JOHN JAMES AUDUBON BRIDGE ELEVATION
soil in the region is soft, and driven piles are impractical because of the lack of lateral support in the upper regions. However, sunken caissons present such inherent risks as limited
control of geometry and lack of flexibility. Other challenges
include working at depth with pressurized air and handling
the delivery of large precast-concrete elements. The request
for proposals allowed the bidders on this project to use either
dredged caissons or drilled shafts. Given the particular conditions of the bridge site and the risks that would be encountered, the design/build team selected drilled shafts.
BUCKLAND & TAYLOR LTD., TOP; PHOTO BY JOE DUNN, COURTESY OF AUDUBON BRIDGE CONSTRUCTORS, BOTTOM
Each tower is supported by 21 drilled
shafts measuring 8 ft in diameter. The
shafts are joined by an 18 ft thick reinforced-concrete pile cap that runs 160 ft
along the longitudinal axis of the bridge
and 64 ft transversely. The bottom of
the pile cap is located at minimum low
water elevation, 5 ft above mean sea level, to prevent vessel impact on an individual shaft. A pedestal that measures
24 ft in the longitudinal axis of the
bridge and 140 ft transversely extends
5 ft above the maximum high-water level to 61 ft above mean sea level to prevent vessel impact on the hollow sections of the towers supported above. The
shafts themselves are not conventional and required sophisticated tools and
procedures for construction. Each shaft
comprises two distinct elements: the
lower portion is of conventionally reinforced concrete, and the upper portion is
reinforced with an integral steel casing.
The casing in the upper portion of the
shaft serves as a form for the concrete when the shaft is placed
in open water or soft soils; it also serves as a load-carrying element to enhance the lateral capacity of the shafts.
The lateral design of the shafts was dictated by the possi-
bility of vessel impact. In anticipation of the combination of
two concurrent extreme events, the shafts were designed for
the maximum anticipated vessel load in combination with
half of the 100-year design scour depth. Live load and wind
were combined with the full 100-year design scour depth. To
resist the high vertical loads on the drilled shafts—a factored
resistance of approximately 5,000 tons—the shafts were ex-
tended deep into the dense sand layer at the bottom of the
Mississippi. A conventional drilled shaft of this type would
normally develop resistance primarily through skin friction.
However, skin friction would have required additional length
that would have extended the shaft into a hard clay layer that
had significantly less bearing capacity. Therefore, the tips of
the shafts were kept in the dense sand layer and a “tip grout-
ing” technique was used to enhance the end bearing capacity.
By injecting the tip of the shafts with pressurized cementi-
tious grout after the shaft concrete had
cured, the soil at the tip was effectively
recompacted to restore the end bearing
capacity lost by excavation of the shaft.
The effectiveness of this tip grouting
was demonstrated by the performance
of several static load tests using an Os-
terberg cell embedded in the concrete.
The final tip elevations of the shafts
were 175 to 180 ft above mean sea level.
The John James Audubon Bridge deck is
supported by 136 parallel-strand stay cables
arranged in a traditional semiharp pattern.
0885-7024/12-0009-0062/$30.00 PER ARTICLE
SEPTEMBER 2012 Civil Engineering