Countermeasures to Protect Bridge Abutments from Scour (2007) / Chapter Skim
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From page 79... ...
79 7.1 Introduction The findings of the DOT survey presented in Chapter 4, the literature review presented in Chapter 5, and the preliminary experiments presented in Chapter 6 indicate that suitably positioned aprons of riprap, cable-tied blocks, or geobags hold promise as an effective scour countermeasure for wing-wall abutments. The present chapter investigates such aprons in further detail.
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From page 80... ...
7.2 Experiments on Aprons of Riprap or Cable-Tied Blocks This section describes the experiments conducted to determine the performance behavior of an apron of riprap or cable-tied blocks placed around a wing-wall abutment under live-bed conditions. The experiments were completed at the University of Auckland, New Zealand.
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From page 81... ...
Figure 7-2. Dimensions of the 1.5-m wide flume.
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From page 82... ...
Bed Sediment Uniform coarse sand was used as the bed material for all the experiments. A sieve analysis was carried out for the sand (Van Ballegooy, 2005)
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From page 83... ...
or synthetic cables to form a mattress. By adopting the McCorquodale et al.
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From page 84... ...
ratio, of which values above 1.0 represent a condition called "live-bed scour." This condition is extreme for scouring of armouring units such as riprap, cable-tied blocks, and geobags because the bed forms travelling past the armouring units can dislodge individual units and therefore cause failure. Figure 7-10 shows the surface velocity distributions across the flume for the four flow velocities at two different flow depths.
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From page 85... ...
Bed Forms Bed forms can be seen in Figure 7-11. After the velocity distributions were measured, the flow was stopped and 10-m long bed profiles were measured longitudinally in the flume at 100-mm spacings.
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From page 86... ...
sediment from between the riprap stones and the cable-tied blocks. The experiments were conducted with two flow depths, ym 100 mm (bank-full)
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From page 87... ...
R2 riprap was used for d50 = 27, R3 riprap was used for d50 = 40, and R4 riprap was used for d50 = 60. For these experiments, a 200-mm wide apron was used, placed at a burial depth db = 1d50, such that one riprap layer was buried flush with the average bed level and the other riprap layer was placed on top.
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From page 88... ...
88 Upstream Downstream ym (m)
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From page 89... ...
89 0.170 1.1 0.300 0.040 0.000 -0.075 0.115 0.150 0.490 -0.075 0.095 0.200 0.470 0.170 1.5 0.300 0.040 0.000 -0.075 0.125 0.150 0.490 -0.075 0.100 0.200 0.480 0.170 1.8 0.300 0.040 0.000 -0.075 0.135 0.090 0.510 -0.075 0.115 0.150 0.490 0.170 2.1 0.300 0.040 0.000 0.045 0.145 - 0.510 -0.075 0.135 0.150 0.530 0.170 1.1 0.200 0.020 0.020 -0.020 0.130 0.110 0.410 -0.020 0.105 0.150 0.360 0.170 1.5 0.200 0.020 0.020 0.065 0.180 - 0.420 -0.020 0.165 0.120 0.460 0.170 1.8 0.200 0.020 0.020 - - - - - - - - 0.170 2.1 0.200 0.020 0.020 - - - - - - - - 0.170 1.1 0.200 0.027 0.027 -0.025 0.105 0.110 0.340 -0.025 0.100 0.145 0.340 0.170 1.5 0.200 0.027 0.027 -0.025 0.140 0.100 0.400 -0.025 0.105 0.120 0.340 0.170 1.8 0.200 0.027 0.027 -0.025 0.140 0.060 0.400 -0.025 0.120 0.100 0.380 0.170 2.1 0.200 0.027 0.027 0.085 0.170 - 0.470 -0.025 0.155 0.070 0.480 0.170 1.1 0.200 0.061 0.061 -0.060 0.105 0.110 0.390 -0.060 0.075 0.120 0.300 0.170 1.5 0.200 0.061 0.061 -0.060 0.115 0.070 0.400 -0.060 0.085 0.120 0.310 0.170 1.8 0.200 0.061 0.061 -0.060 0.130 0.060 0.410 -0.060 0.085 0.060 0.320 0.170 2.1 0.200 0.061 0.061 -0.015 0.155 - 0.440 -0.060 0.115 0.050 0.340 Cable-Tied Block Protection 0.100 1.1 0.100 - 0.000 -0.005 0.065 - 0.071 -0.005 0.060 - 0.076 0.100 1.4 0.100 - 0.000 0.020 0.090 - 0.071 0.015 0.080 - 0.076 0.100 1.8 0.100 - 0.000 - - - - - - - - 0.100 2.2 0.100 - 0.000 - - - - - - - - 0.170 1.1 0.100 - 0.000 0.075 0.145 - 0.071 0.065 0.135 - 0.071 0.170 1.5 0.100 - 0.000 - - - - - - - - 0.170 1.8 0.100 - 0.000 - - - - - - - - 0.170 2.1 0.100 - 0.000 - - - - - - - - 0.100 1.1 0.150 - 0.000 -0.005 0.065 - 0.133 -0.005 0.060 0.000 0.137 0.100 1.4 0.150 - 0.000 0.000 0.085 - 0.124 -0.005 0.075 0.000 0.130 0.100 1.8 0.150 - 0.000 0.000 0.085 - 0.124 -0.005 0.075 - 0.127 0.100 2.2 0.150 - 0.000 - - - - - - - - Upstream Downstream ym (m)
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From page 90... ...
the filter layer could become exposed if riprap shear failure or excessive apron settlement occurred -- that is, for the experiment, d50 27 mm and V/Vc 2.1 shown in Figure 7-20 and the experiment db 0 and V/Vc 1.8 shown in Figure 7-18. Lauchlan (1999)
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From page 91... ...
rial is undermined from the apron by the troughs of the propagating bed forms, increasing the burial depth increases the stability of the apron. The current experiments show that as the burial depth is increased, apron width can be reduced to afford similar levels of scour protection at the abutment.
