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OCR for page 24
24 Figure 4.12. Velocity hydrograph and scour depth versus time curve for Bent 3 of the Sims Bayou River Bridge at SH 35. For predicting the future development of a scour hole at a 6. Read the erosion rate z corresponding to max on the bridge pier over a design life tlife, one can either develop a equivalent erosion function. synthetic hydrograph (much like is done in the case of earth- 7. Calculate the equivalent time te for a given design life quakes) or assume that the hydrograph recorded over the last of the bridge thydro for the design velocity vmax and for period equal to tlife will repeat itself. The time required to per- i value of Step 6 by using Equation 4.7. the z form Step 3 is about 8 hours per Shelby tube sample because 8. Knowing te, z i, and z at max, calculate the scour depth z it takes about eight points to properly describe the erosion the end of the design life by using Equation 4.3. function (z versus curve) and, for each point, the water is kept flowing for 1 hour to get a good average z value. The An example of such scour calculations is shown in Fig- time required to perform all other steps, except for Step 2, is ure 4.13. about 4 hours for someone who has done it before. In order to reduce these 4 hours to a few minutes, a simplified version of SRICOS, called S-SRICOS, was developed. Note that this 4.8 CASE HISTORIES simplified method is only recommended for preliminary In order to evaluate the E-SRICOS and S-SRICOS Methods, design purposes. If S-SRICOS shows clearly that there is no eight bridges were selected (Figure 4.14). These bridges all need for refinement, then there is no need for E-SRICOS; if satisfied the following requirements: the predominant soil type not, an E-SRICOS analysis must be performed. was fine-grained soils according to existing borings; the river For preliminary design purposes, S-SRICOS can be used. bottom profiles were measured at two dates separated by at The method proceeds as follows: least several years, these river bottom profiles indicated any- 1. Calculate the maximum depth of scour z max for the design where from 0.05 m to 4.57 m of scour; a USGS gage station velocity vmax by using Equation 4.2. The design velocity existed near the bridge; and drilling access was relatively easy. is usually the one corresponding to the 100-year flood or The Navasota River Bridge at SH 7 was built in 1956. The the 500-year flood. main channel bridge has an overall length of 82.8 m and con- 2. Collect samples at the site within the depth z max. sists of three continuous steel girder main spans with four 3. Test the samples in the EFA to obtain the erosion func- concrete pan girder approach spans. The foundation type is versus ) for the layers involved. tion (z steel piling down to 5.5 m below the channel bed, which con- 4. Create a single equivalent erosion function by averag- sists of silty and sandy clay down to the bottom of the piling ing the erosion functions within the anticipated depth according to existing borings. Between 1956 and 1996 the of scour. peak flood took place in 1992 and generated a measured flow 5. Calculate the maximum shear stress max around the pier of 1,600 m3/s, which corresponds to a HEC-RAS calculated before scour starts by using Equation 4.1. In Equation mean approach flow velocity of 3.9 m/s at Bent 5 and 2.6 m/s 4.1, use the pier diameter B and the design velocity vmax. at Bent 3. The pier at Bent 3 was square with a side equal to

OCR for page 24
25 Figure 4.13. Example of scour calculations by the S-SRICOS Method. 0.36 m, while the pier at Bent 5 was 0.36 m wide and 8.53 m continuous steel girder main spans with eight prestressed long and had a square nose. The angle between the flow direc- concrete approach spans. The foundation type is concrete pil- tion and the pier main axis was 5 degrees for Bent 5. River ing penetrating 9.1 m below the channel bed, which consists bottom profiles exist for 1956 and 1996 and show 0.76 m of of sandy clay, clayey sand, and sand down to the bottom of local scour at Bent 3 and 1.8 m of total scour at Bent 5. At the piling according to existing borings. Between 1965 and Bent 5, the total scour was made up of 1.41 m of local scour 1998, the peak flood occurred in 1966 and generated a mea- and 0.39 m of contraction scour as explained later. sured flow of 2,600 m3/s, which corresponds to a HEC-RAS The Brazos River Bridge at US 90A was built in 1965. The calculated mean approach velocity of 4.2 m/s at Bent 3. The bridge has an overall length of 287 m and consists of three pier at Bent 3 was 0.91 m wide and 8.53 m long and had a Figure 4.14. Location of case history bridges.