HomeMy WebLinkAboutEX_08_Geotechnical Memo15365 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
March 7, 2022
ES-7126.01
Mr. Huy Nguyen
15400 Southeast 155th Place, Unit 99
Renton, Washington 98058
Subject: Geotechnical Review: Infiltration Trenches
Proposed Short Plat
3804 Meadow Avenue North
Renton, Washington
Reference: Earth Solutions NW, LLC
Geotechnical Engineering Study, ES-7126, dated March 12, 2020
Encompass Engineering & Surveying
Preliminary Short Plat Plans, Drawing No. 20591, dated March 7, 2022
2017 City of Renton Surface Water Design Manual (2017 Renton SWDM)
Greetings, Mr. Nguyen:
As requested by Encompass Engineering & Surveying (Encompass), Earth Solutions NW, LLC
(ESNW) has prepared this letter for the subject project. Based on our understanding of site
conditions and review of COR Maps, a high erosion hazard (EH) area is present along the eastern
margin of the subject site. The City of Renton (City), pursuant to the requirements of the
referenced 2017 Renton SWDM, requires that a geotechnical professional review the referenced
plans because the infiltration trenches are sited within 200 feet of an EH area.
As summarized in the referenced geotechnical report, the site is underlain generally by one-and-
one-half to four-and-one-half feet of fill, with underlying native recessional outwash deposits.
Previous infiltration testing within the recessional outwash (at TP-2, approximately three-and-
one-half feet below grade) resulted in a recommended long-term infiltration rate of 5 in/hr for the
tested location and depth. In our opinion, the previous infiltration testing indicates the native
recessional outwash possesses moderate to high capacity to function as a suitable receptor soil
for infiltration systems.
Mr. Huy Nguyen ES-7126.01
March 7, 2022 Page 2
Earth Solutions NW, LLC
ESNW recommends the infiltration trenches be sited within the well-drained, native recessional
outwash at depth. Topsoil, existing fills, silt-rich soil, or otherwise unsuitable material must be
completed advanced through such that the infiltration trench bottoms are sited entirely within the
native recessional outwash. Over-excavations, as recommended by ESNW at the time of
construction, should be restored using washed drain rock or gravel backfill per City Standard Plan
225.20. Based on our review of the referenced plans, these recommendations have been
incorporated into the infiltration trench designs.
Provided the recommendations outlined in the referenced geotechnical report and this review
letter are incorporated into construction, it is our opinion the infiltration trenches are feasible, from
a geotechnical standpoint, as depicted on the referenced plans. Considering cumulative impacts
from the project and surrounding area under full built-out conditions, it is our opinion the proposed
infiltration trenches will not adversely affect the site and surrounding area.
ESNW should be contacted to observe and document infiltration trench installation.
Supplementary recommendations may be provided at the time of construction based on observed
conditions.
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: Encompass Engineering & Surveying
Attention: Mr. Costa Philippides, P.E. (Email only)
03/07/2022