BLD20120833 approved buffer planting plan.pdfo
UY
fT-
Theresa Neal
1611672 nd Ave
Edmonds, WA 98026
Subject: Assessment of Impacts of Buffer Restoration
1611672 "d Ave
Edmonds, WA 98026
Dear Theresa:
February 15, 2013
Project 1680-3
In response to an email from Mike Clugston, dated December 10, 2012, we have provided our
assessment of the impacts on the slope from allowing the non -conforming vegetated areas
(garden at south end and within the buffer) to remain. We have also provided our assessment of
the impacts on the steep slope from the planting plan submitted by Tom Zachary Landscape
Architects, dated 1-24-2013. The planting plan is part of a long process over the last several
months related to proposed deck improvements under plan check #2012-0833. A brief
discussion of the history of the project is included below.
Histoa of Pr°oiect
A Critical Areas Report was originally prepared for the subdivision by GeoGroup NW in 2000
for the topographic ridge. Our review and summary of pertinent elements of that report was
included in our letter dated September 10, 2012, (Geotechnical Assessment of Impact fi^om Deck
Revision, The Galli Group) and is included as an appendix to this letter. That report
recommended a 15 -foot setback from what was then the top of the bluff. It also recommended
that the residence on this lot be supported on deep foundations to avoid adverse impacts from the
receding bluff. Since that time the bluff has receded including a relatively recent event that
resulted in loss of several feet of the top of the bluff effectively reducing the distance from the
top of slope to the residence. The proposed deck improvements covered an existing patio (with
the exception of approx 45 sq. ft. of area between two patios), but were positioned outside the
original 15 foot buffer.
In the process of review it was discovered that the CCRs for the original subdivision prohibited
lawns in the buffer and recommended maintaining native vegetation. This created a dilemma in
5034 18th Avenue NE, Seattle, WA 98105 Phone: 206.525.5097 Fax: 206.525.5091
Buffer Restoration
1611672 d Ave, Edmonds, WA 98026
February 15, 2013
that the existing vegetation was mostly grasses and non-native blackberries. The buffer area is
now mostly lawn (see Figure 1). The city recommended replacing the lawn with native
vegetation within the buffer in order to try to increase compliance with the CCRs and the ECDC
code.
That brings us to the current submittal which includes a re -vegetation plan for the buffer area
including native plants. The planting plan also accompanies improvements proposed for the
deck revision/addition.
Assessment of Planting Plan
ECDC 23.80.060 and 23.80.070 (Critical Areas code) provides several requirements for
development within steep slope areas. We believe most of these were addressed in the
GeoGroup NW Critical Areas Report and summarized in our subsequent letter dated September
10, 2012. The previous geotechnical report established that a 15 -foot buffer would be adequate
on the site provided that the structures were supported on deep foundations, and that drainage
and other erosion control recommendations were followed (see summary in our letter of 9-10-
2012 and full text of GeoGroup NW Critical Areas Report). The buffer setback was established
from the top of the slope at the time of the platting.
At the time of platting the GeoGroup report concluded that the general requirements of ECDC
23.80.060 were satisfied. Specific development standards (ECDC 23.80.070) related to reduced
buffer, alterations to the buffer and grading, design standards, seasonal restrictions, and point
discharges were addressed in that report. The primary area of concern for this action is whether
the statements in ECDC 23.80.060 are altered as a result of the planting plan and the deck
addition, and dealing with the ECDC 23.80.070(4) requirement related to Vegetation Retention.
The proposed deck addition adds about 45 square feet of new impervious area but in doing so
captures runoff from the new and old (patio) impervious areas in gutters and drains and directs
the runoff to the storm drain collection system. This results in less stormwater directed into the
buffer area from impervious surfaces and thereby decreases the risk of sloughing and bluff
retreat. The new deck columns will be supported on pipe piles in accordance with the
recommendations from GeoGroup NW report for foundation support of structures within 50 feet
of the bluff.
The proposed planting plan removes the non -conforming lawn area and replaces it with native
plantings. It also includes some stepping stones for access within and maintenance of the buffer,
but these stones will be surrounded by vegetation and will not have any significant adverse
impact.
