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Chapter 3 Affected Environment, Impacts, Mitigation Measures, Significant Unavoidable Adverse Impacts Chapter 3

Affected Environment, Impacts, Mitigation
Measures, Significant Unavoidable
Adverse Impacts
3.1 EARTH


This section describes existing soil and geologic conditions on the North Bay site and in the site
vicinity, and evaluates potential impacts from development under the range of Alternatives. This
section is based on the January 2005 Technical Memorandum on Geotechnical Conditions,
prepared by Landau Associates (see Appendix A).

3.1.1
Affected Environment


Information on site and area conditions is based on a review of geotechnical borehole logs, and
published sensitive area maps and surficial geology maps, including information from the US
Geological Survey, Washington Department of Natural Resources, Soil Survey of King County,
Seattle Area Geologic Mapping Project, City of Seattle Sensitive Areas Maps, City of Seattle
Department of Planning and Development (DPD) project files, King County Technical Library,
Washington State Department of Ecology well records, and other sources.

Regional Topography and Geology

The site is located in the central portion of the Puget Lowland. Regional topography is
dominated by a series of north-south trending elongated ridges and glacial uplands. The
uplands are separated by large, glacially excavated troughs that were further modified by
geologic processes following the retreat of the most recent ice sheet, and are now partially
occupied by Puget Sound and other large bodies of water such as Lake Washington.

The geology of the Puget Sound region includes a thick sequence of overconsolidated glacial
and normally-consolidated non-glacial soils overlying bedrock. Glacial deposits were formed by
ice sheets originating in the mountains of British Columbia and from alpine glaciers that
descended from the Olympic and Cascade Mountains, during at least four glacial advances
between approximately 150,000 to 10,000 years ago. Between these glacial advances and
after the last glaciation, portions of the Puget Lowland filled with alluvial sediments deposited by
rivers draining the Cascades and Olympics. Existing topography, surficial geology and
hydrogeology in the project area were heavily influenced by the advance and retreat of the
Vashon ice sheet, the most recent glacial advance.

Site Topography and Geologic Conditions

For this Draft EIS analysis, the North Bay site has been divided into seven areas, as indicated
on Figure 2-3. Topography across the site varies considerably between the seven areas of the
site and in some cases within an individual area. Generally, the site slopes to the south. Area 1
(Existing Industrial Uses) is relatively flat with elevations that range from 10 to 20 feet above
Mean Sea Level (MSL). Area 2 (Uplands/Including Greenbelt) is relatively flat in the lower
portion with steeper slopes and more elevation variation along the greenbelt on the western side
of the site. The elevation of the lower flat portions of Area 2 ranges from approximately 10 to 25
feet MSL with a gradual slope to the south, while the steeper greenbelt portion ranges in
elevation from approximately 25 to 75 feet MSL. Area 3 (City Land) gently slopes to the south
with elevations ranging from approximately 8 to 17 feet MSL. Area 4 (West Yard) also gently
slopes to the south with elevations ranging from about 0 to 10 feet MSL. Area 5 (Armory)
slopes to the southwest with elevations ranging from 10 to 25 feet. Areas 6 and 7 (BNSF) are
both relatively flat with elevations ranging from about 10 to 25 feet MSL.
North Bay Master Plan Draft EIS
3.1-1
Earth
April, 2005 The North Bay site is situated within a north-south trending topographic trough that is locally
referred to as the Interbay Trough. The Interbay Trough is bounded by glacial uplands to the
west (Magnolia) and east (Queen Anne). These uplands are generally comprised of very dense
and hard glacial soils that were laid down during the advance and retreat of several glaciers.
The Interbay Trough is generally underlain by loose beach deposits, alluvium, and very soft to
stiff estuarine deposits laid down since the last glacial ice age. Since the late 19th century, the
Interbay Trough has been partially filled during previous development with various materials,
including landfill refuse. Additionally, previous development at the North Bay site included
certain pile-supported structures that are no longer evident. The piles for some of these
previous structures may have been abandoned in place.

Surficial Geology

Information on the surficial geology of the North Bay site was derived from area geologic maps
from the USGS (1962) and City of Seattle (2002). Generally, the surficial geology of Areas 1, 4,
6 and 7 and the lower elevations of Areas 2 and 5 are mapped as fill. The intermediate upland
portions of Areas 3 and 5 and the steep slope greenbelt area of Area 2 are mapped as glacially
consolidated soil with intermittent overlying landslide debris.

Geologic Units

Various geologic units are reported to have been encountered during the deeper subsurface
explorations that were reviewed for this Draft EIS (see Figure 2 in Appendix A). Some soils with
similar engineering properties that were created in different depositional environments have
been grouped together.

Very few geologic units have precise boundaries or contacts and the geology of an area can
change drastically both horizontally and vertically within a few feet or, in some instances, can
remain fairly consistent for hundreds of feet. Typical descriptions of the geologic units
encountered during previous explorations at or in the vicinity of the site are presented below.
The Vashon-age glacial till and the older units present at the site have been glacially
consolidated and are typically very dense or hard. Over time, the glacially carved Interbay
Trough has been filled with non-glacially over-ridden alluvium, beach deposits, estuarine
deposits, landslide debris, fill and landfill material. Generally, the top of the glaciolacustrine or
glacial outwash deposits delineates the contact between the dense or hard glacially over-ridden
soils and the loose to soft overlying non-glacially over-ridden soils. Figure 3.1-1 depicts the
approximate depth to glacially consolidated soils in each area of the site. See Figures 3 through
7 in Appendix A for cross sections of the site.

Fill / Modified Land

The term modified land is used to describe surficial geologic conditions that have been
modified by human activities such as, but not limited to: cutting, filling, grading, leveling,
sluicing, shoreline protection, and railroad bed construction. Fill material at the site is generally
composed of glacial soils or alluvium that have been relocated from various locations and may
consist of clay, silt, sand, and/or gravel. Dumped rock, construction debris and boulders may
also be present. In some areas, some effort at compaction may have been made during
placement of these fills, whereas in other areas, limited effort at compaction may have been
made. Consequently, the relative density could vary widely and the specific engineering
properties of the fill materials could be very different from location to location.
North Bay Master Plan Draft EIS
3.1-2
Earth
April, 2005
Landfill


Although landfill material generally comes under the term fill/modified land, it has been
expressly called out to reflect material associated with the nearby Interbay Landfill (a now-
closed, approximately 40-acre landfill located near the northeast portion of the site). Typically
this material consists of refuse and construction debris. Landfill material is only located in a
small portion of the site.

Landslide Deposits

Landslide deposits typically consist of intermixed debris from nearby soil units transported
downslope as landslides, slumps, and earth flows. The slides often occur along steep hillsides
and along the sides of steep stream gullies that have eroded headward from shorelines and
valleys. Most landslide deposits consist of a mixture of the overlying material, typically glacial till
and outwash and underlying silts and clays. Organic debris, including brush, logs and tree
stumps, is often embedded in the sli