Quality Geo Letter (12-21-20) - 8112 Talbor Rd Consult (QG20-082) (2)QUALITY
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12/21 /2020
Danielle Hill
8112 Talbot Rd
Edmonds, WA
Subject: 8112 Talbot Rd - Slope Consultation
8112 Talbot Rd, Edmonds, WA
QG Project No.: QG20-082
Dear Ms. Hill:
At your request, Quality Geo, PLLC (QG) has completed a preliminary review of the above
referenced property's existing site conditions, including site visual reconnaissance, slope analysis,
and review of existing geologic literature for the site. The project site consists of a mostly
developed residential property comprising a portion of a regional slope. It is our understanding
that the client intends to complete various site improvements and additions within an area
designated as a potential critical area.
QG understands that the client requests a geotechnical consultation to confirm that current
conditions are favorable, and to provide any additional and necessary recommendation regarding
slope considerations. The following report presents the findings and conclusions of our literature
review, addresses feasibility of proposed site development, and provides additional geotechnical
recommendations for planning and design intended to reduce the inherent risks associated with
site development within a potentially geologically hazardous area.
GEOLOGIC LITERATURE REVIEW
The Washington Geologic Information Portal (WGIP) maintained by the Department of Natural
Resources Division of Geology and Earth Resources provides 1:24,000-scale geologic mapping of
the region. The subject site is mapped as Glacial Till (Qgt). Till typically comprises unsorted, sub -
angular and highly compacted clay, silt, sand, and gravel deposited directly by glacier ice, varying
in thickness from discontinuous layers less than 1-inch thick to more than 30 feet. Soil may be
Quality Geo, PLLC
Geotechnical Engineering & Consulting
Phone: 360-764-8485 1 Web: quality-geo.com I Mail: 420 Golf Club Rd SE, Ste 201, Lacey, WA 98503
8112 Talbot Rd Consult Quality Geo, PLLC
12/21/2020 Project # QG20-082
grey where fresh, and yellowish brown where oxidized. Outwash clay, sand, silt, and gravel may
be included.
According to the regional -scale interactive map, no deep-seated landslides are known to exist
within the site or adjacent vicinity. Available LiDAR imagery of the site did not reveal any
obvious or prominent landslide features within the site or immediate vicinity.
SITE INVESTIGA TION METHODOLOGY
On 12/2/2020 a QG Licensed Geologist visited the site to perform visual reconnaissance of the
surface and topographic features of the subject property and its proximal slope. While on site, we
conducted site surface explorations for a geologic hazard assessment and site feasibility
characterization. Relevant property dimensions and slope topography were documented and
mapped at representative intervals as access allowed. Soil conditions were evaluated through local
exposures. Salient slope features and existing vegetation were documented in order to assess
general site stability as well as observe for signs of local instability of an erosional or subsurface
nature currently or in the past.
A site region and vicinity maps are provided in Appendix A, and a site plan with typical
topographic contours and relevant features is presented in Appendix B. Photos showing general
site features are provided in Appendix C.
SURFACE OBSERVATION
The project site is elongated in shaped, with access to the street at the northern edge, and extending
across the creek to the south. The upland site is generally flat. A concrete retaining wall exists
approximately 60 feet south of the house, and effectively establishes the crest of the local slope.
Slopes descend to the south at approximately 3H:1V for approximately 60 horizontal feet, before
steepening at the creek bank to 1 H:1 V within 20 feet of the water.
QG performed reconnaissance to observe for and document any indications of surface degradation
or large-scale slope instability. No obvious features were observed that would indicate an active
or prior deep-seated slope failure, such as headscarps or downslope accumulations. Topography
was generally consistent, lacking prevalent oversteepened areas, channelized runout zones, or
hummocky deposits. No obvious evidence of rotational or translational failures or major toppling
hazards was observed on the slope in the proximity of the potential building footprint. No obvious
failure features were observed on adjacent slope areas visible from the subject property during our
site visit.
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SUBSURFACE CONDITIONS:
Quality Geo, PLLC
Project # QG20-082
A general characterization of on -site soil units observed through local exposures on site within
fallen tree pits.
• Cover Soils — Sandy Organic Topsoils (SM):
QG encountered an overriding layer of organic rich topsoil over most of the site and slope.
• Native Soil — Silty Sand (SM):
The dominant native soil both shallow and deep, across the upland and slope, appears to
comprise a silty sand, with varying silt content, typically becoming cleaner with depth.
