Scour at Bridge Foundations on Rock (2012) / Chapter Skim
Currently Skimming:
Pages 11-161
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 11... ...
11 Findings and Applications 3.1 Overview The failure of the Schoharie Creek Bridge on Interstate Highway 90 in New York on April 5, 1987, drew attention to potentially dangerous erosion of earth materials thought to be stable and resistant to erosion. This failure led to a mandate from FHWA that all bridges be evaluated for susceptibility to collapse under similar circumstances.
|
From page 12... ...
12 Scour at Bridge Foundations on rock of turbulence in the boundary layer at the channel bed was emphasized in one index method. Index methods were developed for spillway channels of dams that tend to have steep gradients, carry clear water, and exhibit short-duration discharge.
|
From page 13... ...
Findings and applications 13 work, including the Transportation Research Information Services (TRIS) database and the Research in Progress (RiP)
|
From page 14... ...
14 Scour at Bridge Foundations on rock FHWA Memo HNG-31 notes that geologic studies have shown that even the hardest of rock can scour when exposed to moving water for geologic-scale periods of time. Investigation procedures listed in the FHWA Memo are (1)
|
From page 15... ...
Findings and applications 15 P V= t [ .
|
From page 16... ...
16 Scour at Bridge Foundations on rock closure to Annandale's (2005) discussion and included a comment that their results indicated that greater protrusions resulted in lower flow strength for entrainment of the block into the flow, but that Annandale's (1995)
|
From page 17... ...
Findings and applications 17 rock masses and those easily weathered are more susceptible to scour than rock masses that are massive and durable. Mechanical and chemical weathering can degrade rock over time, increasing susceptibility to scour.
|
From page 18... ...
18 Scour at Bridge Foundations on rock Dickenson and Baillie (1999) refer to Akhmedov (1988)
|
From page 19... ...
Findings and applications 19 and C have the same peak power but different durations and serve to demonstrate the essence of Costa and O'Connor's (1995) model.
|
From page 20... ...
20 Scour at Bridge Foundations on rock drying, to have a significant influence on the amount of erosion along the stream channels. This effect was most pronounced in bedrock along stream banks that were submerged during wet-season months and exposed during dry months.
|
From page 21... ...
Findings and applications 21 Dickenson and Baillie (1999) noted that the Slake Durability Test is not commonly used in standard geotechnical practice.
|
From page 22... ...
22 Scour at Bridge Foundations on rock and the stream power of the flow to represent hydraulic turbulence and uplift forces on rock particles, as well as the effects of bedload translating and saltating over the rock-bed channel. They computed the cumulative or integrated stream power that was expended during the time interval between the initial and final cross section surveys, which was the basis for estimating the average amount of erosion that occurred across the cross section.
|
From page 23... ...
Findings and applications 23 advised that the type and condition of the rock formation should be obtained by a site inspection and examination of the boring logs and may require the use of divers to inspect the bed near the piers for existing bridges.
|
From page 24... ...
24 Scour at Bridge Foundations on rock susceptibility of the rock surface to erode. The bedload grain impacts produce fractures within mineral crystals and rock fragments, dislodge individual grains, or break off flakes and fragments from the rock surface.
|
From page 25... ...
Findings and applications 25 In addition to flow-separation vortices being generated by ridges in the rock-bed channel surface, they also can be generated by joints, fractures, and small bed irregularities. Flutes and potholes commonly are associated with these small irregularities.
|
From page 26... ...
26 Scour at Bridge Foundations on rock results in sediment particles accumulating in fractures and joints. They observed clasts ranging from fine sand to boulders wedged very tightly into bedrock joints.
|
From page 27... ...
Findings and applications 27 ing slabs of varying thickness. Vertical joints provide more resistance to quarrying than joints inclined upward in a downstream direction (dipping upstream)
|
From page 28... ...
28 Scour at Bridge Foundations on rock 3.3 Findings from Survey of State and Federal Agencies Four areas of inquiry were identified for the survey of state and federal agencies: 1. Determine the various practices used for estimating the extent and depth of bridge foundation scour in rock; 2.
|
From page 29... ...
