March/April 2024 - 90

Site Description
The project is located in south Seattle
within a former tidal estuary. The
historically meandering Duwamish
River was straightened around the
beginning of the 20th century. The river
itself is a U.S. Environmental Protection
Agency (EPA) Superfund Site for which
clean-up activities are ongoing. The
plant site covers 3 acres (1.2 ha). Prior to
construction, several businesses
occupied the site.
Background
The authors' employer led a consultant
design team that was retained by King
County in 2013 to design a combined
sewer overflow (CSO) treatment system
that would meet strict water quality
requirements. The county had additional
goals for operability, maintainability,
reliability and sustainability, as
well as being good neighbors. Schedule
was a key driver for many project
decisions. Project milestones were
mandated by the EPA consent decree
and contractually enforced by the
designer's contract with the county.
Substantial completion was set for
October 2022 with the facility officially
treating CSO in December of 2022.
Realizing the complexity of the project
and the site conditions, the planned
schedule incorporated 6-12 months of
float into the various design phases and
construction contracts.
One of the decisions made during
predesign was to contract construction
in four separate packages: plant site
demolition and contaminant remediation,
plant site construction, conveyance
construction, and outfall construction.
Project elements
The reasons for having four contracts
instead of a single contract were
primarily related to unique contractor
expertise but secondary benefits were:
* Removal of the existing structures
in the demolition contract allowed
the geotechnical designers to
further explore and define the
subsurface conditions prior to
bidding the much larger plant
construction contract.
* Forced schedule contingencies were
built into the timing of the outfall
and conveyance contracts, a
fortuitous decision because of work
de l ays caused by numerous
subsurface obstructions.
Subsurface Conditions
Before human modification, the Duwamish
River was a natural distributary
channel of rivers that originated on
the flanks of Mount Rainier. After
deglaciation, a fjord canyon occupied
the present location of the Duwamish
River valley. This has rapidly infilled
since the mid-Holocene, largely by
sediment derived from Mount Rainier.
Episodic volcaniclastic debris flows
(lahars) from Mount Rainier caused the
delta front to prograde rapidly towards
its present position at the southeast end
of Elliott Bay. This recent geologic
history, the episodic glaciation that
preceded it and industrial development
since about 1900 have resulted in:
Comparison of actual versus compliance schedule
90 * DEEP FOUNDATIONS * MAR/APR 2024

March/April 2024

Table of Contents for the Digital Edition of March/April 2024

TOC
March/April 2024 - Intro
March/April 2024 - 1
March/April 2024 - 2
March/April 2024 - TOC
March/April 2024 - 4
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