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17522 Bothell Way Northeast
Bothell, Washington 98011
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www.riley‐group.com
TECHNICAL MEMORANDUM
Date: April 24, 2023
To: Andrew Van Gordon
City of Renton
From: Kristina Weller, PE
The Riley Group, Inc.
Subject: Dreamliner Project
511 Airport Way
Renton, Washington
RGI Project No: 2020‐618‐1
RGI previously prepared a geotechnical report for the proposed project. The soils encountered consisted
of interbedded layers of silt, sand, and silty sand. Groundwater was encountered at 10 feet below the
existing grade at the time of drilling. Based on the moisture content of the soils, the water levels may be
as shallow as 8 feet if wells were installed.
The soil conditions and water table represent a liquefaction hazard, therefore; we recommended
installing aggregate piers to mitigate the predicted settlements and allow suitable support for the
building.
We understand the City of Renton has requested the project explore the potential for underground
parking. Based on the water table on the site and the need for ground improvements for support of the
structure; the excavation below grade for a level of parking would require significant dewatering and
shoring, which may affect the adjacent structures, utilities, and streets. In addition, the foundation and
likely the slab would either need to be watertight construction or have an underslab drainage system.
Based on the plans, the temporary dewatering and underslab drainage system would have to be
pumped to the storm system and the system is unlikely to be sized for the additional flow. In many
jurisdictions, permanent groundwater discharge is not allowed to the storm system which would
necessitate watertight construction for a portion of the below grade level.
The additional costs associated with shoring, construction dewatering, permanent waterproofing, and
drainage for a below grade level would likely make the project infeasible from cost and construction
standpoint.
EXHIBIT 12DocuSign Envelope ID: 23F4BD16-5184-4ACD-B93D-53CB9BFD13D7