Findings and applications 29 36 Utah 37 Connecticut 38 California 39 New Jersey 40 Ohio 31 California 32 California 33 California 34 Vermont 35 Virginia 41 Florida 42 Iowa 43 Maryland 44 Mississippi 45 North Carolina Question 2: Does the inventory of bridges in your area identify if the foundation is soil or rock? Of the responses, 10 answered yes; 8 answered no; 3 answered not sure; 23 provided qualified answers; 1 skipped the question.
|
From page 30... ...
30 Scour at Bridge Foundations on rock 27 Yes, but data is not available for many older bridges. Those with unknown foundations are treated as on soil.
|
From page 31... ...
Findings and applications 31 16 17 18 Not sure how many CT has now. 19 20 21 22 We build bridges for forest highway partners, but do not manage bridges directly.
|
From page 32... ...
32 Scour at Bridge Foundations on rock 13 Both spread footings on rock and drilled shafts socketed into rock have been used. 14 15 16 Both spread footings and drilled shafts.
|
From page 33... ...
Findings and applications 33 2 We evaluate each structure for scour. 3 4 5 6 Based upon an evaluation of rock cores, including RQD, and exposed bedrock at the bridge site, we estimate the scourability of the rock on a scale of 1 to 10.
|
From page 34... ...
34 Scour at Bridge Foundations on rock 35 36 37 If rock/foundation "interface" is continuously exposed to stream flow, it may be more of a consideration. Most rock foundations in CT are several feet below stream bed surface.
|
From page 35... ...
Findings and applications 35 19 20 I am not aware of any general problem with rock. We have had concerns at individual bridges with coal seams, etc.
|
From page 36... ...
36 Scour at Bridge Foundations on rock 4 5 6 7 Some cases -- the two that come to mind have been replaced, but I know of others we are watching. 8 9 10 11 12 We keep detailed records not of scour depths, but of channel x-secs, taken every 3 years.
|
From page 37... ...
Findings and applications 37 Question 8: Has your organization tried to evaluate scour of rock quantitatively? Of the responses, 17 answered never; 1 answered once; 7 answered a few times; 2 answered a number of times; 6 answered not sure; 9 provided qualified answers; 3 skipped the question.
|
From page 38... ...
38 Scour at Bridge Foundations on rock 40 41 42 43 SHA has a process for evaluating scour in rock. 44 There is rock in the Tallahatta Formation.
|
From page 39... ...
Findings and applications 39 32 33 Both. 34 35 Annandale's is what we have only recently begun to look into.
|
From page 40... ...
40 Scour at Bridge Foundations on rock 28 29 30 31 32 33 34 35 36 37 38 39 40 41 N/A 42 43 44 45 Question 11: Do knickpoints or waterfalls exist near bridges in your area? Of the responses, 19 answered yes; 10 answered no; 4 answered not sure; 20 provided qualified answers; 6 skipped the question.
|
From page 41... ...
Findings and applications 41 23 Mountainous areas in East Tennessee can have small waterfalls near bridges and knickpoints from headcuts can be found near bridges in West Tennessee. 24 25 26 Not many to my knowledge.
|
From page 42... ...
42 Scour at Bridge Foundations on rock 12e. Has geologic and/or geotechnical field data been collected at a bridge with an eroding rock foundation?
|
From page 43... ...
Findings and applications 43 13 Problem areas have typically been where spread footings have been founded on sedimentary rock types. Non-problem locations are those where we have socketed drilled shafts into competent rock.
|
From page 44... ...
44 Scour at Bridge Foundations on rock type and weathering issues than rock scour. For bridge foundations on rock WITHOUT problems, I would direct you to the statewide foundation engineer.
|
From page 45... ...
Findings and applications 45 in rock-bed channels accumulate on a flood-by-flood basis; if the scour holes are filled during the waning stages of floods, the filling material is sand with much less scour resistance than the rock-bed material. Scour of sand-bed channels creates holes that tend to be filled during waning flood stages with sand that is similar in its scour resistance to the sand that was removed to create the holes; in this regard, scour of sand-bed channels is neither cumulative nor progressive.
|
From page 46... ...
46 Scour at Bridge Foundations on rock River in western Kentucky near Paducah discovered during construction of the Kentucky Dam. The variable scour resistance of the limestone rubble and soil filling the collapsed solution cavity could be a scour issue, even though the solubility of the limestone formation, per se, might not be a concern during the service life of a bridge structure.
|
From page 47... ...
