The report examines the cause of a differential settlement between the highway approach embankments and bridge decks and abutment tilting at a bridge site located on the Bluegrass Parkway in Kentucky. The site contains several design, construction and maintenance features and soil types that are typical of many bridge sites in Kentucky. The approach embankments are side-hill fills. The approach pavements have settled several inches and have been patched on numerous occasions. Each of the four abutments give the appearance of having tilted backward toward the backfill. However, closer examination shows that the abutments have moved laterally toward the bridge ends and are bearing against the steel girder spans. Slope inclinometer readings obtained at each approach embankment over a two-year period shows that the approach fills have moved toward the bridge ends some three inches. The front portion of the western fill failed in 1969, some four years after construction, and exposed several feet of piling. Shear strength of the embankment and foundation soils were established from consolidated isotropic, undrained triaxial tests with pore pressure measurements and consolidated drained, direct shear tests. Slope stability analyses were performed using a computerized solution of Bishop's simplified method of slices. Study results show that the approaches settled as a result of embankment instability due to progressive failure. Based on peak shearing strength of the soils, the long-term safety factors of the approach embankments ranged from 0.96 to 1.03. Based on residual strength, the long-term safety factors were about 0.70. Consequently, the western embankment failed apparently as a result of a (time-conditioned) gradual decrease of the shear strength of the soils. Slope indicator data indicate that progressive failure is spreading; movement of the embankments is continuing. Stability of the approach fills, abutments and piles is precarious. Several remedial solutions are described.

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No. 356

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The contents of this report reflect the views of the author who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Department of Highways or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation.