Cultivating
the Field Museum
Underground Space to Allow Lakefront Facility to Add to its Artifact Collection
by Craig Barner
Insects, dinosaurs and seemingly everything in between are
studied at Chicago's Field Museum of Natural History.
Field's drive to acquire knowledge has resulted in a collection
of more than 22 million artifacts - reportedly the fourth
largest of its kind in the world.
A problem has gradually arisen at the Lake Michigan landmark
because of the decreasing amount of storage space for newly
acquired relics and other uses, said Jim Croft, vice president
of finance and administration for the Field. Several alternatives
were studied.
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In previous years, about 150,000 sq. ft. of area was freed
for storage and other uses by covering over and converting
five light wells into use, but only one light well remains.
Off-site storage was considered, but quick access to the collection
is critical because it is used for ongoing research.
"Getting specimens from point A to B would create the
possibility of damage, and many of them are priceless artifacts,"
Croft added. "You would also have to move scientists
back and forth."
Adding space onto the building could conflict with city ordinances
adopted since the structure was originally erected in 1921
that bar construction within a certain distance of the lakeshore.
Field officials were left with one alternative.
"We chose to go down, outside the building," Croft
said. The $65 million Collections Resource Center is being
built below grade.
The two-level facility will provide 182,000 sq. ft. of space
for the museum's dinosaur bones, totem poles, meteorites and
other objects.
The L-shaped space will wrap around the east facade and the
part of the south facade east of the entry stairs. Construction
started in fall 2002 and is expected to be complete in September.
Other construction, including the $23 million central plant,
$10 million East Entrance and deferred maintenance, means
a total of about $126 million in work is under way.
CRC Site Poses Issues
Planning was needed to design and excavate the CRC due to
its location immediately next to the museum and McFetridge Drive,
and also near the lake.
The hole needed to keep out water because the lake is only a
few hundred feet away, said Frantz Cartright, president of Evanston,
Ill.-based CATH Associates, the project manager.
More important, the lateral strength of the below-grade walls
was critical to resist overturning due to earth pressures.
"If you excavate with conventional excavation, the dirt
will fall in and the piles will be destabilized," he said.
"That would be detrimental to the structure of the building
and the street."
A sheet-piling system that would buttress the CRC walls was
considered, but the sheets would consume a large amount of space
in the hole. Less space would then be available for storage.
A bentonite slurry wall was selected instead, Cartright said.
The slurry, which looks like chocolate, prevents the penetration
of water because bentonite has a higher density, and its properties
thwart the excavation sides from collapsing.
Avoiding Collapse
Coordination of the slurry wall construction was carefully
orchestrated to avoid cave-ins during construction.
The normal wall segments were about 20 ft. long, 2 ft. wide
and 75 ft. deep, and planning ensured against the collapse of
the narrow and deep trenches, said Tom Chlipala, CATH project
manager. Concrete guide walls about 26 in. apart were built
on each side of the slurry wall location to guide excavation.
"So we had two beams that are parallel everywhere the slurry
wall is going to be," Cartright added.
They prevented the trench edges from toppling during excavation
with a rectangular clamshell - a bucket with jaw-like halves.
The trench was completed, and the slurry was pumped in.
A rebar cage with one or two pipes within was fabricated on
the ground, lifted and lowered into the hole. Above at the entrance,
the guide walls prevented the rebar from scraping the trench
walls during insertion.
The pipe in the cage was used to convey concrete into the cavity
bottom, and the slurry was pushed out as the cage filled up.
Above ground, loops captured the slurry so it could be filtered
in a truck and reused for the next wall segment.
A system of keys helped link the wall panels together. Before
the concrete hardened at the top, a V-shaped steel piece was
dropped in near the edge and later pulled out.
Concrete pumped into the adjacent wall filled in the key, and
the two were linked.
"Ultimately, you end up with a bunch of panels with a key
to each other," Cartright said.
Handling the Pressure
Excavation began, but forestalling collapse remained a top
concern because lateral pressure on the wall builds up as the
hole goes deeper.
Whalers, or horizontal beams, were welded to cleats on the wall
interiors at two different precalculated heights. They hold
the ends of the 26-in.-diameter struts, or steel pipes that
in some cases are about 200 ft. long. Occasionally, mid-span
supports hold up the struts.
"The purpose of the whaler beams is to ensure the pressure
they exert against the wall panels is uniformly applied,"
Cartright added.
Peering at the site from above brings to mind sewer grating
because of the immense size and large quantity of struts beneath
the museum.
Some struts would have been excessive in length, so a few tiebacks
tensioned with cables were used to provide lateral support.
The tiebacks are anchored in the ground and hold the walls in
position.
About 155,000 cu. yds. of dirt was removed from the hole, Chlipala
said.
Columns have started to be formed, and they will be followed
by slabs and walls. A slab midway down the space will make up
the separation between the two levels, and another slab at the
top will become the CRC's cover, which will support the landscaping.
Central Plant Improved
Improvements were needed for the museum's nearly complete
central plant partly because some systems dated to the 1950s.
Extra capacity was also incorporated because of the museum's
expected growth and the changing nature of scientific investigation,
Croft said. More energy is needed to support increasingly sophisticated
research.
"Research is moving from the old type of examining specimens,
measuring them and describing them," he added. "We're
doing molecular biology and DNA sequencing."
The reliability of the plant was an issue, too.
"In some cases, scientists have 20 years or their life
tied up in [an experiment in] a freezer," Croft said. "Then
the electricity could go out. If you don't keep the experiment
at a certain temperature, you're just lost 20 years of work."
Newly installed components include chillers - for ice and chilled
water - steam boilers, fire pumps and air-handling units.
Energy-saving initiatives were implemented, such as installing
a system to make ice for the HVAC system at night when the cost
for power is cheap. Demand costs for energy are expected to
go down about 25 percent.
Like the CRC, the central plant was located in two underground
levels, but southwest of the museum building, Cartright said.
The plant's small size allowed sheet piling to be used, rather
than a slurry wall, for retention.
Inside the museum, a switch was made from a two-pipe to a four-pipe
system to provide improved heating and cooling efficiency. The
four-pipe system provides greater flexibility to cool or heat
selected building areas. Fourteen new risers will be needed
because of the greater amount of piping and ductwork, but only
four have been built.
Improving Access
The yet-to-start East Entrance will improve museum accessibility.
A comparatively short distance separates the North Parking Lot
of next-door Soldier Field from the building's east side. By
comparison, the only existing ground-level door, the West Entrance,
is about three to four city blocks from the parking.
The new entrance will become the main way in for guests with
strollers, the disabled and schoolchildren, Croft said.
A marble-clad pavilion will form the entrance and attach to
the building. It will be built over the CRC.
A unique feature is that a skylight composed of tensioned
cables will hold the glazing.
"All the accommodations for the East Entrance are presently
being finalized," Cartright added.
Useful Source
Visit www.fieldmuseum.org/research_collections/
to find out more about the artifact collection at the Field
Museum of Natural History.
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