Findings and applications 47 in their paper containing calculated and estimated flow parameters of these flood events from which the numbers in Figure 3.7 were obtained. Baker and Costa (1987)
|
From page 48... ...
48 Scour at Bridge Foundations on rock Hancock et al.
|
From page 49... ...
Findings and applications 49 of cavitation bubbles, which would inhibit development of cavitation for extended periods of scour. Whipple et al.
|
From page 50... ...
50 Scour at Bridge Foundations on rock perpendicular to the channel bed. Both the ultimate scour depth and the scour threshold flow velocity are determined as a function of (1)
|
From page 51... ...
Findings and applications 51 are the same for rock blocks dipping with, or against, the flow. Rock specific gravity used in all cases was 2.65 (165 lb/ft3 or 2650 kg/m3 density)
|
From page 52... ...
52 Scour at Bridge Foundations on rock Figure 3.12a. Summary of ultimate scour depth normalized to pier diameter for rock blocks of various thicknesses and flow velocity.
|
From page 53... ...
Findings and applications 53 energy. The Headcut Erodibility Index Method spreadsheet is available online as a Microsoft Excel file on the NRCS website for hydraulics and hydrology tools and models at http://www.
|
From page 54... ...
54 Scour at Bridge Foundations on rock The results of the comparative analysis of the two index methods are summarized in Table 3.2. A velocity enhancement factor of 1.7 times the approach velocity developed for design of riprap at rectangular piers (Lagasse et al., 2001b, p.
|
From page 55... ...
Findings and applications 55 ρ ρ τ τ τ τ τ Table 3.2. Comparative analysis of Headcut Erodibility Index and Erodibility Index Methods using durable rock blocks (vertical joints, 1:4 block shape, and no protrusion)
|
From page 56... ...
56 Scour at Bridge Foundations on rock a channel slope of 0.1 (Case 9) were used in the spreadsheet for calculating E
|
From page 57... ...
Findings and applications 57 measured scour or predict future scour during engineering time. Whipple et al.
|
From page 58... ...
58 Scour at Bridge Foundations on rock ESD NIL SGr A i i = × -3 5315 10 3 13 5.
|
From page 59... ...
Findings and applications 59 Table 3.3. Example Modified Slake Durability Test data for a bulk sample of siltstone from the left bank of the Sacramento River in Redding, California.
|
From page 60... ...
60 Scour at Bridge Foundations on rock power. Daily values of equivalent scour and stream power are obtained by multiplying hourly values by 24 hours per day.
|
From page 61... ...
Findings and applications 61 of scour. Linear regression analyses were performed on each group of two samples for each rock type, neglecting initial 60-minute sample results.
|
From page 62... ...
62 Scour at Bridge Foundations on rock The State Materials Office in Gainesville, Florida, provided testing for this research using RETA (Henderson et al., 2000; OEA, 2001)
|
From page 63... ...
Findings and applications 63 air, so the samples were wrapped with polyolefin film and subjected to heated air to shrink the polyolefin film onto the sample. The polyolefin film was crimped at the ends of each core sample with an electric heat sealer to preserve field moisture content.
|
From page 64... ...
64 Scour at Bridge Foundations on rock Attempts by the State Materials Office in Gainesville, Florida, to test the siltstone core samples from the Sacramento River site in the RETA were unsuccessful. The core samples were too fragile to survive the sample preparation requirements for cutting the ends of the core and drilling the hole through the core.
|
From page 65... ...
Findings and applications 65 applied to sand-bed channels since the 1930s with relationships developed by Hjulstrom and Shields (Krumbein and Sloss, 1963; Graf, 1971) that describe hydraulic conditions (e.g., velocity or shear stress)
|
From page 66... ...
66 Scour at Bridge Foundations on rock to emphasize that rock scour is a rock-water interaction problem, meaning that a definition of rock without including the water context is incomplete. The hardest, most durable rock material can be cut in seconds by high-energy water jets (water cutter)
|
From page 67... ...
Findings and applications 67 (Santi, 1998, 2006)
|
From page 68... ...
68 Scour at Bridge Foundations on rock cumulative daily stream power that the bridge site will experience. The third step is for the structural engineer and geotechnical engineer to determine the amount of scour at the bridge site that would be allowable (ultimate scour depth divided by a safety factor)
|
From page 69... ...
