Geotech ReportCOBALT
G E 0 S C I E N C E S
October 29, 2021
Susan Dunn
C/O Maggie Hanna
Maggie.wakedesign (& gmail.com
RE: Limited Geotechnical Evaluation
Proposed Remodel/Addition
315 8th Avenue North
Edmonds, Washington
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
In accordance with your authorization, Cobalt Geosciences, LLC has performed a limited
geotechnical evaluation related to the proposed deck at the referenced property.
Site & Project Description
The site is located at 315 8th Avenue North in Edmonds, Washington. The site consists of one
rectangular shaped parcel (No. 00434208501600) with a total area of about 0.4 acres.
The central portion of the property is developed with a residence with daylight basement and
driveway. There are local patio and deck areas along the north and west sides of the residence,
and a carport attached to the residence. The property is vegetated with grasses, bushes, trees, ivy,
ferns, blackberry vines, and other understory.
The property slopes downward from south to north and northwest at magnitudes of 10 to 40
percent and relief of about 42 feet. The steepest slope is near Sprague Street and includes a cut
slope that is 50 to 8o percent in magnitude with relief of 20 to 25 feet. This slope is well
vegetated.
The original site grading included mostly cuts to create the building pad and yard areas. There
are local walls supporting former cuts within the site and near the south property line. Rockery
walls are generally 4 feet or shorter and concrete walls are generally 6 feet tall or shorter.
The property is bordered to the east, west, and south by residences, and to the north by Sprague
Street. Eighth avenue north terminates at the southeast corner of the property, which provides
access via a driveway from the roadway.
The project includes enclosure and finish framing of the existing carport into a garage along with
about 151 square feet of new/replacement deck areas in existing graded areas and existing deck
areas along the north and west sides of the residence. As noted, this work will occur existing
developed areas.
Site Geology
The Geologic Map of the Edmonds East and West Quadrangles indicates that the site is underlain
by Vashon Advance Outwash.
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October 29, 2021
Page 2 of 4
Limited Geotechnical Evaluation
Vashon Advance Outwash consists of glacially consolidated sand and gravel with variable
amounts of silt deposited in front of the advancing glaciers approximately 15,000 years ago.
These deposits are underlain in some areas by Transitional Beds. This is a silt/clay lacustrine
facies of the outwash sands. These deposits are also typically dense below a weathered zone.
Soil & Groundwater Conditions
As part of our evaluation, we excavated and sampled a hand boring between the house and
moderately steep slope area to the north.
The hand boring encountered approximately 12 inches of topsoil and vegetation underlain by
approximately 3.5 feet of loose to medium dense, silty -fine to medium grained sand trace gravel
(Weathered Advance Outwash). This layer was underlain by medium dense to dense, fine to
medium grained sand trace gravel (Advance Outwash), which continued to the termination
depths of the hand boring.
The slope appears to be underlain by weathered and unweathered Vashon Advance Outwash,
consistent with the geologic mapping in this area.
Geologic Hazards
The site contains steep slope and erosion hazards based on soil composition and slope
magnitudes. These areas are located north and west of the residence and include both natural
slopes and partially re -graded (cut) slopes near Sprague Avenue.
While the soil survey indicates the presence of highly erosive soils, these areas are well vegetated
and generally within a natural condition.
We performed a reconnaissance of the steep slope areas and the subject property. Overall, the
site slopes are stable at this time with no evidence of historic or recent landslide activity,
emergent groundwater, or erosion. The risk of erosion and shallow sloughing will remain low
provided the slope continues to be fully vegetated and not modified. The slope system will not be
affected by the proposed development and similarly, the proposed development will not be
affected by the steep slope area.
This conclusion is based on utilization of proper erosion control measures where the limited
excavation work will occur.
Conclusions and Recommendations
It is our opinion that the steep slope and erosion hazard areas near the residence are stable at this
time with no evidence of landslide activity or severe erosion. The proposed garage and patio areas
may be completed without adversely affecting the nearby critical area provided proper erosion
control measures are in place below and around any areas where grading is proposed.
The hazard areas are located as shown in Figure 1. The development is not located within the
hazard areas. No specific buffer or setback is warranted or required from a geotechnical
standpoint since the areas of the proposed work are within already developed areas, and the slope
has been previously modified.
