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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-
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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
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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.