Findings and applications 69 those depicted in Figure 3.6, to exist at bridge sites. The time-rate of scour in highly variable materials could be estimated based on detailed knowledge of the nature and distribution of materials along a cross section.
|
From page 70... ...
70 Scour at Bridge Foundations on rock Abrasion of degradable rock is approached with scour numbers that related equivalent depth of scour to equivalent stream power. An event-based approach is used to estimate stream power generated by flood flows of different frequency or recurrence interval.
|
From page 71... ...
Findings and applications 71 The general geology of the bridge sites is summarized in Table 3.6. Pertinent information is described for each of the bridge sites in the following sections.
|
From page 72... ...
Figure 3.20a. Location of Schoharie Creek and the I-90 Bridge on a 1:100,000-scale quadrangle map.
|
From page 73... ...
Findings and applications 73 Figure 3.21. I-90 Bridge over Schoharie Creek rebuilt after the former bridge failed in 1987.
|
From page 74... ...
Figure 3.24a. Location of Chipola River and the I-10 Bridge on a 1:100,000-scale quadrangle map.
|
From page 75... ...
Findings and applications 75 extends into Alabama near the Georgia state line. The bridge (Figure 3.25)
|
From page 76... ...
76 Scour at Bridge Foundations on rock to 7.94 MPa) , whereas unconfined tensile strength (UTS)
|
From page 77... ...
Findings and applications 77 Figure 3.28. State Route 273 Bridge over Sacramento River -- view looking toward right abutment.
|
From page 78... ...
78 Scour at Bridge Foundations on rock the 15-year discharge since 1945. Keswick gage is located approximately 3 miles upstream (Fig- ure 3.27a)
|
From page 79... ...
Findings and applications 79 Figure 3.33. State Route 22 Bridge over Mill Creek.
|
From page 80... ...
80 Scour at Bridge Foundations on rock of the State Route 22 Bridge over Mill Creek using a 5-ft-long, HQ size, triple tube, wireline core barrel. Siltstone was encountered at a depth of about 1.5 ft, at which depth coring was begun.
|
From page 81... ...
Findings and applications 81 channel (Figure 3.35, Panel B) indicates that quarrying and plucking is the dominant mode of scour.
|
From page 82... ...
Figure 3.36a. Location of Montezuma Creek and the SR-262 Bridge on a 1:100,000-scale quadrangle map.
|
From page 83... ...
Figure 3.37. State Route 262 Bridge over Montezuma Creek.
|
From page 84... ...
84 Scour at Bridge Foundations on rock upstream from the bridge (HDR, 2004)
|
From page 85... ...
Findings and applications 85 Figure 3.41. Sand grains and gravel fragments wedged tightly into joint- and bedding-plane separations.
|
From page 86... ...
86 Scour at Bridge Foundations on rock exceeded. Sand-bed channels scour rapidly in response to peak discharge that exceeds the erosion threshold.
|
From page 87... ...
Findings and applications 87 reached. Degradable rocks wear away gradually and progressively over time with each flood event contributing to the scour hole.
|
From page 88... ...
88 Scour at Bridge Foundations on rock q = Unit discharge, ft3/s per foot width (m3/s per meter width) Sf = Slope of the energy grade line, ft/ft (m/m)
|
From page 89... ...
Findings and applications 89 energy loss) per unit area (i.e., [lb-ft/s/ft2]
|
From page 90... ...
90 Scour at Bridge Foundations on rock exposed to the flow, but it is subject to abrasion by the coarse bed materials that have become mobile. To illustrate this concept, consider the case where "effective" stream power is associated with a threshold value corresponding to a 2-year flood event, which is considered to be a channelforming flow for a particular site.
|
From page 91... ...
Figure 3.45. Effective stream power versus discharge with an imposed threshold condition.
|
From page 92... ...
92 Scour at Bridge Foundations on rock 3.6.2.3 Long-Term Cumulative Daily Stream Power The prediction of scour in erodible rock must consider the hydraulic loading imposed over many years by many flood events. This is true whether or not a threshold condition must be exceeded before the rock in the streambed is exposed to erosive forces.
|
From page 93... ...
Findings and applications 93 Given a future cumulative hydraulic loading Wfut, the scour number can be used to estimate the future scour caused by the anticipated loading for the particular rock formation. Estimates of future scour may then be made for various purposes, such as • Predicting scour over the remaining service life of the structure, • Predicting scour at other existing structures with foundations in the same (or similar)
|
From page 94... ...