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From page 92... ...
Contrary to expectations, the measured scour depths for clear-water conditions are considerably smaller than the measured scour depths at the outer edge of the apron for livebed conditions. It is apparent that the troughs of the bed forms in the live-bed experimental work were very much deeper than the local scour at the abutment; the former, therefore, dominated the maximum scour depth ds2.
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From page 93... ...
the PTV flow field measurements and acoustic Doppler velocimeter measurements of the vertical velocity distribution. Figure 7-23 shows the riprap size normalized with the flow depth as a function of the Froude number at the bridge section Fr2, as well as the riprap sizing equation from Lagasse et al.
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From page 94... ...
W = 100 mm W = 200 mm (7-5) Where: Scb specific gravity of the blocks and n Manning coefficient.
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From page 95... ...
W = 300 mm W = 400 mm Equation 7-5 provides a simple means of estimating block size to resist failure due to overturning and roll-up of the leading edge. In use of Equation 7-5, care needs to be taken to ensure that the leading edge of the mat remains buried.
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From page 96... ...
downstream end because the bed forms were larger at the upstream end. At the downstream end of the abutment, the flow was fully contracted in the main channel.
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ym (m) V/Vc V2-surf (ms-1)
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From page 98... ...
manner in which the aprons settled. Because the cables prevented the cable-tied block aprons from increasing in width, sand was eroded from beneath the apron, and the outer edge of the apron folded down, retaining the sand at an angle larger than the repose angle of the sand.
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From page 99... ...
99 Figure 7-25. Scour depth at the abutment as a function of the maximum bed-form height for both the upstream and downstream corners of the abutment.
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From page 100... ...
Where the coefficient C5 varies for the upstream and downstream locations depending on the direction of movement of the undermined riprap stones. At the upstream corner of the wingwall abutment, the riprap stones moved both laterally away from the abutment and upstream when rolling into the scour regions.
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101 Figure 7-29. Portion of apron that was undermined (W-Wmin)
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From page 102... ...
that cable-tied block aprons need to be wider than riprap aprons (with two riprap layers) to afford the same level of protection at wing-wall abutments.
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From page 103... ...
and 7-16 tend to underpredict the α2 values slightly, but overall there is a good agreement between the measured and predicted 2 values.Underprediction of the 2 values is conservative,however, because the troughs of the bed forms are predicted to pass closer to the abutment face. Equations 7-15 and 7-16 have a similar structure with the exception of the factor (ncos – C5C6)
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From page 104... ...
Wing-Wall Configurations The bulk of the experiments were conducted using a single wing-wall abutment that replicated, at a scale corresponding to about 1:40, the width of abutments typical of two-lane roads in the United States; the road width is about 12 m (40 ft)
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From page 105... ...
Fr KS n ≤0.8 1.02 2.0 >0.8 0.69 0.1 front of the abutment itself (A) , in front of loose apron protection (B)
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From page 106... ...
The depth parameter Kh is defined as a function of water depth y and equivalent roughness ks. Pilarczyk suggests using ks Dn.
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From page 107... ...
The geobag apron failed to withstand the flow. Some geobags became embedded in the sediment, and some were rolled away from the abutment.
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From page 108... ...
turbulent flow formed around the abutment and jumble of the filter cloth and geobags and resulted in a scour hole that was deeper than the baseline scour hole. The failure process is shown in Figures 7-46 through 7-49.
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From page 109... ...
scour process described above. The resulting scour hole is shown in Figure 7-50.
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This apron proved to be very effective. It completely prevented the scour from occurring at the abutment.
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From page 111... ...
Eventually, scour deepening caused the embankment side slopes to become unstable and to slide into the scour hole, where sediment had been removed by the flow. As the embankment collapsed, the flow passed around the exposed abutment.
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From page 112... ...
riprap apron sliding into the scour hole forming around the apron. As the riprap apron slid, it exposed the pile cap so that embankment sediment was winnowed from beneath the pile cap.
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113 (a) Before scour (b)
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From page 114... ...
that the protection (geobag or riprap) must extend as a mat across essentially the full opening of a bridge waterway.
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115 Figure 7-60. Recommended minimum extent of mat formed from geobags or riprap for single-span bridges.
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116 significant scour can occur readily near the downstream edge of the apron. A possible concern in using an apron is to ensure that shifting of scour does not imperil a nearby pier or portion riverbank.
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From page 117... ...
dunes moving through the channel in the vicinity of the bridge. • The geobags should be placed in a shingled manner, whereby adjoining geobags overlie joints between underlying geobags.
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