Based upon our review of the previous geotechnical report and the alterations to the patios that
include construction of the new deck surface with drainage we concluded that the proposed
alterations and planting plan will not have any net adverse effect on the geologically hazardous
area beyond predevelopment conditions. The four general requirements of ECDC 23.80.060
1680 neal buffer planting assessment 2 The Galli Group
Buffer Restoration
1611672 nd Ave, Edmonds, WA 98026
February 15, 2013
remain satisfied. The proposed vegetation plan removes existing non -conforming vegetation (the
lawn) but provides native plantings in place of the lawn that bring the condition of the buffer
back into compliance with the CCRs of the plat and help preserve the stability of the buffer once
they are established. We also believe that allowing the existing plants at the southwest end of the
lot to remain will not adversely affect the performance of the slope.
Recommendation
In our opinion the proposed activity will not have any adverse impact on the nearby slope
provided the recommendations below are followed during the activity.
No soils should be dumped over the face of the slope. All exposed soils should be
covered with 3" of mulch after the construction is completed. No stormwater runoff from
the deck or work area shall be allowed to flow toward the face of the bluff. All runoff
from the impervious deck surface should be collected in a shallow French drain or gutter
system and routed to the existing downspout system.
® We recommend that no excavation activity should take place during active rainfall and
that immediately following removal of the lawn the area should be protected with a thin
layer of mulch that can be incorporated into the soil during planting.
The gutters/drains conveying the rainfall to the storm drain should be installed
immediately after the deck surface is prepared. The Galli Group or the City of Edmonds
must verify the connection of the deck runoff collection system to the downspout
collection system.
® The Galli Group must monitor the installation of the pipe piles to verify refusal criteria.
After construction is completed we will submit a final report to the city. We trust that this helps
you move forward. Please contact the undersigned if you have any questions about our
assessment or recommendations.
Regards,
THE GALLI GROUP
w (vr
IXANiaoaoIi"01
Paul L. Stoltenberg, P.E.
Geotechnical Engineer
Attached: Copy of 9-10-2012 Letter from The Galli Group
)13
1680 neat buffer planting assessment 3 The Galli Group
If
IQ
,,, J
yX�
4X7
CW
oil,
fZ,
'2'
W. Z
<
JO
fZ,
'2'
fl GG
TIE GALLI GROUP
Geotechnical Consulting
September 10, 2012
Project 1680-1
Theresa Neal
1611672 d Ave
Edmonds, WA 98026
Subject: Geotechnical Assessment of Impact from Deck Revision
1611672 nd Ave
Edmonds, WA 98026
Dear Theresa:
At your request a geotechnical engineer from The Galli Group visited the site referenced above
to provide an assessment of the impact of the proposed deck on the nearby steep slope. The
proposed deck is intended to create a single level surface for access around the house and
connecting to a short flight of stairs along the west side of the house down to the lower portion of
the house. Plans include repair of an existing impervious deck, and would slightly enlarge the
footprint from two smaller concrete patios. The net increase in impervious area associated with
the revision/repair is estimated at about 40 to 70 square feet. At the time of our site visit the City
of Edmonds had issued a stop work order since the deck was being constructed without permit.
The location of the lot is provided on Figure 1, Vicinity Map.
We have addressed the various items in the Earth Subsidence and Landslide Hazard Area
(ESLHA) of North Edmonds permit submittal checklist below. The check list is attached to our
letter report and indicates items that are either to be supplied by others or else considered not
applicable for the project given the limited scope of the deck work.
CRITICAL AREAS STUDY DOCUMENTS
A Critical Areas Study addressing the geologic hazards of the site was prepared by Geo Group
Northwest, Inc., in February, 2000 (Geotechnical Engineering Study, Sequoia Ridge Subdivision,
Edmonds, Washington, February 29, 2000). The study was prepared when the four lots were
platted. The subject property is Lot 3 of the approved subdivision. In that study, pertinent
conclusions related to critical areas designations include the following:
5034 18th Avenue NE, Seattle, WA 98105 Phone: 206.525.5097 Fax: 206.525.5091
Geotechnical Site Assessment
Theresa Neal
1611672 d Ave
Edmonds, WA 98026
• The slope on the west side of the parcel (Lot 3 or 16116 72nd Ave W) consists of steep
slopes with near vertical bluffs. In the study the declination of the bluff was estimated at
45 to 60 degrees or 100 to 173 percent).