94 Scour at Bridge Foundations on rock Using either observed scour depths versus calculated cumulative daily stream power over a specific time period or an erosion-rate relationship based on results of laboratory tests on local rock material, an erosion-rate function for recurrence-interval flood events for a particular site is defined as y f ts i( )
|
From page 95... ...
Findings and applications 95 where ys = Average annual scour depth, ft/yr (m/yr) ; li = Annual frequency of the ith recurrence-interval event, no/yr; where i = 1-, 2-, 5-, 10-, 25-, 50-, 100-, and 500-year event; annual frequency is reciprocal of return period; and (ys)
|
From page 96... ...
96 Scour at Bridge Foundations on rock 3.7.2 Rock Scour Mode Screening Modes of rock scour described in Section 3.4 are evaluated systematically in the rock scour screening procedure outlined in Figure 3.50 and in Table 3.7. Basic information needed at the start of this procedure includes regional climate, geology, and topography data, as well as local geology and topography.
|
From page 97... ...
Findings and applications 97 quartzite formations in the upper drainage basin with siltstone or shale formations at the bridge site)
|
From page 98... ...
98 Scour at Bridge Foundations on rock 3.7.2.2 Cavitation Mode Screening Cavitation is a mode of scour that has been documented in spillway tunnels and is thought to be a process in some natural channels. Mean flow depth and velocity conditions that would be consistent with possible or likely cavitation are illustrated in Figure 3.8.
|
From page 99... ...
Findings and applications 99 Block removal by quarrying and plucking is a process that was described in Section 3.4.4 and Appendix C with a numerical model. It is a threshold-controlled process governed by peak hydraulic loading.
|
From page 100... ...
100 Scour at Bridge Foundations on rock Figure 3.51a. Framework for selecting degradable rock scour models.
|
From page 101... ...
Figure 3.51c. Degradable rock scour Model B for estimating design scour depth and time rate of scour at bridge sites with gage data but without repeated cross sections.
|
From page 102... ...
102 Scour at Bridge Foundations on rock Models A and C (Figures 3.51b and d) make use of repeated cross sections of the channel, typically at existing bridges; it is doubtful that repeated cross sections would be available at locations other than existing bridges.
|
From page 103... ...
Findings and applications 103 into flood event scour depths. The design scour depth is the product of the average annual scour and the design life or service life of the bridge.
|
From page 104... ...
104 Scour at Bridge Foundations on rock also can contribute to non-uniformity, such as damage to foundation rock caused by excavation blasting. The geotechnical aspects of channel geometry and other conditions characterized in an assessment of general stream stability are appropriate to be included in the initial evaluation of rock scour at bridge sites.
|
From page 105... ...
Findings and applications 105 reported to have been similar to those shown on Figure 3.23 (boulder armor layer overlying icecontact, stratified glacial till deposits)
|
From page 106... ...
106 Scour at Bridge Foundations on rock 3.8.3 Geotechnical Laboratory Testing Conventional laboratory testing provides most of the information needed to characterize rock material at bridge sites for scour evaluations. Typically, rock material unit weight (ASTM C127 [concrete aggregate]
|
From page 107... ...
Findings and applications 107 USGS, 1982 [Bulletin 17B]
|
From page 108... ...
108 Scour at Bridge Foundations on rock 3.8.7 Case Study Examples Five bridge sites visited as part of this research project were described in Section 3.5. These sites were analyzed during development of the procedures described in this report.
|
From page 109... ...
Findings and applications 109 each of which exceeded the threshold discharge at which the armor layer became mobile (20,000 ft3/s) during the 33-year life span of the bridge (Resource Consultants and Colorado State University, 1987)
|
From page 110... ...
110 Scour at Bridge Foundations on rock Type III probability distribution. The generalized skew of 0.363 at this location was combined with the observed station skew to produce a weighted skew of 0.050 for use with this probability distribution, for the 69-year period of record of annual instantaneous peak flows.
|
From page 111... ...
Findings and applications 111 Figure 3.56. Flood frequency estimates for the 69-year period of record for the I-90 Bridge over Schoharie Creek, New York.
|
From page 112... ...
