HomeMy WebLinkAboutDrywell for Retaining Wall Drains - Earth Solutions.pdf15365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 • (425) 449-4704 • FAX (425) 449-4711
Earth Solutions NW LLC
Geotechnical Engineering, Construction
Observation/Testing and Environmental Services
June 13, 2022
ES-7334.05
MainVue WA, LLC
121 – 3rd Avenue
Kirkland, Washington 98033
Attention: Ms. Allison Rothstein
Subject: Retaining Wall Footing Drainage
Brennon Landing (Vaughn Trust Property)
1400 Aberdeen Avenue Northeast
Renton, Washington
Reference: Earth Solutions NW, LLC
Geotechnical Engineering Study, ES-7334, dated March 16, 2021
Greetings, Ms. Rothstein:
As requested by Barghausen Consulting Engineers, Inc., Earth Solutions NW, LLC (ESNW) has
prepared this letter for the subject project. The current design for the proposed two-tier Keystone
retaining walls calls for the footing drains to outlet into drywells. We understand the City of Renton
has expressed concerns regarding the current drainage design, and the purpose of this letter is
to summarize the feasibility of using drywells as footing drain outlets from a geotechnical
standpoint. It should be noted that the use of drywells for this purpose was initially recommended
by the undersigned geotechnical engineer.
As summarized in the referenced geotechnical report, the subject site and surrounding area are
underlain by recessional outwash deposits, which were encountered largely as poorly graded
sands. The on-site recessional outwash deposits are considered well drained and possess
excellent capacity to function as an infiltration receptor, as evidenced by our previous infiltration
testing (a long-term design rate of 14.8 in/hr was recommended) and laboratory sieve analyses
(samples were largely “clean” at depth, containing less than 5 percent fines). With respect to the
anticipated volume of groundwater collected by the footing drains, our field observations and
understanding of the local geologic setting suggest a negligible volume of groundwater is likely
to be collected. The footing drains are designed to collect water flowing through the subsurface
(interflow), which is typically heaviest at the contact between relatively permeable and
impermeable soil horizons as the groundwater perches atop the less permeable strata. For the
subject site, there is no evidence (per our June 2020 field investigation) that significant volumes
of interflow and/or perched groundwater seepage zones develop in the native recessional
outwash.
MainVue WA, LLC ES-7334.05
June 13, 2022 Page 2
Earth Solutions NW, LLC
The developed plat condition will include significant areas of new impervious surfaces, e.g.,
roadways and structures. Very little of the current pervious (exposed) surface area will remain in
the developed plat condition, which minimizes the potential for water to enter the substratum as
interflow and potentially be collect by the retaining wall footing drains. Additionally, water
collected by the residential footings and roof drains will be routed to individual-lot drywells,
separate from the drywells proposed for the retaining wall footing drains.
Because the native soil is well drained, and because the anticipated volume of water collected
by the footing drains is anticipated to be negligible, it is our opinion the use of drywells as footing
drain outfalls is feasible from a geotechnical standpoint. ESNW should be retained to observe
drywell installation at the time of wall construction to confirm that subsurface conditions are as
anticipated in both this letter and the referenced geotechnical report.
We trust this letter meets your current needs. Please call if you have any questions about this
letter or if we can be of further assistance.
Sincerely,
EARTH SOLUTIONS NW, LLC
Keven D. Hoffmann, P.E.
Associate Principal Engineer
cc: Barghausen Consulting Engineers, Inc.
Attention: Mr. Trent Skoda, P.E. (Email only)
Mr. Barry Talkington, P.E. (Email only)
Mr. Tom Barghausen, P.E. (Email only)
06/13/2022