• The site is located immediately east of the Meadowdale Landslide Complex. "The steep
west -facing slope on the site forms the eastern margin of the landslide complex, and is
the headwall of the landslide zone. Most recent movements in the landslide complex (in
1946-47, 1955-56, and 1970-1971) have occurred in the middle to western portions of the
complex, away from the site." (page 4)
• The site appears underlain by Advance Outwash and the outwash unit underlain by
transitional beds that are visible farther down the slope. The surficial soils were
described as Alderwood-Everett gravelly sand loam according to the SCS soil survey.
(page 5)
• Groundwater seepage was visible at the presumed contact between the advance outwash
unit and the transitional beds west of the site at approximate elevation 220 — 230, or 30 to
40 feet below the elevation of the top of the bank (page 6).
• The site was identified as having "moderate to high erosion potential" and classified as an
"erosion hazard" critical area (page 8).
• The steep slope was considered to have a "5 percent probability of failure within 25
years" by Geo Engineers and RLA (see page 9 of report). Based on criteria in the ECDC,
"the steep west -facing slope on the site is a landslide hazard." (page 10)
• The Geo Group NW report concluded the "areas of the site proposed for development are
not considered to contain seismic hazard areas." (page 11)
• Geo Group NW concluded that the prescriptive setback of 50 feet could be reduced for
the project. The report recommended a setback from the steep slope to the building
footprint of a minimum of 15 feet on Lot 3. This modified setback was also based upon
other considerations during site development, including supporting the residences on
deep pile foundation systems (see page 12-13).
The above summaries are provided for ease of review. The report includes other important
information related to site development. The entire document is attached as an appendix to this
letter.
1680 heal deck addition Itr report 2 The Galli Group
Geotechnical Site Assessment
Theresa Neal
1611672 nd Ave
Edmonds, WA 98026
NORTH EDMONDS EARTH SUBSIDENCE AND LANSLIDE HAZARD AREA MAP
The ESLHA Map with signature block is included with other supporting documentation.
TOPOGRAPHIC MAP AND SURVEY
A portion of the site plan showing the 15 -foot setback line from the top of the bank is shown on
Figure 2, Site Plan. The original site plan was prepared by Brian Corbite Architects, The Bosch
Residence, Lot 3, Sequoia Ridge, Edmonds, WA, 7-18-2001. This was presumably adapted from
the original survey which we did not have access to at the time of our review. The Site Plan for
the Bosch residence is included in the supporting documentation. The site plan shows the 15 -
foot setback line as originally established. It should be noted that the proposed deck
addition/repair remains outside but adjacent to the prescribed buffer.
GEOTECHNICAL REPORT
As already mentioned the geotechnical report and subsurface investigation for the subdivision
was prepared by Geo Group NW. That report included site specific recommendations related to
grading, erosion control, subgrade stabilization, compaction and backfill, foundations, retaining
walls, modular block walls, rockeries, site drainage, pavement, soil stabilization and
revegetation. We have reviewed the previous geotechnical report and discuss recommendations
that seem pertinent to the proposed deck repair below.
The proposed deck includes a lower level less than 18 inches above existing grade. It also
includes a repair to the upper level deck that is positioned above the lower ground -level deck.
The lower deck is planned to be supported on small pier blocks, beams and deck joists. The
upper deck is supported on a column founded on a new spread footing poured against the
ground.
1. The previous geotechnical report recommended that all structures be "located outside the
steep slope setback line approved by the city during the short plat review process." The
proposed deck plan appears to extend up to the setback line as shown on Figure 2, Site
Plan.
