REVIEWED STM Quality Geo Letter (5-16-2021) - 8112 Talbor Rd - Infiltration Addendum (QG20-082 & QG21-039) (1)LIALITY
EO
12/21 /2020
Danielle Hill
8112 Talbot Rd
Edmonds, WA
Subject: 8112 Talbot Rd — Infiltration Addendum
8112 Talbot Rd, Edmonds, WA
QG Project No.: QG20-082 & QG21-039
Dear Ms. Hill:
At your request, Quality Geo NW, PLLC (QG) has completed a supplementary infiltration
investigation for your project.
QG understands that the city requests a geotechnical consultation to evaluate infiltration potential
of site soils, and to provide any additional and necessary recommendation regarding stormwater
considerations. The following addendum presents the findings and conclusions of our literature
review, addresses feasibility of proposed site development, and provides additional geotechnical
recommendations for stormwater design intended to reduce the inherent risks associated with site
development within a potentially geologically hazardous area.
SITE INVESTIGA TION METHODOL OGY
Site exploration activities were performed on 4/21/2021. Exploration locations were marked in the
field by an QG Project Geologist with respect to the provided map and cleared for public
conductible utilities. Our exploration locations were selected by an QG Project Geologist prior to
field work to provide safest access to relevant soil conditions. The geologist directed the
advancement of 2 excavated test pits (TP). The test pits were advanced within the vicinity of the
anticipated development footprint areas, to depths of 10.0 feet below present grade (BPG) in
general accordance with the specified contract depth.
During explorations QG logged each soil horizon we encountered, and field classified them in
accordance with the Unified Soil Classification System (USCS). Representative soil samples were
collected from each unit, identified according to boring location and depth, placed in plastic bags
to protect against moisture loss, and were transported to the soil laboratory for supplemental
Quality Geo NW, PLLC
Geotechnical Investigations & Engineering Consultation
Phone: 253-202-3675 1 Web: qualitygeonw.com I Mail: 420 Golf Club Rd SE, Ste 203, Lacey, WA 98503
8112 Talbot Rd Consult
5/16/2021
classification and other tests.
SUBSURFACE CONDITIONS:
Quality Geo NW, PLLC
Project # QG20-082 & QG21-039
A general characterization of on -site soil units observed through local exposures on site within
fallen tree pits.
• Cover Soils — Silty Sand (SM):
In both test pits, QG encountered an overriding layer of orange/brown silty sand. The soils
were heavily mottled, suggesting the delayed percolation of stormwater. This unit extended
from the surface to as deep at 3 feet across the site.
• Outwash Sand — Sand (SP):
The dominant underlying native soil across the upland and slope, appears to comprise a
tan/gray sand, typically becoming cleaner with depth. These soils more closely resemble a
recessional glacial outwash soil rather than a dense glacial till. The unit extended from 3
feet beyond 10 feet below present grade.
Groundwater elevation is inferred to exist at or near the creek elevation.
DISCUSSION 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.
Infiltration Rate Determination:
During test pit excavations for general site investigation, QG additionally collected representative
samples of native soil deposits among potential infiltration strata and depths. Soils across the site
were identical. Representative soil samples were selected to characterize the sitewide infiltration
conditions.
We understand the project will be subject to infiltration design based on the Washington
Department of Ecology Stormwater Management Manual for Western Washington (DoE
SMMWW). For initial site infiltration characterization within the scope of this study, laboratory
gradation analyses were completed including sieve and hydrometer tests for stormwater design
characterization and rate determination to supplement field observations. Results of laboratory
testing in terms of rate calculation are summarized below.
8112 Talbot Rd Consult Quality Geo NW, PLLC
5/16/2021 Project # QG20-082 & QG21-039
Laboratory results were interpreted to recommended design inputs in accordance with methods of
the 2019 DoE SMMWW. Gradation results were applied to the Massmann (2003) equation (1) to
calculate Ksat representing the initial saturated hydraulic conductivity.
(1) log10(Ksat) = -1.57 + 1.90*D10 + 0.015*D60 - 0.013*D90 - 2.08*ff
Corrected Ksat values presented below are a product of the initial Ksat and correction factor CFT.
For a generalized site -wide design situation, we have applied a site variability factor of CFv = 1.0
along with typical values of CFt = 0.4 (for the Grain Size Method) and CFm = 0.9 (assuming
standard influent control).
