Risk-Based Management Guidelines for Scour at Bridges with Unknown Foundations (2007) / Chapter Skim
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Pages 43-55
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From page 43... ...
NCHRP 24-25 Page 43 Phase II Final Report 4. MITIGATING ACTIONS FOR SCOUR This section summarizes recent findings from States that have experience in mitigating scour for bridges with unknown foundations, and the three basic types of mitigating actions for scour: 1.
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NCHRP 24-25 Page 44 Phase II Final Report structural, and monitoring. The latest scour monitoring techniques and associated costs are described as well as the cost of armoring of the bed with riprap and a manmade product.
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NCHRP 24-25 Page 45 Phase II Final Report minimum depth of a pier or footing. For example, it may be known that historic practice entailed installing piers down to a specified depth below a certain bedrock (or fill)
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NCHRP 24-25 Page 46 Phase II Final Report other surface methods that are subsequently proven to be accurate based on a comparison with the Parallel Seismic results may then be used with greater confidence to evaluate unknown foundation depths of other abutments and/or piers on a bridge. It should be noted that as local experience is gained with the use of any of the borehole or surface NDE methods for typical local bridge substructure types and subsurface conditions, the accuracy and applicability of the methods will become much better known to DOT engineers.
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NCHRP 24-25 Page 47 Phase II Final Report Table 15 Effectiveness of NDT Methods Ability to Identify Foundation Parameters Sonic Echo (SE) /Impulse Response (IR)
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NCHRP 24-25 Page 48 Phase II Final Report Ability to Identify Foundation Parameters Sonic Echo (SE) / Impulse Response (IR)
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NCHRP 24-25 Page 49 Phase II Final Report 4.3. Scour Monitoring Scour monitoring provides early identification of potential scour problems to reduce the potential for bridge failure.
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NCHRP 24-25 Page 50 Phase II Final Report costs include the basic instrument mounting hardware, power supply data logger, and instrument shelter/enclosure. Adding a cell-phone based telemetry link to the system adds approximately $3,000 to the cost.
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NCHRP 24-25 Page 51 Phase II Final Report used when a greater area coverage is required (multiple bridges and multiple piers) , and geophysical instruments are used as a forensic tool.
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NCHRP 24-25 Page 52 Phase II Final Report Table 19 Comparison of Portable Instrumentation Instrument Best Application Advantages Disadvantages Physical Probes Small bridges and channels Simple technology Accuracy, high flow application Sonar Larger bridges and channels Point data or complete mapping, accurate High flow application Reflection Seismic Profilers Larger bridges and channels and coastal environments Accurate map of the bottom and subbottom in water depths on the order of hundreds of feet Expensive, must be submerged, data contamination by bridge piers etc. Ground Penetrating Radar Small to medium bridges and in freshwater channels Accurate map of the bottom and subbottom on sand bars and to water depths on the order of 30 feet, samples under good conditions Expensive, post processing, water depth exceed 30 ft, saline waters, clay Table 20 Estimated Instrument Cost Instrument Instrument Cost Cost for Installation or Use Operation Cost Physical Probes <$2,000 Varies by use Varies, minimum 2person crew for safety Portable Sonar $500 (fish finder)
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NCHRP 24-25 Page 53 Phase II Final Report 4.4. Scour Countermeasures Guidelines developed for management of bridges with unknown foundations will undoubtedly include some protocol to implement countermeasures against scour.
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NCHRP 24-25 Page 54 Phase II Final Report Table 21 Stone Riprap weights Weight Maximum a kg b [lbs] Weight 50% c kg [lbs]
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NCHRP 24-25 Page 55 Phase II Final Report out from the pier in all directions. Based on the costs presented in Table 23, the costs of riprap and cabled block for various pier widths and lengths are presented in Table 26.
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