Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 26
26
round nose. The pier was in line with the flow. River bottom 5 degrees. River bottom profiles exist for 1993 and 1995 and
profiles exist for 1965 and 1997 and show 4.43 m of total indicate 0.05 m of local scour at Bent 3.
scour at Bent 3 made up of 2.87 m of local scour and 1.56 m The Bedias Creek Bridge at US 75 was built in 1947. This
of combined contraction and general scour as explained later. 271.9-m-long bridge has 29 spans and Bent 26 is founded on
The San Jacinto River Bridge at US 90 was built in 1988. a spread footing. The soil tested from the site varied from low
The bridge is 1,472.2 m long and has 48 simple prestressed plasticity clay to fine silty sand. Between 1947 and 1996, the
concrete beam spans and 3 continuous steel plate girder peak flood occurred in 1991 and generated a measured flow
spans. The foundation type is concrete piling penetrating of 650 m3/s, which corresponds to a HEC-RAS calculated
24.4 m below the channel bed at Bent 43 where the soil con- mean approach flow velocity of 2.15 m/s at Bent 26. The pier
sists of clay, silty clay, and sand down to the bottom of the at Bent 26 is square with a side of 0.86 m. The pier is in line
piles according to existing borings. Between 1988 and 1997, with the flow. River bottom profiles exist for 1947 and 1996
the peak flood took place in 1994 and generated a measured and show 2.13 m of total scour at Bent 26 made up of 1.35 m
flow of 10,000 m3/s, which corresponds to a HEC-RAS cal- of local scour and 0.78 m of contraction and general scour as
culated mean approach velocity of 3.1 m/s at Bent 43. The explained later.
pier at Bent 43 was square with a side equal to 0.85 m. The The Bedias Creek Bridge at SH 90 was built in 1979. This
angle between the flow direction and the pier main axis was 73.2-m-long bridge is founded on 8-m-long concrete piles
15 degrees. River bottom profiles exist for 1988 and 1997 embedded in layers of sandy clay and firm gray clay. Between
and show 3.17 m of total scour at Bent 43 made up of 1.47 m 1979 and 1996, the peak flood occurred in 1991 and gener-
of local scour and 1.70 m of combined contraction and gen- ated a measured flow of 650 m3/s, which corresponds to a
eral scour as explained later. HEC-RAS calculated mean approach flow velocity of 1.55 m/s
The Trinity River Bridge at FM 787 was built in 1976. The at Bent 6. The pier at Bent 6 was square with a side of 0.38 m.
bridge has three main spans and three approach spans with an The angle between the flow direction and the pier main axis
overall length of 165.2 m. The foundation type is timber pil- was 5 degrees. River bottom profiles exist for 1979 and 1996
ing and the soil is sandy clay to clayey sand. Between 1976 and show 0.61 m of local scour at Bent 6.
and 1993, the peak flood took place in 1990 and generated a
measured flow of 2,950 m3/s, which corresponds to a HEC- 4.9 PREDICTED AND MEASURED LOCAL
RAS calculated mean approach flow velocity of 2.0 m/s at SCOUR FOR THE EIGHT BRIDGES
Bent 3 and 4.05 m/s at Bent 4. The piers at Bent 3 and Bent 4
were 0.91 m wide and 7.3 m long, and had round noses. The The data for all bridges is listed in Tables 4.2 and 4.3. For
angle between the flow direction and the pier main axes was each bridge, the E-SRICOS and S-SRICOS Methods were
25 degrees. River bottom profiles exist for 1976 and for 1992 used to predict the local scour at the chosen bridge pier loca-
and show 4.57 m of total scour at both Bent 3 and Bent 4, tion. One pier was selected for each bridge, except for the
made up of 2.17 m of local scour and 2.40 m of contraction Navasota River Bridge at SH 7 and the Trinity River Bridge
and general scour as explained later. at FM 787 for which two piers each were selected. Therefore,
The San Marcos River Bridge at SH 80 was built in 1939. a total of 10 predictions were made for these eight bridges.
This 176.2-m-long bridge has 11 prestressed concrete spans. These predictions are not Class A predictions since the mea-
The soil tested from the site is a low-plasticity clay. Between sured values were known before the prediction process
1939 and 1998, the peak flood occurred in 1992 and gener- started. However, the predictions were not modified once
ated a measured flow of 1,000 m3/s, which corresponds to a they were obtained.