(2) CFT=CFvxCFtx CFm = 1.0 x 0.4 x 0.9 = 0.36
Results were cross-referenced with test pit logs to determine the validity and suitability of unique
materials as an infiltration receptor. Additional reduction factors were applied for logical rate
determination.
Table 1. Results Of Massmann Analysis
TP
Sample
Unit
Soil
Fines
Ksat
Corrected
LT Design
Cation Exchange
Organic
#
Depth
Extent
Type
D10
D60
D90
o
(/o)
(in/hr)
Ksat
Infiltration
Capacity
Content
BPG
t
(in/hr)
Rate in/hr
(meq/100g)
%
1&2
5.0
3 to —1 .5
sand
0.194
1.678
28.75
3.2
34.27
11.25
2.5
2.8
0.8
Beneath sod and cover soils, the lower clean outwash sands were observed to generally exhibit
minimal fines content and minimal oxidation patterns. Based on the proximity of the slope to the
house, it is advised that the project engineer consider infiltration between the house and the street,
rather than between the house and the slope. Based on the thick overriding layer of silty sand we
do not recommend the designer pursue pervious pavements. The amount of overexcavation
required to expose suitable infiltration soils is too excessive for such efforts at this time.
For typical in -ground infiltration features infiltrating into the underlying clean sand (with
their base below 3 feet in dept) we recommend a maximum design rate of up to 2.5 inch/hour
be considered, which is typically suitable for most infiltration galleries, and considers potential
reductions from compaction during construction. These rates are considered applicable to all areas
of the subject site at the specified depths.
QG recommends the facility designer review these results and stated assumptions per reference
literature to ensure applicability with the proposed development, level of anticipated controls, and
long- term maintenance plan. The designer may make reasonable adjustments to correction factors
and the resulting design values based on these criteria to ensure design and operational intent is
ki
8112 Talbot Rd Consult
5/16/2021
Quality Geo NW, PLLC
Project # QG20-082 & QG21-039
met. We recommend that we be contacted if substantial changes to rate determination are
considered.
Treatment Potential:
Depending on stormwater and runoff sources, some stormwater features such as rain gardens or
pervious pavements may require treatment. Stormwater facilities utilizing native soils as treatment
media typically require Cation Exchange Capacities (CEC) of greater than 5 milliequivalents per
100grams (meq/100g) and organic contents greater than 1% (this may vary depending on local
code). Soils at likely infiltration depths did not meet these requirements. If treatment of stormwater
is required, treatment media may need to be added, depending on the project engineer's final
design.
CLOSING. -
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
)21
—Luke Preston MST F G_ I
Licensed Engineering Geologist
Attachments: Limitations
Appendix A. Site Region and Vicinity Maps
Appendix B. Aerial Site Map
4
8112 Talbot Rd Consult
5/16/2021
LIMITATIONS
Quality Geo NW, PLLC
Project # QG20-082 & QG21-039
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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8112 Talbot Rd Consult Quality Geo NW, PLLC
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Appendix B. Aerial Site Map
Talbot Rd
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Site Map Source: King County L1DAR & WA DNR
Quality Geo, PLLC g112 Talbot Rd Scale & Locations are approximate
Not for Construction
Quality Geo NW, PLLC
Geotechnical Investigations & Engineering Consultation
Phone: 253-202-3675 1 Web: qualitygeonw.com I Mail: 420 Golf Club Rd SE, Ste 203, Lacey,
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8112 Talbot Rd Consult Quality Geo NW, PLLC
5/16/2021 Project # QG20-082 & QG21-039
Appendix C. Laboratory Testing Results
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Geotechnical Investigations & Engineering Consultation
Phone: 253-202-3675 1 Web: qualitygeonw.com I Mail: 420 Golf Club Rd SE, Ste 203, Lacey, WA 98503
8112 Talbot Rd Consult
5/16/2021
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FERIALS TESTING
CHRYSL€R DR
urlington , WA 99233
�boratory #: 521-07463
Cation Exchange CEC rneq/100g
Other Tests:
Organic Matter (L01 360}
Quality Geo NW, PLLC
Project # QG20-082 & QG21-039
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Date Received: 4/27/2021
Grower 21SO19 QC
Field: 821-0540 HILLTP-1 AT . _
Sampled By:
Customer Account 4:
Sall Test Results Customer Sample ID:
2.8 PH 1:1
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Est Sat Paste E.C. rn,mhos/cm
Effervescence
S %.
Ammonium - N mg/kg
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