It is therefore likely that the gravel under the
pier was eroded, exposing the piles. Once the
current starts swirling around piles, the rate of
scour quickly increases.
Other evidence indicating the probability of
scour includes:
A replacement bridge was in the design stage
when the temporary bridge failed. The bed
elevations obtained in 1989 as part of the
soil boring tests were 68 in. below the 1964
elevation used in the design of the tempo-
rary bridge. This bed elevation is presumed
to be the bottom of the concrete casing.
Public utilities lines (i.e. water and phone
a. Looking downstream.
lines) immediately downstream of the bridge
were exposed during breakup events in the
past. The utility lines ultimately failed.
Foreign material (i.e. logs and metal) was
found in the bottom of the excavation for
the new pile cap beam 10 ft below the bed.
The possibility that this debris was left over
from previous construction was ruled out
because of the location of the new piers and
the construction technique used on previ-
ous structures.
Floating ice impacting the nose of a pier gen-
erates a shear force and an overturning moment
along the axis of the pier. The reaction to the
shear force is a function of the cross-sectional
b. Looking upstream. The Route 5 bridge is in the
area of the foundation elements, i.e. the piles and
background.
the concrete. For the White River bridge, the weak-
est shear element was the piles. For a pile diam-
Figure 3. Failure of the Bridge Street bridge, January
eter of 12 in. at the mud line and a conservative
26, 1990.
shear strength of 120 psi (American Institute of
Timber Construction 1974), the load capacity of
the nine piles was 122 kips. This capacity is three
times the predicted load for 12-in.-thick ice and
the 100 psi recommended by the AASHTO (1992)
design code. Reaction to overturning is devel-
oped along the length of the foundation. If the
ratio of the water depth to the length of the foun-
dation is less than 1, the pier is usually more than
adequate to resist overturning. The water level
was well within this criterion on January 24. The
thalweg of the White River is skewed to the pier
alignment, and it is likely that the piers were
subjected to side impacts. In the extreme case
(that is, the ice impacting the pier perpendicular
to the pier alignment) the pier is not as stable.
For simplicity, consider the pier as a column with
a side load and the ends restrained by the bridge
deck and the foundation. The deck connection
was just a bearing pad, which allows free rota-
Figure 4. Temporary piers subjected to ice forces in
tion of the pier. The rigidity of the foundation
1969.
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