatively thick colluvial
wedge as shown on the Idealized Geologic Conditions West
Magnolia profile, Figure
3-2. The colluvial wedge overlies a hard surface of
Lawton Clay that commonly slopes toward Puget Sound. This
contact between the colluvial wedge and the hard Lawton
Clay creates groundwater conditions conducive to landsliding.
12.4.4
Shallow Colluvial Landslides
Figure
B-14 shows the distribution of shallow colluvial landslides
in Magnolia and Queen Anne. It is our opinion that groundwater
along southwest Magnolia and southwest Queen Anne significantly
contributes to shallow colluvial landslides as well as groundwater
blowout landslides. Furthermore, based on the spatial distribution
of shallow colluvial and groundwater blowout landslides
along the southwest margins of Magnolia and Queen Anne,
the flow direction of groundwater perched atop the glaciolacustrine
silt and clay may be toward the southwest. Landslides plotted
from the database are conspicuously absent from the north
margins of both Magnolia and Queen Anne even though the
sand-clay contact surrounds both hills. Many landslides
have occurred in Discovery Park, but these were not reported
because they have little to no affect on structures or transportation
routes.
The east side of Queen Anne
represents an area where proper development can increase
the stability of a hillside by incorporating proper buttressing
and consequent drainage improvements. For example, the
undeveloped steep slope west of Westlake Avenue N. is susceptible
to landsliding resulting from uncontrolled drainage. Where
several condominiums were recently built along Westlake
Avenue N., Dexter Avenue N., and Aurora Avenue North, the
potential for shallow colluvial sliding has been reduced
substantially because of the incorporation of tied-back
retaining walls, subsurface drainage, and surface drainage
improvements.
12.5
Landslides with Debris Flows
A map showing the distribution of debris flow
landslides in the Magnolia/Queen Anne area is presented in
Figure B-15.
Areas where debris flows are common include Kinnear Park,
Perkins Lane W., and along Magnolia Way W. Near Perkins Lane
W., the landslides with debris flows generally originate in
the depressions along the undulating slope crest of the upper
lodgement till bluff. Near Kinnear Park and Magnolia Way,
steep slopes with relatively unimpeded runout zones dominate
these areas.
12.6
Timing of Landslides
A map of Magnolia and Queen Anne showing the
distribution of landslides by decade is presented in B-16.
It illustrates that both the west and east sides of Queen
Anne and the Perkins Lane W. area of Magnolia are chronic
landslide areas.
12.7
Severe Storm-Related Landslides
A map illustrating the distribution of landslides
in Magnolia and Queen Anne during the four most notable landslide
winters ( 1933/34, 1971/72, 1986/87, and 1996/97) is presented
in Figure B-17.
The most notable trend in the quantity and distribution of
the severe storm-related landslides in this area is the large
number of 1996/97 landslides in Magnolia. Conversely, while
the 1933/34 precipitation year is believed to be comparable
to that of 1996/97 (based on information received from the
City), very few 1933/34 landslides are documented in the database
in Magnolia. The severity of the 1933/34 storm was partially
responsible for the large number of WPA projects in Seattle
(notice the proximity of the 1933/34 events to the WPA project
locations along Perkins Lane W. and Westlake Avenue N. on
Figure B-10);
therefore, the landslides resulting from this storm may not
be sufficiently documented.
12.8
Potential Slide Areas
A map illustrating the coincidence of historical
landslides in the Magnolia/Queen Anne study area with the
potential slide areas is presented in Figure
B-18. Approximately 81 percent of the historical landslides
in the Magnolia/Queen Anne area fall within the currently
mapped Potential Slide Areas, as described in Section
20.0 of this report. Landslides outside of the Potential
Slide Area occurred along the upper and lower slopes along
the northern portion of Perkins Lane W. in Magnolia and along
the east flank of Queen Anne, west of the existing Potential
Slide Areas as indicated in City documents.
12.9
Stability Improvements
This section presents possible stability improvements
that could be made by the City to protect utilities, drainage
features, streets, and other City facilities in the Magnolia/Queen
Anne area. Furthermore, this section includes measures that
could be made by adjacent property owners in conjunction with
the City to improve the stability of an entire landslide or
unstable slope. We further present comments regarding educating
private property owners on steps they may take to improve
stability.
The Magnolia/Queen Anne area has been divided
into nine smaller Stability Improvement Areas where landslide
activity has been prevalent, in order to describe various
improvements and homeowner education suggestions. As shown
on Figure B-19
(Appendix B, Map Folio), the nine areas are as follows:
-
-
-
-
-
-
-
-
-
For each area, we summarize the general subsurface
conditions, landslide types and causes, and present actions
that could be considered for improving stability.
12.9.1
Perkins Lane North
The Perkins North area, located
as shown on Figure
B-19, is notorious for instability. It consists of
those properties in and north of the 1900-block of Perkins
Lane W. Properties and instability south of the 1900-block
are presented subsequently under the Perkins South Stability
Improvement Area.
In the Perkins Lane North area,
111 landslides have been reported. They have occurred throughout
the years, being first recorded in 1930 and extending through
January 1998. All four types of landslides have been recorded:
high bluff peeloff (11), groundwater blowout (4), deep-seated
(40), and shallow colluvial (56). The high bluff peeloffs
and the groundwater blowouts have been recorded primarily
on the uphill side (east) of Perkins Lane. The other two
types of landslides (deep-seated and shallow colluvial)
have reportedly occurred on both sides of Perkins Lane.