112 Scour at Bridge Foundations on rock transforms for the I-90 Bridge over Schoharie Creek were developed using a HEC-RAS model of the bridge reach; the results are shown in Figure 3.58. The cumulative daily stream power for the 69-year period of record for Schoharie Creek was calculated using the methods described in Section 3.6.2.
|
From page 113... ...
Findings and applications 113 Daily stream power for events equal to or greater than the 2-year flood event was calculated by use of the depth and velocity transform functions for discharges greater than the threshold (2-year) flood event.
|
From page 114... ...
114 Scour at Bridge Foundations on rock used to perform the testing described in the forensic report could not be obtained on which to base estimates of stream power. Therefore, the information in the forensic report was supplemented by making assumptions of the Darcy-Weisbach friction factor (0.02)
|
From page 115... ...
Findings and applications 115 were displayed as erosion volume versus time with lines representing flow velocities in the flume, as shown in Figure 3.61. The flume at Cornell University used for the testing was 8 inches wide and 16 feet long.
|
From page 116... ...
116 Scour at Bridge Foundations on rock Erosion volume in cubic inches was converted to sample erosion depth in feet by assuming that the volume was derived uniformly from the 7-inch by 12-inch surface of the sample. The sample erosion depth was then converted to a daily erosion depth by multiplying the sample erosion depth by 24 hours and dividing it by the cumulative time of the test.
|
From page 117... ...
Findings and applications 117 The scour hole depth described in the forensic report (Wiss et al., 1987) was 14 feet.
|
From page 118... ...
118 Scour at Bridge Foundations on rock the period of record occurred prior to the construction of Shasta Dam; that flood had a mean daily flow of 160,000 ft3/s, with an instantaneous peak of 186,000 ft3/s. Figure 3.64 shows the effect of river regulation provided by Shasta Dam with the annual peak discharge values being close to the corresponding largest mean daily discharge values.
|
From page 119... ...
Findings and applications 119 allows the duration of each recurrence-interval flood to be estimated, as shown in Figure 3.66. The average duration in days for conventional flood frequency recurrence are summarized in Table 3.14.
|
From page 120... ...
120 Scour at Bridge Foundations on rock Figure 3.66. Flood event durations for conventional recurrence intervals for the SR-273 Bridge over Sacramento River, California.
|
From page 121... ...
Findings and applications 121 in the probability-weighted analysis approach for predicting the average annual amount of scour in rock and rock-like materials, as described in Section 3.6.2.1. For the Sacramento River at the SR-273 Bridge in California, the durations of floods greater than the 2-year event were estimated by inspection of the time series of mean daily flows, as shown in Figure 3.66 and Table 3.14.
|
From page 122... ...
122 Scour at Bridge Foundations on rock The estimated long-term average annual scour is calculated according to the procedure described in Section 3.6.3. Table 3.15 indicates that the probability-weighting approach yields an estimated long-term average annual scour depth of about 0.088 ft/yr if the 2-year discharge threshold is applied; the average annual scour depth would be 0.157 ft/yr if flood events less than the 2-year discharge also contribute.
|
From page 123... ...
Findings and applications 123 dent around the approximately 20-year-old bridge foundations, the scour evaluations assumed an erodible bed and used HEC-18 methodology for noncohesive granular bed material to predict total scour depths (contraction scour plus pier scour) in the range of 17 to 19 feet for the 100-year flood.
|
From page 124... ...
124 Scour at Bridge Foundations on rock Table 3.16. Flood frequency analysis for Chipola River at Altha, Florida, as reported in OEA (2001)
|
From page 125... ...
Findings and applications 125 Hydraulic Analysis -- The depth and velocity transforms for the I-10 bridges over the Chipola River were developed using Manning's equation and the assumption that normal depth occurs through the bridge reach. A trapezoidal cross section having a bottom width of 255 feet and 2:1 (H:V)
|
From page 126... ...
126 Scour at Bridge Foundations on rock The cumulative daily stream power for the 35-year period of record was calculated using the methods described in Section 3.6.2. The discharge corresponding to the 2-year event (4,962 ft3/s)
|
From page 127... ...
Findings and applications 127 properly in the probability-weighted analysis approach for predicting the average annual amount of scour in rock, as described in Section 3.6.2.1. For the Chipola River at the I-10 bridges in Florida, the durations of floods equal to or greater than the 2-year event were estimated by inspection of the time series of mean daily flows as described in Figure 3.71 and Table 3.18.
|
From page 128... ...
