Contraction Scour at Bridges Founded on Clay Soils

by W. R. Ivarson,
F. Qadir,
M. Phelps,

Abstract: With the federally mandated bridge scour program deadline of January 1997 approaching, scour evaluations are in full progress in many states around the country. A common physical situation at bridge sites is that bridge piers and abutments are founded on piles or pile-bent foundations, which are driven through alluvial sands and gravels into an underlying clay strata. Most of the well known literature does not address the scourability of such clay material in a quantitative way, and therefore, the engineer performing the scour analysis is left to his own judgement regarding how to estimate scour depths and rate the bridge. This usually results in a conservative assumption that the clay will behave like fine sand with the resulting rating of the bridge as scour critical or scour susceptible. The authors propose the use of, and have successfully used, a 1963 study by the USGS to provide guidance in quantifying the erodibility of clay soils. The USGS study in combination with evaluating bridge contractions similar to Laursen derivation of clear-water contraction scour equation for granular soils, results in a clay soil clear-water contraction scour equation which gives more realistic estimates of contraction scour in clay soils.

Subject Headings: Scour | Granular soils | Hydraulic contraction | Federal government | Bridge tests | Bridge abutments | Bridge foundations

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