2. The previous geotechnical report recommended supporting the residence on deep
foundations. This provides a measure of risk reduction should the top of the bank
continue to erode or migrate headward toward the house. If the slope were to fail back to
the house it would likely remain supported on the deep foundations (±25 feet below
ground surface). The Geo Group NW report likewise recommended that "use of shallow
foundation systems, such as conventional spread footings, should be limited to buildings
which will be located at least 50 feet from the top of the steep west -facing slope" (page
1680 neal deck addition Itr report 3 The Galli Group
Geotechnical Site Assessment
Theresa Neal
1611672 nd Ave
Edmonds, WA 98026
23). Soils sufficient to support the deck are evident near the ground surface. However,
based upon these report recommendations and the fact that there appears to be continued
headward sloughing of the steep slope, we recommend that the proposed elevated deck be
supported on driven pipe piles with a concrete pier cap. The elevated deck is braced in
both directions by the house, so a single 2 -inch diameter pile should be sufficient for the
corner column load. A working load of 6,000 lbs may be assigned to the driven 2 -inch
diameter pipe pile. The pier blocks for the lower deck are supported on top of the
existing concrete patio and appear adequate given the relatively lightly loaded deck and
its proximity to the ground.
3. The Geo Group NW report recommended that "Drainage off impervious areas (building
roofs and pavements) should be directed to catch basins and tightlined to discharge to the
storm drainage line below North Meadowdale Road" (page 22). In addition, "roof
downspouts must be separately (from footing drains) tightlined to discharge into the
storm water collection system." [page 30]. The proposed decks will have impervious
surfaces. The proposed plan includes capturing runoff from the deck surface and routing
it to an existing cleanout for the tightline system that drains to the storm drainage system.
Other elements from the initial report have more to do with overall site development and house
construction. Provided the column footing is supported on pipe pile foundations, that the
impervious surfaces are drained via tightline to the storm water collection system, and that the
proposed deck does not extend beyond the setback line of record, then the proposed deck appears
to comply with the recommendations from the previous report.
Based upon review of other geotechnical reports, it appears that the steep bank west of the house
experienced additional sloughing during 2001, which resulted in headward migration of the top
of the bank toward the house. The house is now measured at 10 feet from the top of the bank at
its closest point. The Geo Group NW report indicated that headward migration of the slope was
likely. A geotechnical engineer from PanGeo visited the site in May of 2008 and concluded the
following: "Based on the setback distance of the building to the slope crest and the fact that the
building is supported by 16 -inch diameter augercast piles extending at least 25 feet below the
building foundation level, in our opinion the steep slope on the west and north sides of the
building will not pose immediate risks to the existing building." They also indicated that
"natural erosion and weathering of the steep slope will likely continue, which may potentially
cause future soil sloughing or small shallow slide." (Preliminary Geotechnical Evaluation,
16116 — 72"d Avenue West, Edmonds, Washington, PanGEO Inc., May 16, 2008.)
1680 neal deck addition Itr report 4 The Galli Group
Geotechnical Site Assessment
Theresa Neal
1611672 nd Ave
Edmonds, WA 98026
CIVIL PLANS
We don't believe civil plans are reasonable given the small scope of the project. The gutter
system should be connected to the existing storm drain and disturbed areas should be replanted
immediately following construction of the deck. Apart from these two requirements
conventional BMPs should be incorporated into project planning and construction.
DESIGN PROFESSIONAL STATEMENT
We have reviewed the proposed deck repair and revision along with the geotechnical reports on
record. We understand the geotechnical recommendations and have incorporated into the design
the established measures to reduce the potential risk of injury or damage from any earth
movement predicted in the report.
GEOTECHNICAL HAZARD IDENTIFICATION/DECLARATION & MITIGATION
STATEMENT OF RISK
The plans conform to the recommendations in the geotechnical report and additional comments
in our letter.
The risk of damage to the proposed development, or to adjacent properties, from soil instability
will be minimized subject to the conditions set forth in the reports of record and our letter herein.
The proposed deck revision will not increase the potential for soil movement on the property.
As stated in previous reports, probability of earth movement has been described as "5 percent
probability of failure within 25 years." These risks of slope movement can be reduced by
vigilant maintenance of stormwater runoff and collection facilities, maintaining vegetation
within the buffer area and on the slope, avoiding water leaks, and avoiding dumping on the slope
surface. However, many factors affecting risk of slope movement remain beyond the control of
the homeowner.
In our opinion the ground disturbance will be minimal during construction. All exposed soils
must be covered with mulch or vegetated prior to the winter season.
