FLOOD04125

• County: Statewide
• Basin: Statewide
• Title: Interim Procedures for Evaluating Scour at Bridges
• Date: 9/1/1988
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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7. Perform the foundation analysis for the bridge on the basis that all stream
bed material in.the scour prism above the total scour line (step 4) has
been removed and is not available for bearing or lateral support. All
foundations should be designed in accordance with the AASHTO Standard
Specifications for Highway ~ridges. In the case of a pile foundation, the
piling should be designed for additional lateral restraint and column
action because of the increase in unsupported pile length after scour. In
areas where the hole formed by local scour is confined to the proximity of
the footing, the lateral ground stresses on the pile length which remains
embedded may not be significantly reduced from the pre-local scour
conditions. The depth of local scour and volume of soil removed from above
the pile group should be considered by geotechnical engineers when
computing pile embedment to resist vertical load.

a. Spread Footinqs On Soil.

o Place the top of the footing below the design scour line from
step 4.

o Check to make sure that the bottom of the footing is at least 6.0
feet below stream bed as per AASHTO standards.

b. Spread Footings On Rock Highly Resistant To Scour.

Place the bottom of the footing directly on the cleaned rock surface
for massive rock formations (such as granite) that are highly resistant
to scour and corrosion. Small embedments (keying) should be avoided
since blasting to achieve keying frequently damages the subfooting rock
structure and makes it more susceptible to scour. If footings on
smooth massive rock surfaces require lateral constraint, steel dowels
should be drilled and grouted below the footing level.

c. Spread Footings On Erodible Rock.

Weathered or other potentially erodible rock formations need to
be carefully assessed for scour. An engineering geologist who
Is familiar with the area geology should be consulted to
determine if rock or soil or another criteria should be used to
calculate the support for the spread footing foundation. The
decision should be based on an analysis of intact rock cores
including rock quality designations and local geology as well
as hydraulic data and anticipated structure life. An important
consideration may be the existence of a high quality rock
formation below a thin weathered zone. For deep deposits of
weathered rock, the potential scour depth should be estimated
(steps 4 and 6) and the footing base placed below that depth.
Excavation into weathered rock should be made with care. If
blasting is required, light, closely spaced charges should be
used to minimize over break beneath the footing level. loose
rock pieces should be removed and the zone filled with lean
concrete. In any event the final footing should be poured in
contact with the sides of the excavation for the full designed'

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