These soils more closely resemble a recessional glacial outwash soil rather than a dense
glacial till. The native sand was loose nearly throughout, becoming somewhat medium
dense with depth.
Groundwater elevation is inferred to exist at or near the creek elevation.
DISCUSSION & CRITICAL SLOPE RECOMMENDATIONS
The findings of QG's site reconnaissance at the subject site do not indicate any excessively
prohibitive conditions exist for the site, assuming appropriate site management efforts are
maintained. In consideration of the available information, and our direct observations, at this time
QG does not consider the site to be within an active deep-seated landslide hazard area.
Erosional hazards related to soil creep and saturated soils may be mitigated per the
recommendations detailed below to increase slope safety factors.
New Slope Retaining Wall Recommendations:
At this time, no new dwellings are proposed. Therefore, no foundation setbacks are anticipated to
be required to inform site improvements.
Small proprietary block walls less than 4 feet in height (measured from bottom of base block to
top of wall) typically do not require engineering, and may be suitable for landscape and cosmetic
improvements along the upper slopes inclining near 3H:1V. We recommend such a walls base
gravel fill section bear on native soils compacted to a firm and unyielding condition, and that they
incorporate a drainage course behind the wall, tightlined in accordance with the Drainage Controls
section of this report. The wall shall be keyed into a minimum depth that at least spaces the base
block 5 horizontal feet from native or newly graded slope face exposure. Block wall construction
shall follow the manufactures printed installation instructions. We do not recommend the
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8112 Talbot Rd Consult Quality Geo, PLLC
12/21/2020 Project # QG20-082
construction of block walls greater than 4 feet in height, as the loose native soils are likely to
experience settlement due to greater soil loads.
Driven Pile Wall Recommendations:
QG recommends that any proposed walls that are to be greater than 4 feet in height be designed
by a structural engineer. Block and cast -in -place walls may likely experience settlement due to the
loose sandy conditions. Such walls would need to bear on piles. The most stable, and economic
solution would then be to design any new retaining walls as soldier pile walls, with driven pile
supports and lagging between.
We recommend the pile embedment and capacities be determined using either the WSDOT Pile
Driving Formula, or proprietary formulas provided by the selected pile/hammer company. Actual
pile locations, sizing, spacing, and materials shall be determined by the project designer.
Except as noted, typical design elements and construction procedures shall be in accordance with
manufacturer standards. Any discrepancies encountered that are not addressed herein shall be
reconciled by the design engineers during construction. All piles shall be driven to refusal per the
manufacturer/installer minimum criteria as determined based on the size of the pile and proposed
mechanism of installation. Piles shall be driven straight and plumb, avoiding eccentricity as much
as feasible. Piles angled near to or greater than 3 degrees may need to be abandoned.
Piles shall be driven to a depth where the pile base is horizontally spaced a minimum of 40 feet
from the slope face, in accordance with IBC slope setback requirements.
If early pile refusal is encountered, pile acceptance shall be evaluated by the retained inspector in
consideration of achieved depth, driving behavior, and adjacent pile conditions. If refusal is
encountered at an excessively shallow depth (within upper deposits), QG recommends an
alternative driving location be attempted at minimum 3*d (three times pile diameter) and at
maximum 5*d on -center from the refused pile. Final acceptance of installed piles shall be at the
discretion of the inspector and project engineer.
Wall Construction:
• Excavations:
The duration of time that excavations behind walls remain open should be limited to only as
necessary to prepare the base pad and placement of the wall features, backfilling with drain
rock and approved fill immediately. Temporary worker protections such as trench boxes or
temporary shoring may be required for entering excavations deeper than 4 feet, and all OSHA
safety regulations should be observed. Extended open cut periods or work proceeding in wet
weather may require surface coverings, lesser cut angles, and/or temporary bracing be applied.
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Project # QG20-082
Shoring may be required to prevent the undermining of nearby existing structures, if
excavations are within 5 feet of them, and should be considered by the designer or earthworker
as needed.
Stockpiling of excavated tailings is to be prohibited above, near, or on slopes. Tailings should
be removed to an inland area of the site, sufficiently away from the crest if temporary storage
of exported/imported materials is required.
• Wall Drainage:
To preclude build-up of external hydrostatic pressure, we recommend a minimum width of 1
foot of clean, granular, free -draining material extend from the footing drain at the base of the
wall to the ground surface immediately behind the wall. Native soils are not considered suitable
as drainage material. Imported wall drain aggregate should conform to WSDOT Standard
Specification 9-03.12(4) Gravel Backfill for Drains or 9-03.12(5) Gravel Backfill for Drywells.
