New Bridges with Known Foundations
Evaluate new bridges with known foundations for potential scour in accordance with the following:
- Guidelines outlined in Evaluating Scour at Bridges (HEC-18).
- Do notuse abutment scour equationsbecause none of the equations to date yield acceptable results.Use contraction scour equations to calculate total scour at abutments.Protect abutments against potential scour through use of a flexible revetment, where possible.
Determine scour at bridges using the following
analysis techniques
:
- Use the following table to determine susceptibility of competent rock to scour when it is present at moderate to shallow depths. Consider materials deemed either not susceptible or mildly susceptible to scour the limit of the maximum scour depth.Table 5-4: Material Susceptibility to ScourMaterialSubtypeTCP ValuesSusceptibilityRockHard (granite, limestone, shale)< 4 in./100 blowsNot susceptibleSoft (shale)< 12 in./100 blowsMildly susceptible but not considered over time span of one flood eventClaysHard (redbed, shaley clays, very stiff clays)< 12 in./100 blowsMildly susceptible but not considered over time span of one flood eventSoft to medium> 12 in./100 blowsSusceptible to scour at a moderate rateSandsAllAllVery susceptible
- Monitor shales and stiff clays for long-term degradation. Shales and stiff clays tend to break down and disintegrate when exposed to repeated wetting and drying, a major problem in northeast Texas where head cutting in the Sulphur River basin has resulted in the channels down-cutting into the shale. The rate of degradation of shale in this situation is typically on the order of inches per year. As a result, most shales and stiff clays are not considered susceptible to scour during a single flood event. Consider long-term history of channel cross sections when evaluating these materials.
- For channels in cohesionless materials, such as sand and gravel, calculate contraction and pier scour using the following methods:
- Contraction scour: use the equations in HEC-18.
- Pier scour: use either the equations in HEC-18, Froelich’s Equation, or Sheppard’s Equations.
- For channels in cohesive materials, such as clay, calculate contraction and pier scour using the following methods:
- Limit d50to 0.2 mm (6.56 x 10-4 ft or 7.87 x 10-3 in). For contraction scour, use the equations in HEC-18. For pier scour, use the equations in HEC-18 with a reduction factor of 0.5 for soils with 11% or more clay.
- Use the SRICOS Method.Refer to NCHRP Research Report 915 for determination of erodibility parameters.
- Use Annandale’s Erodibility Index Method.
- For channels in layered soil, calculate scour using the following methods:
- Conduct a scour analysis layer by layer using themethodsspecified above for individual layers. If thecalculatedscourof a given layeris greater than the thickness of the layer, remove that layer and recalculate the hydraulic variables. Then continue the scour analysis with the next layer.
- Use the SRICOS Method.Refer to NCHRP Research Report 915 for determination of erodibility parameters.
- Use Annandale’s Erodibility Index Method.
Because of conservatism built into
analysis techniques
for calculating scour and limitations and gaps in existing knowledge, apply engineering judgment when using results from scour analyses
.Before using the scour analysis for bridge foundation design, check the scour predictions to ensure:
- the scour calculations account for layered soil/rock profiles.
- the scour calculations account for the soil/rock properties (that is, clay, silt, sand, gravel, rock, etc.)
- thecalculatedscour depths do not extend into competent rock.
Determine if the
calculated scour exceeds
the typical disregard depth of 10 feet from the channel flow line. If so, use the following to evaluate the calculated
scour:
- Performance of the existing structure during past floods (compare historic data of cross section changes at the bridge with the scour predictions).
- Hydrologic characteristics and flood history of the stream and similar streams.
- Recalculation of the scour analysis using a step-wise procedure that incrementally removes material and recalculates the required hydraulic variables. This may decrease the total scour depth.
Upon completion of a scour
evaluation
for a new bridge, the
(Form 2605) is to be completed and entered into the bridge inspection system, along with scour evaluation documentation. All Bridge Division and District contracts for bridge design and plan preparation should include a submission of completed Form 2605 and a full scour evaluation report as a deliverable
. In addition, the results of the scour evaluation
are to be used to determine the Bridge Inspection Coding for Item 113 – Scour. The results of the coding need to be entered into the BIMS
.Do not show scour depths on the Bridge Layout Elevation View. All pertinent scour information must be listed on the Scour Summary Sheet in the bridge inspection system.