128 Scour at Bridge Foundations on rock field observations were made; therefore, the scour number is zero based on observations of the channel response to river flows. Geotechnical Analysis -- The observed scour at the I-10 bridges over the Chipola River was effectively zero.
|
From page 129... ...
Findings and applications 129 erly in the probability-weighted analysis approach for predicting the average annual amount of scour in rock and rock-like materials, as described in Section 3.6.2.1. For the Chipola River at the I-10 bridges in Florida, the durations of floods greater than the 2-year event were estimated by inspection of the time series of mean daily flows, as shown in Figures 3.71 and Table 3.18.
|
From page 130... ...
130 Scour at Bridge Foundations on rock geotechnical scour number for the Chipola limestone (Figure 3.75) by the total daily stream power for each discharge event.
|
From page 131... ...
Findings and applications 131 estimated pier scour that is less than 0.25 feet (<0.005 ft/yr × 50 yr)
|
From page 132... ...
132 Scour at Bridge Foundations on rock relationship from regional regression equations developed by USGS for the Willamette River basin in which the gaging stations are located. The resulting area-weighted relationship equation was QSR22 = 1.187 Qgage.
|
From page 133... ...
Findings and applications 133 95 percent confidence limits for the 15-year record of observed annual peaks and also for the entire 74-year extended period of record. As seen in Figure 3.79, the estimates of the recurrenceinterval flood magnitude, and the corresponding confidence limits, are considerably different for the two periods of record.
|
From page 134... ...
134 Scour at Bridge Foundations on rock the contributing basin (33 mi2) makes such an inspection impossible because the hydrology is a rainfall-driven system.
|
From page 135... ...
Findings and applications 135 analysis. The hydrographs resulting from the watershed modeling are shown in Figure 3.82.
|
From page 136... ...
136 Scour at Bridge Foundations on rock The results of this integration are provided in Figure 3.84. The scour depth for each recurrenceinterval event is estimated by multiplying the scour number for the site (determined by the longterm cumulative daily stream power versus observed scour relationship, Figure 3.81)
|
From page 137... ...
Findings and applications 137 long-term average annual scour at the SR-22 Bridge over Mill Creek multiplied by 62.7 years results in a total estimated pier scour of 4.6 feet if the 2-year discharge threshold is applied, or 6.3 feet if events less than the 2-year discharge also contribute. These estimated scour depths are approximately 66 and 91 percent of the 7 feet of pier scour interpreted from repeated surveys over a 62.7-year period.
|
From page 138... ...
138 Scour at Bridge Foundations on rock ranged in size from about 0.3 to 1.5 feet. The blocks were bounded by joints and fractures that were smooth and planar over the scale of the blocks, but were discontinuous and non-parallel over the scale of the exposure or stream channel.
|
From page 139... ...
Findings and applications 139 founded on fluvial sandstone of Jurassic age, as described in Section 3.5.6 and shown in Figures 3.36 to 3.42. The bridge was built in 1960 over a 50-foot-wide channel excavated across a narrow neck of land in a prominent meander bend (Figure 3.38)
|
From page 140... ...
140 Scour at Bridge Foundations on rock storm produces 2.62 inches of precipitation in the 1,154-square mile drainage basin. The elevation of the bridge site is 4,520 feet and the mean elevation of the drainage basin is 6,000 feet.
|
From page 141... ...
Findings and applications 141 Figure 3.87. Flood frequency estimates for the SR-262 Bridge over Montezuma Creek based on USGS regional regression equations.
|
From page 142... ...
142 Scour at Bridge Foundations on rock estimate scour at the base of the headcut for discharge conditions associated with the 25-, 50-, and 100-year floods; they reported plunge-pool scour holes of 38 to 50 feet.
|
From page 143... ...
Findings and applications 143 Bridge are listed in Table 3.25, along with the calculated values of threshold, input, and applied stream power for the 100-year flood. The sandstone at the SR-262 Bridge over Montezuma Creek has the appearance of durable concrete; the Headcut Erodibility Index in Table 3.24 corresponds to a threshold energy of 1600 kW/ft for scour to occur.
|
From page 144... ...
144 Scour at Bridge Foundations on rock of 12.35 kW/m2. The hydraulic loading used in the Erodibility Index Method is calculated with procedures that differ from those used for the Headcut Erodibility Index; the Erodibility Index Method produces values that have conventional stream power units.
|
From page 145... ...