A filter fabric suitable for use in soil separation and water transmission is recommended to be
placed against retained soil cuts behind the wall (if present) to limit migration of fines into the
drain corridor. Final parameters shall be determined by the wall designer.
• Wall Backfill:
Native material is not considered suitable for wall backfill due to its elevated fines content. For
additional wall backfills as needed, soils should be relatively granular with less than 5 percent
fines (material passing the U.S. No. 200 sieve). QG recommends wall backfill import material
to conform to WSDOT Standard Specification 9-03.12(2) Gravel Backfill for Walls.
• Wall Backfill Compaction:
It is recommended that the upper two feet of wall backfill be compacted to 95 percent of the
modified Proctor maximum dry density per ASTM D1557, and 90 percent below that to avoid
added pressure on the wall. Wall backfill supporting landscaping elements and other non-
structural components should be compacted to a relatively firm and unyielding condition.
Site Grading and Permanent Slopes:
We recommend that fill placed on slopes steeper than 3H:1V be `benched' in accordance with
hillside drives entry of section 2-03.3(14) of the WSDOT Standard Specifications.
QG recommends that any new permanent graded slopes be inclined no greater than 3H:1 V at a
minimum so as to catch natural topography at the top and toe of the new incline. QG also
recommends permanent slopes and undeveloped surfaces be planted with a deep -rooting, rapid -
growth vegetative cover as soon as possible after completion of slope construction. Alternatively,
slopes may be covered with plastic, straw, etc., until they can be landscaped. Cut or uncontrolled
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fill contaminated with organic or manmade debris are not suitable for reuse as structural or grade
fill.
Impervious Pavement
QG understands the existing driveway is failing in many areas, with potholes, birdbaths, slumps,
and cracks. This appears to be due to a combination of 3 factors: lack of proper functioning
drainage, lack of a compact base, and presence of relict organic sub soils. The client has requested
we offer recommendations for proper replacement of the pavement sections on site.
Based on the planned use as a driveway, we assumed relatively low traffic. Table I includes
preliminary recommendations for impervious hot -mix asphalt (HMA) pavement and base course
thickness for the new roadway. This recommendation assumes that the subgrade will be prepared
following the recommendations provided in this report and the traffic assumptions are valid.
Table 1: Preliminary Pavement Design Recommendations for Roadway
Pavement Layer Type
Minimum
Thickness, inches
Hot -Mix Asphalt
3
WSDOT Spec Base Course
9
These sections should be considered preliminary until the site designer verifies the parameters,
traffic loading, and assumed grading are applicable to the final project design. We recommend
pavement sections be reviewed by the project designer, who may apply an alternative section for
final project use based on the conditions reported herein and final design and construction
preferences.
Native subsoils shall be free of organic debris and compacted to a firm and unyielding condition
prior to the placement of the base course. The main entrance/exit drive will likely experience
different traffic volumes than the far end of the pavement areas. As a result, consideration could
be given to increasing the pavement section in the main entrance/exit drive.
One of the important considerations in designing a high quality and durable pavement is providing
adequate drainage. Design of drainage for the proposed pavement section is outside of QG 's scope
of work at this time. It is important that bird baths (leeching basins) and surface waves are not
created during construction of the HMA layer. A proper slope should also be allowed, and drainage
should be provided along the edges of pavements and around catch basins to prevent accumulation
of free water within the edges and base course, which otherwise may result in subgrade softening
and pavement deterioration under exposure and repeated traffic conditions.
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All pavements require regular maintenance and repair in order to maintain the serviceability of the
pavement. These repairs and maintenance are due to normal wear and tear of the pavement surface
and are required in order to extend the serviceability life of the pavement. However, after 10 years
of service, a normal pavement structure is likely to deteriorate to a point where pavement
rehabilitation may be required to maintain the serviceability. The deterioration is more likely if the
pavement is constructed over poor subgrade soils or in area of higher traffic volumes.
Drainage Controls:
During our site visit, it was apparent that there are no functioning site drainage controls. Roof
gutters and downspouts are found to outfall directly around the immediate perimeter of the house
and pavement. QG is not aware of a dedicated drywell or infiltration basin. While soils on site are
generally sandy, it appears that stormwater infiltration is delayed, based on the clients reports of
soggy yard conditions. Therefore, inground stormwater infiltration may not be feasible considering
the current volumes of stormwater.
QG recommends proper drainage controls for stormwater runoff during and after site development
to protect the site. The ground surface adjacent to the building should be sloped to drain away from
the building pad and slope at a 5% minimum to prevent ponding of water adjacent to the house.
