HomeMy WebLinkAboutBLD2021-00602 - 09 Geotech ReportMud Bay Geotechnical Services, LLC• 1001 Cooper Pt Rd SW Ste 140 PMB 108 • Olympia, WA 98502
360-481-9784 • cheathman@mudbaygeotech.com
January 23, 2022 Job: 1592-JEF
Page 1
Subject: Quilcene School District #48
294715 US Hwy 101,
Quilcene, WA 98376
Geotechnical Addendum No. 2
Additional Geotechnical Recommendations for Stormwater Design
Dear Lance Fay,
It is our understanding that the stormwater design will consist of using the very porous gravelly
native material that was observed starting at or just below the ground surface at the site, with
infiltration occuring at a shallow depth within the native material left in place as an infiltration
basin. The purpose of this addendum is to further clarify that there will be at least 3 feet of
clearance from infiltration of stormwater to the groundwater table, and provide a confirmation
that there is not a hardpan infiltration restricive layer at the site.
Groundwater was not encountered during our subsurface exploration of the site on June 26,
2021. The Big Quilcene River runs roughly 1,300 feet south of the subject parcels. As a part of
the project investigation, septic records for nearby parcels were reviewed with the following
findings:
1. On the north side of East Rose St, the Quilcene School is free of ground water to a depth
of 6 feet below ground surface (SEP1998-61, Tillman Engineering, July 1999, parcel #
937200201);
2. To the southeast, at 131 Mcardle Ave, from approximate ground surface of 20 feet, there
was no groundwater observed to elevation 16 feet (SEP1985-00075, parcel #702241003);
Mud Bay Geotechnical Services, LLC• 1001 Cooper Pt Rd SW Ste 140 PMB 108 • Olympia, WA 98502
360-481-9784 • cheathman@mudbaygeotech.com
3. To the northeast, at 83 Mcardle Ave, from approximate ground surface of 19 feet, there
was no groundwater observed to elevation 13 feet (SEP1996-00317, Tom McClanahan,
August 1996, parcel #937200401- now 410);
4. And the attached, to the east, at 121 Mcardle Ave, from approximate ground surface of
20 feet and 19 feet, groundwater was observed at elevation 15.5 feet (SEP2003-00173,
Septic Sence- Janet Welch, July 2003, parcel #937200403).
The site has an average elevation of approximately 29 feet. Based on the septic data, the
groundwater at the site is greater than 10 feet below the existing ground surface even during the
wet time of the year, providing more than the 3-foot minimum clearance requirement from the
bottom of stormwater infiltration facilities to the groundwater.
We reviewed the test pit explorations that were performed for the project, paying particular
attention to the soils that were encountered, and the soils that were observed and samples that
were collected. Based on that review, it should be noted that wherever refusal blow counts were
recorded on the boring logs, these were in all cases inflated blow counts due to the particle size
that the tests were performed, and not indicative of a hardpan infiltration restrictive layer.
Starting at 48 to 72 inches gravelly soils with particle sizes ranging from 8-inch cobbles to 12 to
18-inch boulders were observed. The absence of a hardpan layer was verified with the infiltration
testing that was performed on site. The water was observed to infiltrate downward at a very high
rate through the test pit floor at a very high downward gradient, as opposed to laterally flowing
through the side walls of the test pits.
We appreciate the opportunity to serve your geotechnical needs on this project and look forward
to working with you in the future. Please contact us at your earliest convenience if you have any
questions or would like to discuss any of the contents of this letter.
Sincerely,
Chris J. Heathman, P.E
Mud Bay Geotechnical Services, LLC
Digitally signed by
Chris Heathman, PE
Date: 2022.01.23
20:52:41 -08'00'