By: Wayne Adaska
When the new FedEx hub at Alliance Airport in Fort Worth, Texas, opens in
late 1997, the clearinghouse for packages shipped between the West Coast
and the southwestern U.S. will include more than 300,000 sq yd of reinforced
concrete pavement underlain by a cement-treated base and cement-treated
subgrade.
Unlike most airport terminals, there are no passengers to load and unload;
instead there are packages. In the hours just after midnight, FedEx planes
each day fly into these hubs carrying cargo. Workers then unload the packages,
sort them by their destination and reload the planes with the sorted cargo
in two to four hours.
The Alliance facility will be the smallest of FedEx's hubs even though it
has the long-term potential to expand into the company's second-largest
distribution center.
The initial plan for the FedEx distribution center was for 50 acres (242,000
sq yd) of paving at the Alliance Airport in North Fort Worth, Texas. Additional
scope was added later in the project with the potential for another 50 acres
of paving. The actual total paving is about 330,000 sq yd or about 70 acres.
Five pavement areas will be part of the project:
- Aircraft taxi lanes will support fully loaded aircraft arriving and
departing. There will be 50 acres of taxi lane, aircraft gates and concrete
paving between all the areas. This area is designed to have 9 in. of cement-treated
subgrade, 9 in. of cement-treated base, and 14 in. of portland cement concrete
surface.
- The ramp area where aircraft are parked and serviced will support loading
and unloading of the aircraft. The pavement is designed so any aircraft
that FedEx has or anticipates having can park in any position on the ramp.
It is designed for the heaviest aircraft. There are 242,000 sq yd of aircraft
paving. Huitt-Zollars Inc. designed a pavement that includes 9 in. of cement-treated
subgrade, 9 in. of cement-treated base, and 14 in. of concrete.
- The truck terminal area is where loaded tractor trailers come in to
the facility to the sort building. This pavement must support a high level
of repetition with heavy loads of highly channelized traffic. Truck paving
is 10 in. of jointed-reinforced concrete pavement over 6 in. of cement-treated
clay subgrade.
- The container storage area has a high volume of turnover in a 24-hour
period. Essentially this is where empty freight containers from the aircraft
are stored. Pavement design in this area is still under consideration, but
most of those areas will be similar to the truck-paving section.
- Employee parking has not been designed. But engineers say this will
most likely be a concrete pavement.
In designing the project, FedEx considered a long-range outlook in working
with engineers. FedEx had two concerns. It was looking for a dependable
sorting system and a dependable aircraft pavement with a long design life,
according to Huitt-Zollars the project engineers.
Designers used a 90-day, 750-psi, third-point loading design for the concrete
taxiway and ramp pavements. The third-point loading design was used rather
than a center-loading design because it is thought to be a more conservative
design.
Rone Engineers, the geotechnical and materials testing firm on the project,
chose a cement-treated base and cement-stabilized clay subgrade. The cement-treated
subgrade was less expensive and stronger than the lime alternative, says
Charles Jackson, P.E., vice president of Rone Engineers, Inc.
Jackson says the soil's bearing capacity will be improved, allowing overall
cost saving on the project. The cement-stabilized clay subgrade provided
a good all-weather working platform for construction and weather durability
under construction traffic.
Other considerations that went into designing the pavement were keeping
aircraft wing tips level, providing a separate drainage system for taxi
lanes and gates, and making sure there was no swelling of the soils or pavement
heaving.
Level wing tips aid in the loading and unloading of aircraft. The slope
from nose to tail of the aircraft can be no more than 1¦2%.
In addition to the contractor providing level paving, the design incorporates
tethers into the aircraft pavement to load and unload the wide body aircraft.
Crews tether the nose gear when it is nearly empty to keep the nose from
lifting up when loads are in the rear of the aircraft. Eye hooks are embedded
in the concrete for this purpose. Straps are attached to the aircraft and
concrete mass and dowels in the pavement resist uplift forces of up to 50,000
lb.
Drainage in the taxi lane and gate areas accommodate refueling the aircraft
and deicing at every gate. These chemicals played an important role in selecting
the joint sealant. Designers chose a neoprene joint sealant for its longevity
and for jet fuel resistance.
The difference between this concrete pavement and other airport pavement
is the vast area of concrete and FedEx's requirement that it be essentially
flat in gates and the method of drainage on the site. The pavement has a
50-year design life because FedEx wanted to look at value costs of the project
over time.
The gates are on a separate drainage system. Those areas drain to systems
with oil/water separators and underground containment areas. FedEx will
treat the first flush of drainage to remove the oils and fuels.
Paving contractor, Duininck Brothers, Grapevine, Texas, constructed the
subgrade, base and concrete paving. Concrete was batched on site. Most soils
in this area of Texas are clay with a high plasticity index (PI) of about
40 to 50. "The soils here are about the worst you can build an airport
on," says Rone Engineers' Jackson. "However, cement reduces swelling
in the clays and the cement gives the subgrade its needed strength."
For the subgrade, Duininck pulverized the soil prior to spreading cement.
Crews then spread dry portland cement and blended it with the soil using
a single-shaft CMI pulver mixer. They added water to bring the blended soil
and cement mixture up to optimum moisture content. Crews followed with two
passes with the pulver mixer, mixing and compacting the layer. Specifications
called for the processed material to be pulverized to 100% passing the 11¦2
sieve and 60% passing the No. 4 sieve. Compaction was specified to be a
minimum of 95% of Standard Proctor.
According to Ronnie Rone of Rone Engineers, the firm established a maximum
plasticity index of 12 (in-situ PI's were as high as 38), as well as a strength
of 250 psi for the treated subgrade. A cement content of 7% by dry weight
of soil produced the desired result. Tests with other commonly used stabilizers
failed to produce even 100 psi in the laboratory using as much as 9% addition
of the material.
When a seam of highly plastic montmorillonite clay was encountered during
construction, project engineers adjusted the cement factor in the field
to 9%, which was retained for the duration of the subgrade treatment operations.
At this level, strengths of some of the less plastic stabilized soils exceeded
500 psi after the cement processing.
Select granular material was mixed with cement and water in a pugmill to
produce the cement-treated base material. After mixing, the soil-cement
was placed in dump trucks and spread on grade using a jersey spreader.
Soil-cement was compacted using vibratory steel-drum rollers. Strength of
the cement-treated base was in the 1000 to 1200 psi range, compared to a
750 psi, 28-day strength requirement. Concrete paving followed completion
of the cement-treated base.
"There were no problems except for the weather," said Kyle Duininck,
project manager. "We placed about 2000 cu yd of concrete a day with
a Gomaco 3000 slipform paver. The work area was wide open and that made
the job easier. We had a tight grade tolerance, 1¦2% cross slope for
drainage, but it was no problem."
Holnam, Inc., Midlothian, Texas, supplied the cement. Construction began
in May 1995 and support structures are scheduled for completion next year.
Wayne Adaska is the director of public works for the Portland Cement
Association.