HEC-RAS calculated mean approach flow velocity of 1.9 m/s For each bridge, Shelby tube samples were taken near the
at Bent 9. The pier at Bent 9 is 0.91 m wide and 14.2 m long bridge pier within a depth at least equal to two pier widths
and has a round nose. The pier is in line with the flow. River below the pier base. The boring location was chosen to be as
bottom profiles exist for 1939 and 1998 and show 2.66 m of close as practical to the bridge pier considered. The distance
total scour at Bent 9 made up of 1.27 m of local scour and 1.39 between the pier and the boring varied from 2.9 m to 146.3 m
m of contraction and general scour as explained later. (Table 4.2). In all instances, the boring data available was
The Sims Bayou Bridge at SH 35 was built in 1993. This studied in order to infer the relationship between the soil lay-
85.3-m-long bridge has five spans. Each bent rests on four ers at the pier and at the sampling locations. Shelby tube sam-
drilled concrete shafts. Soil borings indicate mostly clay lay- ples to be tested were selected as the most probable represen-
ers with a significant sand layer about 10 m thick starting at a tative samples at the bridge pier. These samples were tested
depth of approximately 4 m. Between 1993 and 1996, the in the EFA and yielded erosion functions z versus . Figures
peak flood occurred in 1994 and generated a measured flow 4.15 and 4.16 provide examples of the erosion functions
of 200 m3/s, which corresponds to a HEC-RAS calculated obtained. The samples also were analyzed for common soil
mean approach flow velocity of 0.93 m/s at Bent 3. The pier properties (Table 4.3).
at Bent 3 is circular with a 0.76 m diameter. The angle For each bridge, the USGS gage data was obtained from
between the flow direction and the pier main axis was the USGS Internet site. This data consisted of a record of dis-
OCR for page 27
27
TABLE 4.2 Full-scale bridges as case histories
charge Q versus time t over the period of time separating the the mean longitudinal slope of the river at the bridge site
two river bottom profile observations (Figure 4.9). This dis- (obtained from topographic maps, Table 4.2), and Manning's
charge hydrograph was transformed into a velocity hydro- roughness coefficient (estimated at 0.035 for all cases after
graph by using the program HEC-RAS (1997) and proceed- Young et al., 1997). For a given discharge Q, HEC-RAS
ing as follows. The input to HEC-RAS is the bottom profile gives the velocity distribution in the river cross section,
of the river cross section (obtained from TxDOT records), including the mean approach velocity v at the selected pier
TABLE 4.3 Soil properties at the bridge sites
OCR for page 28
28
Figure 4.15. Erosion function for San Jacinto River
sample (7.6 m to 8.4 m depth).
location. Many runs of HEC-RAS for different values of Q Figure 4.17. Profiles of Navasota River Bridge at SH 7.
are used to develop a relationship between Q and v. The rela-
tionship (regression equation) was then used to transform the
Q-t hydrograph into the v-t hydrograph at the selected pier
between the two profiles was calculated with scour being pos-
(Figures 4.10, 4.11, and 4.12).
itive and aggradation being negative. The net area was then
Then, the SRICOS program (Kwak et al., 2001) was used
divided by the Width AB to obtain an estimate of the mean
to predict the scour depth z versus time t curve. For each
contraction/general scour. Once this contraction/general scour
bridge, the input consisted of the z versus curves (erosion
was obtained, it was subtracted from the total scour at the
functions) for each layer at the bridge pier (Figures 4.15 and
bridge pier to obtain the local scour at the bridge pier. In some
4.16), the v versus t record (velocity hydrograph) (Figures
instances there was no need to evaluate the contraction/general
4.10, 4.11, and 4.12), the pier diameter B, the viscosity of the
scour. This was the case of Bent 3 for the Navasota Bridge
water and the density of the water w. The output of the pro-
(Figure 4.17). In this case, the bent was in the dry (flood plain)
gram was the scour depth z versus time t curve for the selected
at the time of the field visit, and the local scour could be
bridge pier (Figures 4.10, 4.11, and 4.12) with the predicted
measured directly. Figure 4.19 shows the comparison between
local scour depth corresponding to the last value on the curve.
E-SRICOS predicted and measured values of local scour at the
The measured local scour depth was obtained for each case
bridge piers. The precision and accuracy of the method appear
history by analyzing the two bottom profiles of the river cross-
reasonably good. Although more than 10 data points may be
section (Figures 4.17 and 4.18). This analysis was necessary
preferable, note that these 10 data points represent 10 full-
to separate the scour components that added to the total scour
scale, real situations.
at the selected pier. The two components were local scour
The S-SRICOS Method was performed next. For each
and contraction/general scour. This separation was required
bridge pier, the maximum depth of scour z max was calculated
because, at this time, SRICOS only predicts local scour. The
by using Equation 4.2. The velocity used for Equation 4.2 was
contraction/general scour over the period of time separating
the maximum velocity, which occurred during the period of
the two river bottom profiles was calculated as the average
time separating the two river bottom profile observations.
scour over the width of the channel. This width was taken as
Then, at each pier, an average erosion function (z versus
the width corresponding to the mean flow level (Width AB
on Figures 4.17 and 4.18). Within this width, the net area
Figure 4.16. Erosion function for Bedias Creek sample
(6.1 m to 6.9 m depth). Figure 4.18. Profiles of Brazos River Bridge at US 90A.
