Bridge Section No. of Piles Frequency (%) Mean Tip
W 01 322
Group I 232 72 82 8.5 0.10
Group II 73 23 107 9.0 0.08
Group III 17 5 141 9.5 0.07
W 02 403
Group I 365 90 80 6.7 0.08
Group II 38 10 110 7.0 0.06
Group III 0 0 N/A N/A N/A
E 02 454
Group I 398 88 57 16 0.28
Group II 45 10 88 23 0.26
Group III 11 2 133 25 0.19
E 01 262
Group I 260 99 38 2.7 0.07
Group II 1 0.5 65 N/A N/A
Group III 11 0.5 146 N/A N/A
tip capacity (mobilized tip capacity), is 350
tons. The boring shows the variability of
the limestone within a vertical reach, and
adjacent borings show significantly different
limestone strength profiles. Reasonably
common interpretations of pile capacity
analyses from borings and pile tests do little
to predict the variability in pile lengths
actually encountered during construction
(Figure 3). The controlling consideration
for pile-tip elevation is the highly variable
subsurface stratigraphy along the length of
the project.
Elevation (ft)
SD of Top
Elevation (ft)
COV of Tip
Elevation
Statistical analytics
Statistical analysis of the historical pile
driving records led to an interesting interpretation
of the mechanisms leading to
the extreme variability of pile lengths, and
potentially, to a basis for identifying risk in
a future bridge project across Tampa Bay.
In planning such a project, pile lengths
are estimated based on a calculation of
the type shown in Figure 3. For a Tampa
Bay crossing, the historical data suggest
the presence of the competent limestone
layer as pile-tip bearing stratum between
TECHNICAL
Figure 3. Boring log and corresponding pilecapacity
calculation based on mobilized end
bearing and side friction. Ultimate capacity is a
24-inch prestressed concrete pile. Frankland
Bridge.
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Els. -100 ft and -117 ft. However, the boring
(Figure 3) shows that the hard limestone is
a potentially thin layer with a risk of piles
punching through it. These shallow bearing
strata appear to be what the test borings
hit, as well as where most of the shorter
piles found bearing.
Table 1 summarizes some of the pile
driving statistics from the existing Tampa
Bay bridge crossing. Geological conditions
under the bridge vary from east to west.
The earlier engineering effort had divided
the bridge piers into four sections,
Table 1. Statistics of pile-tip elevations. The mean tip elevations are calculated globally by section.
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