Footing drains and yard drains should be incorporated for the building and site design to help
maintain a dry yard and building area.
QG recommends roof and footing water sources be tightlined (piped) away from the building to
an existing catch basin, stormwater system, established channel, or down the slope to be released
beyond the base using appropriate energy -dissipating features at the outfall to minimize point
erosion.
Roof and footing drains should be tightlined separately or should be gathered in an appropriately
sized catch basin structure and redistributed collectively. If storm drains are incorporated for
impervious flatworks (driveways, patios, etc.), collected waters should also be discharged
according to the above recommendations. All drainage tightlines should be composed of
appropriately sturdy material (such as rigid PVC), sized adequately according to anticipated flow,
and anchored sufficiently. QG recommends slope tightlines be inspected by the owner periodically
to look for signs of damage or displacement requiring repair.
With permit approval, dispersion within the existing creek may be considered for reasonable
quantities of stormwater, so long as appropriate energy reducing features are established at the
outfall, such as fabric and quarry spalls, or other approved methods, to prevent erosion. QG
recommends the site designer follow guidelines for dispersion set forth in the local stormwater
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design manual, or the WA Department of Ecology 2019 Stormwater Management Manual for
Western Washington.
Vegetation Improvements:
Any trees showing the potential to fall within the building envelope or across the slope should be
considered for removal, if possible. Where revegetation is not possible, stumps should be left in
the ground to offer some stabilization of shallow surface soils. QG ultimately recommends the
client consult a local arborist and the county code when determining which trees may be removed.
Following construction and for long-term site use, maintaining existing downslope vegetation and
installing additional beneficial deep rooting ground plantings within the vicinity of the
improvements and over the slopes is encouraged assuming installation is done in a manner that
minimizes slope face disturbance and erosional hazard in the long term. Adding vegetation will
increase the erosional and hydrologic resistance of the slope and assist in retaining cover soils.
Further information and recommendations for erosion control including typical beneficial native
plantings for sloping areas are provided herein.
Erosion Controls:
Erosion is one of the most common driving forces leading to slope instability. In addition to the
above commentary, the following general recommendations should be implemented in general to
reduce long-term erosion potential of the slope below the project site and maintain slope stability:
• Minimize the volume and velocity of water that travels toward and down the slope face (via
proper choice of site development features including stormwater controls discussed above).
• Avoid accelerating slope erosion and mass wasting due to human activity such as:
✓ Adding side -cast such as dumping landscape debris or fallen trees on or above the slopes.
✓ Using heavy construction equipment on or near steep slopes.
✓ Excavating near adjacent steep slopes toe or on slope face.
✓ Placing excavated soil near the steep slope crest.
• Prior to construction, a silt fence and/or a continuous line of straw bales should be placed on
the slopeward edge of the construction area. Heavy construction equipment, construction
materials, or native and imported soils should not be placed behind the erosion control devices.
Suitable temporary erosion and sediment control measures should be implemented at the
construction site during and immediately after ground disturbance occurs. Temporary areas
bare of vegetation should be protected from erosion via a blanket of straw or rolled erosion
control product (RECP) during prolonged breaks in site work and prior to reseeding or
revegetation.
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• At the end of the project, all bare surfaces and areas of disturbed vegetation should be replanted
and maintained until fully reestablished. Concentrated surface water should not be allowed to
traverse the slope during or after the construction phase of the project. Roof downspouts and
footing drains should be routed into closed separate pipes which outfall into appropriate
drainages. Outlets for these pipes should be protected from erosion through the use of rip -rap
(quarry spalls) or some other energy dissipating device. Similarly, concentrated drainages
should be captured in closed pipe systems and routed down slope to appropriate outfalls.
• Clearing of existing vegetation outside the proposed building area on and adjacent to the
existing slopes should be avoided except as approved by a qualified professional. This provides
additional stability to the loose top soil and minimizes the effects of down -slope water
movement. This is excepting removal of problem, dead, or dying, trees if posing a direct hazard
to site installations or adjacent roadways.
• Grading or excavation of soils during construction should be accompanied by grass reseeding
and re -vegetation as the project is completed. According to "Vegetation Management: A Guide
for Puget Sound Bluff Property Owners" (Manashe, 1993) the following types of vegetation
provide good to excellent erosion control:
Common Name
Botanical Name
Deciduous/Evergreen
Mature Height t)
Bi leaf Maple
Acer macro h llum
Deciduous
60
Douglas Fir
Pseudotsu a menziesii
Evergreen
200+
Evergreen
Vaccinium ovatum
Evergreen
To 8
Oceanspray
Holodiscus discolor
Deciduous
10+
Oregon Grape
Mahonia s .
