Proceedings of the 21st Annual Members' Conference, 1996, San Francisco, CA, (DFI)
Micropiles for Seismic Stabilization of West Emerson Street Viaduct
Tom A. Armour, PE, Thomas M. Gurtowski, PE, Carole L.B. Mitchell, PE & Dennis Gathard, PE
The structural analysis of the West Emerson Street Viaduct in Seattle, Washington, indicated that a seismic retrofit of the existing foundations at Bents 7 through 11 was necessary to restrain excessive overturning moments and lateral forces. Battered micropiles proved to be the most technically feasible means for resisting these overturning moments and lateral forces considering the low headroom and tight access conditions. This project constitutes the first use of micropiles for a bridge seismic retrofit in Washington State. The foundation retrofit of Bents 7 through 11 included a total of 40 micropiles approximately 19 meters (m) in length. The piles were designed to support an ultimate seismic loading of 33.7 kN per pile in compression and tension. The specifications required the load testing of a sacrificial pile in tension only prior to installation of production piles to verify the contractor designed micropile system. The design and construction of the foundation support retrofit were complicated by liquifiable soil conditions, unknown location of the existing timber piles, an adjacent 3.5 m diameter concrete sewer main running parallel along the entire bridge alignment, limited overhead and existing structure clearances and the proximity of the main spur of the Burlington Northern Railroad (BNRR) between Bents 4 and 5. Results of the pile load test program showed that the micropile axial stiffness was less than the specified 15.2 mm at the design tension load of 33.7 kN. The pile creep displacement at 130% of the design tension load was approximately half of the maximum specified value and the test pile successfully supported the maximum test load of 43.8 kN in tension.
|article #93; publication #18 (AM-1996)|