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Renton City Hall is a 5-story steel and glass structure with an integrated, 3-level,
reinforced concrete parking garage. Desired site access improvements included an elevator
shaft and an open-air, monumental staircase extending from the first parking level to the
third level at the northwest corner of the garage, a new driveway, and a two-lane ramp
providing additional access to the second and third parking levels.
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The proposed ramp consisted of an elevated pavement extending from grade (first parking
level) to an elevation of approximately 21.5 ft above grade (third parking level). The ramp
alignment contains two 90-degree curves along the main drive and a "T" extension at
the second parking level.
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Subsurface soil at the site consists of approximately 34 ft of poor soil overlying competent
sands. The existing parking garage is supported on concrete piles extending to the competent
sands. The poor upper soils include loose alluvial sands and approximately 12 ft of
compressible peat. A previous geotechnical investigation predicted about 10 inches of
settlement near the high point of the ramp, and about 8 inches of settlement near the lower
portion of the ramp if a traditional soil embankment supported the ramp. Recognizing the many
problems associated with settlement of this magnitude, EPS geofoam was recommended for use as
lightweight structural fill to substantially reduce the expected ramp settlement.
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The following three alternatives were identified for support of the proposed parking garage
ramp: a pile foundation system and structurally supported span; combination soil fill
embankment, retaining wall, and short bridge span support; and soil/geofoam embankment
support alternatives. The primary advantages and disadvantages of each identified alternative
are summarized below.
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Constructing a pile-supported ramp would result in substantial design and construction costs.
Due to highly unfavorable soil conditions, piles extending approximately 35 to 40 ft or more
below grade were required to achieve acceptable performance. Bridge spans extending between
the pile groups add significantly to the overall cost, rendering this alternative
uneconomical.
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Construction of the parking garage ramp on a conventional soil fill embankment presented
several problems. The weight of the fill placed over the compressible site soil would result
in significant consolidation of the underlying soil and settlement of the ramp. Settlement of
the ramp would require periodic adjustments to the connection between the ramp and the garage
as well as periodic pavement maintenance. Placement of fill in proximity to the existing
structure would impose downdrag loads on the pile foundations supporting the structure.
Retaining walls and short bridge spans would be required to separate the fill from the
parking garage structure. The expected settlement could distort and damage the retaining
walls, the bridge spans, and the existing parking garage. The risk of structural damage to
the parking garage, frequent maintenance and repair of ramp connections and pavement, and the
high costs associated with retaining wall and bridge span design and construction rendered
this alternative undesirable.
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Utilization of geofoam as embankment backfill would essentially eliminate problems associated
with settlement of the proposed ramp. By over-excavating the subgrade soil (removing weight)
and placing geofoam below current grade, a balanced soil/geofoam embankment design would
impose essentially no significant additional loads to the foundation soils. Without
significant additional loading to the foundation soils, the anticipated settlement and the
risk of structural damage would be negligible.
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Construction proceeded through the wet season and the entire project was completed in the
Spring of 2000. The City of Renton, at first skeptical of the use of geofoam, highlighted the
project on their web site and attributed a cost savings of $600,000 to the use of geofoam
(see link below).
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EPS geofoam embankment construction.
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Guardrail construction following geofoam placement and sideslope construction.
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