On a number of occasions, landsliding has damaged the roadway
and frequently debris has come down onto the lane. A number
of houses on both sides of Perkins Lane have been destroyed
by landslides.
In the 1900-block along Perkins
Lane (south end of this designated area), a 110-foot-long
portion of the lane was rebuilt with lightweight fill material
(bottom ash from Centralia, Washington) in 1983. This work
was contracted by homeowners in this area in order to repair
a landslide that destroyed a portion of Perkins Lane and
prevented vehicle access to properties to the south. A
deep subdrain trench was incorporated into this repair effort.
Perkins Lane is located at
the western edge of Magnolia, overlooking Puget Sound (refer
to Figure B-19).
The lane, reportedly constructed in 1926 and 1927, is situated
on an uneven midslope bench. From the top of Magnolia Bluff
to the east, near the location of Magnolia Boulevard W.,
the ground surface slopes steeply to precipitously down
to the west. The midslope terrace on which Perkins Lane
is built slopes moderately to steeply down to the Puget
Sound shoreline on the west. A majority of the shoreline
beaches are protected by rock seawalls or concrete bulkheads.
The right-of-way of Perkins Lane is normally 40 feet wide
(locally 60 feet); however, the asphalt-paved lane is rarely
wider than 20 feet and no sidewalks are present. Drainage
ditches and catch basins are commonly included in the paved
section. To the south of W. Raye Street (approximate center
of this designated improvement area) a rail and concrete
lagging toe wall, about 4 feet high, is locally present.
The subsurface conditions in
this area are illustrated on Figure
3-2, Idealized Geologic Conditions, West Magnolia Bluff.
As shown, there are five geologic units; however, not all
the units are present everywhere and may not be of similar
thickness as indicated. Near the south end of this improvement
area, Vashon till is exposed in the bluff, but toward the
north, the till is absent and advance outwash sand dominates
the hillside. The elevation of the sand-clay contact varies
along Perkins Lane. Draped over much of the hillside is
colluvium (relatively loose), which is commonly thicker
at and to the west of Perkins Lane. On the steep, unvegetated
portions of the bluffs, soil loosened by weathering is present.
With respect to groundwater
seepage, that which occurs at the contact between the recessional
outwash (not always present) and Vashon till is minor.
The more prolific springs emanate from the sand/clay contact.
Seepage can also occur from pervious sand layers within
the till or clay units.
The primary factors that contribute
to instability in this area are steep to moderately steep
slopes, colluvium or weathered soil on the slopes, and high
groundwater levels with associated seepage near the sand/clay
contact. The predominant triggering mechanism is heavy
rainfall with storm/surface water runoff and infiltration.
As a result of the 1996/1997
storms, 16 landslides were identified by the City between
the 1900 and 3400 blocks of Perkins Lane W. As a result
of these landslides, the City contracted for design and
construction of remedial measures, which were made in the
latter part of 1997. The stability improvements consisted
of drainage improvements, rock buttresses, and catchment/
retaining walls. The drainage improvements included finger
drains, intercept trench subdrains, springhead drains, and
directional drains. The improvements apparently are generally
performing as anticipated, although some additional effort
is recommended below.
Additional stability improvements
that could be considered to protect the lane are catchment/retaining
walls at two locations: 3400-block and 2800-block south
of W. Barrett Street. Additional finger drains may be appropriate
in the 2800-block and 2300-2400 blocks. It is recommended
that existing storm drainage facilities be maintained, possibilities
for improving drainage explored, and homeowner education
take place. In particular, drainage from private properties
should be suitably controlled so as not to reduce stability.
12.9.2
Perkins Lane South
This Stability Improvement
Area consists of the 1700 and 1800 blocks along Perkins
Lane W., extending from Magnolia Boulevard W. (to the east)
down to the shore of Puget Sound; refer to Figure
B-19. In this area, 17 landslides have been recorded
in the Seattle Landslide Database: listed as high bluff
peeloff (7), deep-seated (6), and shallow colluvial (4).
The earliest recorded landslide occurred in 1934. Since
then, landslides have been reported in the 1960s through
1990s. The high bluff peeloff landslides have occurred
primarily from the high bluff on the east side of Perkins
Lane. The recorded deep-seated landslides have generally
occurred in colluvium located at and west (downhill) of
Perkins Lane. One of these deep-seated landslides (1972)
damaged the west shoulder of the roadway and was repaired
by the City with pit-run sand and gravel backfill. The
deep-seated landslides that were recorded in 1996 and for
the 1996/1997 storm also involved the movement of bluff
soils uphill from the lane. The shallow colluvial landslides
also occurred in colluvium located on the downhill side
of the residences west of Perkins Lane.
The site topography is generally
similar to that described for the Perkins North area. In
this Perkins South Stability Improvement Area, Perkins Lane
is also situated on a midslope bench. This area slopes
from Magnolia Boulevard steeply to precipitously some 75
to 100 feet downward to Perkins Lane on the west. The private
properties to the west of Perkins Lane slope moderately
to steeply downward an additional 80 to 90 feet (vertical
measurement) to the Puget Sound shoreline. Most of the
shoreline, except toward the north, is protected by some
type of seawall (rocks or timber piles).
The subsurface conditions