The geotechnical engineer should be on site to verify erosion control systems are in place and to
observe soil bearing conditions at foundation areas.
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October 29, 2021
Page 3 of 4
Limited Geotechnical Evaluation
Deck Foundation Support
The proposed deck and any new footings for the remodel (not anticipated) may be supported on
shallow spread footing foundation systems bearing on undisturbed medium dense or firmer
native soils or on properly compacted structural fill placed on the suitable native soils. Any
undocumented fill and/or loose native soils should be removed and replaced with structural fill
below foundation elements. Structural fill below footings should consist of clean angular rock 5/8
to 4 inches in size. We should verify soil conditions during foundation excavation work.
For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively,
for continuous wall and isolated column footings supporting the proposed structure. Provided
that the footings are supported as recommended above, a net allowable bearing pressure of 2,000
pounds per square foot (psf) may be used for design.
A 1/3 increase in the above value may be used for short duration loads, such as those imposed by
wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted
to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing
excavations should be inspected to verify that the foundations will bear on suitable material.
Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or
adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12
inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower.
If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch.
Differential settlement, along a 25-foot exterior wall footing, or between adjoining column
footings, should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most
settlement is expected to occur during construction, as the loads are applied. However, additional
post -construction settlement may occur if the foundation soils are flooded or saturated. All
footing excavations should be observed by a qualified geotechnical consultant.
Resistance to lateral footing displacement can be determined using an allowable friction factor of
0.4o acting between the base of foundations and the supporting subgrades. Lateral resistance for
footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds
per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12
inches below grade in exterior areas). The frictional and passive resistance of the soil may be
combined without reduction in determining the total lateral resistance.
Care should be taken to prevent wetting or drying of the bearing materials during construction.
Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the
footing excavations, should be removed prior to placing concrete. The potential for wetting or
drying of the bearing materials can be reduced by pouring concrete as soon as possible after
completing the footing excavation and evaluating the bearing surface by the geotechnical engineer
or his representative.
Erosion and Sediment Control
Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to
wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment
control measures should be implemented, and these measures should be in general accordance
with local regulations. At a minimum, the following basic recommendations should be
incorporated into the design of the erosion and sediment control features for the site:
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October 29, 2021
Page 4 of 4
Limited Geotechnical Evaluation
• Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance
of the site soils, to take place during the dry season (generally May through September).
However, provided precautions are taken using Best Management Practices (BMP's), grading
activities can be completed during the wet season (generally October through April).
• All site work should be completed and stabilized as quickly as possible.
• Additional perimeter erosion and sediment control features may be required to reduce the
possibility of sediment entering the surface water. This may include additional silt fences, silt
fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration
systems.
• Any runoff generated by dewatering discharge should be treated through construction of a
sediment trap if there is sufficient space. If space is limited other filtration methods will need
to be incorporated.
Closure
The information presented herein is based upon professional interpretation utilizing standard
practices and a degree of conservatism deemed proper for this project. We emphasize that this
report is valid for this project as outlined above and for the current site conditions and should not
be used for any other site.
Sincerely,
Cobalt Geosciences, LLC
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Phil Haberman, PE, LG, LEG
Principal
www.cobaltgeo.com (2o6) 331-1097
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HB-1 Approximate
Hand Boring Location Ref. Sno. County GIS Maps
Proposed Remodel/Landscaping AERIAL
315 8th Avenue North IMAGE
• S C I E N C k S Edmonds, Washington FIGURE 1
NAT
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeoogmail.com
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EXISTING RESIDENCE -
PROPpSED (NO CHANGETO FOOTPRINT)
DECK 1,500 sq ft
444 iq ft
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LOCATION OF EXISTING HWSE AND NEIGHBOR'S
PROPERTY LOCATIONS ARE ESTIMATED AND BASE OFF OF
SNOHOMISH COt1M7 & CITY OF EDMONDS GIS DATA
Proposed Remodel/Landscapi
315 8th Avenue North
Edmonds, Washington
SITE
PLAN
FIGURE 2
NAT
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(2o6) 331-1097
www.cobaltgeo.com
cobaltgeoogmail.com