The most probable form of slope movement one the property would likely consist of shallow
colluvial slides or shallow sloughing near the top of the slope --movements that involve the
looser, wetter, surficial soils. These slides are often associated with wet antecedent conditions
and extreme stormwater runoff events. Other factors contributing to these types of failures
include denuding the slope, failure or rupture of irrigation or water lines, and clogged or
malfunctioning downspout drains or catch basins. Headward erosion of the top of the bank by
means of periodic sloughing over time should also be expected. To mitigate the risk of these
types of failure we recommend the following:
1680 neal deck addition Itr report 5 The Galli Group
Geotechnical Site Assessment
Theresa Neal
1611672 nd Ave
Edmonds, WA 98026
1. Avoid adding fill soils to the ground surface within 25 feet of the top of the slope
2. Avoid dumping yard waste or vegetative debris over the top of the bank
3. Permanent irrigation systems should be prohibited
4. Avoid stormwater runoff or infiltration within 50 feet of the top of the bank
5. We recommend against installation of water features, pools, or hot tubs without including
special safety measures
6. The owner must provide vigilant maintenance of the gutters, downspouts, and stormwater
collection system to avoid overtopping, pooling, or concentrated runoff.
Some hazards associated with steep slopes cannot be completely avoided. At times the slope is
part of a larger slope system or can be influenced by factors far beyond the control of a single
property owner; these factors can are affect stability. There remains risk of failure of the west -
facing steep slope from environmental, weather-related, or human factors. In addition headward
migration of the steep upper portion of the slope should be anticipated. While this appears
unlikely to threaten the house, over time the loss of yard space at the top of the slope is likely
and this could eventually increase the risk to the residence or to appurtenant structures (decks
and patios) and systems (drains, services). At some point it might be prudent to provide
structural measures such as a tangent pile wall to arrest future headward migration of the steep
bank and to preserve yard space.
SUMMARY CONCLUSION
Our conclusions based upon research of the available geotechnical information and subsurface
boring logs prepared by others are similar to those of PanGEO in their letter of May 16, 2008.
We don't believe the house or the newly constructed deck will be immediately at risk from steep
slope failures. Nor do we believe that the proposed repairs/revision of the deck will increase the
risk of damage to the residence or the steep slope. Removing some of the sheet now currently
contributed from the patio and other impervious surfaces by capturing it in the gutter and then
directing it to the storm water collection system should decrease the risk of sloughing at the top
of the slope. As stated in the other reports and as evidenced by the 2001 slide, the owner should
anticipate that continued headward erosion or sloughing will continue. The owner has been
aggressive at planting and maintaining vegetation on the slope and at the top of the slope which
should help mitigate the potential for erosion and sloughing.
Our review and assessment were based upon a reconnaissance of the property and research of
previous geotechnical reports for the subject property. In our opinion the limited scope of the
work (deck repairs and reconfiguring over existing concrete surfaces plus stairway access) did
not warrant additional subsurface investigation or a full geotechnical report. The owner must
recognize that living on or near a steep slope involves risk. Our comments are in no way
1680 neal deck addition Itr report 6 The Galli Group
Geotechnical Site Assessment
Theresa Neal
1611672 nd Ave
Edmonds, WA 98026
intended as a warranty of the slope or subsurface conditions. As we discussed at some future
point it might be necessary to introduce structural means of arresting the headward migration of
the top of the bank which various geotechnical engineers appear to agree is common and should
be anticipated.
We trust that this helps you move forward. Please contact the undersigned if you have any
questions about our assessment.
Regards,
THE GALLI GROUP
..up"�" qy dY
MAIPor�Np'm"
Paul L. Stoltenberg, P.E.
Geotechnical Engineer
9-10-2012
Figures: Figure 1 — Vicinity Map
Figure 2 — Site Plan
Appendix: Geo Group NW geotechnical study, February 2000
Included Separately:
ESLI A Submittal Checklist (2 copies)
Vicinity Map (4 copies)
Portion of Site Plan (4 copies)
Landslide Hazard Map with signature block (1 copy)
Topographic Map for reference (original site plan) by Corbite Architects (4 copies)
1680 neat deck addition Itr report 7 The Galli Group
J
21
uL
4
Z'
_41
M
J