Findings and applications 145 compared to the Veronese equation results (38 to 50 feet) reported by HDR (2004)
|
From page 146... ...
146 Scour at Bridge Foundations on rock 3.9 Design and Construction Guidelines 3.9.1 Overview Guidelines presented in this section are subdivided into (a) those related to evaluating rock scour and developing design parameters and (b)
|
From page 147... ...
Findings and applications 147 for nearby bridges may be appropriate to consider in the evaluation of some bridge sites, particularly for new bridges. Field observations were discussed in Section 3.8.2; the field investigation subtasks are listed in Table 3.28.
|
From page 148... ...
148 Scour at Bridge Foundations on rock Table 3.29. Phase 3: Guideline tasks and subtasks for laboratory testing of rock scour.
|
From page 149... ...
Findings and applications 149 accepted as the channel-forming discharge, could be assumed to be the "effective" discharge (e.g., Sacramento River in Section 3.8.7.2)
|
From page 150... ...
150 Scour at Bridge Foundations on rock Montezuma Creek (GSN = 0.001 ft/unit of stream power in U.S. Customary Units; Figure 3.90)
|
From page 151... ...
Findings and applications 151 Table 3.31. HEC-18 Step 3, Substep 1, Part 4 with rock scour comments.
|
From page 152... ...
152 Scour at Bridge Foundations on rock most appropriate for local pier scour. The repeated cross sections used to estimate the amount of scour for calculating empirical scour numbers typically measured depths to the channel at different places along the bridge for each cross section, leading to approximate scour-depth estimates.
|
From page 153... ...
Findings and applications 153 HEC-18 Step 6 Evaluate the bridge type, size, and location on the basis of the scour analysis performed in HEC-18 Steps 3 through 5. Rock Scour Comments Engineering geologic condition and complexity of the bridge site and the dominant mode of rock scour are primary considerations.
|
From page 154... ...
154 Scour at Bridge Foundations on rock durability test drums should be appropriate for estimating a geotechnical scour number. Consequently, the procedures developed for degradable rock material may be useful for cohesive soil deposits, also.
|
From page 155... ...
Findings and applications 155 3.9.4 Constructing Bridge Foundations on Rock Bridge-foundation construction guidelines used by DOTs were requested during the course of this research. The few responses that were received pertained to guidelines for conducting geotechnical investigations.
|
From page 156... ...
156 Scour at Bridge Foundations on rock Two basic rock conditions can exist at bridge foundations as follows: 1. Rock is durable and strong; if the rock is durable, then • Rock is unfractured or it is fractured into large blocks, or • Rock is fractured into small fragments or 2.
|
From page 157... ...
Findings and applications 157 Construction considerations for rock foundations of all types are included in the USACE Engineering and Design Manual EM 1110-1-2908 (USACE, 1994b, Chapter 11)
|
From page 158... ...
158 Scour at Bridge Foundations on rock Peak hydraulic parameters are needed for assessment of quarrying and plucking of durable rock fragments, which are the same parameters needed for evaluating scour of sand-bed channels. The geotechnical data needed for durable rock scour quarrying and plucking are the sizes and shapes of rock blocks and the nature and orientations of the fracture surfaces in terms of the resistance provided to block removal by turbulence intensity fluctuations.
|
From page 159... ...
Findings and applications 159 will be needed by geotechnical engineers and engineering geologists for implementing the rock scour procedures for evaluating existing bridges than normally is required for the HEC-20 and HEC-18 procedures. An important task to facilitate future evaluation of scour performance is careful measurements of the channel position on a repeated basis (e.g., annual, biennial, or triennial)
|
From page 160... ...
160 Scour at Bridge Foundations on rock TRB can play a leading role in disseminating the results of this research to the target audience through its annual meetings and committee activities, publications such as Transportation Research Record, and periodic bridge conferences. AASHTO is the developer and sanctioning agency for standards, methods, and specifications.
|
From page 161... ...
Findings and applications 161 3.10.6 Criteria for Success The best criteria for judging the success of the implementation plan will be acceptance and use of the guidelines and methodologies that result from this research by state highway agency engineers and others with responsibility for design, maintenance, rehabilitation, or inspection of highway facilities. As of mid-2011, two states (West Virginia and Wisconsin)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.