Evergreen
To 6
Pacific Madrone
Arbutus menziesii
Evergreen
70
Red huckleberry
Vaccinium parvifolium
Deciduous
To 12
Rose
Rose spp.
Deciduous
2-10
Salal
Gaultheria shallon
Evergreen
To 4
Salmonberry
Rubus s ectabilis
Deciduous
To 12
Serviceberry
Amelanchier alnifolia
Deciduous
12+
Snowberry
Symphoricarpos albus
Deciduous
3+
Vine Maple
Acer cricinatum
Deciduous
10+
Willow
t Salix spp.
Deciduous
10+
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CLOSING.
Quality Geo, PLLC
Project # QG20-082
We trust this letter satisfies your project needs currently and thank you for the opportunity to be
of service. QG wishes you the best while completing the project.
Respectfully Submitted,
Quality Geo, PLLC
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LUKE PRESTON MCCAN14
Luke Preston McCann, L.G.
Principal Geologist
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Nick Taylor, P.E.
Supervising P.E. Review
Attachments: Limitations
Appendix A. Site Region and Vicinity Maps
Appendix B. Aerial Site Map
Appendix C. Site Photos
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12/21/2020
LIMITATIONS
Quality Geo, PLLC
Project # QG20-082
Upon acceptance and use of this report, and its interpretations and recommendations, the owner
shall agree to indemnify and hold harmless QG, including its owners, employees and
subcontractors, from any adverse effects resulting from development and occupation of the subject
site. Ultimately, it is the owner's choice to develop and live in such an area of possible geohazards
(which exist in perpetuity across the earth in one form or another), and therefore the future
consequences, both anticipated and unknown, are solely the responsibility of the owner. By using
this report for development of the subject property, the owner must accept and understand that it
is not possible to fully anticipate all inherent risks of development. The recommendations provided
above are intended to reduce (but may not eliminate) such risks.
This report does not represent a construction specification or plan and shall not be used or
referenced as such. The information included in this report should be considered supplemental to
the requirements contained in the project plans & specifications and should be read in conjunction
with the above referenced information. The selected recommendations presented in this report are
intended to inform only the specific corresponding subjects. All other requirements of the above -
mentioned items remain valid, unless otherwise specified.
Recommendations contained in this report are based on our understanding of the proposed
development and construction activities, field observations and explorations, and laboratory test
results. It is possible that soil and groundwater conditions could vary and differ between or beyond
the points explored. If soil or groundwater conditions are encountered during construction that
differ from those described herein, or If the scope of the proposed construction changes from that
described in this report, QG should be notified immediately in order to review and provide
supplemental recommendations.
The findings of this study are limited by the level of scope applied. We have prepared this report
in substantial accordance with the generally accepted geotechnical engineering practice as it exists
in the subject region. No warranty, expressed or implied, is made. The recommendations provided
in this report assume that an adequate program of tests and observations will be conducted by a
WABO approved special inspection firm during the construction phase in order to evaluate
compliance with our recommendations.
This report may be used only by the Client and their design consultants and only for the purposes
stated within a reasonable time from its issuance, but in no event later than 18 months from the
date of the report. It is the Client's responsibility to ensure that the Designer, Contractor,
Subcontractors, etc. are made aware of this report in its entirety. Note that if another firm assumes
Geotechnical Engineer of Record responsibilities they need to review this report and either concur
with the findings, conclusions, and recommendations or provide alternate findings, conclusions
and recommendation.
Land or facility use, on- and off -site conditions, regulations, or other factors may change over time,
and additional work may be required. Based on the intended use of the report, QG may recommend
that additional work be performed and that an updated report be issued. Non-compliance with any
of these requirements by the Client or anyone else will release QG from any liability resulting
from the use of this report. The Client, the design consultants, and any unauthorized party, agree
to defend, indemnify, and hold harmless QG from any claim or liability associated with such
unauthorized use or non-compliance. We recommend that QG be given the opportunity to review
the final project plans and specifications to evaluate if our recommendations have been properly
interpreted. We assume no responsibility for misinterpretation of our recommendations.
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Appendix B. Aerial Site Map
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SCALE (FEET)
Site Map Source: King County UDAR & WA DNR Quality Geo, PLLC Scale & Locations are approximate Figure 2
8112 Talbot Rd Not for Construction
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